COMPARISON OF HYDRATION BEHAVIOR OF BOVINE AND CAPRINE CASEINS AS DETERMINED BY O-17 NUCLEAR-MAGNETIC-RESONANCE - EFFECTS OF SALT

Oxygen-17 nuclear magnetic resonance (NMR) transverse relaxation measu rements 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 concentrat ion dependences were observed for the oxygen-17 MMR transverse relaxat ion rates, but the fitting of the data did not require any higher orde r virial coefficients; only Bo was needed. This indicates that long-ra nge, charge-charge repulsive interactions dominate entirely the protei n activity in solution. Estimates of the bovine casein average ''net'' charge were obtained from the virial coefficient (B-o) and the partia l specific volume; these are in accord with published amino acid seque nce data. For bovine casein, the alpha(s1)- and beta-components occur in nearly equal amounts, whereas in caprine casein, beta-casein is the predominant species and alpha(s1)-casein varies from high to low valu es, suggesting altered protein-protein interactions. Hydration levels of caprine casein high in the alpha(s1)-casein component when compared with those of bovine casein (intermediate hydration) and the low alph a(s1)-caprine casein (low hydration) are interpreted in terms of ''tra pped water'' and ''preferential interactions'' with water on the basis of quantitative differences in casein monomer contents.