Density functional calculations on alanine-derived radicals: Influence of molecular environment on EPR hyperfine coupling constants

The amino acid L-alpha -alanine and its associated radiation-induced radica ls display particular characteristics in solid-state that make it very appr opriate for use in Electron Paramagnetic Resonance (EPR) dosimetry. In cont rast to the number of experimental studies. relatively few theoretical stud ies have been published concerning the EPR parameters of these radicals. Ho wever. these studies inadequately account for the molecular environment of the alanine radicals in the crystalline lattice. Here. we present Density F unctional Theory (DFT) calculations on one of the stable radiation-induced radicals of L-alpha -alanine both in molecular cluster models and in period ic models. An extensive investigation is presented on the various geometric al ingredients which have a substantial impact on the hyperfine coupling co nstants as the planarity of the radical backbone and the internal rotations of the final methyl and amino group vary. It is found that the accurate mo deling of the hydrogen bonds with neighboring molecules is of utmost import ance for an adequate reproduction of the experimental data.