pH and ionic strength effects on electron transfer rate constants and reduction potentials of the bacterial di-heme protein Pseudomonas stutzeri cytochrome c4.

We have investigated the ionic strength (0.1-0.5 M NaCl) and pH dependence (4.0-7.5) of the electron transfer (ET) rate constants for oxidation and reduction of the bacterial di-heme protein cytochrome c4 (cyt c4; Pseudomonas stutzeri, ATCC No. 11607) by [Co(bipy)3]3+/2+ (bipy = 2,2'-bipyridine). The kinetics is bi- or tri-phasic, and a mechanism based on cooperative ET at both hemes, slow intramolecular ET and electrostatically dominated inter-heme interaction is presently best in line with all the available data. The ionic strength and pH dependence of the rate constants and reduction potentials is weak. The rate constants mostly decrease by 0-50% in the ionic strength range 0.1-0.5 M. The macroscopic potentials decrease by < 10 mV. Three of the microscopic potentials increase by 10-25 mV, while the fourth one decreases by 50 mV, but the accuracy of the microscopic reduction potential values is low. There is no pH dependence of the rate constants in the range 6.0-7.5, but most rate constants drop to half the 6.0-7.5 value in the range 4.0-6.0, leaving the reduction potentials almost unaffected. The small effects are unexpected in view of the highly charged and strongly dipolar character, and the many hydrogen bond contacts of cyt c4. These small effects must be related to the detailed rather the overall charge distribution of cyt c4.