Characterization of the interactions between apple condensed tannins and biologically important metal ions [Fe2+ (3d6), Cu2+ (3d9) and Zn2+ (3d10)]
Abstract To provide information regarding the mechanisms of interactions between apple condensed tannins (ACT) and biologically important metal ions (Fe 2+ , Cu 2+ , Zn 2+ ) as well as characterize ACT-metal ion coprecipitates, spectroscopy methods, inductively coupled plasma mass spectrometry (ICP-MS), thermogravimetry analysis (TGA) and field emission scanning electron microscopy (FE-SEM) studies were utilized in the present study. Our results showed that ACT from unripe apple (cv. Golden Delicious) fruits were a mixture of B-type procyanidin oligomers with tetramers and pentamers as the main components. The (−)-epicatechin was the dominant unit of oligomer structures. Metal ions quenched the fluorescence of ACT via a static mechanism, while ACT interacted with metal ions mainly via catechol group-ion exchange. The binding constants, binding sites, the absolute values of Gibbs energy changes and effectiveness of precipitation of metal ions by ACT followed the order of Fe 2+  > Cu 2+  > Zn 2+ . Besides, the characteristics of ACT-metal ion coprecipitates confirmed the interactions of ACT with metal ions. The metal content of ACT-Fe 2+ , ACT-Cu 2+ or ACT-Zn 2+ coprecipitates was approximately 1.94%, 1.71% or 1.25%, respectively. Therefore, the findings of our investigation were able to help in the developments of ACT-metal nanoparticles, ACT-based sensors and sorption materials for metal ions.