An FTIR spectroscopy study of the interaction between αs-casein-bound phosphoryl groups and chitosan

Abstract Fourier-transform infrared spectroscopy was used to study the nature of the linkage and interactions of phosphate ester bonds in α s -casein under precipitation by chitosan. We have found that the dianionic stretching band of the covalently bound phosphate in α s -casein at 976 cm −1 is sensitive to the ionization state and the binding of Ca 2+ or chitosan. Thus, the neutralization of the negative charges of carboxylates and phosphates by lowering the pH of α s -casein solution from 6.8 to 2.0 led to a dramatic reduction of this signal. Precipitating amounts of Ca 2+ caused a shift in the phosphate signal from 976 to 986 cm −1 indicating a direct electrostatic interaction between Ca 2+ and phosphate. The interaction of α s -casein with low molecular weight chitosan showed a small shift (ca. 2 cm −1 ) in the phosphate peak position as compared with pure α s -casein with a pronounced reduction in the phosphate peak amplitude that was about a half of that of casein alone. When α s -casein was precipitated with high molecular weight chitosan, a more noticeable effect occurred as this complex showed only around 25% of the phosphate peak amplitude. The interactions between the phosphate groups covalently bound to α s -casein and the amino groups in chitosan seem to induce changes similar to those observed upon protonation of the negative charges of phosphate.