Dissociation energies of C a – H bonds in amino acids – a re - examination 3

The Ca–H bond dissociation energies (BDE) in glycine and alanine peptide models have been assessed using selected theoretical methods from the G3 and, in part, G4 family. The BDE values (and thus the stability of the respective Ca peptide radicals) are shown to depend significantly on the level of theory, the size of the model system and the coverage of conformational space. For the largest dipeptide models chosen here, BDE(Ca–H) values of +363.8 kJ mol 21 (glycine) and +372.3 kJ mol (alanine) have been obtained at G3B3 level. This reconfirms earlier findings that glycyl peptide radicals are more stable than radicals derived from alanine or any other amino acid carrying substituents at the Ca position.