Theoretical Investigation on the Absence of Spore Photoproduct Analogue at Cytosine-Thymine Site

As a well known DNA photolesion product, the special UV induced pyrimidine dimmer called spore photoproduct (SP), has aroused strong research interests. The SP formation was reported solely between two adjacent thymidine residues. It remains unclear in perviousexperimental observations why there is an absence of the cytosine-derived SP-like photoprod-uct formation at the cytosine containing DNA strand, although the cytosine residue holds great similarity to thymine in terms of molecular structure. From a theoretical perspec-tive, we have explored this issue in this work by means of density functional theory at the B3LYP/6-311++G(d,p) //B3LYP/6-31G(d,p) level for the DNA dinucleotide fragment, cy-tosine phosphate thymine (CpT). Key factors blocking the formation of the SP-like product between two adjacent cytosine and thymidine residues are revealed. Instead of undergoing photochemical SP reaction, a photophysical deactivation pathway back to the ground state turns out to be favorable for the CpT dinucleotide fragment.