PHYSICOCHEMICAL VERIFICATION OF NONEXISTENCE OF αs1‐CASEIN‐κ‐CARRAGEENAN INTERACTION IN CALCIUM‐FREE SYSTEMS

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Abstract
The interaction between αs1, ‐casein and K‐carrageenan was investigated by sedimentation velocity and sedimentation equilibrium ultra‐centrifugation, frontal chromatography, fluorescence polarization, viscosity, and turbidity experiments. Schlieren patterns of αs1‐casein‐K‐carrageenan mixtures during sedimentation velocity ultracentrifugation experiments (pH 6.6, μ= 0.08) revealed a large αs1 ‐casein containing peak followed by a slower sedimenting peak which was thought to be residual, uncomplexed k‐carrageenan. The S2 0, w of the interaction peak was greater than the S2 0, w of αs1 ‐casein alone under identical conditions. The effects of pH, ionic strength, temperature and 6.OM urea suggested that the interaction observed during sedimentation velocity ultracentrifugation and viscometry of αs1 ‐casein and K‐carrageenan was mediated by hydrogen bonding. Fluorescence polarization, frontal chromatography, and molecular weight distributions calculated from sedimentation equilibrium data, however, showed that αs1 ‐casein and K‐carrageenan did not interact in calcium‐free systems (pH 6.6, μ= 0.08). DNS‐αs1 ‐casein and DNS‐K‐carrageenan were employed as the labelled components during the fluorescence polarization experiments. αs1 ‐Casein‐K‐car‐rageenan mixtures eluted from a controlled pore glass column (170 Å pore diameter) as the individual components with elution volumes identical to those obtained when αs1 ‐casein and K‐carrageenan were chromatographed separately. The molecular weight distributions of αs1 ‐casein‐K‐carrageenan mixtures (pH 6.6, μ= 0.08), subjected to sedimentation equilibrium ultracentrifugation, contained a major peak in the molecular weight range corresponding to unreacted αs1, ‐casein. Thus the “interaction” revealed by sedimentation velocity and viscosity data was not a chemical interaction but, rather a physical entrapment of αs1 ‐casein by K‐carrageenan. Fluid flow through the capillary during viscosity measurements and the intense gravitational fields generated during sedimentation velocity ultracentrifugation probably induced physical entanglement of the K‐carrageenan. Under these conditions, αs1 ‐casein‐K‐carrageenan mixtures flowed as a “porous‐plug” where αs1 ‐casein, larger than the pores of the K‐carrageenan network, was trapped giving rise to the observed “interaction” peak. Physical entanglement of αs1 ‐casein within the K‐carrageenan system which causes a pseudo‐interaction was not detected during frontal chromatography, sedimentation equilibrium, and fluorescence polarization measurements. Thus, αs1 ‐casein and K‐carrageenan do not chemically interact in calcium‐free systems.