Rate constants k 1 2/M-1 s-1 (22 °C) for the reduction of the oxidised form of horse-heart cytochrome c by eight organic radicals (OR), e.g., methyl viologen MV•+, determined by pulse radiolysis and/or stopped-flow methods, correlate well in a plot of log k 1 2 vs reduction potential E°1 for the OR. In the pulse radiolysis experiments the ORs were generated in situ by rapid reduction of the parent form by the formate radical CO2 •-, and in the stopped-flow experiments prior reduction of the parent was with dithionite. In both procedures it was necessary to consider formation of double-reduced parent forms. Rate constants k 1 2 are in the range 109 to 104 and E°1 values from −0.446 to +0.194V. The Marcus free-energy plot of log1 0 k 1 2 − 0.5 log10 f vs E°1/0.059 gives a slope of 0.49, in excellent agreement with the theoretical value of 0.50, with an intercept at −E°1/0.059 = 0 of 6.55 in good agreement with a calculated value of 6.51 from known parameters. The latter includes the reduction potential E°2 (0.263V) and self-exchange rate constant k 22 (0.36 × 103 M-1 s-1) for the cytochrome c(III)/(II) couple, and assumes a common self-exchange rate constant k 11 of 1.0 × 106 M-1 s-1 for the ORs.

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A kinetic study of water exchange and NCS complexation on the mixed oxo-sulphido aqua complex [Μο3(μ3-5)(μ-0)3(ΟΗ2)9] 4+ is reported along with structural (Mo Kedge EXAFS) and electronic ( 95 MO NMR) data. The complex possesses nonequivalent water ligands with those labelled 'd' (two per Mo), located approximately trans to μ-οχο, undergoing exchange ~3 χ 10 4 times faster than those labelled 'c' (one per Mo), located approximately trans to the capping μ3-βυ1ρ1^ο group. The faster rate for the former arises, as with the all μ-sulphido cluster, [MO3^3-S)^-S)3(OH2)9] 4+ , from a conjugate-base labilising pathway involving the monohydroxy species, [Μο3(μ3-5)(μ-0)3(ΟΗ2)8(ΟΗ)] 3+ (iCaM = 0.58M estimated from the [Η + ] dependence of the anation reaction with NCS -). Deprotonation at an adjacent water'd' on the same Mo atom is suggested. Taken together, the substitution and water exchange data gathered on this family of cluster ions is consistent with dissociative ligand substitution preferentially at a water'd' on the monohydroxy species in all cases. Substitution of sulphido for oxo in the two core positions affects mainly the exchange at water'd'; substitution at the μ3^3ρρϊ^ position retarding the rate of exchange by a factor of ~4 (no effect on water 'c') whereas similar substitution at all three μ-bridging positions increases the rate of the water 'd' exchange ~200-fold but that on water 'c' only ~ll-fold. These along with the 95 Mo NMR with Mo Kedge EXAFS data suggest that the observed kinetic * Corresponding author. effects stemming from changes to the capping μ3-ligand may be largely structural in nature whereas a direct electronic influence (affecting most noticeably water'd') is implied for the μ-bridging ligands.

Cross-reaction rate constants k 12 (22°C) at pH 7.0 have been determined for the reduction of Fe III 2 and tyrosyl-radical-containing active-R2 from E. coli ribonucleotide reductase with eight organic radicals (OR), e.g., MV •+ from methyl viologen. The more reactive OR's were generated in situ using pulse radiolysis (PR) techniques, and other OR's were generated by prior reduction of the parent with dithionite, followed by stoppedflow (SF) studies. In both procedures it was necessary to include consideration of doubly-reduced parent forms. Values of k 12 are in the range 10 9 to 10 4 M -1 s -1 and reduction potentials E o 1 for the OR vary from -0.446 to +0.194 V. Samples of E. coli active-R2 also have an Fe III 2 met-R2 component (with no Tyr • ), which in the present work was close to 40%. From separate experiments met-R2 gave similar k 12 rate constants (on average 66% bigger) to those for active-R2, suggesting that reduction of the Fe III 2 center is the common rate-limiting step. A single Marcus free-energy plot of log k 12 -0.5 log f vs -E o 1 /0.059 describes all the data, and the slope of 0.54 is in satisfactory agreement with the theoretical value of 0.50. It is concluded that the ratelimiting step involves electron transfer. In addition, the intercept at -E o 1 /0.059 ) 0 is 5.94, where values of the reduction potential and self-exchange rate constant for met-R2 contribute to this value. To maintain electroneutrality at the ∼10 Å buried active site H + uptake is also required. For both e -and H + transfer the conserved pathway Trp-48, Asp-237, His-118 to Fe A is a possible candidate requiring further examination.