# Origin of Biological Homochirality by Crystallization of an RNA Precursor on
a Magnetic Surface

S. Furkan Ozturk  Ziwei Liu  MRC Laboratory of Molecular Biology, Cambridge
Biomedical Campus, Cambridge CB2 0QH, UK  John D. Sutherland  MRC Laboratory
of Molecular Biology, Cambridge Biomedical Campus, Cambridge CB2 0QH, UK
Dimitar D. Sasselov  Department of Astronomy, Harvard University, Cambridge,
MA 02138, USA

###### Abstract

Homochirality is a signature of life on Earth yet its origins remain an
unsolved puzzle. Achieving homochirality is essential for a high-yielding
prebiotic network capable of producing functional polymers like ribonucleic
acid (RNA) and peptides. However, a prebiotically plausible and robust
mechanism to reach homochirality has not been shown to this date. The chiral-
induced spin selectivity (CISS) effect has established a strong coupling
between electron spin and molecular chirality and this coupling paves the way
for breaking the chiral molecular symmetry by spin-selective processes.
Magnetic surfaces can act as chiral agents due to the CISS effect and they can
be templates for the enantioselective crystallization of chiral molecules.
Here we studied the spin-selective crystallization of racemic ribo-
aminooxazoline (RAO), an RNA precursor, on magnetite (Fe3O4) surfaces,
achieving an unprecedented enantiomeric excess of about 60$\%$. Following the
initial enrichment, we then obtained homochiral crystals of RAO after a
subsequent crystallization. Our work combines two necessary features for
reaching homochirality: chiral symmetry-breaking induced by the magnetic
surface and self-amplification by conglomerate crystallization of RAO. Our
results demonstrate a prebiotically plausible way of achieving systems level
homochirality from completely racemic starting materials.
Keywords: Homochirality, CISS effect, ribo-aminooxazoline, magnetite,
conglomerates

Understanding the origins of biomolecular homochirality is essential for
understanding the origins of life, and the origin of homochirality remains a
long-standing mystery since Pasteur discovered the molecular asymmetry of
organic compounds in 1848 [1]. Achieving a homochiral state early in the
prebiotic synthesis of the monomers would be very beneficial for successful
polymerization and the overall robustness of the entire synthetic network [2,
3]. The prebiotic need for high yields combined with high selectivity requires
a persistent and well-matched pair of a chiral symmetry-breaking agent and an
amplification mechanism.

One of the central molecules for prebiotic synthetic networks emerged in 1970
from the work of Sanchez and Orgel [4], who identified the aminooxazolines
(ribo, arabino, xylo and lyxo) as novel organic intermediates useful for the
synthesis of a wide variety of nucleotides, the monomers of nucleic acids (RNA
and DNA). Follow-up studies [5, 6] culminated in the prebiotic synthesis of
pyrimidine nucleotides [7] with aminooxazolines as the precursors, and the
consequent development of an entire synthetic network for nucleotides and
amino acids by the Sutherland lab [8]. Blackmond and coworkers emphasized
ribo-aminooxazoline (RAO)’s centrality in solving the origin of homochirality
due to its crystallization properties [9] but the search for a prebiotically
plausible mechanism that can induce and amplify a chiral bias has remained to
be an open problem.

Biological systems comprise many homochiral molecules. In principle such a
state could be achieved by separately resolving each individual chiral
compound. However, a more attractive solution would be to establish the
homochirality in a compound from which propagation of homochirality to the
whole system could occur. Emergence of homochirality at the stage of RAO sets
the stage for the propagation of homochirality through RNA to peptides and
thence through enantioselective catalysis to metabolites.

In our recent work, we proposed a symmetry-breaking agent that can trigger
such an amplification in the reduction reactions leading up to the production
of the chiral 3-carbon sugar glyceraldahyde (itself the precursor to RAO). Our
proposed mechanism employs the strong coupling between the electron spin and
molecular chirality as established by the chiral induced spin selectivity
(CISS) effect [10]. We identified evaporative lakes with authigenic iron-oxide
sediments as prebiotically plausible settings [11] in which enantioselective
reduction reactions can occur close to the magnetized surface by spin-
polarized photo-electrons released from the magnetite by solar UV light.

In this work, we demonstrate that the spin-polarized surfaces themselves are
chiral agents breaking the chiral symmetry at the surface, in a further
expansion of the role of the CISS effect, and the simultaneous conglomerate
crystallization provides the necessary amplification to reach a homochiral
state (Figure 1). Here we report the spin-selective homochiral crystallization
of the ribonucleotide precursor, ribo-aminooxazoline (RAO), on magnetite
surfaces, from its completely racemic solution (Figure 2).

sec1.sectionsec1.sectionHomochirality of RNA and the Central Role of
RAOHomochirality of RNA and the Central Role of RAOsec1.section

## Homochirality of RNA and the Central Role of RAO

RNA is thought to have played two major roles in the origin of life. The
sequence of the nucleobases attached to the sugar phosphate backbone
constitutes genetic information which can be passed from generation to
generation by replication via a complementary strand through Watson-Crick base
pairing. The sequence of an RNA also dictates its shape, and it is the
adoption of a wide variety of shapes that endows RNA with its catalytic
ability. Enantiomeric purity of the nucleotide components of RNA is crucial to
both its roles. Replication via a complementary strand proceeds by way of an
A-form duplex and this is not possible if the nucleotide components deviate
significantly from enantiomeric purity. Incorporation of a nucleotide of
opposite handedness into an RNA strand changes its shape and this might endow
a new or improved catalytic ability, but the chirality switch cannot be passed
to subsequent generations by replication, so the potentially beneficial change
is non-hereditable. Establishing a mechanism whereby the nucleotide building
blocks of RNA might have been synthesized enantiomerically pure is thus
crucial to understanding how Earth’s RNA-based life originated. Peptides too
depend on the enantiomeric purity of their component amino acids. In extant
biology, the correlation of D-ribonucleotides with L-amino acids is
established in two principal ways. Ribonucleotides and chiral amino acids are
biosynthesized with high stereoselectivity and L-amino acids are attached to
transfer RNA (tRNA) composed of D-ribonucleotides by stereoselective
aminoacyl-tRNA synthetases. The idiosyncratic behavior of amino acids makes
the synthesis of all L-amino acids under early Earth conditions a daunting
challenge, but recent findings suggest that this hurdle might be side-stepped.
Prebiotically plausible chemical means of attaching amino acids to tRNA
analogs have been discovered and proceed with high-level control of relative
stereochemistry. Thus, for example, the L- over D- stereoselectivity for
attachment of alanine to a tRNA acceptor stem mimic composed of
D-ribonucleotides is of the order of 10:1 [12]. This suggests that the overall
homochirality problem of prebiotic chemistry might be reduced to the problem
of making ribonucleotides in enantiopure form—racemic amino acids might
suffice.

