STRUCTURE AND ESR FEATURES OF GLYCINE RADICAL

The structure, conformational behavior, and ESR features of the glycin e radical have been investigated by an established quantum-mechanical protocol with the aim of better elucidating the role of intrinsic and environmental effects in determining the physicochemical properties of amino acid radicals involved in biological systems. From a structural point of view, extraction of a hydrogen atom from glycine modifies on ly the local environment of the C(alpha)atom. The conformational freed om of the radical is, however, severely restricted with respect to its closed-shell parent. In particular, only planar or nearly planar stru ctures are energetically accessible. These are characterized by very s imilar hyperfine splittings, which ate in agreement with experiment fo r C-alpha and N, but are significantly too large for H-alpha. Although the average value of H(N) splittings is not far from the experimental value, the two protons are strongly not equivalent. The computed tors ional barrier around the N-C-alpha bond is too high to allow an effect ive rotational averaging and also inversion of the NH2 moiety, which i s governed by a low-energy barrier (approximate to 3 kJ/mol), cannot r estore agreement with experiment. Inclusion of solvent-induced structu ral modifications significantly improves matters for H-alpha, whereas the equivalence of H(N) atoms in acidic solution can be explained in t erms of a mixture between the neutral species and a nonclassical catio nic form.