On the internal photorelaxation mechanism of DNA

TL;DR

This paper models DNA's photo-deactivation process, showing ultrafast delocalization and re-localization of excitation states that explain DNA's inherent photostability, supported by quantum chemical and molecular dynamics evaluations.

Contribution

It introduces a detailed quantum chemical and molecular dynamics model for DNA's internal photorelaxation mechanism, aligning with recent ultrafast experimental findings.

Findings

Excitation delocalizes over 3-4 bases rapidly.

Charge-transfer state re-localizes within 10-100 ps.

Deactivation occurs via conical intersection near tautomeric geometries.

Abstract

We propose a model for the photo-deactivation mechanism for DNA based upon accurate quantum chemical and molecular dynamical evaluations of model Watson/Crick nucleoside pairs and stacked pairs. Our results corroborate recent ultrafast experimental studies on DNA oligonucleotides and suggest that following photo-excitation to a local $\pi-\pi^*$ state, the excitation is rapidly delocalized over several (3-4) bases on an ultrafast time-scale. However, this delocalized state is unstable with respect to the motions of the protons involved in hydrogen-bonding between Watson/Crick pairs and rapidly re-localizes to a charge-transfer state on a longer time-scale ranging from 10 to 100 ps. This state, too, is unstable and relaxes via a conical intersection with the ground state near the geometry of the enol- and imino-tautomeric form. We suggest that this internal deactivation mechanism is responsible for the intrinsic photostability of DNA.