Methylmercury compounds, formation

The in vivo formation of toxic methylmercury compounds probably involves methylation by methylcorrinoids [see Section VII,D and Chapter 17 of ref. 136)1... 

Besides thermolysis, the photochemical decomposition of solid trihalo-methylmercury compounds RHgCCli, CF3HgOCOCF3 and Hg(OCOCF3)2 has been studied (Scheme 2). The irradiation of samples placed in an evacuated quartz tube, which was connected to a helium cryostat, was carried out at -50 to +10°C. Thus, a desorption into the gas phase of the primary products of the photolysis occurred, and consequent low-temperature matrix stabilization of them was made. As a result, the formation of only the radicals CCI3 (1 3 898 cm" ) and CF3 (vi 1084, V2 702, P2+ V4 1205, r>3 1249 cm ) or of products of their secondary reactions was observed (Mal tsev et al., 1974, 1975, 1977b). 

Yamanada, M. and Tonomura, K., 1972a. Formation of methylmercury compounds from inorganic mercury by Clostridium cochlearium. J. Ferment. Technol., 50 159—166. 

Methylmercury Compounds.— The mechanism of methylation of HgCla by methyl-cobalamin (30) involves transfer of a methyl carbanion from base on (30). Electrochemical formation of McaHg was reported using a Hg anode in a NaCl,MeAlCla melt. 

The unexpectedly high concentrations of mercury found in water and in fish tissues result from the formation of soluble monomethylmercury ion, CHjHg, and volatile dimethylmercury, (CH3)2Hg, by anaerobic bacteria in sediments. Mercury from these compounds becomes concentrated in fish lipid (fat) tissue and the concentration factor from water to fish may exceed 10. The methylating agent by which inorganic mercury is converted to methylmercury compounds is methylcobal-amin, a vitamin Bj2 analog ... 

The formation of butyl derivatives of methylmercury and ethylmercury was studied as a means of providing stable, volatile derivatives for the gas chromatographic determination of these compounds (Quimby and Sullivan, 1990). The resulting dialkylmercury species were separated satisfactorily on a capillary... 

Uncontrolled species transformations during analysis form another source of error. For methylmercury determinations in sediments it was demonstrated that errors of up to 80% resulted from the formation of the compound from inorganic mercury during separation and analysis [28, 29], For the study of possible species transformations during analysis multiple isotope dilution could be used as a diagnostic tool for identifying the error and bias inherent in specific methods of storage, sample preparation and measurement [30, 31]. 

The rate of methylmercury formation by this process is largely determined by the concentration of methyl cobalamine compounds, inorganic mercuric ions, and the oxygen concentration of the water, with the rate... 

MeHg" " is distributed throughout the body, and easily penetrates the blood-brain and blood-placental barriers (Clarkson 1993, Hansen et al. 1989, Suzuki et al. 1984). The transport of MeHg" " into tissues is mediated by the formation of a MeHg-cysteine complex (Aschner and Aschner 1990, Tanaka et al. 1991, Kerper et al. 1992). Soon after application, McHg" " is found in the blood, predominantly in the red cells. In humans, the ratio of MeHg in red blood cells to serum is approximately 20 1 (Kershaw et al. 1980). Short-chain alkyl-mercury compounds such as methylmercury or ethylmercury are rather stable in the body, whereas long-chain alkylmercury or arylmercury compounds such as phenylmercury may be metabolized relatively quickly to Hg " " ions (Roberts et al. 1979) and, therefore, show similar behavior to the Hg " " ion (Pfab et al. 

The main difficulty in preparing a sediment reference material for interlaboratory studies on chemical species is to achieve the stability of the relevant compounds [12]. With respect to methylmercury, the main source of instability is due to bacteria, either by demethylation [88,89] or formation of volatile dimethylmercury [90]. The conversion is indirectly provoked by the biological activity of various types of bacteria, such as (i) aerobic mesophilic heterotrophic microorganisms, (ii) anaerobic sulfate-reducing bacteria and (iii) anaerobic spore-forming bacteria. In order to control the remaining bacteria present after different irradiation treatments, a bacterial enumeration was performed on samples which were dehydrated after homogenization and irradiated at various 7-ray doses (0, 4, 8, 12, 25 and 50 kGy). The determination of the sulfate-... 

GC is widely used to separate volatile organometallic species. The favored approach involves derivatization (e.g., ethylation or hydride formation), preconcentration, chromatographic separation followed by element-specific detection using AAS, AES, or AES. For species such as methylmercury and methylarsenic compounds, derivatization is normally necessary prior to GC in order to form volatile species. The most common approach has been to convert both inorganic and organometal species via hydridization with sodium borohydride, or alkylation, e.g., using sodium tetraethylborate, to form volatile hydrides or alkylated species, respectively. Not all species of an element are amenable to derivatization. For example, the application of hydridization... 

Uchida et al 2 have shown that the mercury compound in the shellfish that caused the Minimata disease (Japan) was methyl (methylthio) mercury. Westoo concluded that it is reasonable to assume that methylmercury, if present in fish, should at least to some extent be present as a methylthio derivative. The Hg-S bond is stronger than Hg-NH or Hg-OH bonds. Accordingly, the former bond would prevent the formation of the latter types of bonds in the presence of ammonium hydroxide solution and increase the solubility in water. Any methylthio group present should therefore be removed before the extraction with alkali. 

An appreciation of the strong influence of complexation and chelation on heavy metals behavior in natural waters and wastewaters can be gained by reading Section 11.9, which deals with that subject. Methylmercury formation is also discussed in Section 11.9. Both topics involve the combination of metals and organic entities in water. The interaction of metals with organic compounds is of utmost importance in determining the role played by the metal in an aquatic system. 

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