Comparison of Hydration Behavior of Bovine and Caprine Caseins As Determined by Oxygen-17 Nuclear Magnetic Resonance: Effects of Salt

Oxygen-17 nuclear magnetic resonance (NMR) transverse relaxation measurements at 4.7 T were used to study the hydration properties of bovine and caprine casein solutions with and without NaCl. Measurements were carried out at 21 degrees C and pD 6.95. Nonlinear protein concentration dependences were observed for the oxygen-17 NMR transverse relaxation rates, but the fitting of the data did not require any higher order virial coefficients; only B0 was needed. This indicates that long-range, charge-charge repulsive interactions dominate entirely the protein activity in solution. Estimates of the bovine casein average "net" charge were obtained from the virial coefficient (B0) and the partial specific volume; these are in accord with published amino acid sequence data. For bovine casein, the alphas1- and beta-components occur in nearly equal amounts, whereas in caprine casein, beta-casein is the predominant species and beta-casein varies from high to low values, suggesting altered protein-protein interactions. Hydration levels of caprine casein high in the alphas1-casein component when compared with those of bovine casein (intermediate hydration) and the low alphas1-caprine casein (low hydration) are interpreted in terms of "trapped water" and "preferential interactions" with water on the basis of quantitative differences in casein monomer contents