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AN INVESTIGATION OF THE CHEMICAL STABILITY OF ARSENOSUGARS IN BASIC ENVIRONMENTS USING IC-ICP-MS AND IC-ESI-MS/MS
Citation:
Gamble, B. M., P A. Gallagher, J A. Shoemaker, A N. Parks, D M. Freeman, C A. Schwegel, AND J T. Creed. AN INVESTIGATION OF THE CHEMICAL STABILITY OF ARSENOSUGARS IN BASIC ENVIRONMENTS USING IC-ICP-MS AND IC-ESI-MS/MS. ANALYST 128(12):1458-1461, (2003).
Impact/Purpose:
The goal is to develop an extraction protocol that mimics the human digestive tract and then to use it to assess the bioavailable fraction of arsenic from complex dietary mixtures such as a daily composite -- to move current methods toward a better human physiologically-based exposure estimate method which approximates the "true" bioavailability of arsenic within an environmental or dietary matrix.
Description:
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)-b-ribofuranosyloxy]-2-hydroxypropylene glycol - As(328), 3-[5'-deoxy-5'-(dimethylarsinoyl)-b-ribofuranosyloxy]-2-hydroxypropanesulfonic acid - As(392), 3-[5'-deoxy-5'-(dimethylarsinoyl)-b-ribofuranosyloxy]-2-hydroxypropyl hydrogen sulphate - As(408), and 3-[5'-deoxy-5'-(dimethylarsinoyl)-b-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 0C over a period of up to 8 hours. Two arsenosugars, As(328) and As(392), were found to be relatively stable in this solution for up to 8 hours. However, As(408) and As(482) formed detectable quantities of dimethylarsinic acid (DMAA) and As(328) within 0.5 and 2 hours, respectively. It was found that 97% of As(408) degrades after 8 hours 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 hour treatment at 60 0C. 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 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 SN2 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 SN2 attack at the arsenic atom.