Abstract
The presented studies focused on the specificity binding of particular casein fractions: αS1-, β- and κ-casein (αS1CN, 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-αS1CN, 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-αS1CN, 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.
Highlights
Casein is recognized as the main cow’s milk protein, accounting for about 80% of the total milk protein content [1]
This study describes for the first time the mechanism of binding zinc ions to individual casein fractions, causing the formation of complexes, and indicating the precise contribution of individual fractions to the binding
The initial stage is associated with rapid initial sorption, the second stage with moderate sorption and the third stage with gradual achievement of the sorption equilibrium
Summary
Casein is recognized as the main cow’s milk protein, accounting for about 80% of the total milk protein content [1]. The main fractions are α (αS1, αS2)-, β- and κ-casein (αCN, βCN and κCN) They form colloidal aggregates called micelles in combination with calcium phosphate [2]. The main components of casein (αCN, βCN and κCN) form a strong micellar complex stabilized by van der Waals forces, hydrophobic effects, hydrogen bonding and electrostatic and steric stabilization [2]. Micelles are stabilized by physicochemical properties such as the ability of phosphorylated serine residues to bind to calcium ions and the amphiphilic nature of κCN, which is a fraction that is glycosylated and responsible for stabilizing micelles on the surface [4]. It turns out that caseins have a natural ability to bind to other metal ions such as silver [7], iron [8] and zinc [9,10]. There are still many uncertainties in the mechanism of binding metal ions to casein fractions, especially their quantitative contribution and the role of the active functional group in this process
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