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Kinetic and Mechanistic Study of Oxidation of Dopamine by Manganese (IV) Clusters with Relevance to Photosystem II

Golesorkhi, Bahman | 2013

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 44702 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Mohammadi Boghaei, Davar
  7. Abstract:
  8. Studying of synthetic models which have many similarities to Oxygen Evolving Complex (OEC) in Photosystem II (PS II) can play a very significant role in gaining an insight to mechanism of oxidation of water in PS II. The aim of this study is to investigate the mechanistic and kinetic aspects of oxidation of dopamine as an important neurotransmitter in body by tri and tetranuclear oxo-bridge manganese clusters which have been reported in the past to get some information about the influence of protonation and deprotonation of oxo-bridges and water molecules bonded to these clusters on properties of them in redox reactions.In aqueous media, the trinuclear complex [Mn3(μ-O)4(phen)4(H2O)2](NO3).2.5¬H2O behaves like a monobasic acid, equilibrates with its deprotonated conjugate base. Both are reactive oxidants in the title redox. An aqueous solution of MnIV3 in the pH range 2.0–4.2 smoothly oxidizes dopamine; itself being reduced to MnII. In its kinetics, we observed that the reaction rate increases on increasing pH of the reaction medium until pH 3.2. Since, above pH 3.2 the rate of reaction increases. Protonated oxidantsare better oxidants than their deprotonated analogues from both thermodynamic and kinetic points of view. A likely and intuitive explanation is that the charge interaction is responsible for this observation as all the reacting species are positively charged and on increasing pH, deprotonated oxidant with lesser positive charge interacts with positively charged reductant.The title complex [Mn4(μ-O)6(bipy)6](ClO4)4.H2O is one of the most remarkable tetranuclear clusters with relevance to PS II. Below pH 3.0, protonation on the oxo-bridge of 14+ results in the formation of an additional oxidant. Both are reactive oxidants in the title redox. An aqueous solution of MnIV4 in the pH range 2.0–5.5 smoothly oxidizes dopamine; itself being reduced to MnII. Analysis of the rate data clearly established that the oxo-bridge protonated oxidant is kinetically far more superior in oxidizing dopamine. It was found observed rate increases on increasing acidity of the reaction medium in the pH region 2.0-3.5. The observed rate dependence on pH, as found in the low pH region could only be explained if we assume a single protonation in MnIV4 as protonated oxidants always react faster than their conjugate bases.However, above pH 3.5, oxidation rate was found to increase on increasing pH of the reaction medium. A likely and intuitive explanation is that the charge interaction is responsible for this observation. Rates of both reactions are substantially lowered in D2O-enriched media in comparison to that in H2O media. An initial one electron one proton transfer electroprotic rate step could be mechanistically conceived.
    The kinetics were monitored using UV-Vis spectroscopy and common methods of collecting pseudo first-order rate constants. UV-Vis and FT-IR spectroscopies have been used in order to characterize the complexes and products of reactions
  9. Keywords:
  10. Photosystem II ; Oxygen Evolving Complex ; Oxo-Brige Manganese Clusters ; Redox Reactions ; Electroprotic Mechanism

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