Methionine-driven covalent chelation of Zn²⁺ by linusorb: a paradigm in hydrophobic cyclic peptide metal binding

This study elucidates the covalent coordination mechanism underlying Zn²⁺ chelation by linusorbs (LOs) with different redox stages, highlighting the critical role of methionine (Met) residue. Fluorescence quenching, circular dichroism, and differential scanning calorimetry analyses demonstrated that Zn²⁺ interacts with LOs via a static quenching mechanism, forming stable complexes while reinforcing the secondary structural order and stability of LOs. The unoxidized LOs exhibit superior Zn²⁺-binding capacity (Ka = 20.3 L/mol), while oxidation to methionine sulfoxide/sulfone (MetO/MetO₂) significantly diminishes chelation efficacy. ¹H NMR analysis identifies Zn²⁺-induced deshielding of the S-linked γ-CH₂ protons (Δδ = +0.03 ppm), with signal perturbations localized exclusively to the sulfur atom of Met, confirming exclusive coordination to the Met. Density functional theory and quantum topology analyses further validate the covalent characteristics of the S-Zn²⁺ bond (∇²ρ = -0.0696) and polarization of the S-(γ)CH₂ bond triggered by sulfur's electron donation to Zn²⁺. LOs inhibit Zn²⁺-catalyzed lipid oxidation through specific covalent chelation.