INTRAPROTEIN ELECTRON-TRANSFER IN A RUTHENIUM-MODIFIED TYR83-HIS PLASTOCYANIN MUTANT - EVIDENCE FOR STRONG ELECTRONIC COUPLING

A site-directed mutant of spinach plastocyanin, Pc(Tyr83-His), has bee n modified by covalent attachment of a photoactive [Ru(bpy)(2)(im)](2) complex to the His83 residue. The residue is surface exposed and loc ated about 10-12 Angstrom from the copper ion at the entrance of a pro posed natural electron transfer pathway from cytochrome f. Electron tr ansfer within the Ru-Pc complex has been studied with time-resolved op tical spectroscopy using two different approaches. In the first, the f ully reduced [Cu(I), Ru(II)] protein was photoexcited and subsequently oxidized by an external quencher, forming the [Cu(I), Ru(III)] protei n. This was followed by an electron transfer from reduced Cu(I) to Ru( III). In the second method, the initially oxidized Cu(II) ion acted as an internal quencher for excited Ru(II) and the photoinduced reductio n of the Cu(II) ion was followed by a thermal recombination with the R u(III) ion. The reoxidation of the Cu ion, which has an estimated driv ing force of 0.56 eV, occured with a rate constant k(et) = (9.5+/-1.0) X 10(6) s(-1), observed with both methods. The results suggest a stro ng electronic coupling (H-DA>0.3 cm(-1)) along the Ru-His(83)-Cys(84)- Cu pathway.