Molecule of the Month

Diahopane (Hopane “X”)

This C₃₀ rearranged hopane had been widely observed before its structure was determined (by Moldowan et al., 1991).  Originally labelled as “compound X” (after Philp & Gilbert, 1986), it was initially considered a possible terrestrial marker, but when Moldowan et al. (1991) recognised a series of diahopanes with similar isotopic composition to the regular hopanes, a bacterial origin was established. 

Diahopane differs from the regular C₃₀ hopane in having the methyl group at C-14 rearranged to the C-15 position on the D-ring.  This rearrangement is likely to be a clay-catalysed process, and results in a very thermally stable product.  The Diahopane Index (diahopane/(diahopane + C₃₀ab) primarily serves as a maturity parameter, increasing to high maturity levels.

References:

Moldowan J.M., Fago F.J., Carlson R.M.K., Young D.C., van Duyne G., Clardy J., Schoell M., Pillinger C.T. & Watt D.S. (1991). Rearranged hopanes in sediments and petroleum. Geochimica et Cosmochimica Acta 55, 3333-3353.

Philp R.P. & Gilbert T.D. (1986). Biomarker distributions in Australian oils predominantly derived from terrigenous source material. Organic Geochemistry 10, 73-84.