Identification of 3, 6, 9, 12, 15‐octadecapentaenoic acid in laboratory‐cultured photosynthetic dinoflagellates

Abstract

Polar and nonpolar chromatograms of fatty acid methyl esters derived from 11 species of photosynthetic, marine dinoflagellates cultured in modified Erd-Schrieber medium contained a component (4–23%) not identifiable by conventional graphic or arithmetic methods. Hydrogenation followed by gas liquid chromatography of the product showed the unknown component to be a straight chained 18 carbon fatty acid methyl ester. Chemical (CH₄) ionization mass spectrometry of the isolated ester gave a spectrum characteristic of methyl esters and a mol wt of 288, indicating an 18 carbon molecule with 5 double bonds, or equivalent unsaturation. The IR spectrum showed that the double bonds are nonconjugated, and all arecis in geometry. Electron impact mass spectrometry of the pyrrolidide derivative provided evidence that double bonds are located in the 3, 9, 15 positions and probably also in the 6 and 12 positions of the molecule. These double bond positions were confirmed by NMR spectrometry. Data obtained by quantitation of the algal methyl esters suggest the possibility that these dinoflagellates synthesize 18∶5ω3 (shorthand) notation for chain length: number of double bonds and position of final double bond counted from the terminal methyl group) by a 2 carbon chain shortening of 20∶5ω3, rather than by the insertion of a †3 bond into 18∶4ω3.