Identification of novel widely distributed sedimentary acyclic sesterterpenoids

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Abstract
Isoprenoids, the general class of natural products biosynthesized from isoprene units, are abundant in nature. Many isoprenoids are also found in sediments, sedimentary rocks and crude oils where they have proved useful as 'chemical fossils' of biological activity and as indicators of geothermal stress1. However, acyclic isoprenoids with 25 carbon atoms-so-called sesterterpenoids-have been reported only rarely in the biosphere and the geosphere. Indeed, the only acyclic sesterterpenoids of confirmed structure reported in sediments are of pentamethyleicosane structural isomers (Fig. 1, carbon skeleton I). For example, 2,6,10,14,18-pentamethyleicosane has been proposed as a marker for halophilic bacteria and the 2,6,10,15,19-isomer (I) as an indicator of methanogens2. We have now identified a group of novel sesterterpenoids that are major components of the hydrocarbons of recent freshwater, marine and hypersaline sediments from many different parts of the globe. Because they have unusual structures, these sesterterpenes show promise as a new class of biological marker compounds.