The Photophysics and Photochemistry of Epigenetics and “Natural” Noncanonical Bases: Insights from Quantum Mechanical Calculations

This chapter concisely reviews the computational studies describing the photophysical and photochemical processes occurring in the most important “natural” noncanonical nucleobases, from light absorption to the recovery of the ground electronic state. We thus treat epigenetic bases, i.e. those dynamically modified in the genome in order to regulate gene expression and silencing, focusing on the cytosine derivatives substituted in position 5 (e.g. 5-methyl-cytosine and its derivatives) and on methylated purines. Then, we describe the bases most commonly produced in cells by endogenous and exogenous oxidative processes, with special focus on 8-oxo-guanine. Particular attention is devoted to briefly discussing how the different covalent modifications of the canonical bases alter their photoactivated dynamics.