Proton mobility in protonated glycylglycine and N-formylglycylglycinamide: a combined quantum chemical and RKKM study.

Theoretical model calculations were performed to investigate the degree of validity of the mobile proton model of protonated peptides. The structures and energies of the most important minima corresponding to different structural isomers of protonated diglycine and their conformers, as well as the barriers separating them, were determined by DFT calculations. The rate coefficients of the proton transfer reactions between the isomers were calculated using the RRKM method in order to obtain a quantitative measure of the time scale of these processes. The proton transfer reactions were found to be very fast already at and above the threshold to the lowest energy decomposition pathway. Two possible mechanisms of b2+-ion formation via water loss from the dipeptide are also discussed. The rate-determining step of the proton migration along a peptide chain is also investigated using the model compound N-formylglycylglycinamide. The investigations revealed that this process very possibly occurs via the protonation of the carbonyl oxygens of the amide bonds, and its rate-determining step is an internal rotation-type transition of the protonated C=O-H group between two adjacent C=O-HellipsisO=C bridges. Copyright 2001 John Wiley & Sons, Ltd.