Liquid chromatography–mass spectrometry of spiroketal stereoisomers of pectenotoxins and the analysis of novel pectenotoxin isomers in the toxic dinoflagellate Dinophysis acuta from New Zealand

The acid-catalyzed inter-conversion of spiroketal isomers of pectenotoxins PTX1, PTX6 and PTX2 were studied by liquid chromatography–electrospray ionization-tandem mass spectrometry (LC–MS–MS). Using a C8-silica reversed-phase column and a mobile phase of aqueous acetonitrile containing 2 mM ammonium formate and 50 mM formic acid, the known spiroketal stereoisomers of PTX1 eluted in order of PTX1, PTX4 and PTX8, while those of PTX6 eluted in the order PTX6, PTX7 and PTX9. Acid treatment of PTX2 yielded two novel spiroketal stereoisomers, which have been named PTX2b and PTX2c. LC–MS–MS spectra obtained for the [M+NH4]+ ions of PTX2, PTX2b and PTX2c were essentially identical. As an application of the LC–MS–MS methodology, a sample of the toxic dinoflagellate Dinophysis acuta collected from the coast of New Zealand was analyzed for pectenotoxins. PTX2 and a new pectenotoxin, which has been named PTX11, were detected as the most predominant compounds. Novel PTX2 and PTX11 isomers were also found in the D. acuta although the levels of these compounds were low.