Are the five natural DNA/RNA base monomers a good choice from natural selection?: A photochemical perspective

In order to prevent the damaging effects of sun radiation in the genetic material, its constituent chromophores, the five natural DNA/RNA nucleobases cytosine, thymine, uracil, adenine, and guanine, should be able to efficiently dissipate absorbed radiation, UV specifically, avoiding as much as possible photoreactions leading to lesions. It has been established experimentally and theoretically that efficient internal conversion channels, still open and relevant in the oligomer-stacked strands, exist in the monomers allowing an effective waste of the initial energy. Previous evidences cannot explain, however, why minor differences in the molecular structure modify drastically the photochemistry of the systems, leading for many derivatives to slower decays, sometimes to intense fluorescence, and also to reactivity. Using the accurate CASPT2//CASSCF quantum chemical method and the Photochemical Reaction Path Approach it is determined that the five natural nucleobases display barrierless paths from the allowed excited state toward accessible conical intersection seams with the ground state. Such features are known to be the funnels for efficient energy decay and fluorescence quenching. Modified nucleobases, except the methylated ones, are predicted less photostable because they display energy barriers along lowest-energy paths and hence restricted accessibility of the internal conversion channel. This specificity speaks in favor of the choice of the biological nucleobases by natural selection based on their resistance to photochemical damage. Whereas natural and methylated nucleobases, also frequent in the genetic code, are photostable and cannot be photochemically discarded, other non-natural nucleobases may have been eliminated at early stages of the natural selection process.