Molecular and Electronic Structure of the Electron-Transfer Probe Analogue [trans-(NH₃)₄Ru(imidazole) (isonicotinamide)](CF₃CO₂)₃2-propanol

Kinetic and thermodynamic studies of long-range electron transfer in cytochrome c, azurin, and myoglobin have exploited the modification of these metalloprotein surfaces with ruthenium ammine probes attached to surface histidine imidazole groups. The authors combined structural, polarized electronic spectrosocopic, and calculational study of the (L-histidinato)(NH₃)₅Ru^III chromophore characterized the strong d_p/p_p overlap between the Ru(III) d_p vacancy and the histidine imidazole p-system. One implication of this study for the use of (NH₃)₅Ru^III(his) probes as surface electron acceptors is that the effective electron-transfer pathway from the protein active site should be that calculated to the edge of the probe imidazole unit rather than to the metal center. Use of the modified surface probe trans-(NH₃)₄(isonicotinamide)Ru^III/II(his) in the cytochrome c system results in an approximate reversal of the driving force for electron transfer. No information has been obtained on the angular relationship between the histidine imidazole and the isonicotinamide groups in Ru^III(NH₃)₄(his)(isonicotinamide)- modified cythochrome c and its effects on the electronic spectrum of this chromophore. Depending upon the mutual orientations and electronic properties of the isonicotinamide and imidazole rings in the modified probe, coupling of the Ru(III) d_p vacancy to the imidazole could be substantially different from that of the parent (NH₃)₅Ru^III(his) probe and thus might constitute an additional barrier in the electron-transfer pathway. These questions prompted the authors to examine in detail the structure and electronic spectroscopic properties of trans-(NH₃)₄Ru^III(isonicotinamide)(imidazole).