HIGHLY BRANCHED ISOPRENOIDS (HBIS): IDENTIFICATION OF THE MOST COMMON AND ABUNDANT SEDIMENTARY ISOMERS

Abstract

Tri- and tetraunsaturated highly branched isoprenoid (HBI) alkenes are widespread sedimentary geochemicals but few have been isolated from sediments in sufficient quantities for rigorous identification. However, two C25 trienes, four C25 tetraenes and two C25 pentaenes have now been isolated from the diatom Pleurosigma intermedium following bulk scale culture, and these have been purified by column chromatography and fully characterised by NMR spectroscopy and mass spectrometry. The compounds have been used to identify the previously unknown, but common and abundant HBIs found previously in many studies of sediments, particles and biota from around the world. These HBIs are structurally different to those reported from other diatoms. For example, unlike HBIs from the diatoms Haslea ostrearia and Rhizosolenia setigera, the alkenes in P. intermedium are unsaturated at the major branch point of the carbon skeleton and E/Z isomerism is observed for one of the trisubstituted double bonds. There is no evidence for the presence of configurational diastereoisomerism. The distributions of HBIs in P. intermedium (including the E/Z ratios) also show a dependence on the growth conditions within the five cultures studied. The positions of the double bonds in the HBIs of P. intermedium, and by inference, of the sediments, are consistent with the positions of sulphur incorporation in some of the HBI thiolanes and thiophenes which have been reported previously in some sediments and oils.