Deuterium exchange in arsenobetaine and dimethylarsinoylacetic acid

Arsenobetaine occurs naturally in almost all marine animals and it is assumed to be the unreactive end-product of a detoxification pathway. To investigate the properties of arsenobetaine and its likely immediate biogenic precursor, dimethylarsinoylacetic acid, we studied the exchanges of the C-2 methylene protons of these compounds in D2O solution and showed them to be pH dependent first-order reactions. For arsenobetaine, the rate of exchange was highest at high pH values although exchange also occurred at low pH values. For dimethylarsinoylacetic acid, the rate was highest at low pH values although there was also exchange at high pH values. The half-life of the reaction was maximum for arsenobetaine at pH values of 5-6, and for dimethylarsinoylacetic acid at 6.5-8.5. Mechanisms are suggested for the exchange reactions involved.