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 struct ural isomers of protonated diglycine and their conformers, as well as the b arriers separating them, were determined by DFT calculations. The rate coef ficients of the proton transfer reactions between the isomers were calculat ed using the RRKM method in order to obtain a quantitative measure of the t ime scale of these processes. The proton transfer reactions were found to b e very fast already at and above the threshold to the lowest energy decompo sition pathway. Two possible mechanisms of b(2)(+)-ion formation via water loss from the dipeptide are also discussed. The rate-determining step of th e proton migration along a peptide chain is also investigated using the mo del compound N-formylglycylglycinamide. The investigations revealed that th is 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-H . . .O=C bridges. Copyright (C) 2001 John Wiley & Sons, Ltd.