Electron tunneling in ruthenium-modified cytochromec

Distant Fe²⁺-Ru³⁺ electronic couplings have been extracted from intramolecular electron-transfer rates in Ru(2,2′-bipyridine)₂(imidazole)(histidine-X)²⁺ (X = 33,39,62,72,79) derivatives of cytochromec. The rates are 〉 1 · 10⁸ (79); 3.2(4) · 10⁶ (39); 2.6(3) · 10⁶ (33); 9.0(3) · 10⁵ (72); 1.0(2) · 10⁴ s⁻¹ (62); the couplings increase according to 62 (0.006) < 72 (0.057) < 33 (0.097) < 39 (0.11) < 79 (〉 0.6 cm⁻¹). The rates (and the couplings) correlate with the lengths of σ-tunneling pathways comprised of covalent bonds, hydrogen bonds, and through-space jumps from the histidines to the heme group.