Electron-transfer chemistry of Ru-linker-(Heme)-modified myoglobin: Rapid intraprotein reduction of a photogenerated porphyrin cation radical

We report the synthesis and characterization of RuC7, a complex in which a heme is covalently attached to a [Ru(bpy)(3)](2+) complex through a -(CH2)(7)- linker. Insertion of RuC7 into horse heart apomyoglobin gives RuC7Mb, a Ru(heme)-protein conjugate in which [Ru(bpy)(3)](2+) emission is highly quenched. The rate of photoinduced electron transfer (ET) from the resting (Ru2+/Fe3+) to the transient (Ru3+/Fe2+) state of RuC7Mb is >10(8) s(-1); the back ET rate (to regenerate Ru2+/Fe3+) is 1.4 x 10(7) s(-1). Irreversible oxidative quenching by [Co(NH3)(5)Cl](2+) generates Ru3+/Fe3+: the Ru3+ complex then oxidizes the porphyrin to a cation radical (P.+); in a subsequent step, P.+ oxidizes both Fe3+ (to give Fe-IV=O) and an amino acid residue. The rate of intramolecular reduction of P.+ is 9.8 x 10(3) s(-1); the rate of ferryl formation is 2.9 x 10(3) s(-1). Strong EPR signals attributable to tyrosine and tryptophan radicals were recorded after RuC7MbM(3+) (M = Fe, Mn) was flash-quenched/frozen.