RAO emerged as a potentially important ribonucleotide precursor over 50 years
ago, but there remained several issues to be dealt with before its true
potential was realized. Sanchez and Orgel showed that RAO (which they
synthesized from ribose and cyanamide) was a highly crystalline compound which
underwent reaction with cyanoacetylene to generate $\alpha$-cytidine, but
subsequent conversion to the natural $\beta$-anomer was very inefficient [4].
Furthermore, the synthesis of ribose is one of the persistent challenges of
prebiotic chemistry – even though the formose reaction is now very well
understood, it still cannot be controlled to produce more than a trace of
ribose as part of a complex mixture. Nevertheless, the attractiveness of RAO
prompted further research and Joyce showed that it could be sequestered from
simpler mixtures of sugars than those made by the formose reaction by reaction
with cyanamide followed by crystallization [5]. Joyce also reported more on
the crystallization behavior of RAO and showed that it crystallized in the
chiral $P2_{1}2_{1}2_{1}$ space group and that crystals grown from solutions
of rac-RAO were composed of twinned clusters with individually homochiral
domains. The problem with the provenance of ribose was solved when Sutherland
showed RAO could be obtained, along with lesser amounts of the other pentose
aminooxazolines, by reaction of glyceraldehyde with 2-aminooxazole, the latter
itself deriving from the reaction of glycolaldehyde with cyanamide [6]. The
same group also found that non-racemic solutions of RAO gave rise to crystals
with increased enantiomeric excesses (ee’s) and above a threshold ee,
enantiopure crystals resulted. This behavior is consistent with a conglomerate
susceptible to twinning. The conglomerate nature of RAO was finally proven by
Powner [13]. Then high-yielding prebiotic syntheses of ribonucleotides from
aminooxazolines were reported by Sutherland. First, it was shown that arabino-
aminooxazoline could be elaborated to pyrimidine nucleoside cyclic phosphates
[7], then it was shown that RAO could be converted to pyrimidine nucleotides
by a reaction sequence involving a photochemical inversion of the anomeric
stereocentre [14]. In the meantime, Blackmond had shown that enantiomerically-
enriched proline could participate in the reaction of 2-AO with glyceraldehyde
and give enantiopure RAO after crystallization [9]. What has remained elusive
however is how racemic compounds could give rise to enantiopure RAO purely by
means of a process controlled only by the environment.

Figure 1: The mechanism of spin-selective crystallization due to the CISS
effect and the experimental setup. a. As molecules approach a surface, they
transiently acquire an induced charge polarization. Due to the CISS effect,
transient charge polarization of a chiral molecule is accompanied by spin
polarization. The spin state associated with the charge poles is determined by
the handedness of the chiral molecule. Because the magnetic surface itself is
spin polarized it kinetically favors (akin to a seed crystal) the enantiomer
whose transient spin state results in a lower energy spin-exchange
interaction. The lower energy overlap with the magnetic surface is singlet-
like (red, $\uparrow\downarrow$) and the higher energy overlap is triplet-like
(blue, $\uparrow\uparrow$). The energy difference between these two
configurations is higher than the room temperature, $k_{B}T$, therefore the
effect robustly manifests itself. b. Schematic of the setup used in the
crystallization experiments and a sample microscope image of the RAO crystals
on a magnetite surface from a direct crystallization experiment. The image
shows the magnetite surface as the black background and the needle-shaped
conglomerate crystals of RAO formed on the surface, as well as the twinned
crystals with stochastically arranged needles of D and L-RAO, and racemic RAO
in the form of a flaky powder suspended in the water column above the surface.

sec2.sectionsec2.sectionChiral Induced Spin Selectivity and Spin Selective
ChemistryChiral Induced Spin Selectivity and Spin Selective
Chemistrysec2.section

## Chiral Induced Spin Selectivity and Spin Selective Chemistry

The CISS effect has established a robust coupling of electron spin to
molecular chirality. The initial experiments have shown that the electron
transfer through a chiral monolayer is spin-dependent and the preferentially
transferred spin state depends on the handedness of the monolayer [15, 16].
The experiments have achieved near-perfect spin filtering at room
temperature—showing the robustness of the coupling [17]. Later work has proved
that the coupling established by the CISS effect can be manifested in various
ways from spintronic applications to long-range electron transfer in biology
[18, 19, 20]. More recent studies have shown that spin selective behavior
exists for freely diffusing molecules near magnetic surfaces [21, 22], and
even for achiral reagents [23, 24]. These studies have motivated us to
consider the spin-selective processes near magnetic surfaces due to the CISS
effect to impose a chiral bias on prebiotic chemistry.

Recently, we proposed closed-basin evaporative lakes with authigenic
ferrimagnetic sediments (e.g. magnetite, greigite) as plausible prebiotic
environments in which enantiospecific processes can be carried out due to the
CISS effect [10]. Our mechanism employed UV-ejected photo-electrons from
magnetite surfaces as chiral agents in reductive synthesis because of the
helical character of such electrons in the close vicinity of the magnetic
surface (Fig. 1. in [10]). We proposed that these helical electrons can
achieve kinetic resolution in the reduction of chiral molecules for which the
reaction rates for enantiomers differ by
$\exp\left(\frac{2H_{\text{SO}}}{k_{B}T}\right)$. Here, $H_{\text{SO}}$ is
defined as the effective coupling due to a combination of spin-orbit and spin
exchange interactions of a chiral molecule with an electron, $k_{B}$ is the
Boltzmann constant, and $T$ is the temperature (Fig. 2. in [10]). As a
suitable chemistry to apply our idea, we proposed cyanosulfidic chemistry
which uses photo-ejected hydrated electrons to drive the reductive synthesis
of sugars [8, 25]. In particular, we focused our attention on the spin-
selective reduction of the cyanohydrin of glycolaldehyde that is producing
glyceraldehyde, the first chiral sugar, and an RNA precursor—after the
hydrolysis of the resultant imine. Although this proposed reduction is still a
worthy experiment, the isomerization of glyceraldehyde to dihydroxyacetone and
the cyanide liberating equilibrium of the reagent cyanohydrin, make it a
harder reaction to study. Moreover, freely diffusing molecules with a single
chiral center are likely to display less enantioselective behavior due to the
reduced coupling of the electron spin to the molecular frame. Therefore, in
this work, we studied the crystallization of the stable RNA precursor, RAO, as
a static process with higher selectivity, on a magnetic surface.

