Redox Potential and Electrostatic Effects in Competitive Inhibition of Dual-Path Electron Transfer Reactions of Spinach Plastocyanin

Redox inactive ions with high positive charges lower the rate constant for oxidation of several plant plastocyanins (PC) by small positively charged inorganic reaction partners. The rate constant decrease is commonly attributed to competitive inhibition where the redox inactive ions are bound to the negatively charged remote electron transfer (ET) site of PC and block this site sterically. We have investigated the effects of the inhibitor [(NH3)5Co(NH2)Co(NH3)5] on the ET reactions of spinach PC with [Co(phen)3]3+ (phen = 1,10-phenanthroline) and the electrically neutral analogue [Co(phen-SO3)3] (phen-SO3 = 5-sulfonato-1,10-phenanthroline) at the ionic strengths μ = 0.1 M and 0.03 M. Inhibition of the [Co(phen)3]3+ reactions is notably smaller for PC(II) reduction than for PC(I) oxidation. This is indicative of a redox potential increase of PC(II)/PC(I) on inhibitor attachment. The effect amounts to 16 mV at μ = 0.1 M and 31 mV at μ = 0.03 M. These data, and analysis in terms of ET theory show that inhibition cannot be caused solely by steric blocking. Driving force and interreactant electrostatic work terms are equally important. The PC(I)/ [Co(phen-SO3)3] reaction exhibits a more entangled pattern. The rate constant first increases slightly with increasing inhibitor concentration, then drops, and approaches a constant value not far from the original value. This pattern is in line with association between the negatively charged -SO−3 groups of the Co(III) complex and the inhibitor, and ET of the associate at both ET sites of PC.