Anion-regulated binding selectivity of Cr(III) in collagen

We present a mechanism for the selectivity of covalent/electrostatic binding of the Cr(III) ion to collagen, mediated by the kosmotropicity of the anions. Although a change in the long-range ordered structure of collagen is observed after covalent binding (Cr(III)-OOC) in the presence of SO₄ ^2- at pH 4.5, the ν_sym (COO^- ) band remains intense, suggesting a relatively lower propensity for the Cr(III) to bind covalently instead of electrostatically through Cr(H₂ O)₆ ³⁺ . Replacing SO₄ ^2- with Cl^- reduces the kosmotropic effect which further favors the electrostatic binding of Cr(III) to collagen. Our findings allow a greater understanding of mechanism-specific metal binding in the collagen molecule. We also report for the first time, surface-enhanced Raman spectroscopy to analyze binding mechanisms in collagen, suggesting a novel way to study chemical modifications in collagen-based biomaterials.