An investigation of the chemical stability of arsenosugars in basic environments using IC-ICP-MS and IC-ESI-MS/MS

Abstract

This paper evaluates the chemical stability of four arsenosugars using tetramethylammonium hydroxide (TMAOH) as an extraction solvent. This solvent was chosen because of the near quantitative removal of these arsenicals from difficult to extract seafood (oysters and shellfish). Four arsenosugars (3-[5′-deoxy-5′-(dimethylarsinoyl)-β-ribofuranosyloxy]-2-hydroxypropylene glycol—As(328), 3-[5′-deoxy-5′-(dimethylarsinoyl)-β-ribofuranosyloxy]-2-hydroxypropanesulfonic acid—As(392), 3-[5′-deoxy-5′-(dimethylarsinoyl)-β-ribofuranosyloxy]-2-hydroxypropyl hydrogen sulfate—As(408), and 3-[5′-deoxy-5′-(dimethylarsinoyl)-β-ribofuranosyloxy]-2-hydroxypropyl-2,3-hydroxypropyl phosphate—As(482)) were evaluated. The stability of these four arsenosugars were studied independently in a solution of 2.5% TMAOH at 60 °C over a period of up to 8 h. Two arsenosugars, As(328) and As(392), were found to be relatively stable in this solution for up to 8 h. However, As(408) and As(482) formed detectable quantities of dimethylarsinic acid (DMAA) and As(328) within 0.5 and 2 h, respectively. It was found that 97% of As(408) degrades after 8 h of treatment producing 3.4 times as much DMAA as As(328). This is contrary to As(482), which produces 13 times as much As(328) as DMAA and only 37% of the As(482) was converted by the 8 h treatment at 60 °C. These degradation products led to the investigation of weaker TMAOH extraction solvents. Three different concentrations (2.5%, 0.83% and 0.25%) were used to determine the effect of TMAOH concentration on the degradation rate of As(408). By reducing the TMAOH concentration to 0.83%, the conversion of the arsenosugar to As(328) and DMAA is nearly eliminated (less than 5% loss). Arsenosugars, As(408) and As(482), were also studied in 253 mM NaOH to verify the degradation products. The NaOH experiments were conducted to investigate a possible hydroxide based reaction mechanism. Similar degradation plots were found for each arsenosugar when compared to the 2.5% TMAOH data. A mechanism has been proposed for the formation of As(328) from As(408) and As(482) in base via an S_N2 reaction (hydroxide attack) at the side chain carbon adjacent to the inorganic ester. The formation of DMAA is observed in all arsenosugars after prolonged exposure. This probably occurs via an S_N2 attack at the arsenic atom.