Jack R. Pladziewicz

Stereoselective electron transfer between Cu(II) superoxide dismutase and Λ- and Δ-[Fe(pdta)]2−

Abstract The kinetics of reduction of human Cu(II), Zn(II) superoxide dismutase (SOD) by Λ-, Δ- and racemic-[Fe(pdta)] 2− have been studied as a function of pH, ionic strength and temperature. The reaction is observed to be first order with respect to both the concentration of SOD and [Fe(pdta)] 2− . At 25 °C, pH 6.0 (0.050 MES) and ionic strength 0.03 mol dm −3 , the second-order rate constants, k , k Λ , k Δ , are 13.1 ± 0.4, 16.6 ± 0.7 and 9.5 ± 0.5 dm 3 mol −1 s −1 for the racemic mixture, lambda and delta isomers, respectively. At the same temperature and ionic strength and pH 5.0 and 7.0, k for reduction of SOD by the racemic mixture is 20.0 ± 2 and 10.0 ± 0.3 dm 3 mol −1 s −1 , respectively. At an ionic strength of 0.094 mol dm −3 (25 °C and pH 6.0) k for reduction of SOD by the racemic mixture is 8.0 ± 0.2 dm 3 mol −1 s −1 . The magnitude and sense of the stereoselectivity is observed to be independent of pH and ionic strength over the ranges studied and at 25 °C, k Λ / k Δ is 1.7. The rate of reaction at pH 6.0, ionic strength 0.03 mol dm −3 was measured as a function of temperature between 5 and 50 °C yielding the activation parameters of 62.5 ± 0.5 and 71.9 ± 0.8 kJ mol −1 for Δ H ‡ and −12 ± 2 and 16 ± 3 J mol −1 K −1 for Δ S ‡ for SOD reduction by the lambda and delta isomers, respectively. These activation parameters are compensatory in that the enantiomeric preference of SOD for the lambda isomer at 25 °C results from a lower activation enthalpy which compensates for a more negative activation entropy relative to reduction by the delta isomer. At sufficiently high temperatures, the stereoselectivity is predicted to change in favor of the delta isomer due to its more positive activation entropy.

Comparison of gas and solution phase intrinsic rate constants for electron transfer of tetraalkylhydrazines

Rate constants for 40 gas-phase electron transfer reactions at 550 K between 22 tetraalkylhydrazines are fitted using Marcus cross rate theory, and compared with previous studies in acetonitrile solution. The relative reactivities are surprisingly similar in the gas phase and in solution. Strong cation–neutral association in the gas phase appears to lessen striking electronic effects previously reported for similar reactions in solution. The relative reactivities of 1,2-dimethylhexahydropyridazine ([6]Me2) and 1,2-dimethyl-1,2,3,6-tetrahydropyridazine ([u6]Me2) switch between solution and the gas phase. It is suggested that this may occur because gas-phase ion–dipole complexes are much more tightly bound than are encounter complexes in solution, allowing a conformational change to the more reactive conformation of [u6]Me2 in the gas phase, but not in solution.