On the Deactivation Mechanisms of Adenine–Thymine Base Pair

• Published in: The journal of physical chemistry. B Vol. 116; no. 13; pp. 4089 - 4097
• Main Authors: Gobbo, João Paulo, Saurí, Vicenta, Roca-Sanjuán, Daniel, Serrano-Andrés, Luis, Merchán, Manuela, Borin, Antonio Carlos
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 05.04.2012
• Subjects: adenine; Adenine - chemistry; Adenines; Base Pairing; Bridges (structures); Conversion; energy; Excitation; hydrogen; Hydrogen - chemistry; Maintenance; mathematical theory; physical chemistry; Protons; Repairing; Tautomers; Thymine - chemistry; ultraviolet radiation; Ultraviolet Rays; Wave functions
• ISSN: 1520-6106; 1520-5207; 1520-5207
• DOI: 10.1021/jp300583h

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.