The role of pyrimidine nucleobase excimers in DNA photophysics and photoreactivity

Abstract

Quantum chemical studies using the accurate CASPT2//CASSCF procedure show that π-stacked interactions in biochromophores such as pyrimidine (Pyr) DNA/RNA nucleobases pairs yield excimer-like situations which behave as precursors of processes like charge transfer (CT) or photoreactivity and are the source of the emissive properties in DNA. Examples are the CT between adjacent DNA nucleobases in a strand of oligonucleotides and the photodimerization taking place in cytosine (C) pairs leading to cyclobutanecytosine (CBC) mutants. These processes take place through nonadiabatic photochemical mechanisms whose evolution is determined by the presence and accessibility of conical intersections (CIs) and other surface crossings between different electronic states.

Can the Hexagonal Ice-like Model Render the Spectroscopic Fingerprints of Structured Water? Feedback from Quantum-Chemical Computations

“…The repulsive states lead to crossing regions with the ground state at large R(O-O) distances. The corresponding conical intersections may mediate in this way an effective nonradiative decay channel towards the repopulation of the ground state, as it occurs in DNA/RNA nucleobases [48][49][50][51][52][53][54]. The well found on the 3 From an experimental standpoint, the wavelength of the emission peak ( max ) depends on the wavelength of the excitation applied ( exc ) in a variety of aqueous solutions employing different solutes (salts, Nafion 117 solution/film, L-lysine, D-alanine, D-glucose, and sucrose) [2].…”

Photodynamic behavior of electronic coupling in a N-methylformamide dimer

“…It has been shown in the studies on the excited state interactions of two nucleobases that the inclusion of dynamical correlation effects is necessary to correctly describe the chargetransfer as well as excimer states and, thus, the CASPT2 method has to be used for a reliable description of such systems. [29][30][31][32][33][34] It has been also shown that the basis set superposition error (BSSE) must be accounted for to properly characterize the potential energy surface of the excited state dimers and their stabilities. In these studies the states of pp* character were considered.…”