31P-NMR studies of bovine beta-casein

Highly resolved 31P nuclear magnetic resonance spectra of the phosphoprotein bovine beta-casein and one of its phosphopeptides (residues 1-25) have been obtained with samples treated to remove paramagnetic metal ions. Spin-lattice (T1) and spin-spin (T2) relaxation times indicate little or no restriction to the segmental motion in the phosphoserine cluster region for both beta-casein and its phosphopeptide. The spectrum of beta-casein consists of four peaks over the pH range 5.7--8.0. The low-field peak has been assigned to Ser(P)-35 by comparison of the beta-casein and the phosphopeptide spectra. Interpretation of the NMR titration curves of 31P chemical shifts against pH in terms of a simple proton dissociation model is unsatisfactory. A two-site model in which it is assumed that the ionization of a phosphoserine residue is affected by the ionization neighbouring residues gives excellent fits to the titration data. The phosphoserine cluster residues (residues 15--19) have been assigned according to this interactive model.