Coordination of WPI Amyloid Fibrils/Hydrolysates with Zinc and Copper Ions: A Comparative Study on the Effect of Accompanying Anion

Abstract

Metal-protein complexes are commonly used to supplement foods and feeds with essential minerals. This study investigated the coordination of Cu²⁺ and Zn²⁺ with WPI nanofibrils/hydrolysates, with a focus on the impact of the accompanying anion. The results showed that nanofibrils had a significantly higher binding ability compared to hydrolysates (50% versus 39%, respectively). Furthermore, Cu²⁺ exhibited a greater binding affinity towards proteins than Zn²⁺, with an approximately 25% higher binding efficiency. Among the accompanying anions, SO₄²⁻ was found to increase the coordination of metal ions more effectively than Cl⁻, achieving a binding efficiency of 50% compared to 37% for Cl⁻. Electrical conductivity measurements provided evidence of Cu²⁺ and Zn²⁺ binding to the proteins. UV–vis spectroscopy revealed that the metal-to-ligand charge transfer transition was the driving force behind the coordination reaction. The formation of metal-protein complexes was further supported by displacement and intensity changes of FTIR peaks, particularly the amide I, II, and III bands, as well as the N-H stretching vibration peak. SEM images visualized the binding of metal ions to the proteins, resulting in disruption of their original microstructure. SEM-coupled energy dispersive X-ray spectroscopy confirmed the presence of metal ions in the surface elemental composition of the chelates, which was also verified by the X-ray diffraction pattern spectra. These findings demonstrate the promising potential of nanofibrils to effectively coordinate metal ions.