We should emphasize that, if realized, enantioselective synthesis of
glyceraldehyde reinforces the spin-selective crystallization of RAO as
glyceraldehyde’s chirality directly determines the chirality of RAO as can be
seen in the blue box in Figure 3b. However, the crystallization process does
not rely on starting from an enantioenriched solution of RAO, therefore our
results presented here are self-sufficient to achieve a homochiral RNA world.

sec3.sectionsec3.sectionMagnetic Surfaces as Chiral AgentsMagnetic Surfaces as
Chiral Agentssec3.section

## Magnetic Surfaces as Chiral Agents

Magnetic surfaces themselves can be chiral agents due to the CISS effect and
surface processes such as crystallization or adsorption on a magnetic surface
can be enantioselective. As demonstrated by the early CISS experiments,
electron flow from a surface through a chiral monolayer is spin-dependent. The
same effect manifests itself when a chiral molecule acquires an induced charge
polarization because the latter is nothing but a transient electron flow
inside a chiral potential. And due to the CISS effect, charge polarization is
accompanied by spin polarization. This transient, induced charge polarization
can be due to intermolecular interactions among chiral molecules or between a
chiral molecule and a surface [19, 26].

When a chiral molecule approaches a surface, the electron density of the
molecule re-distributes itself which gives rise to an induced charge dipole.
And for a chiral molecule, this transient electron flow is spin-dependent due
to the CISS effect and gives rise to a transient spin polarization as shown in
Figure 1a. This spin polarization is realized along the chiral molecular axis
and it is dependent on the handedness of the chiral molecule. In Figure 1a,
the right-handed (D) enantiomer has the minority spin ($\downarrow$) on its
positively charged pole ($\delta^{+}$), whereas the left-handed (L) one has
the majority spin ($\uparrow$). This is how the chiral symmetry is broken by a
magnetic surface which is itself spin polarized: the spin exchange interaction
between the surface electron spins and the transiently spin-polarized chiral
molecules is higher or lower depending on the handedness of the molecule. The
favorable, lower energy interaction with the surface corresponds to a singlet-
like ($\uparrow\downarrow$), and the penalized, higher energy interaction
corresponds to a triplet-like ($\uparrow\uparrow$) overlap of the spins.
Therefore, a spin-polarized surface kinetically traps an enantiomer based on
its spin polarization and breaks the chiral symmetry.

Enantioselective behavior on magnetic surfaces due to the CISS effect has been
demonstrated with the adsorption of chiral molecules like double-strand DNA
and L-cysteine on magnetized ferromagnetic films [27]. Moreover,
enantioseparation on magnetic surfaces by conglomerate crystallization has
been realized with several amino acids with high efficiency [28, 29].
Especially, these studies lay the groundwork for the presented results. We
apply the same idea using a different molecule on a different magnetic
substrate relevant to the prebiotic conditions. And similarly, we use the
magnetic surface analogous to a crystal seed, breaking the chiral symmetry by
promoting the crystallization of one enantiomer. It should be noted that this
is a kinetic-entrainment-like phenomenon and not a thermodynamic effect.
Therefore, if all molecules are allowed to crystallize the selective effect
cannot be observed.

Naturally, the CISS-driven interaction between chiral molecules and magnetic
surfaces works both ways around: a chiral molecule can also selectively spin
polarize a magnetic surface and the direction of this polarization is
determined by the handedness of the chiral molecule. This effect has also been
demonstrated by the chirally selective magnetization switching in a
ferromagnet upon the adsorption of chiral molecules [30]. In that work, Ben-
Dor et al. have shown that chiral molecules can robustly affect the
magnetization of surfaces due to the long-range spin-exchange interaction.
Chirality-induced magnetization switching combined with the presented results
pave the way for positive feedback between chiral molecules and magnetic
surfaces which can purify the magnetization of the iron-oxide sediments and
thus promote the homochiral crystallization of RAO.

sec4.sectionsec4.sectionFeedback Between the Magnetic Surface and RAOFeedback
Between the Magnetic Surface and RAOsec4.section

## Feedback Between the Magnetic Surface and RAO

Utilizing the CISS-based interaction between magnetic surfaces and chiral
molecules, it is possible to envision a positive feedback loop between the
magnetite surfaces and RAO in a prebiotic setting as shown in Figure 3b. In a
wide range of natural depositional environments, authigenic magnetite
sediments form and magnetize under the Earth’s field, resulting in a chemical
remanent magnetization (CRM) [31]. Sediments that acquired a chemical
remanence after the most recent geomagnetic reversal are expected to have a
statistically uniform remnant magnetization on a hemisphere scale within a
shallow depth interval [32]. This initial natural bias of the magnetic domains
can break the chiral symmetry if racemic RAO crystallizes on the magnetic
surface. However, due to the imperfect alignment of the magnetic domains, the
first crystallization attempt can only induce a small enantiomeric imbalance.
However, after the enriched crystals are dissolved due to a sporadic water
flow and re-crystallize on the magnetic surface again, the selectivity will be
higher due to the asymmetric crystallization of RAO [6] and further magnetic
seeding due to the CISS effect (See Extended Fig. 1a). When these nearly pure
RAO conglomerates grow on the surface, they can simultaneously interact with
the surface spins and switch the magnetization of the surface. This process
can purify the magnetic domains along the chiral molecular axis of RAO. And
because these sedimentary rocks are no longer small superparamagnetic
particles, this CISS induced magnetic diagenesis can permanently lock the
surface spins along one direction, unless a large (much larger than Earth’s
field) coercive field is applied. When the surface spins are locked once and
for all, they can induce efficient enantioselectivity for the upcoming
crystallizations and the magnetized surface can separate homochiral RAO
crystals in just a few cycles.

This feedback mechanism can be more effectively realized over a small area
such as the scouring zone of an incoming stream as shown in Figure 3a. At the
scouring zone, the authigenic magnetic sediments are exposed, wet-dry cycles
are more frequently realized (like a shore), and an incoming stream can
constantly feed the surface with racemic RAO. With the described feedback
mechanism, magnetized magnetite sediments at the scouring zone can filter out
homochiral RAO crystals from the incoming racemic stream.

