On the Deactivation Mechanisms of Adenine–Thymine Base Pair

Abstract

In this contribution, the multiconfigurational second-order perturbation theory method based on a complete active space reference wave function (CASSCF/CASPT2) is applied to study all possible single and double proton/hydrogen transfers between the nucleobases in the adenine-thymine (AT) base pair, analyzing the role of excited states with different nature [localized (LE) and charge transfer (CT)], and considering concerted as well as step-wise mechanisms. According to the findings, once the lowest excited states, localized in adenine, are populated during UV irradiation of the Watson-Crick base pair, the proton transfer in the N-O bridge does not require high energy in order to populate a CT state. The latter state will immediately relax toward a crossing with the ground state, which will funnel the system to either the canonical structure or the imino-enol tautomer. The base pair is also capable of repairing itself easily since the imino-enol species is unstable to thermal conversion.

Proton/Hydrogen Transfer Mechanisms in the Guanine–Cytosine Base Pair: Photostability and Tautomerism

“…In a recent study on the proton/hydrogen-transfer processes in the AT base pair, in which the same methodology and computational strategy as in the present study was used, the CI crossing between both ground and S 1 -CT states was optimized and it was found very close in energy and geometry to the corresponding intermediate. 105 Other computational works have also pointed out to the existence of this CI in AT. 114,115 The present study indicates that the situation is the same for GC.…”

Solvent effects on the excited state characteristics of adenine–thymine base pairs

“…76 The driving force of EDPT is the CT state, where charge separation between the fragments leads to exited state proton transfer. A series of ab initio studies have identied CT states in AT-WC, 3,37,77 guanine-cytosine WC 19 as well as in AT-S 35 base pairs. Here we identify the CT states of the AT-S base pair both in the gas phase and in solution.…”

“…The identied CT state could lead to viable non-radiative deactivation pathways via EDPT. 37,76,77 Overall, hydration affects the charge de-localization in the excited states of the AT-S and AT-WC congurations in different ways. The number of inter-fragment de-localized states (i.e.…”

Is UV-Induced Electron-Driven Proton Transfer Active in a Chemically Modified A·T DNA Base Pair?

“…Previous TEAS and TVAS studies of WC G•C pairs provided compelling evidence for sequential EDPT dynamics. 14 28 which predict that, although bound with respect to potential energy, 22,23,28 A*•T* tautomers are not stable in Gibbs free energy space. This behavior contrasts both theoretical predictions and experimental findings for G•C, which together show that the corresponding G*•C* tautomer can be generated through a twostep EDPT mechanism and is subsequently stable on a timeframe of ≥ 1 ns.…”

“…Analogous to G•C, theoretical studies have also predicted that both single and/or sequential double EDPT pathways may be active in individual adenine-thymine (A•T) WC pairs after UV excitation. 5,[22][23][24][25][26][27] However, unlike G•C, there is some debate regarding the stability of the imino-enol tautomer pair of A•T resulting from double Hatom transfer 23,28 and whether EDPT is enhanced when A•T is placed in polar solvents. 6 Zhang et al invoked hybrid EDPT pathways to be active in DNA duplexes containing only A•T, 13 Herein, we report the first TEAS and TVAS measurements on a chemically modified A•T WC base pair, solvated in chloroform solution, which eliminates formation of undesired Hoogsteen-type structures.…”