Electron-Transfer Chemistry of Ru−Linker−(Heme)-Modified Myoglobin: Rapid Intraprotein Reduction of a Photogenerated Porphyrin Cation Radical

Abstract

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 −(CH_2)_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 (Ru^(2+)/Fe^(3+)) to the transient (Ru^(3+)/Fe^(2+)) state of RuC7Mb is >10^8 s^(-1); the back ET rate (to regenerate Ru^(2+)/Fe^(3+)) is 1.4 × 10^7 s^(-1). Irreversible oxidative quenching by [Co(NH_3)_5Cl]^(2+) generates Ru^(3+)/Fe^(3+):  the Ru^(3+) complex then oxidizes the porphyrin to a cation radical (P^(•+)); in a subsequent step, P^(•+) oxidizes both Fe^(3+) (to give Fe^(IV)═O) and an amino acid residue. The rate of intramolecular reduction of P^(•+) is 9.8 × 10^3 s^(-1); the rate of ferryl formation is 2.9 × 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.