Evaluation of quantitative probes for weaker Cu(i) binding sites completes a set of four capable of detecting Cu(i) affinities from nanomolar to attomolar

Abstract

Copper plays essential roles in biology, but abnormal interactions are damaging. Reliable quantification of copper–protein interactions will underpin the molecular understanding of copper nutrition and toxicity. We have previously established two high affinity probes, Bathocuproine disulfonate (Bcs) and Bicinchoninate (Bca) anions, that are capable of in vitro quantification of Cu(I) binding with affinities from pico- to atto-molar concentrations. Quantitative probes are required for Cu(I) binding of lower affinity for proteins and peptides typically associated with neurodegenerative diseases. The present work evaluates two classic Fe(II) ligands Ferene S (Fs) and Ferrozine (Fz) as quantitative probes for Cu(I). Both react with Cu(I) quantitatively to yield well-defined complex anions [Cu^I(Fs)₂]³⁻ (λ_max = 484 nm, ε = 6700 cm⁻¹ M⁻¹) and [Cu^I(Fz)₂]³⁻ (λ_max = 470 nm, ε = 4320 cm⁻¹ M⁻¹). These complexes are sensitive to aerial oxidation (E_1/2 ∼ +0.36 V vs. SHE) and to substitution by other ligands (e.g., Cl⁻, MeCN). However, they can be protected effectively under anaerobic conditions by suitable reductants and an excess of the free probe ligands. Formation constants β₂ were determined by two approaches: direct metal ion titration and ligand competition. They provided estimates which differed by ∼3 orders of magnitude. The sources of these differences were examined carefully to consolidate the affinities of the two probes to a unified standard (10^15.1 M⁻² for Fz and 10^13.7 M⁻² for Fs). It is apparent that application of direct metal ion titrations to quantification of Cu(I) binding affinities is problematical and should be avoided. The four ligands Bcs, Bca, Fz and Fs in combination form a set of versatile probes for ligand competition experiments and are capable of detecting and differentiating an extended spectrum of Cu(I) binding affinities from nano- to atto-molar concentrations. Selected examples of quantification of weaker Cu(I) binding in proteins and peptides are provided, including that of an amyloid-β peptide.