A study of proton transfer in models of a single peptide unit (N-methylacetamide) and diamide (N-1-acetyl-N-1-methylglycinamide) as well as the influence of a single water molecule on proton transfer is presented here. Three proton pathways in protonated N-methylacetamide are considered: isomerization, inversion, and 1,3-proton shift. The isomerization step exhibits the lowest energy barrier. When a single water molecule was added, no significant influence on proton isomerization was observed. In the diamide model, the isomerization-jump mechanism of proton transfer along diamide carbonyl oxygens was inspected, and the proton isomerization steps were found to be the most energy-demanding processes (similar to17 kcal mol(-1)). The presence of a single water molecule leads to a different, lower-energy-barrier proton-transfer mechanism with proton exchange. The highest energy barrier is only 7.6 kcal mol(-1). Possible competing pathways are also discussed.