Study on zinc ions binding to the individual casein fractions: alpha S1-, beta- and Kappa- casein

The presented work is focused on the isotherm study on the Zn2 + ions binding to the individual ca-sein fractions: alpha S1-, beta- and Kappa-casein (alpha S1CN, beta CN and Kappa CN). The experimental isotherms were evaluated using Freundlich and Langmuir models. Subsequently, the stability of the obtained complexes in the re-spective Zn2 + ion concentrations (120, 350, 600 mg/L) was determined by carrying out zeta potential measurements. Capillary electrophoresis combined with inductively coupled plasma mass spectrometry (CE-ICP-MS) confirmed the occurring binding process. Additionally, physicochemical characteristics of the obtained metal-protein complexes was performed including scanning electron microscopy (SEM) in two modes (SE and Z-contrast) and the binding sites of caseins to Zn2 + ions were indicated using attenu-ated total reflectance infrared spectroscopy (FTIR-ATR) and Raman analysis as well as mass spectrometry technique (MALDI-TOF MS). Isothermal studies indicated a heterogeneously complex zinc ion adsorption process, and a stability study showed that the zeta potential is strongly related to the hydrophobicity, size and structure of the casein isoforms studied. Electron microscopy confirmed the modification of ca-sein surfaces due to the addition of Zn2 + ions. Spectroscopic techniques indicated the interaction of zinc ions with polar amino acids of casein, such as glutamic acid (Glu) and aspartic acid (Asp), but also His, Cys. The influence of phosphate groups was also observed. Finally, the study culminated in a molecular docking study of the Zn2 + ion binding process, which confirmed the presence of the listed amino acids responsible for the binding process.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )