supramolecular self - assembly represents a key technology for the spontaneous construction of nanoarchitectures and for the fabrication of materials with enhanced physical and chemical properties [ 14 ] .
in addition , a significant asset of supramolecular self - assemblies rests on their reversible formation , thanks to the kinetic lability of their noncovalent interactions .
this dynamic nature can be exploited for the development of  self - healing  and  smart  materials towards the tuning of their functional properties upon various external factors [ 58 ] .
one particular intriguing objective in the field is to reach a high level of control over the shape and size of the supramolecular architectures , in order to produce well - defined functional nanostructures by rational design . in this direction , many investigations have been pursued toward the construction of self - assembled objects from numerous low - molecular weight scaffolds [ 1018 ] , for instance , by exploiting multiple directional hydrogen - bonding interactions .
in particular , nucleobases have been used as supramolecular synthons as a result of their efficiency to code for noncovalent interaction motifs . among nucleobases
, guanine represents the most versatile one , because of its different h - bond donor ( nh and nh ) and acceptor sites ( o , n , and n ) which display self - complementary patterns of interactions . interestingly , and depending on the environmental conditions , guanosine derivatives can form various types of structures .
most of the supramolecular architectures reported so far from guanosine derivatives require the presence of a cation ( alkali metal , earth - alkali , or lanthanide ions ) [ 2031 ] which stabilizes , via dipole - ion interactions , the macrocyclic g - quartet that can , in turn , stack in columnar g - quadruplex arrangements ( figure 1(a ) ) .
in addition , guanosine can polymerize via hydrogen bonding to give a variety of supramolecular networks including linear ribbons such as a- and b - types ( figures 1(b ) and 1(c ) ) .  
this complex supramolecular behaviour confers to the guanine - guanine interactions their upper interest among all the homonucleobases studied [ 20 , 32 ] .
they have been subjected to intense investigations in various areas ranging from structural biology and medicinal chemistry  guanine - rich sequences are abundant in telomeric ends of chromosomes and promoter regions of dna and are capable of forming g - quartet based structures [ 3335]to material science and nanotechnology [ 36 , 37 ] .   in 1998 ,
it was reported   that , even in the absence of metal cations , lipophilic guanosine derivative 1 ( figure 2 ) is able to self - assemble in ribbon - like structures leading to the formation of a lyotropic liquid crystalline phase .   in the following years , the structures of the ribbons obtained from different guanosine derivatives have been characterized in solution ( by nmr and esi - ms ) , in the solid state ( by x - ray diffraction and nmr ) , and at graphite - solution interface ( by stm ) [ 4046 ] .
two different types of ribbons , with different patterns of hydrogen bonds , have been resolved in solution .
the first species ( ribbon - a , figure 1(b ) ) is characterised by nho and nhn hydrogen bonds , and the second one ( ribbon - b , figure 1(c ) ) by nho and nhn hydrogen bonds . in the case of 1 ,
ribbon - a is detected in anhydrous chloroform solutions ( c > 10  m ) soon after dissolving the polycrystalline powder but subsequently undergoes a structural transition towards a thermodynamically more stable ribbon - b .
interestingly , in the case of the closely related chemical structure of 2 , only ribbon - b has been detected in chloroform solution ( even soon after dissolution ) . for derivative 1 ,
the critical concentration for the formation of the gel - like phase was measured 0.596  m in chloroform , and for derivative 2 0.125  m in the same solvent . in both ribbon - like polymers a and b
however , the two ribbons possess a different symmetry and , as a consequence , while ribbon - a does present a permanent dipole moment , it is not the case for ribbon - b .
this feature has been exploited in organic electronic prototype devices [ 4749 ] , and more generally , linear ribbons from guanosine derivatives are considered as potential scaffolds for functional materials [ 5054 ] .
although the formations of the linear ribbons and their h - bonding patterns have been demonstrated in solution [ 40 , 41 ] ( as well as in the crystal state and on graphite surface ) , only very few investigations have been performed so far toward the determination of the length of these supramolecular polymers , as well as on their possible hierarchical self - assemblies in higher scale structures .
