A Study of the Interaction of Calcium Ions with Bovine αs1-Casein Using Fluorescence Spectroscopy.

The interaction of αs1-casein with calcium ions at pH 7.0 has been studied, and it is shown that there is a stepwise increase in the fluorescence of the tryptophan residues of the protein as increasing amounts of Ca2+ are added, leading to eventual precipitation of the calcium-caseinate complex. The first of these steps appears to result from a conformational change induced by Ca2+, without which the protein cannot precipitate. Further steps arise from interaction between protein molecules to produce particles of about 120 nm in diameter, then final aggregation of these particles to give a precipitate. All of these changes result in changes in the fluorescence emission spectra of one or both tryptophan residues in the casein, although most of the change in fluorescence behaviour in the first step of the interaction probably arises from tryptophan-164 in the protein: [ABSTRACT FROM AUTHOR]