Hopanoids diagenesis

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

The resin fractions of organic sulfur-rich bitumens are for a substantial part composed of monomers with linear, isoprenoid, steroid, hopanoid and carotenoid carbon skeletons connected to each other by (poly)sulfide linkages. These structural units may contain additional intramolecular sulfur linkages. This sulfur-rich geopolymer is also formed by sulfur incorporation into functionalised lipids in an intermolecular fashion during early diagenesis. 

It also appears that further clues about hopanoid origins can be drawn from the polar side-chains which carry different numbers and types of substituents. This, in turn, affects their subsequent diagenesis and the types of hopane hydrocarbon, ketone, and other products that are recorded in sediments. In addition to the diagnostic 3j8-methyl substituents, hexafunctionalized side-chains are prevalent in Type 1 methanotrophic bacteria (Neunlist and Rohmer, 1985 Zundel and Rohmer, 1985a). The hydroxy substituent at C-31 of these compounds appears to assist oxidative loss of this carbon or the one at C-30, resulting in a predominance of C30 hopane and 30-norhopane products where methanotrophs are prevalent or even dominant (e.g., Burhan et aL, 2002 Rohmer et ai, 1992). Anomalous C-depletion of these hopanoids often observed in sediments and oils is quite consistent with this interpretation (e.g.. Summons et ai, 2002). 

Hopanoids (alkenes, acids, ketones and alcohols) are ubiquitous components of sediments and soils, and are more widespread than higher plant derived triterpenoids. Polyhydroxy-bacteriohopanes are major precursors, yielding ketones and carboxylic acids during early diagenesis. The latter are the chief products, with... 

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