Covalent epigenetic modifications contribute to the regulation
of important cellular processes during development and differentiation,
and changes in their genomic distribution and frequency are linked
to the emergence of genetic disease states. Chemical and enzymatic
methods that selectively target the orthogonal chemical functionality
of epigenetic markers are central to the study of their distribution
and function, and considerable research effort has been focused on
the development of nondestructive sequencing approaches which preserve
valuable DNA samples. Photoredox catalysis enables transformations
with tunable chemoselectivity under mild, biocompatible reaction conditions.
We report the reductive decarboxylation of 5-carboxycytosine via a
novel iridium-based treatment, which represents the first application
of visible-light photochemistry to epigenetic sequencing via direct
base conversion. We propose that the reaction involves an oxidative
quenching cycle beginning with single-electron reduction of the nucleobase
by the photocatalyst, followed by hydrogen atom transfer from a thiol.
The saturation of the C5–C6 backbone permits decarboxylation
of the nonaromatic intermediate, and hydrolysis of the N4-amine constitutes
a conversion from a cytosine derivative to a T-like base. This conversion
demonstrates selectivity for 5-carboxycytosine over other canonical
or modified nucleoside monomers, and is thereby applied to the sequencing
of 5-carboxycytosine within modified oligonucleotides. The photochemistry
explored in this study can also be used in conjunction with enzymatic
oxidation by TET to profile 5-methylcytosine at single-base resolution.
Compared to other base-conversion treatments, the rapid photochemical
reaction takes place within minutes, which could provide advantages
for high-throughput detection and diagnostic applications.