Seafood frequently contains high concentrations of arsenic (approximately 10-100 mg/kg dry weight). In marine algae (seaweed), this arsenic occurs predominantly as ribose derivatives known collectively as arsenosugars. Although it is clear that arsenosugars are not acutely toxic, there is a possibility of arsenosugars having slight chronic toxicity. In general, trivalent arsenicals are more toxic than their pentavalent counterparts, so in this work we examine the hypothesis that trivalent arsenosugars might be significantly more toxic than pentavalent arsenosugars in vitro. We compared the in vitro toxicity of (R)-2,3-dihydroxypropyl-5-deoxy-5-dimethylarsinoyl-beta-D-riboside, a pentavalent arsenosugar, to that of its trivalent counterpart, (R)-2,3-dihydroxypropyl-5-deoxy-5-dimethylarsino-beta-D-riboside. The trivalent arsenosugar nicked plasmid DNA, whereas the pentavalent arsenosugar did not. The trivalent arsenosugar was more cytotoxic (IC50 = 200 microM, 48 h exposure) than its pentavalent counterpart (IC50 > 6000 microM, 48 h exposure) in normal human epidermal keratinocytes in vitro as determined via the neutral red uptake assay. However, both the trivalent and the pentavalent arsenosugars were significantly less toxic than MMA(III), DMA(III), and arsenate. Neither the pentavalent arsenosugar nor the trivalent arsenosugar were mutagenic in Salmonella TA104. The trivalent arsenosugar was readily formed by reaction of the pentavalent arsenosugar with thiol compounds, including, cysteine, glutathione, and dithioerythritol. This work suggests that the reduction of pentavalent arsenosugars to trivalent arsenosugars in biology might have environmental consequences, especially because seaweed consumption is a significant environmental source for human exposure to arsenicals.

The reaction of dimethylarsinic acid (DMAV) with hydrogen sulfide (H2S) is of biological significance and may be implicated in the overall toxicity and carcinogenicity of arsenic. The course of the reaction in aqueous phase was monitored, and an initial product, dimethylthioarsinic acid, was observed by using LC-ICP-MS and LC-ESI-MS. Dimethylarsinous acid was observed as a minor product. A second slower-forming product was identified, and the electrospray mass chromatograms for this species produced ions at m/z 275, 171, and 137 in positive mode. To aid in the identification of this slower-forming product, crystalline standards of sodium dimethyldithioarsinate and dimethylarsino dimethyldithioarsinate were prepared and re-characterized by using improved spectroscopic and structural analysis techniques. An aqueous solution of sodium dimethyldithioarsinate produced a single major chromatographic peak that matched the retention time (7.6 min) of the slower-forming product and contained similar molecular ions at m/z 275, 171, and 137 via LC-ESI-MS. The dimethylarsino dimethyldithioarsinate standard produced four aqueous phase species one of which coeluted with the slower forming product. This coeluting peak also produced the identical ESI-MS ions as the slower-forming product of DMAV + H2S. ESI-MS/MS experiments conducted on sodium dimethyldithioarsinate in deuterated water produced molecular ions at m/z 276, 173, and 137. Subsequent collisionally activated dissociation (CAD) experiments on m/z 276 did not produce a product ion at m/z 173. These data indicate that two different species are present in solution, while NMR data indicate that only dimethyldithioarsinic acid exists in aqueous solutions. This discrepancy was investigated by conducting NMR studies on the acidic solution of sodium dimethyldithioarsinate after taking this solution to dryness. The resolubilized solution produced a proton NMR signal characteristic of dimethylarsino dimethyldithioarsinate. Therefore, it was concluded that the ESI-MS ion at m/z 275 associated with the slowly forming second reaction product and the sodium dimethyldithioarsinate compound is a product of the ESI desolvation process.

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The NHC‐RhI complexes [RhCl(COE)(NHC)]2 1 and 2 [COE = cyclooctene, NHC in 1 = N,N‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) and, in 2, N,N‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes)] react with H2 in hexane to give the dimeric, mono‐carbene dihydrido species [Rh(H)2Cl(NHC)]2 (NHC = IPr (3), IMes (4)). In the presence offurther NHC, the bis‐carbene dihydrido species Rh(H)2Cl(NHC)2 are formed; a crystal structure of the IPr complex 5 is analogous to that of the known IMes analogue. The dihydride‐mixed‐carbene species Rh(H)2Cl(IPr)(IMes) (5a) was also observed but not isolated. A benzene solution of 5 under D2 slowly generates the corresponding dideuteride. Reactions of the mono‐carbenes (1/3, or 2/4) with CO in hexane afford the respective dicarbonyl complexes RhCl(CO)2(NHC) [NHC = IPr (6), IMes (7)], while CO reactions with the bis‐carbene dihydrides give, respectively, the mono‐carbonyl complex RhCl(CO)(IPr)2 (8) and the known IMes analogue. All the complexes are characterized by elemental analysis, 1H and 13C{1H} NMR and IR spectroscopies and, in the case of 5, by X‐ray crystallography. The catalytic activities of 5 and the previously reported Rh(H)2Cl(IMes)2 for hydrogenation of COE and 1‐octene (and isomerization of the latter) are shown to be poor.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)