Barrierless Proton Transfer within Short Protonated Peptides in the Presence of Water Bridges. A Density Functional Theory Study

We have used density functional theory at the B3LYP/6-31++G(d,p) level of theory to investigate proton transfer in protonated N(2)-acetyl-N(1)-methylglycinamide and N-acetylglycyl-N(1)-methylglycinamide with multiwater assistance and to determine the structures and energies of the most important minima and transition states corresponding to the proton-transfer pathways. We propose mechanisms for proton transfer between adjacent and nonadjacent carbonyl oxygen atoms with water bridge assistance. The presence of a two-water bridge connected to the two carbonyl oxygen atoms provides a proton-transfer mechanism having such a low-barrier that the excess proton is almost freely mobile.