The characteristic base structure of a sterane (the degraded and saturated version of a steroid), shown above, has three cyclohexane rings and one cyclopentane ring, and a side chain emerging from C17. Eukaryotic cells utilize certain types of sterols in their cell walls (as opposed to prokaryotes, who use certain types of hopanoids).
Sterane nomenclature and stereoisomerism
During diagenesis and catagenesis, the biological stereospecificity of sterols, particularly at C-5, C-14, C-17 and C-20 is usually lost, and a large range of isomers is generated (see above for numbering). The term alphabetabeta sterane (sometimes just alphabeta) is commonly used as short-hand to denote steranes with the 5alpha(H), 14beta(H), 17beta(H) configuration, while alphaalphaalpha sterane would denote 5alpha(H), 14alpha(H), 17alpha(H) stereochemistry. The notation 14alpha(H) indicates that the hydrogen is located below the plane of the paper, whereas in 14beta(H) it is above the plane. In steranes, if no other carbon number is cited, S and R always refer to the stereochemistry at C-20. The prefix ‘nor’, as for example in 24-norcholestane, indicates that the molecule is formally derived for the parent structure by loss of the indicated carbon atom, i.e. C-24 is removed from cholestane. The term ‘desmethylsteranes’ is sometimes used to distinguish steranes that do not possess an additional alkyl group at ring A, i.e. at carbon atoms C-1 to C-4. Diasteranes are rearranged steranes that have no biological precursors, and are most likely formed during diagenesis and catagenesis.
Diasterane: rearranged steranes Steranes may be rearranged into diasteranes during diagenesis. Thus the diasterane / sterane ratio may be a signal of the maturity of the source rock.
Norcholestane: C27 to C30, a sterane without the R group on its Norcholestane, shown above, a cholestane with one carbon missing, has some interesting uses as a biomarker. Only three series of these C26 steranes are known: 21-, 24- and 27-norcholestane. 24-norcholestane has a particular source or depositional environment meaning, whereas 21- and 27- are markers for maturity.