Dynamics of the excited-state hydrogen transfer ina (dG)$(dC) homopolymer: intrinsic photostabilityof DNA

摘要

The intrinsic photostability of nucleic acids is intimately related to evolution of life, while its understanding atthe molecular and electronic levels remains a challenge for modern science. Among the different decaypathways proposed in the last two decades, the excited-state hydrogen transfer between guanine–cytosine base pairs has been identified as an efficient non-reactive channel to dissipate the excess ofenergy provided by light absorption. The present work studies the dynamics of such phenomena takingplace in a (dG)$(dC) B-DNA homopolymer in water solution using state-of-the-art molecular modellingand simulation methods. A dynamic effect that boosts the photostability of the inter-strand hydrogenatom transfers, inherent to the Watson–Crick base pairing, is unveiled and ascribed to the energyreleased during the proton transfer step. Our results also reveal a novel mechanism of DNA decaynamed four proton transfer (FPT), in which two protons of two adjacent G–C base pairs are transferredto form a biradical zwitterionic intermediate. Decay of the latter intermediate to the ground state triggersthe transfer of the protons back to the guanine molecules recovering the Watson–Crick structure of thetetramer. This FPT process is activated by the close interaction of a nearby Na+ counterion with theoxygen atoms of the guanine nucleobases and hence represents a photostable channel operative innatural nucleic acids.