Detection and quantification of a thio-arsenosugar in marine molluscs by IC-ICP-MS with an emphasis on the interaction of arsenosugars with sulfide as a function of pH

The sulfur analog of As(328) (2,3-dihydroxypropyl-5-deoxy-5-dimethylarsinoyl-beta-(D)-riboside), abbreviated As(328-S), was detected and quantified in five species of marine shellfish using IC-ICP-MS with structural verification via IC-ESI-MS/MS. The CAD spectra produced from the parent ion, m/z 345, in the extract contained two major daughter ions, m/z 253 and 235, closely matching the CAD spectrum of synthetic As(328-S). The ability of the oxide and sulfide forms of the arsenosugar to interconvert led to a series of fundamental studies in ideal solutions containing both the arsenosugar and sulfide. The conversion of As(328) to As(328-S) was found to be pH sensitive and promoted in the pH range where HS- is converted to H2S (pK(1) = 7). The conversion was observed in both shellfish extracts and ideal solutions with comparable sulfide and arsenosugar concentrations. The conversion was further studied over a range of sulfur/arsenic molar ratios. At a 15-fold molar excess of sulfide at pH 4.8, the lowest pH experienced by an extract, > 90% conversion to the sulfide was observed. In the context of shellfish, a molar ratio greater than 200:1 sulfide to arsenic was detected in all five extracts. These trends should prove useful in improving confidence in thio-arsenosugar speciation and predicting the extent of conversion under a given set of conditions.