Equilibrium constants for the adsorption of the first water molecule on six protonated dipeptides (Gly-Gly+H(+), Gly-Ala+H(+), Ala-Gly+H(+), Ala-Ala+H(+), Pro-Gly+H(+), and Gly-Trp+H(+)) have been measured as a function of temperature, and DeltaH(o) and DeltaS(o) determined. Density functional theory calculations were performed for both the unsolvated peptides and the peptide water complexes at the B3LYP/6-311++G level. MP2/6-311++G** calculations were also carried out for Gly/Ala peptides. The calculations suggest that adsorption of a water molecule by these simple dipeptides is a complex process, both the unsolvated peptide and the peptide-water complexes have multiple conformations with similar free energies. Average DeltaH(o) and DeltaS(o) values derived from the calculations are in reasonable agreement with the experimental results. According to the calculations, the dominant water adsorption process involves a significant conformational change to accommodate a bridging water molecule. DeltaH(o) is diminished for Pro-Gly+H(+) mainly because the water interacts with a secondary amine, whereas for Gly-Trp+H(+), DeltaH(o) is significantly decreased by the loss of cation-pi interactions upon water adsorption. For unsolvated peptides the proton affinities of the N-terminus and the backbone carbonyl groups are known to be similar. Addition of a single water molecule causes a significant stabilization of the N-terminus protonation site.
