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Kinetics of the laccase-catalyzed oxidation of aqueous phenol

Laccase (E.C catalyzes the oxidation of aromatic substrates with the simultaneous reduction of molecular oxygen to water. It has significant potential for use in many applications due to its high reaction rates, broad substrate-specificity, and use of oxygen as an inexpensive co-factor. The objective of this research was to investigate the ability of laccase from Trametes versicolor to catalyze oxidation reactions under a variety of reaction conditions and to model the kinetics of these transformations. Phenol was selected as a model substrate. / Laccase was very stable when incubated at temperatures less than 30°C and pHs between 6 and 7. The optimum pH for phenol transformation was 6, but when present in sufficient quantities, laccase was able to significantly transform phenol at pHs from 4 to 7 and temperatures from 10 to 60°C. Laccase stability was negatively impacted by the presence of four common redox mediators. Of these, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) and 2,2',6,6'-tetramethylpiperidine-N-oxyl (TEMPO) significantly enhanced phenol transformation but large quantities were required, which may limit the feasibility of the use of these mediators in many applications. / A series of kinetic models was developed in order to achieve a better understanding of the mechanisms and kinetics of laccase-catalyzed reactions and to eventually assist in the choice and design of suitable reactor systems. These models were designed to predict the transient oxygen and phenol concentrations during laccasecatalyzed reactions at pH 6 and 25°C. Over the course of developing and validating these models, it was observed that: (1) the rate-limiting step in the catalytic reactions is the reaction between the oxidized form of laccase and phenol; (2) the stoichiometric ratio, which is defined as the molar ratio of phenol transformed to oxygen consumed in the catalytic reaction, was found to increase with phenol concentration in the reaction mixture from a theoretical lower limit of 1 and to approach a theoretical upper limit of 4; and (3) laccase inactivation occurs over the course of the reaction and was found to be dependent on the rate of substrate transformation. / Ultimately, these findings were incorporated into a comprehensive kinetic model to predict transient species concentrations in an open-system environment where the degree of substrate transformation was not limited by oxygen availability. The model accounts for enzyme kinetics, oxygen mass-transfer, variable reaction stoichiometry, and inactivation related to reaction products. Excellent agreement was observed between measured and modeled phenol and oxygen concentrations for a wide range of initial phenol concentrations and enzyme activities. Simplified models were also developed by incorporating an assumption, referred to as the pseudo-steady-state assumption, that at any instant during the reaction, the enzyme achieves an approximate steady-state distribution of its various forms around the catalytic cycle. The pseudo-steady-state assumption had the advantage of reducing the complexity of model equations without sacrificing their predictive abilities and allowing enzyme quantities to be expressed in activity units instead of molar concentrations.
Date January 2006
CreatorsSoegiaman, Selvia Kurniawati.
PublisherMcGill University
Source SetsLibrary and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation
CoverageDoctor of Philosophy (Department of Civil Engineering and Applied Mechanics.)
Rights© Selvia Kurniawati Soegiaman, 2006
Relationalephsysno: 002601010, proquestno: AAINR32242, Theses scanned by UMI/ProQuest.

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