nmr spectroscopic indications of the supramolecular polymerisation only come from the shifting and broadening of the resonance signals upon concentration of the solution .
in addition , the observation of negative enhancements ( or positive cross - peaks ) in noe ( or noesy ) experiments indicates that guanosine derivatives behave as large molecules with mw > 1000 ( c > 1 ) [ 3840 ] .  
in this paper we report on the investigation of the supramolecular polymerisation of 1 and 2 in chloroform by using light scattering technique as well as transmission electronic microscopy ( tem ) .
this line of studies reveals the formation of supramolecular polymers with high molecular weights that produce hierarchical structuring of very soft self - assemblies displaying either fibrillar ( 1 ) or lamellar ( 2 ) organizations .
the multiangle laser light scattering is the common technique [ 55 , 56 ] for determining the shape of the polymers through the mean square radius of gyration rgz and their structure through the particle scattering factor pz(q ) , the molecular weight mw , and the second virial coefficient a2 .
theory took great interests long time ago giving statistical sense to these parameters [ 5561 ] .
when a2 is near zero and solutions considered as athermal according to general polymer theories   one may deal with the apparent molecular weight mw , app and use the so - called statistical dimensions determined at discrete concentrations [ 59 , 60 ] instead of the one extrapolated at c  0 .
looking at the variation of mw , app versus concentration or temperature allows equilibrium constant k0 to be evaluated which open fields to dynamics of the scaffolds and to their assembly ability .
the form factor   pz(q ) may be also useful to determine structural changes of the objects during the diffusion process of elementary unimers [ 59 , 60 ] .
the form factor may derive from calculations especially when the structure of the coil is well known .
theoretical aspects are shortly reviewed at the end of the paper ( section 4 ) .
there is shown typical processes for data calculations leading to zimm plot which yields to the molecular dimensions and solution thermodynamics [ 5558 ] .
all the data obtained for guanosines 1 and 2 are summarized in table 1 . the zimm plot obtained from guanosine 1 at 25c in chcl3 for different concentrations
, the apparent molecular weight was determined to be relatively stable below the gelation threshold .
the linear regression reveals a modest molecular weight of 3620 ( corresponding to an aggregation number of 6 to 7 units ) but with a radius of gyration of 57  nm which indicates the presence of large objects .
the discrepancy between these two parameters is explained by the high polydispersity of the supramolecular polymer with an average of small ribbons together with a smaller population of large objects .
indeed , the gyration radius being determined from mw , the importance of the large structures becomes predominant .
the molecular weight of the polymers depends largely on the concentration range and a study between 2.88 10 and 2.88 10  gml reveals a power law of 0.75 ( see figure 4 ) which is higher than the value ( 0.6 ) for regular swollen coils and which is the signature for rigid objects of about 200  nm in length [ 57 , 62 ] .
finally , the association constant k0 between two monomers in the supramolecular b - type ribbon ( see figure 1(c ) ) was determined by plotting the apparent molecular weight as a function of the concentration ( figure 5 ) and shows a relatively small value k0 = 400  l  mol [ 59 , 60 ] . when the polymer is homogeneous , flory krigbaum   derived the more realistic model for a2 introducing the  enthalpy and  entropy parameter , respectively . by comparison with the flory - huggins theory   these parameters
are seen to be related by    = ( 1/2 )     where  is the flory huggins interaction parameters . for ideal behaviour  =    and
a2 = 0 so the theta point is reached , with theta temperature being given by  = t(/ ) where t is the absolute temperature .
the theory leads to some basics which are the equivalent size of the solvent and monomers , respectively , and a number of association for statistical calculations .  
these statistical basics hold for guanosine 2 but comments on guanosine 1 may lie in the expression of idealized lattice model for second virial coefficient .
