Calculated bond dissociation energies and enthalpy of formation of α-amino acid radicals

Abstract

Bond dissociation energies (BDEs), radical stabilization energies (RSEs) and the enthalpies of formation (Δf H°( $${\text{AA}}^{\bullet}$$ )) for the Cα-centered radical of 20 common α-amino acids were calculated at the G3MP2 level of theory. In addition, BDEs and Δf H°( $${\text{AA}}^{\bullet}$$ ) for the Cα-centered radicals were calculated via atomization, homolytic, isogyric and isodesmic reactions using 31 reference compounds. RSEs for the α-amino acid radicals reveal a strong captodative effect due to the resonance stabilization. Intramolecular H-bonding and steric repulsion between the side chains and –NH2 and ‒COOH also have an effect on the stability of α-carbon-centered radicals. For the key small reference compounds investigated, the BDEs (Δf H°( $${\text{AA}}^{\bullet}$$ )) computed with isodesmic reactions were found to be quite consistent with each other in terms of mean absolute deviations (MADs) and root-mean-square deviations between 2.0 (2.6) and 3.2 (4.4) kJ mol−1 from experimental data.