Oxygen-17 nuclear magnetic resonance studies of bovine and caprine casein hydration and activity in deuterated sugar solutions

The hydration of bovine and genetically variable caprine caseins in D2O solutions of sucrose and lactose was investigated by oxygen-17 NMR and fitted by nonlinear regression analysis. A charge-charge interaction model was employed to analyze the transverse relaxation (1/T-2) data. Lactose caused increased hydration of the bovine casein and the caprine casein naturally low in alpha(s1)-casein, whereas sucrose led to increased hydration of the caprine casein naturally high in alpha(s1)-casein. At pD 7.20 and 21 degrees C the effect of charge-charge repulsive interactions on the native caseins generally leads to decreased protein stability in bovine and caprine caseins. However, addition of sugars causes stronger (attractive) interactions yielding more stable casein complexes with increased hydration. The calculated preferential binding term -(partial derivative g(s)/partial derivative g(p)) for casein mixtures suggests that sucrose and lactose are "preferentially" excluded from these milk proteins, yielding greater access to much of the aqueous compartment. This is consistent with the view that sugars lead to the stabilization of proteins in nonfrozen, aqueous systems.