A NEWLY DISCOVERED NORISOPRENOID, 2,6,15,19-TETRAMETHYLICOSANE, IN CRETACEOUS BLACK SHALES

A novel C-24 acyclic isoprenoid, 2,6,15,19-tetramethylicosane (TME), h as been unambiguously identified, using a synthesised standard, in the saturated hydrocarbon fractions and in the kerogen pyrolysates of sam ples from a Lower Albian black shale from southeastern France. In addi tion, a novel C-26 acyclic isoprenoid, 10-ethyl-2,6,15,19-tetramethyli cosane (ETME), was tentatively identified. These compounds appear to b e structurally strongly related to the relatively well-known C-25 isop renoid 2,6,10,15,19-pentamethylicosane (PME) and their co-occurrence i n these sediments and similar C-13-contents indicates a common origin. This suggests an alternative biosynthetic pathway for the biosynthesi s of PME via the methylation of TME rather than the tail-to-tail coupl ing 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 uncert ain. Copyright (C) 1998 Elsevier Science Ltd.