Colloidal Calcium Phosphates in Casein Micelles Studied by Slow-Speed-Spinning ³¹P Magic Angle Spinning Solid-State Nuclear Magnetic Resonance

¹ Laboratory for Biomolecular NMR Spectroscopy, Department of Molecular and Structural Biology, Science Park, University of Aarhus, DK-8000 Aarhus C, Denmark
² Protein Chemistry Laboratory, Department of Molecular and Structural Biology, Science Park, University of Aarhus, DK-8000 Aarhus C, Denmark

The composition of bovine casein micelles was analyzed by ³¹P magic angle spinning solid-state nuclear magnetic resonance spectroscopy. By looking at isotropic and anisotropic ³¹P chemical shift parameters, resonance line shapes, the combination of single-pulse and ¹H to ³¹P cross-polarization spectra, and comparison with spectra for various model compounds combined with multiple-component simulation and iterative fitting procedures, we were able to identify and quantify a variety of inorganic and organic phosphates in the micelles. These include phosphates from mobile and immobile inorganic hydroxyapatite-type phosphates as well as phosphates from -casein and the Ca²⁺-binding phosphoserines from _s1, _s2, and ß-casein. This information is discussed in relation to previous knowledge and various models for the colloid formation.

Key Words: casein micelle • ³¹ P solid-state nuclear • magnetic resonance • anisotropic chemical shifts • calcium phosphate

Submitted on July 31, 2000
Accepted on December 1, 2000

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