NMR studies of the phosphoserine regions of bovine alpha s1- and beta-casein. Assignment of 31P resonances to specific phosphoserines and cation binding studied by measurement of enhancement of 1H relaxation rate.

(1) High-resolution 31P-NMR was used to study the environment of the phosphoserine residues of the phosphoproteins, alpha s1-casein B, beta-casein A2 and beta-casein C. For reference purposes 31P-NMR spectra of phosvitin and ovalbumin were also collected. (2) 31P resonances were assigned to specific phosphoserine residues as a result of comparisons of the high-resolution 31P-NMR spectra for alpha s1- and beta-caseins and for peptide fragments of these proteins obtained by cyanogen bromide and trypsin cleavage. (3) Measurements of the enhancement of the relaxation rate for water protons (1H) on addition of Mn2+ to alpha s1-casein B and to a fragment alpha s1-CN3, obtained by cyanogen bromide cleavage, gave approximate pK values for the binding groups and suggest the possibility of a conformational change induced by varying the concentration of divalent cation.