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.