Binding mechanism of metal ions (Ca2+, Cu2+ and Mg2+) with tetrapeptide FFDR: A combined experimental and quantum chemistry approach

A previous study demonstrated that a tetrapeptide FFDR derived from coix seed possesses antioxidant properties. In continuation of the study, Density Functional Theory (DFT) was employed to investigate the molecular-level complexation behaviour of FFDR with Ca²⁺, Cu²⁺, and Mg²⁺. DFT predictions were validated using spectroscopy and cellular model. The electronic properties revealed that Mg-FFDR, with its lower energy gap (1.733 eV), exhibits higher reactivity compared to Ca-FFDR which displayed higher stability (8.180 eV). The Quantum Theory of Atoms in Molecules (QTAIM) showed positive Laplacian values for all metal‑oxygen bonds, indicating the presence of coordination bonds characteristic of closed-shell interactions. Results from ¹H NMR spectra revealed J-coupling patterns consistent with metal coordination for Mg and Ca-peptide complexes. FTIR spectra displayed distinct changes in the vibrational frequencies of functional groups involved in metal binding for all complexes. Both Mg-FFDR and Ca-FFDR demonstrated significant ROS scavenging activities, and enhanced SOD and CAT activities in HepG2 cells. These findings serve as a baseline for the rational design of metal-peptide complexes as functional foods or nutraceuticals.