Mechanism of the Outer-sphere Oxidation of Aqueous L-cysteine and of Iodide in Acetonitrile by a Series of Iron(III) Complexes
Type of DegreeDissertation
Chemistry and Biochemistry
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Trace copper catalysis was observed in the oxidation of L-cysteine by [Fe(bpy)2(CN)2]+ and [Fe(bpy)(CN)4]- in anaerobic aqueous solution. The copper catalysis was effectively inhibited with the addition of dipicolinate (abbreviated asdipic2-) for the reduction of [Fe(bpy)2(CN)2]+, and completely suppressed with the addition of EDTA (pH ? 10.0) or 1,4,8,11-tetraazacyclotetradecane (abbreviated as cyclam) (pH ? 10.0) for that of [Fe(bpy)(CN)4]-. 1H-NMR and UV-vis spectra show that the products of the reaction are L-cystine and the corresponding Fe(II) complexes, with the stoichiometric ratio 1:2 for ?[L-cystine]/?[Fe(II)]. The kinetics for the direct oxidation of L-cysteine by [Fe(bpy)(CN)4]- were studied over the pH range 5.98 ~ 11.9, at ? = 0.10 M and 25.0 °C. The kinetics for the direct oxidation of L-cysteine by [Fe(bpy)2(CN)2]+ were studied over the pH range 3.48 ~ 7.89, at ? = 0.10 M and 25.0 °C, with the addition of 0.20 mM N-tert-butyl-?-phenylnitrone (PBN). The mechanism of the reactions was proposed. The rate-limiting step is electron transfer to form cysteine radicals and Fe(II). Only the thiolate forms of cysteine are reactive. Applying Marcus theory, the self-exchange rate constants (k11) of ?SCH2CH(NH3+)CO2–/–SCH2CH(NH3+)CO2– and ?SCH2CH(NH2)CO2–/–SCH2CH(NH2)CO2–, were obtained. The relatively smaller k11 value of ?SCH2CH(NH2)CO2–/–SCH2CH(NH2)CO2– is ascribed to negative-negative charge repulsion. The redox reactions may occur through one-electron outer-sphere transfer reactions. The oxidations of iodide by [FeIII(bpy)2(CN)2]+, [FeIII(dmbpy)2(CN)2]+, [FeIII(CH3Cp)2]+ and [FeIII(5-Cl-phen)2(CN)2]+ at 25.0 oC and ? = 0.10 M in acetonitrile were studied. Trace copper catalysis was observed for the above four redox reactions. The kinetics of the direct oxidation of iodide were studied with the addition of 2,2’-bipyridine (bpy). According to 1H-NMR and UV-vis spectra, the products of the reaction are I3- and the corresponding Fe(II) complexes, with the stoichiometric ratio of 3:2 between I- and Fe(III). Linear Free-Energy Relationships (LFER) were obtained for both log k1 and log k2 vs E1/2, implying that the reaction follows a one-electron outer-sphere mechanism. The standard-potential (E1/2) of I?/I- was derived from the kinetic inhibition by FeII(bpy)2(CN)2, with the value of (0.60 ? 0.01) V (vs [Fe(Cp)2]+/0). However, the exchange rate constant of I?/I- could not be obtained due to the diffusion-control rate constant of k-1.