Computational Study of UV-Induced Excitations of DNA Fragments

Abstract

The recent experimental and theoretical results in elucidating the structures and properties of ultraviolet (UV)-induced electronic excitations of DNA fragments and related analogs are discussed. Although, the electronic absorption maxima of nucleic acid bases are in the UV region of the energy spectrum, these genetic molecules are highly photostable. The observed photostability is the outcome of the extremely short excited state life-times. This fundamental characteristic of nucleic acid bases on the other hand is attained by the ultrafast nonradiative decay through internal conversion. Recent theoretical investigations unambiguously show that excited state geometries are generally nonplanar, though the amount of nonplanarity depends on the level of theory used in the calculation. It is also evident that conical intersections involving ground and excited state potential energy surfaces are instrumental for such nonradiative deactivation. Though, theory and experiments are complementary to each other, but the experimental progress in studying excited state properties are far ahead compared to the theoretical methods. For example, it is still very challenging for an extensive investigation of excited state properties of systems like nucleic acid bases at multi-configurational theoretical levels and with large basis sets augmented with diffuse functions. The theoretical and computational bottleneck impedes the investigation of effect of stacking interaction, which is of the fundamental importance for DNA, at the reliable theoretical level. However, we hope that with the theoretical and computational advances such investigations will be possible in near future