Characterisation and isomerisation of mono- and di-unsaturated highly branched isoprenoid (HBI) alkenes: Considerations for palaeoenvironment studies

We report the structures and analytical data for several highly branched isoprenoid (HBI) mono- and di-unsaturated alkenes (monoenes and dienes). This has been achieved by isolating several well characterised HBI dienes from diatom cultures and sediments, and transforming these to structurally related monoenes and other dienes using partial hydrogenation and clay-catalysed, or diphenyl disulphide induced, isomerisation reactions. HBI double bond (DB) positions have been determined through analysis of a combination of GC-MS and NMR data, along with RuO4 oxidation of HBI monoenes that produce characteristic ketones. Our structural identifications have also enabled the assignments of a number of previously reported HBI monoenes and dienes to be confirmed or corrected. Monoenes and dienes undergo clay-catalysed isomerisation in the laboratory, which likely has implications for the sedimentary occurrences of such biomarkers. Under our conditions, alkenes with C6/17 DBs are particularly sensitive to isomerisation, yielding isomers with C5/6, C6/7, C7/8 and C7/20 DBs. Isomers with C5/6 and C7/20 DBs also isomerise with each other and to isomers with C6/7 and C7/8 DBs, but not to ones with a C6/17 DB. In contrast, we see no evidence for isomerisation of the Arctic sea ice biomarker IP25 which has a DB at C23/24. These observations may have implications for the use of certain monoenes (e.g. IP25) and dienes for paleo sea ice reconstruction. A scheme to summarise the (monoene) isomerisation reactions is proposed. On the basis of the usual DB positions in HBIs obtained from diatom cultures, the outcomes from our isomerisation reactions, and of previous sedimentary reports, we discuss the potential origins (biogenic versus sedimentary transformation) of HBIs with different DB positions. However, confirmation of such assignments will require further investigation.