Robert A. Binstead

NMR and kinetic characterization of the interaction between French bean plastocyanin and horse cytochrome c

French bean plastocyanin is shown by stopped-flow kinetics to oxidize horse cytochrome c with k (298 K, I = 0.10 M) = 5.1 X 10(6) M-1 X s-1. The activation parameters demonstrate a satisfactory isokinetic correlation with those previously reported for plastocyanin-cytochrome f reactions. NMR line broadening and shifts of the hyperfine shifted resonances of cytochrome c(III) reveal that strong 1:1 complexes are formed with plastocyanin. The negative patch of plastocyanin and the heme edge region of cytochrome c are shown to be the interacting sites by the hyperfine shift perturbations and competitive binding experiments with Gd3+, which associates selectively with the negative patch of plastocyanin. Complexation of plastocyanin and cytochrome c causes a small change in the heme electronic structure, but there is no NMR or optical evidence for significant conformational changes at either metal center. The rate of the reverse electron-transfer reaction within the plastocyanin-cytochrome c complex has been directly measured by NMR line broadening (krev (298 K) = 87 s-1). A rate for the forward intracomplex electron-transfer reaction (kf (298 K) = 4.8 X 10(3) s-1) has been calculated from krev and the optically measured equilibrium constant.

DYNAMICS OF COBALT(II) SPIN-EQUILIBRIUM COMPLEXES

Abstract Ultrasonic absorption measurements have been made on solutions of three cobalt(II) spin-equilibrium complexes. For the bis(terpyridine)cobalt(II) ion, no excess absorption was observed either in aqueous or methanol solutions, which implies a spin-equilibrium relaxation time of less than 2 ns, consistent with previous laser T-jump observations. For the bis(2,6- N -NHCH 3 -pyridinedicarboxaldimine)cobalt(II) ion again no excess sound absorption occurs in aqueous solution. Laser T-jump observations show a spectral change within the 13 ns heating risetime. For this complex in methanol and acetonitrile, however, a large excess sound absorption was observed which is described by overlapping double relaxation curves. A similar absorption was found as well for methanol solutions of the bis(2,6-t-butylpyridinedicarboxaldimine)cobalt(II) ion which is fully high-spin. This implies that the observed relaxations do not arise from the spin equilibrium but from some other process, possibly the dechelation of the ligand. Variable temperature solution magnetic moments and associated thermodynamic parameters are reported for the two spin-equilibrium complexes: for bis(terpyridine)cobalt(II) ion the thermodynamic parameters were calculated to be ΔH ° = (2.085±0.007) kcal mol −1 and ΔS ° = (6.33±0.03) cal deg −1 mol −1 in methanol, and ΔH ° = (3.93±0.02) kcal mol −1 and ΔS ° = (13.12±0.07) cal deg −1 mol −1 in water; for bis(2,6- N -NHCH 3 -pyridinedicarboxaldimine)cobalt(II) ion the thermodynamic parameters are ΔH ° = (2.74±0.01) kcal mol −1 and ΔS ° = (7.29±0.04) cal deg −1 mol −1 in methanol, and ΔH ° = (4.11±0.05) kcal mol −1 and ΔS ° = (10.85±0.15) cal deg −1 mol −1 in water.

Potassium Tris(oxalato)ruthenate(III)

Abstract The structure of the title compound has been found by single-crystal X-ray analysis to be a 4.5 hydrate, isomorphous with the rhodium and iridium analogs (triclinic space group P 1 , a = 6.813(5),b = 10.519(1), c = 12.470(2) A, α = 76.28(1)°, β = 84.26(1)°, γ = 85.60(1)°, Z = 2). The structure was refined to a conventional R value of 0.025 by using 2271 significant reflections. The structure contains the expected tris-bidentate oxalatoruthenate(III) anion with an average RuO bond length of 2.028(7) A. Two methods of preparing this salt free of chloride ions are described. The powder diffraction pattern is tabulated with the three most intense lines of 6.05(100), 13.62(46), and 3.033(38) A. The reduction potential was measured by cyclic voltammetry to be −0.456 V, which differs from that previously reported.