Spectroelectrochemistry of cytochrome b559 in the D1–D2–Cyt b559 complex from spinach

The redox potential of cytochrome b559 (Cyt b559) in the D1–D2–Cyt b559 complex from spinach has been determined to be +90 ± 2 mV vs. SHE at pH 6.0, by thin-layer cell spectroelectrochemistry for the first time. The redox potential, corresponding uniquely to the so-called “low-potential form”, exhibited a sigmoidal pH-dependence from pH 4.0 to 9.0, ranging from +115 to +50 mV. An analysis of the pH-dependence based on model equations suggests that two histidine residues coordinating to the heme iron in the protein subunits may exert electrostatic influence on the redox potential of Cyt b559.