Spectroelectrochemical and electrochemical determination of ligand binding and electron transfer properties of myoglobin, cyanomyoglobin, and imidazolemyoglobin

Electrochemistry and spectroelectrochemistry were used to study solutions of horse myoglobin, both by itself and complexed with cyanide or imidazole, at indium-tin oxide electrodes. In the absence of ligands other than water, myoglobin exhibited slow quasi-reversible heterogeneous electron transfer kinetics, with a formal heterogeneous electron transfer rate constant (k(o')) value of 1.6(+/- 0.4) x 10(-5) cm s(-1). Differences from simple simulations in the reverse sweep (anodic) cyclic voltammograms are proposed to be due to residual amounts of dioxygen in solution and/or in the protein molecule. Those differences were not seen in the anodic waveforms of simultaneously acquired derivative cyclic voltabsorptometry (DCVA) experiments. Cyanomyoglobin exhibited faster heterogeneous electron transfer kinetics, with a k(o') value of 6.0(+/- 1.0)x 10(-4) cm s(-1). Values for the homogeneous rate constants for dissociation (k(f)) and association (k(b)) of cyanide from and to the electrochemically reduced protein were determined to be 0.08(+/- 0.03) s(-1) and 0.06(+/- 0.03) M-1 s(-1), respectively. Imidazolemyoglobin was found to transfer electrons faster than cyanomyoglobin, with a k(o') value of 2.0(+/- 0.5) x 10(-3) cm s(-1). The values for the homogeneous rate constants k(f), and k(b) were 0.40(+/- 0.15) s(-1) and 1.1(+/- 0.1) M-1 s(-1), respectively. (C) 1998 Elsevier Science S.A. All rights reserved.