κ-Carrageenan interaction with bovine and caprine caseins as shown by sedimentation and NMR spectroscopic techniques implication of surface charge by a homologous three-dimensional model for α S2-casein: κ-carrageenan-casein interaction

Abstract

The solubility and hydration characteristics of κ-carrageenan-casein systems from bovine and caprine milk with incorporated salt (NaCl) were determined by means of sedimentation and 17O nuclear magnetic resonance (NMR) experiments. Relative salt interaction parameters both for caseins alone and in mixtures with κ-carrageenan were assessed by nonlinear regression analysis from the characteristics of solubilization of the systems. The κ-carrageenan-casein interactions appear to depend largely on the ratios of κ- to α S1-casein and possibly α S2-casein. Second virial coefficients (B o values) and hydration products derived from 17O NMR data suggest that while soluble at high salt, the caprine casein mixtures exhibit strong interactions, whereas the bovine counterparts do not. At lower salt concentrations the solubility data and the 17O NMR data are in agreement. Thus, a structural dependence upon protein components in salt containing κ-carrageenan-casein solutions from bovine and caprine milk has been demonstrated. The evidence suggested a role for α-casein in the interactions observed. A homologous three dimensional model for α S2-casein was developed to test this hypothesis. The model was produced by the use of a template model derived from a crystal structure of a human chloride intracellular channel (CLIC)-I and demonstrates a large positively charged surface potential for interactions with the negatively charged the negatively charged κ-carregeenan. © 2006 American Chemical Society