Figure 2: Stereoselective and enantioselective crystallization of RAO. a. A
microscope image of the nearly enantiopure RAO crystals formed on a magnetite
surface from their racemic solution. b. Circular dichroism spectra of the
crystals formed on the magnetite surface. The red (blue) spectrum corresponds
to D-RAO (L-RAO) crystals formed when the magnetite surface is magnetized
parallel (anti-parallel) to the surface normal. Both spectra are obtained when
racemic RAO is re-crystallized and they show a cumulative ee of about $60\%$
for the whole surface. The black spectrum corresponds to crystallization on a
non-magnetic, silicon surface in the presence of a magnetic field and it shows
no selectivity. The latter control experiment shows that the enantioselective
effect is due to the spin-exchange interaction, not due to the applied
magnetic field. c. 1H NMR spectrum (400 MHz, H2O/D2O 90:10) before and after
the direct crystallization of RAO. Before the crystallization, the solution is
a mixture of four aminoxazolines: RAO/AAO/LAO(f for furanose, p for
pyranose)/XAO (42:30:18:10). After the crystallization, the redissolved
crystals only contain RAO within the limits of 1H NMR detection. Thereby, mere
crystallization on the magnetic surface stereoselectively and
enantioselectively purifies RAO.

sec5.sectionsec5.sectionFlow ChemistryFlow Chemistrysec5.section

## Flow Chemistry

The mechanism we have shown here benefits from the presence of flow and
streams as previously considered by Ritson et al.[33]. As such, we conceive of
the enantioselective crystallization of RAO on the scouring zone of an
evaporative lake by an incoming stream carrying the racemic solution of sugar
aminooxazolines. In this scenario, the crystallization takes place on the
shallow shores of the magnetite lake which can undergo multiple wet-dry cycles
allowing several crystallization-dissolution cycles of RAO. The flow also
scours the soft, muddy material off the surface and exposes the authigenic
magnetite sediments on which enantioselective processes can take place.

Moreover, while the flow feeds the magnetic surface with a racemic solution of
RAO it simultaneously washes the other enantiomer in the solution and the
racemic powder suspended in the bulk away from the zone and allows for higher
enantioselectivity.

The presence of flow also keeps the area around the crystal surface locally
racemic such that the depletion of one enantiomer does not lead to an increase
in the concentration and subsequent crystallization of the other. Therefore,
the flow maintains the enantioselective crystallization out of equilibrium,
preventing the system from reaching thermodynamic equilibrium. This is
crucial, as the enantioseparation mechanism we have shown is a kinetic effect
and the system would reach its racemic equilibrium if the solution is not
maintained racemic.

Finally, it is more convenient to conceive of the feedback loop we propose on
a smaller region, rather than the whole surface of the lakebed. With selective
crystallization and magnetization cycles, the spin purity of the magnetic
surface around the scouring zone can be locked with less material, and this
small bottleneck region over which the material flows can act as a chiral
filter for the incoming RAO. Nevertheless, achieving our enantioseparation
mechanism is not a fine-tuned geochemical scenario and one can come up with
other (or more refined) prebiotically plausible scenarios compatible with this
process. We, therefore, emphasize the robustness and the strength of our
mechanism rather than the specific geochemical scenario that can accommodate
the enantioseperation process.

Figure 3: An evaporative lake with magnetic sediments can accommodate spin-
selective processes between an RNA precursor and a magnetized surface. a. An
evaporative lake contains authigenic magnetite sediments magnetized by the
Earth’s magnetic field. An incoming stream with racemic aminooxazolines scours
the muddy material off the surface. As the lake evaporates D-RAO selectively
crystallizes on the magnetic surface. b. An incoming stream carries the
reaction products of racemic glyceraldehyde and 2-aminooxazole: racemic RAO,
AAO, XAO, and LAO. Initially, the magnetic sediments are authigenically
magnetized and carry a bias along the Earth’s field. This natural bias enables
the enantioselective crystallization of RAO which also sterically purifies RAO
from the mixture of aminooxazolines. After a small enantiomeric imbalance is
induced, sporadic water flow redissolves the RAO crystals and in a subsequent
dry phase, RAO recrystallizes on the surface with higher ee. As these nearly
pure conglomerate crystals accumulate more material, they cover a larger area
of the surface and flip the spin of magnetic domains along the chiral
molecular axis. This chirality-induced magnetization process increases the
magnetization of the surface and allows for a positive feedback loop between
the magnetic surface and the chiral crystals. As a result, the surface with
higher magnetization induces higher ee for the upcoming crystallizations, and
eventually, enantiopure D-RAO is obtained.

sec6.sectionsec6.sectionEnantioselective Crystallization of RAO on
MagnetiteEnantioselective Crystallization of RAO on Magnetitesec6.section

## Enantioselective Crystallization of RAO on Magnetite

We have studied the crystallization of RAO from its racemic solution on
magnetite surfaces. We fabricated the magnetite surfaces as thin films (200
nm) on silicon substrates following the procedure by Jubb and Allen [34]
(Supplementary Information Section 5). We used electron beam evaporation to
deposit 100 nm iron on Si (100) surface and then heated the sample at 175∘C
for 4 hours to promote the oxidation of iron, producing a magnetite film of
about 200 nm. We characterized the samples by Fourier-transform infrared
spectroscopy (FTIR) and confirmed the complete conversion of iron to magnetite
(Figure S5). We further analyzed the surface roughness and magnetic properties
of the samples by atomic force microscopy (AFM) and superconducting quantum
interference device (SQUID) (Supplementary Information Section 6). We placed
the magnetite surfaces horizontally in a Petri dish and placed a magnet just
below the surface as shown in Figure 1b. We used the magnet to spin-polarize
the magnetite surface and measured the magnetic field to be 325 mT at the
surface location. It is important to place the magnet such that the
magnetization direction is parallel to the surface normal to maximize the
helical character of the surface electrons.

The 1H NMR spectra confirmed the stereoselective crystallization of RAO, as
has been previously reported [5, 6]. Having identified the composition of the
crystals by NMR, we measured the chiro-optical properties by circular
dichroism (CD) spectroscopy. We obtained a non-zero CD signal for the
individual needle-shaped crystals of RAO, however, the rosette-shaped crystals
did not consistently give a CD signal. We further analyzed the rosette-shaped
crystals by X-ray crystallography and found that the individual arms of the
rosettes contain only one enantiomer yet the crystal as a whole is racemic.
Therefore, we found that the rosettes form due to the twinning of needle-
shaped homochiral domains. When we analyzed rosettes with less homochiral
domains we obtained a non-zero CD signal with a randomly varying sign,
however, the rosettes with more branching did not give a CD signal. We also
did a control experiment by crystallizing the enantiopure compound and still
observed the formation of rosette-shaped crystals. Therefore, we concluded
that the twinned, rosette-shaped crystals form due to a stochastic arrangement
of homochiral domains. Because this stochastic twinning occurs on an existing
crystal face, it nullifies the enantioselection seeded by the magnetic
surface. For the experiments in which we primarily obtained needles on the
surface (Figure S11), we observed a surface-wide ee (in addition to the ee of
the individual crystals), however, for most of the cases, we could not
consistently avoid the formation of the twinned crystals. Therefore, we did
not observe consistent selectivity for direct crystallization experiments on
magnetite surfaces. However, if the rosette formation can be avoided by the
change of conditions, such as concentration, buffer, or pH, it should be
possible to observe selectivity for direct crystallization.

