Infrared Spectroscopic Evidence for Calcium Ion Interaction with Carboxylate Groups of Casein

Examination of the Fourier-transform infrared difference spectra of lyophilized whole caseins with and without Ca 2+ ions provides direct empirical evidence for the interaction between these divalent ions and the carboxylate groups of glutamate and as partate residues. In the absence of Ca L+, the O-C-O stretching vibrations of these carboxylates give two characteristic infrared absorption bands near 1400 and 1575 cm -1. When Ca 2 ions are present, this pair of bands was observed to shift about 10 cm -1 to approximately 1410 and 1565 cm -1, respectively. In addition, the difference in the frequency between the two peaks decreased. Such changes in the position of the carboxylate bands typically indicate that significant interaction has occurred between the carboxylates and the metal ion. This observation suggests that carboxylate groups may play an important role along side that of the serine phosphates as sites for Ca 2+ ion binding in caseins.