Calorimetric measurements on a series of dipeptides having methionine as first common term enabled us to determine the thermodynamic quantities related to the proton transfer processes at the free α-amino and α-carboxyl groups of the dipeptides using methionyl-methionine (MetMet) as reference compound.
The drawing forces were found to be the solvation processes of zwitterions, ions and undissociated molecules of dipeptides, hydrophobic effects of side-chains playing an important role. The results were interpreted in terms of the reciprocal influence of the dipeptides components.
Relative and absolute scales related to the enthalpic contribution to the acidity and basicity of the dipeptides studied are supplied.