Spectroscopy and Electrochemistry of the Covalent Pyridine-Cytochrome c Complex and a Pyridine-Induced, “Alkaline-like” Conformation

Spectroscopic studies demonstrate that at neutral pH and in the absence of specific pyridine binding, pyridine induces a conformational change in cytochrome c grossly similar to the alkaline-induced transition observed at high pH. This pyridine-induced conformational change has confounded previous efforts to characterize the electrochemistry of the covalent pyridine-cytochrome c complex. By employing cyclic voltammetry at a variety of time scales to characterize both kinetically and thermodynamically favored species, we demonstrate that, in the presence of pyridine, this pyridine-induced conformation coexists in a solution with native protein and a specific, covalent cytochrome c−pyridine complex. At low scan rates or in the presence of the nonbinding analogue 2-methyl pyridine, reduction of the pyridine-induced conformation is observed at potentials similar to those previously reported for the alkaline form populated at high pH in the absence of pyridine. At high scan rates and/or at slightly acidic pH the true redox reaction of the covalent pyridine-cytochrome c complex is resolved. Digital simulations of the voltammograms are quantitatively consistent with the proposed mechanisms, providing further support for the assigned redox rates and potentials.