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Title: A new analytical method for methylmercury speciation and its application for the study of methylmercury-thiol complexes
Authors: Lemes, Marcos Jose de Lima
Supervisor: Wang, Feiyue (Chemistry)
Examining Committee: Hunter, Norm (Chemistry) Perrault, Helene (Chemistry) Hanson, Mark (Environment & Geography) Belzile, Nelson (Chemistry and Biochemistry, Laurentian University)
Graduation Date: May 2010
Keywords: methylmercury-cysteine
Issue Date: 9-Apr-2010
Abstract: Monomethylmercury (CH3Hg+ and its complexes; hereafter referred to as MeHg) in the intracellular environment is known to be predominantly bonded to thiol-containing biomolecules but the identities of these target biomolecules remain unknown. Some evidence suggests that binding with glutathione acts as a detoxification mechanism for MeHg, while binding with L-cysteine permits MeHg transport across the blood–brain barrier resulting in neurotoxicity. However, the occurrence of these complexes in biological tissues has not been confirmed analytically, and little is known about their kinetic stability. In this thesis, methylmercury L-cysteinate (CH3HgCys) and methylmercury L-glutathionate (CH3HgGlu) were synthesized and structurally characterized by proton nuclear magnetic resonance (1H NMR), electrospray ionization mass spectrometry (ESI-MS), and X-ray crystallography. A new analytical method was developed combining high performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICPMS). The method was capable of separating and analyzing CH3HgCys and CH3HgGlu complexes, as well as CH3HgX and inorganic HgX (X = H2O, OH-, or Cl-), with detection limits at the sub-micromolar levels. Using a new enzymatic hydrolysis method to isolate MeHg species in biological tissues, the HPLC-ICPMS method was successfully applied for the determination of MeHg speciation in the muscle tissue of dogfish (Squalus acanthius). These results provide the first analytical evidence for the presence and dominance of CH3HgCys in fish muscle. The analytical method was also used to study the kinetic stability of CH3HgCys and CH3HgGlu under a range of environmental and intracellular conditions. In general, CH3HgGlu was more stable than CH3HgCys under light exposure or darkness. The stability of both compounds decreases dramatically with increasing ionic strength (I). Half-life of CH3HgCys decreases from 34.1 h (I = 0.01 M) to 5.9 h (I = 0. 5 M) and the half-life of CH3HgGlu decreases from 259 h (I = 0.01 M) to 35.9 h (I = 0. 5 M). Suggesting major differences in their cycling in freshwater (I < 0.01M), seawater (I ≈ 0.7M) and body fluids (I ≈ 0.16 M). The analytical technique and the findings from this thesis research provide a new analytical framework for the study of MeHg speciation in natural waters, and the metallomics of MeHg in biological systems.
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