Long‐range intramolecular electron transfer in Rhus vernicifera stellacyanin: A pulse radiolysis study

The (NH3)5Ru‐modified stellacyanin Ru(III)2‐St[Cu(II)], where two ruthenium ions are coordinated to imidazole residues of His‐32 and 100, has been synthesized. CO− 2 radicals, generated by pulse radiolysis, react with the modified protein yielding an intermediate Ru(II)‐St[Cu(II)]. The rate of molecular electron transfer within this complex Ru(II)‐St[Cu(II)] k Ru(III)‐St[Cu(I)] has been determined, k u = 0.05 s−1 at 18°C, pH 7.0. The Ru(His‐32/100) to Cu(II) separation distance is relatively large (∼ 18 Å), and the driving force is low (10 kJ·mol−1), hence the correspondingly slow rate. This result is compared with those obtained in earlier studies of intramolecular fixed‐distance electron transfer reactions in blue copper proteins.