Complex Formation Followed by Internal Electron Transfer: The Reaction Between Cysteine and Iron(III)

Article abstract of DOI:10.1007/BF00823418

A kinetic study of the anaerobic oxidation of cysteine (H2L) by iron(III) has been performed over the pH-range 2.5 to 12 by use of a stopped-flow high speed spectrophotometric method.Reaction is always preceded by complex formation.Three such reactive complex species have been characterized spectrophotometrically: FeL⁺ (λ_max = 614 nm, ε = 2820 M^-1cm^-1); Fe(OH)L (λ_max = 503 nm; shoulder at 575 nm, ε = 1640 M^-1cm^-1); Fe(OH)L2^2- (λ_max = 545 nm; shoulder at 445 nm, ε = 3175 M^-1cm^-1).Formation constants have been evaluated from kinetic data: Fe³⁺ + L^2- <*> FeL⁺: log K₁^M = 13.70 +/- 0.05; Fe(OH)²⁺ + L^2- <*> Fe(OH)L: log K₁^MOH = 10.75 +/- 0.02; Fe(OH)L + L^2- <*> Fe(OH)L2^2-; log K₂^MOH = 4.76 +/- 0.02.Furthermore the hydrolysis constant for iron(III) was also obtained: Fe(OH)²⁺ + H⁺ <*> Fe_aq³⁺: log K^FeOH = 2.82 +/- 0.02).Formation of the mono-cysteine complexes, FeL⁺ and Fe(OH)L, is via initial reaction of Fe(OH)²⁺ with H2L (k = 1.14*10⁴ M^-1s^-1), the final product depending on the pH.FeL⁺ (blue) formed at low pH decomposes following protonation with a second-order rate constant of 1.08*10⁵ M^-1s^-1.Fe(OH)L (purple) decomposes with an apparent third order rate constant of k = 3.52*10⁹M^-2s^-1 via 2 Fe(OH)L + H⁺ -> products, which implies that the actual (bimolecular) reaction involves initial dimer formation.Finally, Fe(OH)L2^2- (purple) is remarkably stable and requires the presence of Fe(OH)L for electron transfer.A rate constant of 8.36*10³M^-1s^-1 for the reaction between Fe(OH)L and Fe(OH)L2^2- is evaluated. Keywords.Cysteine oxidation; Iron(III)-complexes; Electron transfer reactions.