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 SO42− at pH 4.5, the νsym(COO−) band remains intense, suggesting a relatively lower propensity for the Cr(III) to bind covalently instead of electrostatically through Cr(H2O)63+. Replacing SO42− 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.