it is obvious that one of the optimal conditions for association is an athermal solution which is what we have found for guanosine 2 ( table 1 ) . in the case of guanosine 2
, the light scattering reveals a quite different behavior of the self - assembly process . the zimm plot in figure 6 shows a smaller polidispersity than what is measured for guanosine 1 together with higher molecular weights ( mw = 5.93 10 , and the degree of polymerisation dp = 1250 ) , but with a close radius of gyration ( 43  nm ) which might correlate with the formation of more compact objects
this expectation is confirmed by plotting the mw in the range of concentration 3.4 10 and 6.82 10  g  ml .
the experimentally determined power law of 0.3 ( see figure 7 ) is the signature for very compact objects such as spheres [ 57 , 62 ] .
finally , the determination of the equilibrium constant [ 59 , 60 ] for guanosine 2 shows a strong association process ( k0 = 6.75 10  l  mol ) and a complex behaviour of a sigmoid type ( figure 8) .
this nonlinear effect indicates a cooperative process and the hierarchical association between the linear supramolecular ribbons with their organization in higher - ordered self - assembled structures that in turn stabilize the primary association .
structural observation of self - assembled system can easily be achieved by tem approaches , as widely used and described in the literature [ 65 , 66 ] .  
guanosine derivative 1 in bromoform ( 1.44 10  g  ml ) forms small short fibers ( figure 9(a ) ) .
bromoform was used for technical reasons regarding the evaporation rate at room temperature which is too high for chloroform thus changing the concentration of the sample during the preparation step .
the dielectric constants are very close to each other for chloroform and bromoform ( 4,8 and 4,4 at 20c , resp . ) , and the gelation concentration was observed to be similar in both cases .
these fibers present a diameter of 6  nm and a length of approximatively 200  nm .
indeed , the precise length can not be determined as the fibers present a high entanglement ratio .
this length corresponds to the distance between the nodes of the networks and was also observed from time to time on individual filaments ( see figure 9(b ) ) .
the small individual fibers self - assemble in larger ones to form bundles of 30  nm .
these latter can be formed during the preparation step as the diameter of the bundles varies from one experiment to another .
the distribution of the fibers within a clear network illustrates the organogelator properties of guanosine 1 .
guanosine derivative 2 in bromoform ( 4 10  g  ml ) forms small aggregates and small lamellar structures . on figure 10 ,
large aggregates consisting of the superposition of small ( 100  nm ) lamellar structures are observed ( see arrows ) .
these structures show no define contours and are the results of the aggregations of the smaller domains .
no internal features inside the small domains are visible at the resolution of the technique .
the aggregation within highly compact lamellar structures is also in good agreement with the light scattering experiments that indicate mainly the presence of pseudospherical objects with a strong cooperative effect occurring between the ribbons and their higher - scale self - assemblies .
we have determined the hierarchical polymeric natures of the self - assemblies obtained from the ribbon forming [ 40 , 41 ] guanosine derivatives 1 and 2 .
the light scattering measurements appeared as an appropriate technique for the molecular weight determination versus the structure of these objects that are both concentration sensitive .
lipophilic guanosine 1 forms very soft fibrillar objects of 6  nm of diameter and 200  nm in length and that in turn produces bundles of networked fibers with 30  nm of diameter . despite
its closely related structure , guanosine 2 forms much longer ribbons with molecular weights up to 6 10 that in turn fold in very compact aggregates with lamellar structures .
this process has been shown to be highly cooperative by the determination of the molecular weight as a function of the concentration .
interestingly , while both derivatives show the same motif of h - bonding in the formation of the first level of supramolecular assembly ( the b - type ribbon ) , their hierarchical organization is quite different .
this finding points out the role of other structural aspects in addition to the h - bond recognition pattern ( e.g. , the number and length of tails ) in determining the shape and dimension of the object obtained at the nanoscale level .     
these investigations furnish an explanation to the gelation properties of these two derivatives and can be used to rationalize the synthesis of functional guanosine - based soft materials .

