On the thermodynamics of peptide oxidation: anhydrides of glycine and alanine1

The α-amino C–H bond dissociation enthalpies of glycine anhydride and alanine anhydride and the C–O bond dissociation enthalpies of the corresponding anhydride peroxyl radicals have been measured by photoacoustic calorimetry in aqueous solution. Furthermore, the one-electron oxidation potentials of the C-centered radicals formed upon hydrogen abstraction from glycine anhydride and alanine anhydride have been measured by photomodulation voltammetry in aqueous solution. In addition, the C–H bond dissociation enthalpies of glycine anhydride and alanine anhydride and one-electron reduction potentials of the corresponding radicals have been calculated by ab initio methods [at the B3LYP/6-31G(D) level]. The resulting experimental gas-phase C–H bond dissociation enthalpies are 340 and 325 ± 15 kJ mol–1, the C–O bond dissociation enthalpies are 56 and 64 ± 15 kJ mol–1 and the one-electron oxidation potentials are 0.175 and 0.086 V vs. NHE for the glycine anhydride and alanine anhydride related species, respectively. The calculated C–H bond dissociation enthalpies are 351.1, 334.7 and 332.7 ± 10 kJ mol–1 for glycine anhydride, L-alanine anhydride and D,L-alanine anhydride, respectively. The one-electron reduction potentials of the corresponding radicals are 1.19, 1.00 and 0.99 V vs. NHE. The thermochemical properties of amino acid anhydrides and the corresponding C-centered radicals and peroxyl radicals determined in this work are consistent with each other and with previously published observations on the radical chemistry of amino acid anhydrides and related species.