Molecule of the Month

2-Methylhopane - by Paul Farrimond

Hopanes are widely employed in petroleum geochemistry and have a near-ubiquitous occurrence in oils and source rocks, being derived 

from bacteria.  2-Methylhopanes have an additional methyl group at the C-2 position and are less abundant than hopanes, but offer useful biomarker potential, being derived from a more limited range of bacterial types, and particularly associated with cyanobacteria in shallow marine environments.

2-Methylhopanes occur as a series (C₂₈-C₃₆) with variable length of the side chain at C-21, and as both 22S and 22R isomers for the C₃₂₊ homologues.  Whilst immature sediments contain 17b(H),21b(H) isomers, these are absent in oils and mature source rocks where the 17a(H),21b(H) isomers dominate.   2-Methyhopanes are identified and quantified from the m/z 205 mass chromatogram, but often only the most abundant members (usually C₃₁) of the series are reported in analytical reports.  Nevertheless, useful parameters can be calculated using these compounds and used in oil-oil and oil-source rock correlations (Farrimond et al., 2004).

Reference:

Farrimond P., Talbot H.M., Watson D.F., Schulz L.K. & Wilhelms A. (2005). Methylhopanoids: Molecular indicators of ancient bacteria and a petroleum correlation tool. Geochimica et Cosmochimica Acta68, 3873-3882.