ELECTROSTATIC INTERACTIONS AND REDUCTION POTENTIAL EFFECTS FROM IONIC-STRENGTH AND PH PROFILES IN ELECTRON-TRANSFER REACTIONS OF SPINACH PLASTOCYANIN

We have investigated the ionic strength and pH dependence of the rate constants for electron transfer between spinach plastocyanin and the t wo structurally closely related tris-1,10-phenanthroline and tris-5,su lfonato-1,10-phenanthroline cobalt (III) complexes. The former complex is triply positively charged, the latter electrically neutral. By con sideration of the combined electron-transfer pattern the observed ioni c strength and pH variation can be resolved into reduction potential a nd inter-reactant interaction (work term) effects. Comparison of the l atter with simple models for electrostatic interaction between cobalt( III) and charged protein surface groups shows that electron transfer b etween the negatively charged plastocyanin and the positively charged cobalt(III) is likely to occur at both adjacent and remote sites. This supports previous views of dual-path plastocyanin electron transfer b ased on other kinds of data.