C25 highly branched isoprenoid alkenes from the marine benthic diatom Pleurosigma strigosum

Abstract

The hydrocarbon composition of the marine diatom Pleurosigma strigosum isolated from coastal Mediterranean sediments is described. A suite of five C25 highly branched isoprenoid (HBI) alkenes with 2–5 double bonds were detected together with n-C21:4 and n-C21:5 alkenes and squalene. The analysis by 1H and 13C NMR spectroscopy of two isolated HBI alkenes allowed the structural identification of a novel C25 HBI triene (2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadeca-5E,13-diene) and the first identification in diatom cells of 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5E-ene, an HBI previously detected in marine sediments and particulate matter. The other minor C25 HBIs detected were a tetraene and a pentaene that have been previously identified in other diatoms from the genera Haslea and Rhizosolenia, and one other C25 tetraene that could not be structurally identified. The structures of the HBI alkenes of P. strigosum were compared with those of C25 homologues previously identified in three other Pleurosigma sp. (Pleurosigma intermedium, Pleurosigma planktonicum and Pleurosigma sp.). Unlike most structures previously reported, none of the HBI alkenes produced by P. strigosum showed an unsaturation at C7–C20, or E/Z isomerism of the trisubstituted double bond at C9–C10 (whenever present).