β-Lactoglobulins A and B: The environment of the asp/gly difference residue

The environment of the aspartic acid-glycine substitution has been examined. A pair of peptides of differing mobility, showing the known asp-gly substitution of the native variants, has been isolated from tryptic digests of S-sulfonated β-lactoglobulins A and B. The amino acid composition of the peptides shows a large concentration of carboxylic residues in the neighborhood of the difference amino acid. Comparison with the physicochemical properties of the variants indicates that this area of the primary structure is probably implicated in the low-temperature aggregation, since differences between the two species in degrees of this reaction correlate with the density of carboxyl groups in the vicinity of the substitution.