We proceeded with the re-crystallization of RAO. We synthesized racemic RAO by
the reaction of D- and L-ribose with cyanamide on a large scale. We confirmed
that synthesized RAO is fully racemic with a CD measurement before the
crystallization experiments (Figure S17). We then prepared a 65 mM solution of
the racemic RAO in pure water and placed it on the magnetized magnetite
surface. We followed the same crystallization procedure described above and
obtained diamond-shaped crystals on the magnetite surface as shown in Figure
2a. We collected the crystals with tweezers, dissolved them in pure water, and
obtained their CD spectra. First individually and after for the whole surface.
We confirmed that diamond-shaped crystals of RAO are individually homochiral
by CD and X-ray diffraction measurements. These diamond-shaped crystals and
the individual needle-shaped ones both gave identical X-ray diffraction
patterns and they all belong to the $P2_{1}2_{1}2_{1}$ chiral space group.
However, with the diamond-shaped crystals, we did not observe twinning when
the crystallization is stopped soon after the crystals are observed. When we
waited longer, as the crystals grew in size we observed that a crystal face
became a seed for the other enantiomer. However, these twinned crystals mostly
contained two to three macroscopic, homochiral domains unlike the rosettes
containing many arms, for which each arm is an individual, homochiral domain.
Also, for the re-crystallization experiments, twinned diamond crystals
appeared well after the individual diamonds were formed and visible, therefore
we could stop the experiment before the twinning started to take over.
However, for the direct crystallization experiments, rosettes appeared almost
simultaneously with the needles so there was no slow progression of the
twinning we could control.

Having confirmed that diamond-shaped crystals are individually homochiral, we
dissolved all the crystals on the magnetite surface together in water and
obtained the CD spectrum for the whole surface. To our delight, not only the
individual crystals showed optical activity but also the entire surface did.
As the smoking gun of the CISS-driven phenomenon, when we reversed the
magnetic field direction (therefore the spin state of the surface electrons)
the observed CD signal reversed in sign. As shown in Figure 2b, left-handed
RAO crystals (blue curve) dominated the down-spin ($\downarrow$, south pole)
surface and the right-handed ones dominated the up-spin ($\uparrow$, north
pole) surface. We obtained an ee of about $60\%$ for the entire surface, for
both spin directions starting from a completely racemic solution of RAO. For
this step, it was crucial to collect the crystals early, as the
enantioselectivity of the magnetic surface is a kinetic entrainment effect and
beyond a certain point, ee decreased with the increasing crystallization
yields and due to enantiomorphous twinning. Crystals were usually visible
after several hours and we collected them after about 6 hours to a day. We
collected 48 and 16 individual crystals for the up-spin and down-spin
experiments respectively for the data shown in Figure 2b. Due to the high
number of crystals and the repeatability of the results (Extended Data Fig.
1b), we can rule out the presence of a net ee due to statistical fluctuations
of homochiral crystals.

We investigated the reason why re-crystallization of RAO gives crystals with
different morphology (diamond-shaped) compared to direct crystallization of
the reaction products of glyceraldehyde and 2-aminooxazole (needle-shaped) and
crystallized RAO with added aminooxazolines (AAO, LAO, and XAO) one by one. We
found that in the presence of XAO, RAO crystallizes as needle-shaped crystals
whereas AAO and LAO do not visibly modify the crystallization habit of RAO.
Moreover, we observed a concentration-dependent formation of the rosette-
shaped twinned crystals only when RAO is crystallized in the presence of XAO
(Extended Data Fig. 2a). At higher concentrations, when XAO is present in the
solution, RAO forms rosette-shaped crystals and the crystal morphology is
altered by the relative amount of XAO (Extended Data Fig. 2c), therefore we
conclude that XAO is a crystal habit modifier for RAO (See Scheme 1 in [35]).
We also found that at higher relative concentrations (e.g. 1:1) XAO is
embedded in the crystal lattice structure of RAO as an impurity with
$4.8(4)\%$ abundance, as detected by X-ray diffraction analysis (Figure S29).
Finally, we found that XAO affected the RAO crystallization only when the
relative stereochemistries matched (e.g. D-RAO with D-XAO)—we did not observe
any effect of XAO when we crystallized L-RAO with D-XAO. These findings show
the necessity of re-crystallization in order to observe enantioenrichment to
enantiopurity, as by removing XAO from the solution racemizing stochastic
twinning is circumvented.

As a control experiment, we recrystallized RAO on a non-magnetic, silicon,
surface in the presence of a magnetic field of the same strength and we
collected the crystals from the non-magnetic surface and obtained the black
spectra in Figure 2b. As seen, the surface with no net spin polarization does
not induce any enantioselectivity in the presence of a magnetic field, as
shown by Pasteur [1]. The control experiment confirms that the observed
selectivity is not due to the magnetic field but due to the spin-exchange
interaction. The control experiments on the non-magnetic surface and the
consistent flipping of the ee (See Extended Data Fig. 1b) with the flipping
magnetic pole direction ensure that the obtained ee is physical and not due to
the contamination of surfaces with chiral impurities.

We repeated the re-crystallization experiment on magnetite for both pole
directions multiple times and found that on average we can get around $35\%$
ee for the entire magnetite surface. We then considered this value as a
typical outcome from a racemic solution and then crystallized the enriched
crystals on the magnetic surface one more time. We found that above a starting
ee of about $25\%$ we can obtain completely enantiopure crystals (See Extended
Data Fig. 1a). Therefore, in just two crystallization steps on the magnetic
surface, we could achieve homochirality from a completely racemic mixture.

sec7.sectionsec7.sectionDiscussionDiscussionsec7.section

## Discussion

We have demonstrated an efficient mechanism to resolve racemic RAO, an
important RNA precursor, on a prebiotically available mineral surface,
magnetite $\ce{Fe3O4}$, and obtained homochiral crystals of RAO in two
crystallization steps.

Our mechanism features the two requisites to reach homochirality: chiral-
symmetry breaking by the spin-polarized surface due to the CISS effect and
self-amplification by conglomerate crystallization. The symmetry breaking by
magnetic surfaces is prebiotically plausible, non-destructive, and robust at
room temperature and in solution. Although the demonstrated symmetry-breaking
is a surface effect (2D), conglomerate crystallization allows for the
extension into the bulk (3D). Therefore, the attained selectivity at the
surface level can be seamlessly carried into the bulk of the solution.

Our mechanism requires a well-defined magnetic surface and the crystallization
on colloidal magnetic particles is not enantioselective. A detailed discussion
on this can be found in Supplementary Information Section 7.1. In light of
this fact, we consider sedimentary rock surfaces similar to those found in the
Gale crater [36], as opposed to colloidal muds, as likely sites to realize our
mechanism.

With the feedback effect we suggested, our mechanism offers a persistent and
deterministic chiral bias to prebiotic chemistry as opposed to a singular and
stochastic trigger. Also, by combining the symmetry-breaking with a well-
matched amplification mechanism we have shown a direct way to reach the
homochiral state from a completely racemic starting point.

