Structure and ESR features of glycine radical

The structure, conformational behavior, and ESR features of the glycine 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 only the local environment of the C(alpha)atom. The conformational freedom of the radical is, however, severely restricted with respect to its closed-shell parent. In particular, only planar or nearly planar structures are energetically accessible. These are characterized by very similar hyperfine splittings, which ate in agreement with experiment for 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 torsional barrier around the N-C-alpha bond is too high to allow an effective rotational averaging and also inversion of the NH2 moiety, which is governed by a low-energy barrier (approximate to 3 kJ/mol), cannot restore agreement with experiment. Inclusion of solvent-induced structural modifications significantly improves matters for H-alpha, whereas the equivalence of H(N) atoms in acidic solution can be explained in terms of a mixture between the neutral species and a nonclassical cationic form.