Oleanane

Oleanane is a natural triterpenoid. It is commonly found in woody angiosperms and as a result is often used as an indicator of these plants in the fossil record. It is a member of the oleanoid series, which consists of pentacyclic triterpenoids (such as beta amyrin and taraxerol) where all rings are six membered.

Oleanane is a pentacyclic triterpenoid, a class of molecules made up of six connected isoprene units. The naming of both the ring structures and individual carbon atoms in oleanane is the same as in steroids. As such, it consists of a A, B, C, D, and E ring, all of which are six membered rings. The structure of oleanane contains a number of different methyl groups, that vary in orientation between different oleananes. For example, in 18 alpha oleanane contains a downward facing methyl group for the 18th carbon atom, while 18 beta oleanane contains an upward facing methyl group at the same position. A and B rings of the oleanane structure are identical to that of hopane. As a result, both molecules produce a fragment of m/z 191. Because this fragment is often used to identify hopanes, oleanane can be mis identified in hopane analysis.

Like other triterpenoids, are formed from six combined isoprene units. Because of this, its presence is the fossil record has also been used to trace angiosperms through the fossil record. For example, the ratio of 18 alpha oleanane + 18 beta oleanane:17 alpha hopane in rock extracts (and associated petroleums/oils) has been found to correlate (at least broadly) to the presence of angiosperms in the fossil record. In this study, the combination of alpha and beta oleanane were used as indicators for the presence of angiosperms. They are normalized to hopanes, which are broadly present in almost all rock extracts coming from petroleum. Furthermore, because of the structural similarities between hopanes and oleananes, it is assumed that they will react similarly to the various weathering processes that degrade the biomarkers present. As such, the ratio of hopanes to oleananes should be similar to the initial ratio, and unaffected by processes occurring in the rock after fossilization. There is some delay in the accepted increases in taxonomic diversification of angiosperms (which occurred during the mid Cretaceous period) and the increase of oleanane concentrations in the fossil record (which occurred in the late Cretaceous or even after). This could be due to a number of factors, one being that the early angiosperms were more herbaceous than woody and that woody angiosperms only appeared after further taxonomic diversification. This evidence comes from the fact that various indicators of marine influence (C27/C29 sterane ratios, changes in elemental composition in the downstream direction that are indicative of the infiltration of water into the system and the homophane index). Despite this, it is still unclear as to how marine influence enhances the expression of oleananes (thus increasing observed concentration). Some ideas include the changes in pH, Eh and the microbial environment that come with the interaction with seawater.