Thermodynamics of dipeptides in water: Part 3. Strengthening and weakening effects of different side chains on the free α-amino and free α-car☐yl groups

Abstract

Abstract A calorimetric study has been made of the dissociation processes in water of the free α-amino group of the amino terminal residue and the free α-car☐yl group(s) of the car☐yl terminal residue in various dipeptides. The influence of the structure of glycine, which was one of the residues in each of the dipeptides, upon a number of other α-amino acids as second components, and the influence of these other α-amino acids upon the structure of glycine were investigated using glycyl glycine and individual free α-amino acids as reference structures. A comparison was made with the reciprocal influence of valine structure on some α-amino acids in a series of valyl dipeptides. Differences in behaviour were found, according to the different lengths of the side-chains of the two reference structures. The aliphatic chain of valine deactivates the car☐yl groups of most of the α-amino acids considered in a series of dipeptides, while the positive charge of the NH 3 + group of glycine activates them in another series. The α-amino acids deactivate the free α-amino group of valine in the first series of dipeptides, although this does not happen for the amino group of glycine in the second series of dipeptides.