COMPARISON OF HYDRATION BEHAVIOR OF BOVINE AND CAPRINE CASEINS AS DETERMINED BY OXYGEN-17 NUCLEAR MAGNETIC RESONANCE - TEMPERATURE DEPENDENCE OF COLLOIDAL STABILITY

摘要

Oxygen-17 nuclear magnetic resonance (NMR) transverse relaxation rates for bovine and caprine casein micelles at various temperatures were analyzed by nonlinear regression analysis and a protein activity model. The dependence of the NMR transverse relaxation rates was markedly nonlinear due to interactions between protein molecules. Temperature dependences of the hydration parameters of the bovine and caprine casein micelles were in accordance with the hypothesis that hydrophobic interactions are the predominant forces responsible for the self-association of the caseins. Relaxation differences between reconstituted micelles of bovine and caprine caseins strongly suggest that important structural dissimilarities exist between these milk proteins that are due to differences in the ratios of alpha(s1)- to beta-casein. A higher degree of hydration, characteristic of a more open and looser structure, is observed for caprine casein micelles high in alpha(s1)-casein at 21 and 37 degrees C. The observed hydration behavior of bovine casein micelles at all three temperatures is consistent with the hydration values determined previously by deuterium NMR studies of bovine casein micelles in D2O containing 1,4-piperazinediethanesulfonic acid. The correspondence between the deuterium and oxygen-17 results suggests that both experiments detect exchangeable water ''trapped'' within the casein micelles. The dependence of the second virial coefficient B-0 on temperature was different for bovine and caprine casein micelles, suggesting the importance of ''net'' electrostatic charges of these milk proteins in their interactions with calcium and water.