The mechanism is a kinetic-entrainment-like effect and the magnetic surface
acts as a chiral seed based on its spin polarization direction. Due to the
kinetic nature of the effect beyond a certain point enantioselectivity will
decrease with increasing crystallization yields. Also, the effect is
concentration dependent and it works best around or below the solubility limit
of the compound. At higher concentrations, the selectivity goes down as the
eutectic equilibrium forces the crystallization. Therefore the slower the
crystallization process and the lower the concentration, the higher the
enantioselectivity. The low solubility of RAO in water allowed for obtaining
its crystals at low concentrations, therefore, we could achieve high
enantioselectivity in just one crystallization in comparison to previous work
by Tassinari et al. with highly soluble amino acids in water [28]. Our results
can be further improved if the magnetic surface is placed vertically such that
the crystals forming in the bulk do not fall on the magnetic surface and
reduce the selectivity, as demonstrated by Bhowmick et al. [29].

In addition, we used magnetite as the magnetic substrate with near unity spin
polarization at its Fermi level [37] in comparison to commonly used Ni/Au
substrates with lower intrinsic spin polarization (around $20\%$) and
protective gold layer further reducing the spin polarization [28, 29].
Therefore, we have shown that magnetite surfaces can be used effectively in
the CISS experiments due to their optimal spin-polarization properties.

Our work differs from the previous work by Tassinari et al. and Bhowmick et
al. in multiple ways. First, we achieve much higher ee’s due to the low
solubility of RAO and the higher spin polarization of magnetite. High ee’s
under pristine laboratory conditions are necessary to reach homochirality
under realistic conditions. Moreover, the crystallization of aminoacids
require their supersaturated solutions ($\sim 1$g/mL) in contrast to RAO that
is much less soluble ($\sim 0.01$g/mL) in water. Additionally, they achieve
the selective crystallization of glutamic acid and threonine only in
hydrochloric acid (HCl), asparagine being the exception. This harsh condition
is prebiotically implausible and HCl is known to dissolve magnetite, creating
iron chlorides. Finally, they show that for a given pole (e.g. north)
direction the selective crystallization of asparagine (L) yields the opposite
handedness for the cases of glutamic acid (D) and threonine (D). Therefore,
the pioneering work by Tassinari et al. and Bhowmick et al. establish the
feasibility of the enantioseperation mechanism using magnetic substrates,
however, they do not account for the origin of homochirality.

Although we used stronger magnetic fields to spin-polarize the magnetite
surfaces than what was likely available on early Earth, similar spin-
polarizations under weak fields can be achieved if magnetite forms as small
($\sim 100$ nm in diameter) superparamagnetic particles. Such authigenic
magnetite particles have been shown to possess remanent magnetizations of
above 10 kA/m even when they form under weak magnetic fields ($0.1$ mT
$\approx 0.08$ kA/m) comparable to the Earth’s [38]. Because our mechanism is
based on spin-exchange interaction rather than a magnetic field effect, the
figure of merit is not the magnetic field strength but the degree of spin
alignment (magnetization) at the active surface. Hence one should pay
attention to the prebiotically relevant remanent magnetizations available in
natural magnetic minerals rather than early Earth’s magnetic field strength.
Having said that, with the described feedback mechanism between the chiral
molecules and the magnetic surface even higher magnetizations can be realized.

RAO is not just a chiral molecule, it has a central role in the synthesis of
ribonucleotides and its chirality directly determines the chirality of RNA. It
is very stable against isomerization unlike sugars like ribose or
glyceraldehyde as well as against thermal degradation and UV damage. Therefore
it is a very suitable molecule to lock the chirality in prebiotic chemistry.
Combining this with the fact that it crystallizes as a conglomerate, RAO is an
ideal molecule to apply the described enantioseparation mechanism. However,
even though we chose RAO as the compound and magnetite ($\ce{Fe3O4}$) as the
magnetic surface, our mechanism is more versatile. It is applicable to any
chiral molecule forming conglomerate crystals (e.g asparagine, peptides, etc.)
and with any prebiotically available magnetic sediment (e.g. maghemite,
greigite, etc.).

sec8.sectionsec8.sectionAcknowledgmentsAcknowledgmentssec8.section

## Acknowledgments

The authors thank Oren Ben-Dor, Debkumar Bhowmick, Donna Blackmond, Roger Fu,
Stephen Mojzsis, Ron Naaman, and Jack Szostak for helpful discussions,
suggestions, and feedback. We thank Victor Loi for the AFM measurements. We
acknowledge the Center for Macromolecular Interactions at Harvard Medical
School for the use of their CD spectrometer and Dr. Kelly L. Arnett for
assistance. We acknowledge the Center for Nanoscale Systems (CNS) and the
NSF’s National Nanotechnology Coordinated Infrastructure (NNCI) for the use of
the Sharon EE-3 E-Beam Evaporator. We acknowledge the Laukien-Purcell
Instrumentation Center for the use of their NMR facility and Dongtao Cui for
the assistance. We acknowledge Harvard X-ray Laboratory supported by the Major
Research Instrumentation (MRI) Program of the National Science Foundation
(NSF) under award number 2216066 and Dr. Shao-Liang Zheng for their help with
the X-ray data collection and structure determination. We also acknowledge
other members of the Simons Collaboration on the Origins of Life and the
Harvard Origins of Life Initiative for fruitful discussions that shaped the
ideas behind this work. This work was supported by a grant from the Simons
Foundation 290360 to D.D.S.

## Author contributions

S.F.O., J.D.S., and D.D.S. designed the research; S.F.O. performed the
crystallization experiments, collected and analyzed the data; Z.L. synthesized
the aminooxazolines; and S.F.O., J.D.S., and D.D.S. wrote the paper.

## Competing interests

The authors declare no competing interests.

