Selective photostabilisation of guanine by methylation

Structural and dynamic properties of N7-methyl and N9-methyl keto guanine in the lowest singlet pipi* electronically excited state, S-1, have been investigated using a density functional restricted open-shell Kohn - Sham approach. The excited state geometry of N7-methyl guanine deviates little from the unmethylated N7H species, whereas N9-methyl guanine surprisingly exhibits large qualitative structural differences compared to the N9H tautomer. In particular the strongly out-of-plane bent amino group of N9-methyl guanine is expected to significantly reduce its optical absorption cross section in this spectral region. Furthermore, on-the-fly molecular dynamics simulations of S1 vertically excited N9-methyl keto guanine reveal that in the first roughly 10 fs after photoexcitation a local minimum corresponding to a quasi-planar geometry is populated which is, however, thermally unstable and decays to the heavily distorted global minimum structure on the timescale of 100 fs. In contrast, vertically excited N7-methyl keto guanine is seen to relax to its comparatively mildly distorted S1 global minimum structure within approximately 10 fs. These findings offer a possible explanation for the absence of a N9-methyl keto signal in recent molecular beam IR-UV spectral measurements. In addition, we propose that the special photophysical properties of the biologically relevant N9-substituted keto guanine tautomers may indeed have played an important role in the evolutionary selection of DNA building blocks.