Understanding DNA Radicals Employing Theory and Electron Spin Resonance Spectroscopy

Abstract

Abstract Ionizing radiation, such as γ‐rays, X‐rays, or ion beam radiation, results in the formation of cation and anion radicals in DNA and neutral radicals by secondary reactions. These DNA radicals are stable at low temperatures and, therefore, can be studied by electron spin resonance (ESR) spectroscopy. In these DNA radicals, the unpaired spin on the radical site interacts with nuclei with a magnetic moment, such as, 1 H, 2 H, and 14 N. This electron spin–nuclear spin interaction gives rise to hyperfine coupling, which is characterized by the hyperfine coupling constant (HFCC) values. The HFCC values along with the g ‐values determine the ESR spectra of these radicals. Calculations based on density functional theory (DFT) allow for identification of these DNA radicals via the prediction of stabilization energies and, most importantly, accurate values of the HFCCs. These calculations elucidate radical conformational structures as well. Thus, the combination of ESR with DFT calculations is a powerful approach to the investigation of DNA radicals and is the main focus of this article. This is illustrated in this article by specific examples—such as the prediction of sites of deprotonation in the DNA base cation radicals, intra‐base pair proton transfer processes in DNA base radicals, and excited state‐induced reactions of the DNA base cation and anion radicals leading to sugar radical formation.