User:Arsenokotoi/sandbox

=Natural Abundance=

=Analysis= Chemical analysis of arsenosugars found in nature is complicated by the fact that arsenosugars must be purified from biological samples that may contain a number of different organoarsenic compounds and those compounds are not just limited to the arsenosugar category. Arsenobetaine and inoragnic arsenic, for example, can also be present in these samples.

Typically, biological sample purification is done using solvent extraction, most commonly with a mixture (definitive ratios vary) of water and methanol. This is followed by HPLC techniques which further separate the different arsenosugar species in the sample. Possible HPLC techniques include ion-exchange chromatography, reversed-phase chromatography, size-exclusion chromatography, though single step separation techniques are associated with high rates of error in terms of full separation of compounds. Due to this fact, it is often preferable to carry out more than one HPLC step on the sample, which increases the resolution of each species. It should be noted that ion-exchange chromatography is often quite useful for arsenosugar separation, since, depending on the pH of the solution, arsenosugars may be charged. Reversed-phase chromatography and size exclusion chromatography can be used subsequently to further separate the arsenosugar species in solution.

Commonly used methods of arsenosugar detection are atomic spectroscopy and mass spectrometry. Both offer particular advantages to the characterization of arsenosugar species, since atomic spectroscopy is more sensitive than mass spectrometry, but mass spectrometry yields pertinent information on the structure of the arsenosugar species. Usually, HPLC techniques are coupled with detection by inductively coupled plasma mass spectrometry (ICP-MS), since this detection technique is both sensitive and selective for arsenic species. Specifically, ICP-MS has a low limit of detection, which is necessary when attempting to characterize biological samples with minimal concentrations of arsenosugars.

However, it should be noted that the storage and/or purification processes may result in undesired arsenosugar modification through oxidation of trivalent arsenosugars to pentavalent arsenosugars. Oxidized arsenosugars then readily undergo oxo-thio interconversion.



The likelihood of oxidation makes characterization of biological arsenosugar species difficult, since once oxidation has occurred, the species is no longer what is found in situ and it is impossible to discern whether a species has or has not been oxidized from its naturally occurring state. The process of arsenosugar characterization would also be aided by the characterization of arsenosugar standards: species that are known to be pure and have specific retention times and detection spectra. These standards would allow for comparison with purified biological samples and help determine which species are actually in the sample.

=Synthesis= Synthesis of arsenosugar species is relevant to the preparation of standards for chemical analysis (see above). An artificial synthesis scheme of glycerol arsenoylriboside:
 * DMAsSugarGlycerol Synthesis.jpg

Compound 1, (S)-1,2,-dibenzyl-oxy glycerol, and compound 2, tetraacetyl-β-D-ribofuranose, are both commercially available. In compound 1, Bn indicates a benzyl substituent, and in compound 2, Ac represents an acetyl substituent. In synthesis step a, compounds 1 and 2 are reacted in the presence of boron trifluoride etherate (BF3*Et2O), a Lewis acid. Anomerically pure compound 3 is formed in 60% yield, with retention of stereochemistry due to neighboring group participation from the carbonyl oxygen of the acetyl group.

Compound 3 is then deacetylated in synthesis step b by the addition of ammonia in methanol to form compound 4. This is followed by synthesis steps c, where hydrogen, palladium on carbon (Pd/C), p-Toluenesulfonic acid (p-TsOH), and 5 equivalents of dimethoxypropane are used to both debenzylate the glycerol chain and add protecting groups onto the hydroxyl substituents that result from debenzylation. These reaction steps give compound 5 in 61% yield. In step d, the hydroxyl group is converted to a mesyl group using methanesulfonyl chloride, 4-dimethylaminopyridine (DMAP), and triethylamine (TEA). This forms compound 6 in 100% yield, which is then converted to compound 7 in synthesis step e, using Bu4NBr in N,N-dimethylformamide (DMF) to convert the mesyl to a bromide in 62% yield.

In step f, the bromide is converted to a dimethylarsine to give compound 8 in 51% yield. This is done using 5 equivalents of dimethylarsenic iodide and excess sodium in tetrahydrofuran (THF). Compound 8 is then converted to compound 9 through synthesis step g, where peroxide is used to oxidize the arsenic from its trivalent to its pentavalent state. This results in the double bond between the arsenic and the oxygen. Compound 9 is formed in 81% yield.

Finally, in synthesis step h, the protecting groups on the hydroxyls are removed by aqueous trifluoroacetic acid (TFA) and the resulting compound is neutralized using 2 M ammonium hydroxide to form the arsine oxide substituent on the ribofuranose. The final product is collected in 86% yield after separation on a Sephadex-20 ion exchange column.

=Toxicity=

Because arsenosugars are present in comparatively high concentrations in marine algae, food products made from algae also contain comparatively high concentrations of arsenosugars. The toxicity of most arsenosugars in the human body is unknown. Unlike inorganic arsenic, it's assumed that most consumed arsenosugars exit the body through oxidized metabolites, specifically dimethylarsenate (DMA), in the urine. However, a recent study has shown that arsenosugar consumption may cause oxidative stress and DNA damage in rats. The mechanism for this is unknown, as is the effect that arsenosugar consumption has on the human brain.

In population consumption studies, is difficult to distinguish effects due to arsenosugars alone vs. effects due to the variety of organoarsenic compounds present in seafood and sea weed. Many studies report an increased urine concentration of arsenic, arsenobetaine, and other organoarsenic compounds in addition an increased urine concentration of arsenosugars, so any observed toxicity cannot explicitly be said to occur because of arsenosugar consumption.

=References=