Studies of Electrode Reactions and Coordination Geometries of Cu(I) and Cu(II) Complexes with Bicinchoninic Acid

Abstract

AbstractBicinchoninic acid (BCA) is widely used for determining the valence state of copper in biological systems and quantification of the total protein concentration (BCA assay). Despite its well‐known high selectivity of Cu(I) over Cu(II), the exact formation constants for Cu(I)(BCA)23− and Cu(II)(BCA)22− complexes remain uncertain. These uncertainties, affect the correct interpretations of the roles of copper in biological processes and the BCA assay data. By studying the voltammetric behaviors of Cu(I)(BCA)23− and Cu(II)(BCA)22−, we demonstrate that the apparent lack of redox reaction reversibility is caused by an adsorption wave of Cu(II)(BCA)22−. With the adsorption wave identified, we found that the Cu(I)/Cu(II) selectivity of BCA is essentially identical to another popular ligand, bathocuproinedisulfonic acid (BCS). Density functional theory calculation on the geometries of Cu(I)(BCA)23− and Cu(II)(BCA)22− rationalizes the preferential Cu(I) binding by BCA and the strong adsorption of the Cu(II)(BCA)22− complex at the glassy carbon electrode. Based on the shift in the standard reduction potential of free Cu(II)/Cu(I) upon binding to BCA, we affirm that the formation constants for Cu(I)(BCA)23− and Cu(II)(BCA)22− are 1017.2 and 108.9, respectively. Therefore, BCA can be chosen among various ligands for effective and reliable studies of the copper binding affinities of different biomolecules.