Electron-tunneling pathways in cytochrome C.

Distant Fe(2+)-Ru(3+) electronic couplings have been extracted from intramolecular electrontransfer rates in Ru(histidine(x)) (where X = 33, 39, 62, and 72) derivatives of cytochrome c. The couplings increase according to 62 (0.0060) < 72 (0.057) < 33 (0.097) < 39 (0.11 per wave numbers); however, this order is out of line with the histidine to heme edge-edge distances [62 (14.8) > 39 (12.3) > 33 (11.1) > 72 (8.4 angstroms)]. The rates (and the couplings) correlate with the lengths of sigma-tunneling pathways comprised of covalent bonds, hydrogen bonds, and through-space jumps from the histidines to the heme group. Space jumps greatly decrease couplings: One from Pro(71) to Met(80) extends the sigma-tunneling length of the His(72) pathway by roughly 10 covalent-bond units.