Solution thermochemistry of the radicals of glycine

Abstract

Using gas phase thermochemical data, the following Gibbs energies of formation in aqueous solution (in kJ mol–1) have been estimated for radicals of glycine: H3N+CH2CO2˙– 93, H2N˙+CH2CO2H –163, H3N+CH˙CO2H –198, H2NCH˙C(OH)2+–268, H3N+CH2CO2––371, H2NCH2CO2˙–95, H2N˙+CH2CO2––158, HN˙CH2CO2H –148, H2NCH˙CO2H –246, HN˙CH2CO2––147 and H2NCH˙CO2––208. The uncertainty in these values is estimated to be ±20 kJ mol–1.In accord with earlier EPR studies, the H2NCH˙CO2H and H2NCH˙CO2– radicals are predicted to be the most stable. Non-equivalence of the NH protons of the latter can be rationalized by a strong internal H ⋯–OCO bond. Formation of the H3N+CH2CO2˙ and H2NCH2CO2˙ acyloxyl species is expected to require very strong oxidants (E° > 3 V). Production of H2N˙+CH2CO2H and H2N˙+CH2CO2– is proposed as a better explanation of H2NCH2˙ formation in SO4˙– oxidations. The H2N˙+CH2CO2– radical, which is also susceptible to loss of CO2, would lie above H2NCH2CO2˙ in the gas phase, but its Gibbs energy of formation in aqueous solution will be ca. 0.65 V less than that of H2NCH2CO2˙. E°(H2N˙+CH2CO2–/H2NCH2CO2–) is estimated to be near 1.6 V. This is in keeping with observed one electron oxidations of H2NCH2CO2– by triplet states of organic molecules with reduction potentials in the region of 1.5–1.8 V.