## Data availability

The data that support the findings of this study are available within the
paper and its Supplementary Information

## References

  * [1] Louis Pasteur  “Sur les relations qui peuvent exister entre la forme crystalline, la composition chimique et le sens de la polarization rotatoire”  In _Annales Chimie Phys._ 24, 1848, pp. 442–459
  * [2] GF Joyce et al.  “Chiral selection in poly (C)-directed synthesis of oligo (G)”  In _Nature_ 310.5978 Nature Publishing Group, 1984, pp. 602–604
  * [3] Donna G Blackmond  “The origin of biological homochirality”  In _Cold Spring Harbor perspectives in biology_ 2.5 Cold Spring Harbor Lab, 2010, pp. a002147
  * [4] Robert A Sanchez and Leslle E Orgel  “Studies in prebiotic synthesis: V. Synthesis and photoanomerization of pyrimidine nucleosides”  In _Journal of molecular biology_ 47.3 Elsevier, 1970, pp. 531–543
  * [5] Greg Springsteen and Gerald F Joyce  “Selective derivatization and sequestration of ribose from a prebiotic mix”  In _Journal of the American Chemical Society_ 126.31 ACS Publications, 2004, pp. 9578–9583
  * [6] Carole Anastasi, Michael A Crowe, Matthew W Powner and John D Sutherland  “Direct Assembly of Nucleoside Precursors from Two-and Three-Carbon Units”  In _Angewandte Chemie International Edition_ 45.37 Wiley Online Library, 2006, pp. 6176–6179
  * [7] Matthew W Powner, Béatrice Gerland and John D Sutherland  “Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions”  In _Nature_ 459.7244 Nature Publishing Group, 2009, pp. 239–242
  * [8] Bhavesh H Patel et al.  “Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism”  In _Nature chemistry_ 7.4 Nature Publishing Group, 2015, pp. 301–307
  * [9] Jason E Hein, Eric Tse and Donna G Blackmond  “A route to enantiopure RNA precursors from nearly racemic starting materials”  In _Nature chemistry_ 3.9 Nature Publishing Group, 2011, pp. 704–706
  * [10] S. Ozturk and Dimitar D. Sasselov  “On the origins of life’s homochirality: Inducing enantiomeric excess with spin-polarized electrons”  In _Proceedings of the National Academy of Sciences_ 119.28, 2022, pp. e2204765119  DOI: 10.1073/pnas.2204765119
  * [11] Dimitar D Sasselov, John P Grotzinger and John D Sutherland  “The origin of life as a planetary phenomenon”  In _Science Advances_ 6.6 American Association for the Advancement of Science, 2020, pp. eaax3419
  * [12] Long-Fei Wu et al.  “Interstrand aminoacyl transfer in a tRNA acceptor stem-overhang mimic”  In _Journal of the American Chemical Society_ 143.30 ACS Publications, 2021, pp. 11836–11842
  * [13] Saidul Islam, Dejan-Krešimir Bučar and Matthew W Powner  “Prebiotic selection and assembly of proteinogenic amino acids and natural nucleotides from complex mixtures”  In _Nature Chemistry_ 9.6 Nature Publishing Group, 2017, pp. 584–589
  * [14] Jianfeng Xu et al.  “A prebiotically plausible synthesis of pyrimidine $\beta$-ribonucleosides and their phosphate derivatives involving photoanomerization”  In _Nature chemistry_ 9.4 Nature Publishing Group, 2017, pp. 303–309
  * [15] K Ray, SP Ananthavel, DH Waldeck and Ron Naaman  “Asymmetric scattering of polarized electrons by organized organic films of chiral molecules”  In _Science_ 283.5403 American Association for the Advancement of Science, 1999, pp. 814–816
  * [16] B Göhler et al.  “Spin selectivity in electron transmission through self-assembled monolayers of double-stranded DNA”  In _Science_ 331.6019 American Association for the Advancement of Science, 2011, pp. 894–897
  * [17] Ron Naaman and David H Waldeck  “Chiral-induced spin selectivity effect”  In _The journal of physical chemistry letters_ 3.16 ACS Publications, 2012, pp. 2178–2187
  * [18] Ron Naaman, Yossi Paltiel and David H Waldeck  “Chiral Induced Spin Selectivity and Its Implications for Biological Functions”  Annual Reviews, 2022
  * [19] Karen Michaeli, Nirit Kantor-Uriel, Ron Naaman and David H Waldeck  “The electron’s spin and molecular chirality–how are they related and how do they affect life processes?”  In _Chemical Society Reviews_ 45.23 Royal Society of Chemistry, 2016, pp. 6478–6487
  * [20] Ron Naaman and David H Waldeck  “Spintronics and chirality: Spin selectivity in electron transport through chiral molecules”  In _Annu. Rev. Phys. Chem_ 66.1, 2015, pp. 263–281
  * [21] Tzuriel S Metzger et al.  “The electron spin as a chiral reagent”  In _Angewandte Chemie_ 132.4 Wiley Online Library, 2020, pp. 1670–1675
  * [22] BP Bloom et al.  “Asymmetric reactions induced by electron spin polarization”  In _Physical Chemistry Chemical Physics_ 22.38 Royal Society of Chemistry, 2020, pp. 21570–21582
  * [23] Tzuriel S Metzger et al.  “Dynamic Spin-Controlled Enantioselective Catalytic Chiral Reactions”  In _The Journal of Physical Chemistry Letters_ 12.23 ACS Publications, 2021, pp. 5469–5472
  * [24] Deb Kumar Bhowmick et al.  “Spin-induced asymmetry reaction—The formation of asymmetric carbon by electropolymerization”  In _Science advances_ 8.31 American Association for the Advancement of Science, 2022, pp. eabq2727
  * [25] Jianfeng Xu et al.  “Photochemical reductive homologation of hydrogen cyanide using sulfite and ferrocyanide”  In _Chemical Communications_ 54.44 Royal Society of Chemistry, 2018, pp. 5566–5569
  * [26] Ron Naaman, Yossi Paltiel and David H Waldeck  “Chiral molecules and the electron spin”  In _Nature Reviews Chemistry_ 3.4 Nature Publishing Group, 2019, pp. 250–260
  * [27] Koyel Banerjee-Ghosh et al.  “Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates”  In _Science_ 360.6395 American Association for the Advancement of Science, 2018, pp. 1331–1334
  * [28] Francesco Tassinari et al.  “Enantioseparation by crystallization using magnetic substrates”  In _Chemical science_ 10.20 The Royal Society of Chemistry, 2019, pp. 5246–5250
  * [29] Debkumar Bhowmick et al.  “Simultaneous High-Purity Enantiomeric Resolution of Conglomerates Using Magnetic Substrates”  In _Crystal growth & design_ 21.5 ACS Publications, 2021, pp. 2925–2931
  * [30] Oren Ben Dor et al.  “Magnetization switching in ferromagnets by adsorbed chiral molecules without current or external magnetic field”  In _Nature communications_ 8.1 Nature Publishing Group, 2017, pp. 1–7
  * [31] David J Dunlop and Özden Özdemir  “Rock magnetism: fundamentals and frontiers”  Cambridge university press, 2001
  * [32] Robert Karlin, Mitchell Lyle and G Ross Heath  “Authigenic magnetite formation in suboxic marine sediments”  In _Nature_ 326.6112 Nature Publishing Group, 1987, pp. 490–493
  * [33] Dougal J Ritson, Claudio Battilocchio, Steven V Ley and John D Sutherland  “Mimicking the surface and prebiotic chemistry of early Earth using flow chemistry”  In _Nature communications_ 9.1 Nature Publishing Group, 2018, pp. 1–10
  * [34] Aaron M Jubb and Heather C Allen  “Vibrational spectroscopic characterization of hematite, maghemite, and magnetite thin films produced by vapor deposition”  In _ACS Applied Materials & Interfaces_ 2.10 ACS Publications, 2010, pp. 2804–2812
  * [35] Isabelle Weissbuch, Ronit Popovitz-Biro, Meir Lahav and Leslie Leiserowitz  “Understanding and control of nucleation, growth, habit, dissolution and structure of two-and three-dimensional crystals usingtailor-made’auxiliaries”  In _Acta Crystallographica Section B: Structural Science_ 51.2 International Union of Crystallography, 1995, pp. 115–148
  * [36] Joel Hurowitz et al.  “Redox stratification of an ancient lake in Gale crater, Mars”  In _Science_ 356 American Association for the Advancement of Science, 2017, pp. eaah6849
  * [37] Matthias Schmitt et al.  “Bulk spin polarization of magnetite from spin-resolved hard x-ray photoelectron spectroscopy”  In _Physical Review B_ 104.4 APS, 2021, pp. 045129
  * [38] David J Dunlop and Jafar Arkani-Hamed  “Magnetic minerals in the Martian crust”  In _Journal of Geophysical Research: Planets_ 110.E12 Wiley Online Library, 2005

