Photochemistry and identification of photodegradation products of the marine toxin domoic acid

The photochemistry of the marine toxin domoic acid (DA) and its related compounds in aqueous solution was investigated under environmentally relevant concentrations and radiation conditions. DA was readily phototransformed under simulated sunlight, showing exponential decay as a function of irradiation time. Major identifiable intermediate products were three geometric isomers identified as isodomoic acids D, E, and F. Isodomoic acids D and E formed much more quickly and in higher concentrations relative to isodomoic acid F when DA was irradiated. When pure solutions of either isodomoic acids D, E, or F were irradiated, there was concomitant production of the two other isomers as well as production of DA. These results are significant because they indicate that the three isomers produced upon photolysis of DA are also photolabile and are capable of regenerating the original form of the toxin. A group of less polar products all with a molecular weight of 267 Da was also detected in irradiated solutions of DA. These products are tentatively identified as decarboxylated derivatives of DA. These results demonstrate that exposure of DA to simulated sunlight modifies the chemical structure of DA and produces a suite of isomers and decarboxylated derivatives that can potentially affect the toxicity and metal binding capacity of the original toxin. It is also shown that the photochemistry of DA is a complex system involving simultaneous reversible (isomerization) and irreversible (e.g. decarboxylation) reactions. Therefore, results from this study suggest that isodomoic acids D, E, or F might act as photo-intermediates in the photochemical degradation of DA in the ocean.