The Study of Zinc Ions Binding to α(S1)-, β- and κ-Casein

The presented studies focused on the specificity binding of particular casein fractions: α(S1)-, β- and κ-casein (α(S1)CN, βCN, κCN), with zinc ions. The binding mechanism was determined by kinetic modeling using results of batch sorption. For this goal, models of zero-order kinetics, pseudo-first-order, pseudo-second-order and Weber–Morris intraparticle diffusion were used. The formation of Zn-α(S1)CN, Zn-βCN and Zn-κCN complexes was additionally monitored using spectroscopic methods such as Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy, characterizing active functional groups involved in the binding process. Additionally, a mass spectrometry technique—matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS)—was used to characterize respective protein fractions and obtained complexes. Spectroscopic and spectrometric studies were carried out both before and after binding the protein with zinc ions. The obtained results showed the difference in Zn-α(S1)CN, Zn-βCN and Zn-κCN complexes created at separate kinetic stages. On the basis of instrumental studies, a significant influence of acidic (glutamic acid (Glu), aspartic acid (Asp)) and aromatic (tryptophan (Trp), phenylalanine (Phe), tyrosine (Tyr)) amino acids on the formation of metal complexes was proven. In turn, spectrometric studies allowed determining the molecular masses of casein isoforms before and after binding to zinc ions.