Study of calcium binding to phosphoserine residues of β-casein and its phosphopeptide (1–25) by ³¹P NMR

Summary The effect of Ca 2+ . binding and ionic strength on the physicochemical characteristics of phosphoserine residues was studied on O -phospho-DL-serine, bovine β -casein and its phosphopeptide (1–25) using 31P NMR. pK in various experimental conditions were determined. The pK of the phosphoserine residues of β -casein and its phosphopeptide (1–25) respectively ranged from 6·46 to 7·21 and from 6·57 to 7·10. pK of O -phospho-DL-serine, β -casein and its phosphopeptide decreased when Ca 2+ was bound to the phosphoserine residue or when ionic strength was increased. The binding of Ca 2+ to the phosphopeptide (1–25) took place at first on phosphoserine residues 17, 18, 19 which had the highest pK, then, when these were saturated, on residue 15 whose pK was the lowest. When the four sites had bound Ca 2+ , peaks corresponding to a different complex form appeared in the spectrum.