ESR study of aqueous dispersions of β-lactoglobulin and spin-labelled glyceryl monostearate

Abstract

From the ESR spectra of aqueous dispersions of synthetic glyceryl monostearate (spin labelled at C-12) a critical micelle concentration of 30 μmol/l at room temperature was obtained, which agrees with that deduced from surface tension measurements. At monoglyceride concentrations smaller or larger than the critical micelle concentration, the monomers show increased motional restriction with increasing molar ratio of β-latoglobulin to monoglyceride up to a value of 10, as determined from calculated rotational correlation times. A similar progressive interaction was deduced from spectral changes observed on equimolar dispersions of β-lactoglobulin and monoglyceride on raising the temperature to 55°C at which the protein and monoglyceride coprecipitate. The relevance of these findings for non-labelled monoglyceride dispersions is indicated by the similarity of the pH-dependent flocculation behaviour of labelled and non-labelled monoglycerides, both in the absence and presence of β-lactoglobulin. In addition, proton magnetic resonance and mechanical stability measurements suggest that spin-labelled glyceryl monostearate behaves analogously to non-labelled glyceryl monooleate.