Graduate Thesis Or Dissertation

Synthesis and oxidation of cysteine- and penicillamine- containing Fe(III) and Co(III) complexes. Analogs for the active metal sites of the cysteine and cysteamine dioxygenases

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  • The first part of this thesis describes a study of the characteristics of oxidation of a 1:2 iron(III):thiolate system in methanolic solution and in water. This system was investigated as a potential analog for the cysteine- and cysteamine-dioxygenase catalyzed 0₂-oxidation of cysteine and cysteamine, respectively, to the corresponding sulfinic acids. In the neutral pH range, oxidation by 0₂ and H₂0₂ of a stoichiometric mixture of 1:2 iron(III):cysteine in aqueous solution produced cysteinesulfinic acid, (CyS0₂H) and cysteic acid, (CyS0₃H) , reported as CyS0x⁻ (with yields of 4.1-24.6%), in addition to the principal product, cystine (CySSCy). All amino acid material was separated by ion exchange chromatography and detected quantitatively by standard ninhydrin colorimetric analysis or gravimetric analysis. The primary mechanism pertaining to oxidation of CyS⁻ to CyS0x⁻ involves the proposal of an initial four atom "cluster", [-(FeII)₂(SCy)₂-1]n, which on oxidation with 0₂ generates H₂0₂ and [-(FeIII)₂(SCy)₂-1]n⁺². The active oxidant would then in turn react with an available Fe(III)-thiolate bond forming the S-bonded sulfenate ligand. An IR spectrum of the oxidized solid from the product mixture supports this proposal. An alternative mechanism would be the direct 0₂-oxidation of an Fe(III) coordinated thiolate to (Fe-S(0)₂Cy]. This oxidative mode seems to be active at low Fe(III) concentration in the presence of excess thiolate in slightly alkaline medium. Early iron-thiolate studies were carried out using the thiol, penicillamine. Syntheses of penicillaminesulfinic acid were explored. The syntheses of K₃[(S- PenS-N,S)₃] and of yellow K₃[(S-PenS0₂-N,S)₃] were performed. The ¹H nmr and UV-visible spectra are reported. The decomposition of K₃[(S-PenS0₂-N,S)₃] with ethylenediamine provided the best yield of the sodium salt of the penicillaminesulfinate ion. The molecular acid was unstable in acidic or near neutral, aqueous solution, and therefore was not isolated. The second part of this thesis details the synthesis of bis-penicillaminato-N,S,0-cobaltate(III) complex and its stoichiometric H₂0₂-oxidation resulting in isolation of Na[Co(S-PenS-N,S,0)- (S-PenSO-N,S,0)] and Na[Co(S-PenSO-N,S,O)₂]. The UV-visible, IR and ¹H nmr spectra of these cobalt(III) complexes are reported. Techniques were developed and refined during the study which made possible the separation and identification of the oxidation products, using anion exchange chromatography and gel filtration. Determination of the chirality of each sulfenato sulfur atom was made using applications of circular dichroism. Structures for the several complexes are postulated, based on the combined spectral results. The second-order rate constants for oxidation of the thiolato and the sulfenato complexes were measured under pseudo-first-order conditions. By analogy with the results of the oxidation of Fe(III) and Co(III) complexes studied, proposals on the mechanism and stoichiometry of the reaction at the active site of the cysteine and cysteamine dioxygenases are suggested.
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