ISOLATION AND CHARACTERIZATION OF SEDIMENTARY AND SYNTHETIC HIGHLY BRANCHED C20 AND C25 MONOENES

Argentation chromatography (TLC, HPLC) followed by gas chromatographic (GC), spectroscopic (GCMS, and in two cases, NMR) and degradative (ozonolysis) analyses of pure isolates, has allowed the double bond positions of several synthetic highly branched C20 and C25 monoenes to be established. A similar isolation and characterization of a C20 monoene from sediments of the Tamar estuary (UK) and of a C25 monoene (hydrogenation product of a diene) from McMurdo Sound (Antarctica) sediments, showed that they both contained methylene double bonds, identical to those previously found in monoenes from Shark Bay (Western Australia). Comparison of the GC and GCMS data of the synthetic monoenes with those obtained for a sedimentary C20 monoene from Gluss Voe (Shetland Isles, UK) and two C25 monoenes from the Tamar estuary, showed that the double bonds in these compounds were probably in non-methylenic positions. These findings may have important implications. The differences in double bond positions may reflect contributions of alkenes from different source organisms, or from the same organisms living under different environmental conditions. In time the compounds may prove to be useful biological markers of recent and palaeoenvironments. Also, since it has been suggested that reactions between the alkenes and sedimentary inorganic sulphur species may be controlled by the position and extent of unsaturation, a knowledge of the double bond positions will further our understanding of the diagenetic fate of these unusual compounds.