 3,5-o - didecanoyl-2-deoxyguanosine 1compound 1 was prepared in 94% yield starting from 2-deoxyguanosine ( fluka ) ( 1  mmol ) and decanoic anhydride ( 2.2  mmol ) according to the literature procedure for 3,5-o - dipropanyl-2-deoxyguanosine .
h nmr ( 300  mhz , [ d6]dmso ) :  = 0.84  0.86 ( tt , 6h ; 2 ch3 ) , 1.18  1.40 ( m , 24h ; 12 ch2 ) , 1.40  1.60 ( m , 4h ; 2 ch2ch2co ) , 2.35 - 2.36 ( tt , 4h ; 2 ch2co ) , 2.42 and 2.93 ( mm , 2h , h-2/h-2 ) ,   4.19  4.38 ( m , 3h ; h-4/h-5/h-5 ) , 5.35 ( m , 1h ; h-3 ) , 6.17 ( m , 1h ; h-1 ) , 6.45 ( bs , 2h ; nh2 ) , 7.9 ( s , 1h ; h-8 ) , 10.65 ( s , 1h ; nh ) ; es - ms : m / z ( % ) : 576.8 ( 100 ) [ 1+h ] . 
 
compound 1 was prepared in 94% yield starting from 2-deoxyguanosine ( fluka ) ( 1  mmol ) and decanoic anhydride ( 2.2  mmol ) according to the literature procedure for 3,5-o - dipropanyl-2-deoxyguanosine .
h nmr ( 300  mhz , [ d6]dmso ) :  = 0.84  0.86 ( tt , 6h ; 2 ch3 ) , 1.18  1.40 ( m , 24h ; 12 ch2 ) , 1.40  1.60 ( m , 4h ; 2 ch2ch2co ) , 2.35 - 2.36 ( tt , 4h ; 2 ch2co ) , 2.42 and 2.93 ( mm , 2h , h-2/h-2 ) ,   4.19  4.38 ( m , 3h ; h-4/h-5/h-5 ) , 5.35 ( m , 1h ; h-3 ) , 6.17 ( m , 1h ; h-1 ) , 6.45 ( bs , 2h ; nh2 ) , 7.9 ( s , 1h ; h-8 ) , 10.65 ( s , 1h ; nh ) ; es - ms : m / z ( % ) : 576.8 ( 100 ) [ 1+h ] . 

2,3-o - isopropylidene-5-decanoylguanosine 2compound 2 was prepared in 86% yield starting from 2 , 3-o - isopropylideneguanosine ( sigma ) ( 0.77  mmol ) and decanoic anhydride ( 1.2  equiv ) according to the literature procedure . 
 
compound 2 was prepared in 86% yield starting from 2 , 3-o - isopropylideneguanosine ( sigma ) ( 0.77  mmol ) and decanoic anhydride ( 1.2  equiv ) according to the literature procedure .
the light scattering   experiments were carried out with an in - house apparatus   equipped with ( i ) a red he - ne laser of wavelength 0 = 632.8  nm in vacuum , ( ii ) a discrete - angle goniometer acting within the range from 20 to 155 , ( iii ) a hamamatsu type photomultiplier as detector , ( iv ) a photocounting device , and ( v ) a toluene matching bath .
the vertical polarization of the incident beam with respect to the scattering plane has been used and is written v. the analyzer , arranged between the measuring cell and the photomultiplier , is vertically oriented and is written vv .
this optical setup allows measurement of the isotropic vv scattering intensity .   the excess of light scattering intensity vv(q ) = vvsolution  vvsolvent was measured as a function of scattering vector q = ( 4n/0)sin  ( /2 ) with an accuracy of 1% (    the scattering angle , n the solvent refractive index ) .
the values of rayleigh excess scattering intensity r(q ) were obtained through the calibration of vv(q ) with a benzene standard .
the intensities vv(q ) , after normalization of the raw data , can be written rv   for the vertically polarized scattering light through an analyzer .
