Methylated arsenic compounds

Trimethyl arsine [593-88-4] C H As, has been identified as the toxic volatile arsenical, once known as "Gosio gas," produced by the reaction of certain molds that grow on wallpaper paste and react with inorganic arsenic compounds present in the paper. A number of microorganisms can methylate arsenic trioxide and other arsenic-containing compounds to yield trimethylarsine. These microorganisms include Scopulariopsis brevicaulis Candida humicola and Gliocladium roseum (72). 

Wallschlager, D. and London, J. (2008) Determination of methylated arsenic-sulfur compounds in groundwater. Environmental Science and Technology, 42(1), 228-34. 

Cullen et al. (1994) have proposed a possible mechanism of arsenic methylation after the study in which arsenite, arsenate, monomethyl-arsonate or dimethylarsinic acid were added to the growth medium in the presence of the unicellular alga Polyphsa peniculus. Evidence of arsenic biomethylation by the micro-organism Apiotrichum humicola in the presence of L-methionine-methyl-d3 has come from the same laboratory (Cullen et al., 1995). Their findings point to the role of S-adenosylmethionine, or a related sulfonium compound as possible methyl donors. Arsenic biomethylation and biotransformation has also been demonstrated in a freshwater environment (Kuroiwa et al., 1994). 

The discoverj( of the stereoselective biomethylation of a prochiral arsinic acid by a microorganism opens up an exdting new route to optically active tertiary arsines. Numerous microorganisms reductively methylate arsenic(V) compounds . The biological synthesis of (+)-126 by Scopulariopsis brevicaulis was described in 1936. At that time, however, it was not recognized that simple tertiary arsines chiral at arsenic were configurationally stable and amenable to optical resolution. 

While the plots of enthalpies of formation indicate that the arsenic, antimony and bismuth data are problematic, they cannot be used to recommend alternative values since both the methyl and ethyl are thought to be affected by large errors. If one of these were known to be reliable, it could be used to predict the second one. The difference between the enthalpies of formation of methyl and ethyl derivatives relates to the electronegativity of the affixed atom or group, wherefore the more electronegative it is, the less relatively stable is the methyl compared to the ethyl derivative. This stability reasoning extends to the relative stability of methyl and ethyl cation, and antithetically to methyl and ethyl anion, hence we conclude that only for extremely electropositive elements such as lithium is it at all conceivable that the methyl compound has the more... 

Gudzinowicz and Martin used the same method for quantitative studies of organoarsenic (triphenylarsine and trivinylarsine) and organobromo-arsenic (methyl, ethyl and butyl bromoarsines) compound mixtures. 

A considerable number of organometallic species of arsenic, antimony and bismuth have been detected in the natural environment in different manners. A number of these are nonmethyl compounds which have entered the environment after manufacture and use [e.g. butyltin and phenyltin compounds for antifouling paints on boats, and arsanilic acid (Figure 2, 5) and phenylarsonic acids (Figure 2, 6-8) for animal husbandry]. Only a few methyl compounds are now manufactured and used (e.g. methyltin compounds for oxide film precursors on glass and methylarsenic compounds for desiccants or defoliants). 

The linear terdentate hybrid ligands (74) with the donor set N—N—As, also yield compounds of the types [MnXjCN—N—As)] (X = Cl, Br or I), [Mn(N—N—As)2](C104)2 and [Mn(NCS)2-(N—N—As)], in which all three donor atoms appear to be bonded. Similarly, all four donors of the quadridentate N—N—N—As ligand (75) appear to be coordinated in the compound [Mn-(N—N—N—As)], as in which both an imine proton and an arsenic methyl group are lost upon reaction of the ligand with Mn2(CO)io. ... 

Until recently (93-96), little was known about the arsenic methylation intermediates, MMA(lll) and DMA(lll), in the human system, a result of the lack of techniques for the determination of these arsenic species. Recent developments of more sensitive and improved arsenic speciation techniques contribute to the discovery of these intermediary metabolites in human urine (93-96). Figure 3 shows typical chromatograms obtained from the analyses of arsenic compounds in deionized water and in urine samples. Coinjection of the urine sample with authentic MMA(lll) standard (Fig. 3c) demonstrates the coelution of the suspected MMA(lll) in the sample with that of the standard MMA(III), confirming the identity of MMA(lll) in the urine sample. Similarly, coinjection of the urine sample with standard DMA(III) (Fig. 3d) and As(V) (Fig. 3e) confirms the presence of DMA(lll) in the sample (96). Two other research groups have recently also found MMA(lll) and DMA(llI) in human urine samples (123,124). 

Chiu et al. (53) reported that lowering of soil redox potential increased the ratio of As(III) and promoted arsenic methylation. Methylation of arsenic compounds by yeast and bacteria under oxic conditions plays a significant role, whereas methanogenic bacteria are important under anoxic conditions in releasing volatile arsenic from the soil to the atmosphere (39,41). Woolson and Kearney... 

Thermochemical measurements have been made on a number of arsenic compounds. The heats of formation of trimethyl- and triphenylarsine have been used to calculate the mean dissociation energy of the As—C bonds in these compounds. The values found, 51.5 kcal/mole for the methyl compound and 60.3 kcal/mole for the other, are significantly different (506,507). 

Although it is beyond the scope of this article to consider detailed collection and sample preparation procedures for environmental samples, here are some general considerations for speciation analysis of air, water, biological material, and soils/sediments. The collection of samples in air has focused upon the determination of hydrides and methylated compounds of elements such as arsenic, lead, mercury, and tin. The sample may be introduced into a preevacuated container, or with a pump. The analytes may be isolated using cryotrapping techniques or adsorbent cartridges. For best results, it is recommended to collect the entire gas sample in a container, coated... 

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