A newly discovered norisoprenoid, 2,6,15,19-tetramethylicosane, in Cretaceous black shales

Abstract

A novel C24 acyclic isoprenoid, 2,6,15,19-tetramethylicosane (TME), has been unambiguously identified, using a synthesised standard, in the saturated hydrocarbon fractions and in the kerogen pyrolysates of samples from a Lower Albian black shale from southeastern France. In addition, a novel C26 acyclic isoprenoid, 10-ethyl-2,6,15,19-tetramethylicosane (ETME), was tentatively identified. These compounds appear to be structurally strongly related to the relatively well-known C25 isoprenoid 2,6,10,15,19-pentamethylicosane (PME) and their co-occurrence in these sediments and similar 13C-contents indicates a common origin. This suggests an alternative biosynthetic pathway for the biosynthesis of PME via the methylation of TME rather than the tail-to-tail coupling of farnesyl and geranyl. These irregular isoprenoids are likely to have originated as parts of ether lipids in some unknown archaea, but exactly which organisms biosynthesised these compounds remains uncertain.