title: 31P-NMR studies of bovine beta-casein.
abstract: 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.
mesh_terms: D000818:Animals; D002364:Caseins; D002417:Cattle; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008961:Models, Structural; D010748:Phosphopeptides / Q000032:analysis; D011487:Protein Conformation
chemical_list: D002364:Caseins; D010748:Phosphopeptides