Structural identification of sedimentary C21 and C22 highly branched isoprenoid alkanes

C21 and C22 highly branched isoprenoid (HBI) alkanes occurring in high relative abundance in lagoonal sediments of Abu Dhabi have been unambiguously identified as 2,6,10-trimethyl-7-(3-methylpentyl)dodecane and 3,7,11-trimethyl-6-(3-methylpentyl)tridecane, respectively, using NMR spectroscopy. A second C21 HBI isomer is tentatively identified as 3,7,11-trimethyl-6-(3-methylbutyl)tridecane, on the basis of comparison of its mass spectral fragmentation with those of fully identified HBIs. The structures of these three components are formed by extension of the “parent” C20 HBI alkane, first identified in Rozel Point Oil by Yon et al. [Yon, D.A., Maxwell, J.R., Ryback G., 1982. 2,6,10-trimethyl-7-(3-methylbutyl)dodecane, a novel sedimentary biological marker. Tetrahedron Letters 23, 2143–2146], by one or two carbon atom(s), respectively, at the terminal carbon atom of one or two of the T-branches. In contrast to the C20 HBI alkane present in the oil, 13C NMR spectroscopy did not indicate the presence of diastereoisomers for the C21 and C22 HBIs in sediments, indicating stereospecific enzyme-controlled biosynthesis of these components by an unidentified biological source. The stable carbon isotopic compositions of the major C20–C22 HBIs are identical, suggesting a common biological source.