Myoglobins (Mbs) reconstituted with rotatable octamethylheme and non-rotatable etioheme were prepared and their electrochemical behavior was studied. The redox potential of octamethylheme reconstituted Mb (OMe-Mb), of which hume pops around iron-histidine (F8 -His) bond, shifted negatively by ca. 30 mV compared with non-rotatable etioheme reconstituted Mb (Etio-Mb). On the other hand, the redox potentials of octamethylheme and etioheme themselves were very similar to each other. Due to the similarity of the distal histidine side of the heme of these two reconstituted Mbs, the shifts of the redox potential would be attributable to the drastic change of the orientation of proximal histidine imidazole ring to the heme plane by heme rotation. The dissociation rate constant of cyanide ion from the ferrous heme iron (II) for OMe-Mb form at 5 degrees C and pH 7.5 was three times faster than that of Etio-Mb. The electron transfer kinetics of these Mbs showed that the heme rotation causes faster electron transfer rates in both electrode reaction and chemical reduction in solution with dithionite. The obtained heterogeneous electron transfer rates constants at an In2O3 electrode and first-order rate constants of the chemical reduction were 12(+/- 0.5) x 10(-4) cm s(-1), 9.8(+/- 1.0) s(-1) for OMe-Mb and 6.0(+/- 0.5) x 10(-4) cm s(-1), 4.5(+/- 1.0) s(-1) for Etio-Mb under the present experimental conditions.