Direct Oxidation of l-Cysteine by [Fe(III)(bpy)(2)(CN)(2)](+) and [Fe(III)(bpy)(CN)(4)](-).

Direct Oxidation of l-Cysteine by [Fe(III)(bpy)(2)(CN)(2)](+) and [Fe(III)(bpy)(CN)(4)](-). Research Abstract Details 

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Direct Oxidation of l-Cysteine by [Fe(III)(bpy)(2)(CN)(2)](+) and [Fe(III)(bpy)(CN)(4)](-). Abstract Text:

Xiaoguang Wang,David M Stanbury,Xiaoguang Wang,David M Stanbury,

The oxidation of l-cysteine by the outer-sphere oxidants [Fe(bpy)2(CN)2]+ and [Fe(bpy)(CN)4]- in anaerobic aqueous solution is highly susceptible to catalysis by trace amounts of copper ions. This copper catalysis is effectively inhibited with the addition of 1.0 mM dipicolinic acid for the reduction of [Fe(bpy)2(CN)2]+ and is completely suppressed with the addition of 5.0 mM EDTA (pH < 9.00), 10.0 mM EDTA (9.0 < pH 10.0) for the reduction of [Fe(bpy)(CN)4]-. 1H NMR and UV-vis spectra show that the products of the direct (uncatalyzed) reactions are the corresponding Fe(II) complexes and, when no radical scavengers are present, l-cystine, both being formed quantitatively. The two reactions display mild kinetic inhibition by Fe(II), and the inhibition can be suppressed by the free radical scavenger PBN (N-tert-butyl-alpha-phenylnitrone). At 25 degrees C and mu = 0.1 M and under conditions where inhibition by Fe(II) is insignificant, the general rate law is -d[Fe(III)]/dt = k[cysteine]tot[Fe(III)], with k = {k2Ka1[H+]2 + k3Ka1Ka2[H+] + k4Ka1Ka2Ka3{/}[H+]3 + Ka1[H+]2 + Ka1Ka2[H+] + Ka1Ka2Ka3}, where Ka1, Ka2, and Ka3 are the successive acid dissociation constants of HSCH2CH(NH3+)CO2H. For [Fe(bpy)2(CN)2]+, the kinetics over the pH range of 3-7.9 yields k2 = 3.4 +/- 0.6 M-1 s-1 and k3 = (1.18 +/- 0.02) x 106 M-1 s-1 (k4 is insignificant in the fitting). For [Fe(bpy)(CN)4]- over the pH range of 6.1-11.9, the rate constants are k3 = (2.13 +/- 0.08) x 103 M-1 s-1 and k4 = (1.01 +/- 0.06) x 104 M-1 s-1 (k2 is insignificant in the fitting). All three terms in the rate law are assigned to rate-limiting electron-transfer reactions in which various thiolate forms of cysteine are reactive. Applying Marcus theory, the self-exchange rate constant of the *SCH2CH(NH2)CO2-/-SCH2CH(NH2)CO2- redox couple was obtained from the oxidation of l-cysteine by [Fe(bpy)(CN)4]-, with k11 = 4 x 105 M-1 s-1. The self-exchange rate constant of the *SCH2CH(NH3+)CO2-/-SCH2CH(NH3+)CO2- redox couple was similarly obtained from the rates with both Fe(III) oxidants, a value of 6 x 106 M-1 s-1 for k11 being derived. Both self-exchange rate constants are quite large as is to be expected from the minimal rearrangement that follows conversion of a thiolate to a thiyl radical, and the somewhat lower self-exchange rate constant for the dianionic form of cysteine is ascribed to electrostatic repulsion.

Direct Oxidation of l-Cysteine by [Fe(III)(bpy)(2)(CN)(2)](+) and [Fe(III)(bpy)(CN)(4)](-). Publishing Authors By Initials

X Wang,DM Stanbury,X Wang,DM Stanbury,

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Direct Oxidation of l-Cysteine by [Fe(III)(bpy)(2)(CN)(2)](+) and [Fe(III)(bpy)(CN)(4)](-). Journal Published:

PUBLICATION TYPE: Journal Article

Journal: Inorganic chemistry

VOLUME: 47

Page Numbers: 1224-36

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ISSN: 0020-1669

DAY: 5

MONTH: 01

YEAR: 2008

Direct Oxidation of l-Cysteine by [Fe(III)(bpy)(2)(CN)(2)](+) and [Fe(III)(bpy)(CN)(4)](-). Information

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LANGUAGE: eng

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AFFILIATION: Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849.

Country: United States

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MEDLINETA: Inorg Chem

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