SOLID-STATE MAGIC-ANGLE-SPINNING P-31-NMR STUDIES OF NATIVE CASEIN MICELLES

Solid-state magic-angle spinning P-31-NMR spectroscopy was used to cha racterize the structure and composition of native casein micelles. The features of the magic-angle spinning P-31-NMR spectra, including over lapping resonances from mobile/immobile phosphorylated serine residues and inorganic calcium phosphates, have been determined using differen t experimental techniques and assigned by comparison with spectra of t he presumed constituents within the casein micelle. Comparison with P- 31-NMR spectra of alpha(s1)-, alpha(s2)-, and beta-caseins in dissolve d and freeze-dried forms demonstrated that a major fraction of the pho sphoserines in these proteins was in an immobilized state within the m icelle. Likewise, from P-31-NMR spectra of the C-terminal part of kapp a-casein, it was shown that this region of the micelle has a considera ble conformational mobility. Finally, magic-angle spinning P-31-NMR sp ectra for a series of inorganic calcium phosphates and mineralized bon e tissue revealed that the micellar inorganic calcium phosphates exhib it structural similarities to hydroxyapatite and hence resemble minera lized bone tissue.