Pyrolytic formation of alkylsteranes - Assigning geological orphans to their biological parents

Abstract

Steranes alkylated at position C-3 occur in significant concentrations in many geological samples (e.g. Fig. 1). However, biological equivalents are not known from any living organisms and the formation pathway remains equally enigmatic, rendering them some of the most prominent orphan biomarkers. In some geological samples, the presence of sulphur functionalities indicated that the 3-alkyl group was originally functionalised, which together with a dominance of pentyl-derivatives pointed towards origins form C5 sugars. 3-alkylsteranes were therefore inferred to represent an entirely new class of natural products. Classified as putative ‘bacteriosteroids’ they were thought to reflect the bacterial fusion of eukaryotic (dietary) steroids with sugars to yield steroids with hopanepolyol-like side-chains (Dahl al., 1992; 1995). Other hypotheses encompass the likely bacterially mediated alkylation of stereneintermediates (Summons and Capon, 1988) or algal sources (Schaeffer et al., 1993). We simulated the geological maturation of regular 3-hydroxylated sterols by laboratory-based thermolysis and pyrolysis in the presence of carbon-catalysts and observed the formation of significant quantities of C-3 alkylated products that exhibit alkyl chain lengths of up to eight carbon units—similar to distributions in many geological extracts. Co-elution with an extract of the Ediacaran Araras group, previously shown to contain a series of 3β-n-alkyl steranes (Sousa Jr. et al., 2016), reveals that the lower members correspond to geological αααR isomers of steranes that have a straight hydrocarbon chain added to the 3β-position (Fig. 1). Our results show that 3-alkylsteranes readily form via carbon-catalysed geological process acting on regular (3-OH) sterol precursors. Considering that 3-alkylated steroids have never been identified in any living organism, there is thus no reason to assume that any of the geological 3-alkyl steroids have direct biosynthetic origins. Instead, regular sterols or their diagenetic intermediates are likely abiogenically alkylated in proportions that may be related to the diagenetic and catagenetic conditions, as well as the composition of the bitumen, kerogen and mineral matrix.