Reaction of Scenedesmus obliquus plastocyanin with excess [Ru(trpy)(L)(H2O)]2+ (trpy = 2,2‘:6‘,2‘‘-terpyridine; L = 2,2‘-bipyridine, 4,4‘-(CH3)2-2,2‘-bipyridine, 4,5,4‘,5‘-(CH3)4-2,2‘-bipyridine) affords Ru(trpy)(L)(His59)Pc as the main product. These RuPc derivatives are luminescent with λmax(emission) ∼ 650 nm and lifetimes of (Cu+) in the range 110−140 ns. Photogenerated *Ru2+PcCu2+ is quenched by *Ru2+ → Cu2+ electron transfer (ET) to produce Ru3+PcCu+; intramolecular ET was monitored by transient absorption at 590 (Cu+ → Cu2+) and 424 nm (Ru3+ → Ru2+). The Cu+ to Ru3+ ET rate constants (k ET) are as follows:  2.9(2) × 107 s-1 (L = bpy); 2.3(2) × 107 s-1 (L = dmbpy); and 1.9(2) × 107 s-1 (L = tmbpy). Activationless rates (−ΔG° ∼ λ ∼ 0.70−0.75 eV) are consistent with coupling-limited tunneling through a β sheet at an estimated Cu−Ru distance of 15.6 Å (calcd k ET = 107 s-1 for a tunneling decay constant of 1.1 Å-1). Biphasic Cu+ → Ru3+ ET kinetics (k ET > 107 and ∼104 s-1) were observed after flash-quench generation of Ru3+PcCu+ in acidic solutions. The slow phase kinetics are markedly temperature and pH dependent:  the activation parameters (ΔH ⧧ = 43.1 kJ/mol; ΔS ⧧ = −17 J/(K·mol) for L = bpy) suggest that the trigonal low-pH form of Cu+ reorganizes to the tetrahedral form prior to oxidation to the blue Cu2+ state.

[a5Ru]3Mb (a = NH3), which exhibits novel catalytic properties, and its model compound [a5RuImH] 3•2 20 (ImH = imidazole) have been investigated by 360-MHz *H NMR. The model compound in 2H20 exhibits three nonlabile single-proton peaks corresponding to 2'-H, 5'-H, and 4'-H of the coordinated imidazole at -31.2, -2.5, and near 4.7 ppm at 25 °C and pH 6.1, respectively. A pH titration gives a single pK 8.4 for deprotonation of the imidazole, with resulting increases of imidazole contact shifts. Ruthenium-labeled Mb and Ru-labeled apo-Mb also exhibit peaks at -36 ppm, which are assigned to the 2'-H of the imidazoles of His-12, His-81, and His-113. Integration of these 2'-H peaks relative to a heme methyl allows determination of the extent of reaction of surface histidines with pentaammineruthenium(III). Detailed comparison of the hyperfme shifted resonances for metaquo, methydroxy, metcyano, metazide, and deoxy forms of the proteins show negligible influence of the Ru chromophores on shifts, indicating protein folding essentially unaltered from that in the native protein. The difference in the high-spin/low-spin separation in the metazide form for the native and Ru-labeled proteins is estimated at only 24 cal, indicating a slightly weaker axial ligand field for the latter derivative. The decreased proton spin-lattice relaxation times and increased line widths of Ru-labeled Mb relative to native Mb complexes indicate the presence of metal-metal interactions which influence the electron-spin relaxation of the iron center.

Distant heme-Ru electronic couplings have been extracted from intramolecular electron-transfer rates in Ru(histidine-X) (X=33,39,62) derivatives of cytochrome c. The rates (and the couplings) correlate with the lengths of u-tunneling pathways comprised of covalent bonds, hydrogen bonds, and through-space jumps from the histidines to the heme group.The electron-transfer (ET) reactions that occur within and between proteins typically involve prosthetic groups separated by distances that are often greater than 10 A. An understanding of how the intervening medium, driving force, and nuclear reorganization energetics and dynamics modulate protein ET reactions has been a central goal of our research program. In recent years, we have been examining the rates of electron transfer between surface-bound ruthenium complexes and metalloprotein active sites.' This work has provided a considerable capacity for predicting protein ET rates.We have been guided by semiclassical ET theory, which describes the rate constant for nonadiabatic reaction between a donor and acceptor held at fixed distance and orientatiox2The tunneling matrix element HAD is a measure of the electronic coupling between the reactants and the products at the transition state. The magnitude of HAD depends upon donor-acceptor separation, orientation, and the nature of the intervening medium. The exponential term in Eq. 1 reflects the interplay between reaction driving force (-AGO) and nuclear reorganization energy (A). Various approaches have been used to test the validity of Eq. 1, and to extract the ET parameters HAD and A. Driving-force studies have proven to be a reliable approach, and such studies have been emphasized in our own work.In the nonadiabatic limit, the probability is quite low that reactants will cross over to products at the transition-state configuration.' This probability depends upon the electronic hopping frequency (determined by HAD) and upon the frequency of motion along the reaction ~oordinate.~ When solvent reorientation dominates A, the nuclear reorientation timescale is believed to be given by the solvent longitudinal dielectric relaxation time, T~. The nonadiabatic limit for ET results when HA; a {A,fi/4?r7L}'*.3 Water reorients very rapidly ( T~ = 0.5 ps4) and the solvent-controlled adiabatic limit results when HAD B 80 cm-'. Conversely, when HAD 4 80 cm-', Eq. 1 should adequately describe the ET kinetics. Reorientation of the peptide matrix introduces complications in protein ET. Timescales for this nuclear motion are much slower than the 7L for water.s In situations where slow peptide motions dominate A, much smaller values of HA, are necessary to achieve the "solvent-controlled'' adiabatic limit.In simple models, the electronic-coupling strength is predicted to decay exponentially with increasing donor-acceptor separation (Eq. 2):286In Eq. 2, HADo is the electronic coupling at close contact (do), and P is the rate of decay of coupling with distance (a). Studies of the distance dependence of ET rates in donor-acceptor...

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