STEREOSELECTIVE ELECTRON-TRANSFER BETWEEN CU(II) SUPEROXIDE-DISMUTASEAND LAMBDA-[FE(PDTA)](2-) AND DELTA-[FE(PDTA)](2-)

The kinetics of reduction of human Cu(II), Zn(II) superoxide dismutase (SOD) by Lambda-, Delta- 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 S OD and [Fe(pdta)](2-). At 25 degrees C, pH 6.0 (0.050 MES) and ionic s trength 0.03 mol dm(-3), the second-order rate constants, k, k(Lambda) , k(Delta), 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 red uction 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 degrees C and pH 6.0) k for reduction of SOD by the racemic mixtur e is 8.0+/-0.2 dm(3) mol(-1) s(-1). The magnitude and sense of the ste reoselectivity is observed to be independent of pH and ionic strength over the ranges studied and at 25 degrees C, k(Lambda)/k(Delta) is 1.7 . The rate of reaction at pH 6.0, ionic strength 0.03 mol dm(-3) was m easured as a function of temperature between 5 and 50 degrees C yieldi ng the activation parameters of 62.5+/-0.5 and 71.9+/-0.8 kJ mol(-1) f or Delta H double dagger and -12+/-2 and 16+/-3 J mol(-1) K-1 for Delt a S double dagger for SOD reduction by the lambda and delta isomers, r espectively. These activation parameters are compensatory in that the enantiomeric preference of SOD for the lambda isomer at 25 degrees C r esults from a lower activation enthalpy which compensates for a more n egative 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 activa tion entropy.