Electron transfer reactions between Clostridium pasteurianum flavodoxin semiquinone and various oxidants (horse heart cytochrome c, ferricyanide, and ferric EDTA) have been studied as a function of ionic strength using stopped-flow spectrophotometry. The cytochrome c reaction is complicated by the existence of two cytochrome species which react at different rates and whose relative concentrations are ionic strength dependent. Only the faster of these two reactions is considered here. At low ionic strength, complex formation between cytochrome c and flavodoxin is indicated by a levelling-off of the pseudo-first order rate constant at high cytochrome c concentration. This is not observed for either ferricyanide or ferric EDTA. For cytochrome c, the rate and association constants for complex formation were found to increase with decreasing ionic strength, consistent with negative charges on flavodoxin interacting with the positively charged cytochrome electron transfer site. Both ferricyanide and ferric EDTA are negatively charged oxidants and the rate data respond to ionic strength changes as would be predicted for reactants of the same charge sign. These results demonstrate that electrostatic interactions involving negatively charged groups are important in orienting flavodoxin with respect to oxidants during electron transfer. The effects of structural modifications of the FMN prosthetic group of C. pasteurianum flavodoxin on the kinetics of electron transfer to the oxidized form (from 5-deazariboflavin semiquinone produced by laser flash photolysis) and from the semiquinone form (to horse heart cytochrome c using stopped-flow spectrophotometry) have been investigated. The analogs used were 7,8-dichloroFMN, 8-chloroFMN, 7-chloroFMN and 5,6,7,8-tetrahydroFMN. The ionic strength dependence of cytochrome c reduction was not affected by chlorine substitution, although the specific rate constants for complex formation and decay were appreciably smaller. On the other hand, all of the chlorine analogs had the same rate constant for deazariboflavin semiquinone oxidation. The rate constants for tetrahydroFMN flavodoxin semiquinone reduction of cytochrome c were considerably smaller than those for the native protein. The results for the chlorine analogs indicate the important roles that the polarity of the exposed flavin edge and the substitution of the 8 position play in electron transfer. The data obtained with the tetrahydroFMN analog indicates that the (pi) electron system of the flavin is necessary for rapid electron transfer. These implications are discussed for the electron transfer mechanism of flavodoxin
