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), and 1.0 mM cyclam (pH > 10.0) for the reduction of [Fe(bpy)(CN)(4)](-), H-1 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-tea-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 = {k(2)K(a1)[H+](2) + k(3)K(a1)K(a2)[H+] + k(4)K(a1)K(a2)K(a3){/}[H+](3) + K-a1[H+](2) + Ka1Ka2[H+] + Ka1Ka2Ka3}, where K-a1, K-a2, and K-a3 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 k(2) = 3.4 +/- 0.6 M-1 s(-1) and k(3) = (1.18 +/- 0.02) X 10(6) M-1 s(-1) (k(4) is insignificant in the fitting). For [Fe(bpy)(CN)(4)](-) over the pH range of 6.1-11.9, the rate constants are k(3) = (2.13 +/- 0.08) X 10(3) M-1 s(-1) and k(4) = (1.01 +/- 0.06) x 10(4) M-1 s(-1) (k(2) 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 center dot SCH2CH(NH2)CO2-/-SCH2CH(NH2)CO2- redox couple was obtained from the oxidation of L-cysteine by [Fe(bpy)(CN)(4)](-), with k(11) = 4 x 10(5) M-1 s(-1). The self-exchange rate constant of the center dot SCH2CH(NH3+)CO2-/-SCH2CH(NH3+)CO2- redox couple was similarly obtained from the rates with both Fe(III) oxidants, a value of 6 x 10(6) M-1 s(-1) for k(11) 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.