##  Appendix A Methods

We obtained and used the samples without additional purification unless
otherwise specified. We performed all of the experiments under ambient
conditions unless otherwise specified.

Method for Synthesizing RAO, AAO, XAO, and LAO

We synthesized ribo-, arabino-, xylo-, and lyxo-aminooxazolines by the
reaction of two equivalents of cyanamide with one equivalent of the
corresponding aldopentose sugar. We separately purchased the L and D sugars
and synthesized the L and D aminooxazolines in large amounts (above 250 mg
each). We then weighed the enantiomer on the scale, ground them into powder,
and mixed them in equal amounts to make the racemic pentose aminooxazolines.
We confirmed that the aminooxazolines are racemic by CD measurements.

Method for Fabricating Magnetite Surfaces

We fabricated the magnetite films by evaporating a 100 nm iron layer on 0.625
mm thick silicone (100) wafers using electron beam evaporation under a high
vacuum of $5\times 10^{-6}$ Torr. Following the evaporation, we baked the
samples at 175∘C for 4 hours in the air and promoted the oxidation of iron
($\ce{Fe}$) to magnetite ($\ce{Fe3O4}$). We then cleaned the sample surfaces
with acetone and subsequently in ethanol before every experiment.

Method for Crystallization Experiments

For the direct crystallization experiments we prepared a 0.5M:0.5M solution of
racemic glyceraldehyde and 2-aminooxazole in 2 mL water. We incubated the
solution at 40∘C for 12 hours and obtained a yellow-brown solution of
aminooxazoles.

For the re-crystallization experiments, we prepared a 65 mM solution of
racemic RAO in 2 mL water.

We placed the magnetite surfaces horizontally in a polystyrene Petri dish (35
by 10 mm) on a magnet such that the surface normal is parallel to the
magnetization direction. The magnetic field strength at the sample position
was measured to be 325 mT with a Hall probe. We then filled the Petri dish
with the incubated solution and made sure that the magnetite surface is
covered with the liquid. We then placed the setup in the fridge kept at 12∘C
and waited until the first crystals appear. This process can take several
hours to a few days. Afterward, we slowly filtered out the mother liquid and
washed the surface and crystals with pure water three times such that the
racemic liquid is washed away. We then collected the crystals with tweezers
under a stereo-microscope. We discarded the crystals formed on the plastic
surface of the Petri dish and on the rough edges of the silicon substrate and
collected the rest formed on the magnetite surface. We fully dissolved all of
the collected crystals in pure water and analyzed the solution.

Method for Circular Dichroism Measurements

We took the CD measurements in a quartz cuvette after diluting the solution in
2 mL water until the UV/Vis absorption peak is below OD = 1 for accurate
measurements. We used a Jasco J-815 Circular Dichroism Spectropolarimeter with
an active temperature control connected to a water bath with a temperature set
to 20∘C. The temperature feedback is performed by a Jasco PFD-425S/15
controller with a Peltier control unit. Before the measurements, we took a
baseline measurement of the water and cuvette background. We simultaneously
measured the circular dichroism, and UV/VIS absorption of the sample together
with the photo-multiplier voltage to ensure that the spectrometer is not
operating beyond its specified voltage range of 600 units. We took the
measurements in the 185-210 nm wavelength range and used the auto-baseline
subtraction feature. We used a data pitch of 0.2 nm, a bandwidth of 1 nm, a
data integration time of 1 second, and a scanning speed of 20 nm/minute. We
averaged each measurement 5 times. We normalized the CD signal amplitude based
on the UV/Vis absorption and calculated the enantiomeric excess using the
calibration procedure described in the Supplementary Information Section 9.

Extended Data Fig. 1: Re-crystallization of enantioenriched and racemic RAO on
magnetite a. When we crystallized enantioenriched solutions of RAO we observed
a drastic increase in the ee and from about $25\%$ ee we could reach a
homochirality state. The $25\%$ enriched D-RAO (red) is crystallized on up-
spin magnetite and we obtained homochiral D-RAO. Similarly, $20\%$ enriched
L-RAO (blue) is crystallized on down-spin magnetite and we obtained nearly
homochiral ($80\%$) L-RAO. b. Repeated re-crystallization of racemic RAO is
done to accumulate statistics. DL-RAO is crystallized on up- and down-spin
magnetite. Red curves correspond to up-spin and blue curves to down-spin.
Black lines are the control experiments done on a silicon surface in the
presence of the magnetic field. The ee for the down spin experiments is
calculated to be 59, 36, 18 (mean = $38\%$). The ee for the up spin
experiments is calculated to be: -10, -22, -41, -54 (mean = $-32\%$). The ee
for the control experiments is calculated to be: 5.5, 0.9, 0.6, -3.4 (mean =
$0.9\%$). We estimate an error of $\pm 5\%$ for the ee calculations. Extended
Data Fig. 2: Crystal habit modification of RAO by XAO a. RAO crystallization
is modified with XAO. In the presence of XAO and at high concentrations, RAO
forms rosette-shaped twinned conglomerates. b. Direct crystallization of RAO
gives rosette-shaped twinned clusters, and as the concentration is decreased
the conglomerate crystals become more angular and the rosette formation is
suppressed. At lower concentrations for direct crystallization, the crystal
morphology becomes identical to re-crystallized RAO in the presence of XAO. c.
XAO modifies the crystal shape of RAO by inhibiting the growth of the side
face and thus promoting the growth vertically. This makes the RAO crystals
more elongated and eventually needle-shaped with increasing XAO abundance. On
the top-center, the ratio between D-RAO and D-XAO; on the bottom-right, the
sketch of the crystal shape is shown.