use the following formula for stray light rv : 
								 ( 1)rv = kvcmwp(q)s(q ) , 
							 where mw is the molecular weight , c is the concentration of the polymer , p(q ) is the form factor , and s(q ) the long range interference from distant scatterers , where for dilute solution s(q ) ~ 1 and kv is the optical factor for the system including the refractive index increment of the polymer as follows , where kv = kvbenzene(dn / dc ) is the optical contrast calibrated with benzene standard ( the suffix gives the polarization of incident beam and stray light ) .
the calibration of the spectrometer was made by evaluating the optical constant kv   or kv , v   as follows : 
								 ( 2)kv=42nref2rv , refna04.na is avogadro number , 0    is the wavelength in vacuum , nref    is the benzene refractive index , and rv , ref is the benzene rayleigh ratio for vertically polarised incident light . the chloroform refractive index and the average refractive index increment of the assembly for 1 to chcl3 are equal to n = 1.4459 and dn / dc = 0.0769  ml / g , respectively .   the chloroform refractive index and the average refractive index increment of the assembly for 2      to chcl3 are equal to n = 1.4459 and dn / dc = 0.0830  ml / g respectively .  
according to yamakawa    the ratio kvc / rv may be read versus q as 
								 ( 3)kvcrv=1mwp(q)(1+a2mwc)2 , 
							 where rv is the normalized scattering intensity from vertically incident beam analyzed vertically , and a2 is the second virial coefficient . using the formalism of debye   and zimm , ( 2 ) gives the following : 
								 ( 4)kvcrv = mw1p1(q)+2a2c .
the inverse of the form factor   p(q ) leads after a maclaurin transform at low q vector to ( 5 ) . here
we get rgz the second moment of the mass distribution which is the statistic z average mean radius of gyration [ 57 , 58 ] as follows : 
								 ( 5)kvcrv = mw1(1+q2r2z3+)+2a2c . for extrapolation at q
 0 and c  0   ( 4 ) leads to the determination of the molecular mass and the second virial coefficient a2 .
when the thermodynamic forces applied on the coil equilibrate , we can write a2 = 0 .
this athermal condition for binary mixture allows the determination of the apparent molecular mass mw , app   for each concentration [ 59 , 60 ] : 
								 ( 6)kvcrv = mw , app1(1+q2r2z3). equation ( 6 ) contains all information on the shape and the conformation of the isolated polymer in solution .
the determination of the radius of gyration is only valid in the guinier range qr < 1 where the architectures of the polymers are barely distinguishable .
the plot of ( 5 ) versus kq + kc , called zimm - plot , allows a simultaneous extrapolation to q = 0 and c = 0 , which yields mw(g  mol ) as the ordinate intercept and   rgz   as the initial slope of ( 6 ) and the second viral coefficient a2    from ( 5 ) .  
the value of the second viral coefficient a2 is in our case of an athermal binary mixture and thus neglected : 
								 ( 7)lim(q0 )  kcrv = mw , app1 . 
							
so we get a good evaluation of the variation for mw , app versus the concentration c. following the calculation of huglin   and elias   we obtain 
								 ( 8)mw , app2mw , app , i2=4000(k0mw , app , i)c . 
							 here  
mw , app , i is the molecular weight of the initial monomer and mw , app is the weight average molecular weight determined through ( 7 )
. measurements of each sample have been carried out after various dilutions with precise volume of clarified solvent obtained by filtration through millipore filters size 0.45  m . all the samples are directly processed in the measurements vial to avoid dust pollution .
 here we investigate the supramolecular polymerizations of two lipophilic guanosine derivatives in chloroform by light scattering technique and tem experiments . 
 the obtained data reveal the presence of several levels of organization due to the hierarchical self - assembly of the guanosine units in ribbons that in turn aggregate in fibrillar or lamellar soft structures . 
 the elucidation of these structures furnishes an explanation to the physical behaviour of guanosine units which display organogelator properties . 
1. Introduction
2. Results and Discussion
3. Conclusions
4. Materials and Methods