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Oleanane
[edit]Oleanane is a natural triterpenoid. It is commonly found in woody angiosperms and is often used as an indicator of these plants in the fossil record. It is a member of the oleanoid series, which includes a number of other pentacyclic triterpenoids.
Structure
[edit]As mentioned above, oleanane is a pentacyclic triterpenoid. The naming of the 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.[1] A picture of this structure is shown below to the right.
The structure of oleanane contains a number of different methyl groups, which 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.
Of note is that fact that the A and B rings of the oleanane structure are identical to that of hopane. As a result, both molecules produce a fragment of mass 191. Because this fragment is often used to identify hopanes, one must be careful to check for oleanane contaminants among the hopanes.
Synthesis
[edit]Like other triterpenoids, are formed from six combined isoprene units[1]. These isoprene units can be combined via a number of different pathways though in eukaryotes, such as plants, this pathway is the MVA pathway. For the formation of steroids and other triterpenoids the isoprenoids are combined into a precursor known as squalene, which then undergoes enzymatic cyclization to produce the various different triterpenoids, including oleanane.[1]
Once the oleananes have been transported into rocks or sediments they will also undergo further alteration before they are measured. There is evidence that both oleananes and taxeranes (another member of the oleanoid series) convert to oleananes during diagenesis.
Uses as a Biomarker
[edit]Oleanane has been identified as a compound in modern day angiosperms.[2]
Because of this, its presence is the fossil record has also been used to 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.[3] 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.[3]
Lastly, the study introduced the idea of an "oleanane parameter," which could be used in assessing angiosperm input to petroleum sources. This, in turn, gives some idea of the age of said petroleum sources.[3]
That being said, the presence of angiosperms may not be the only thing affecting the oleanane content of sediments, rock extracts and petroleum. For example, there is evidence that contact with seawater during early sedimentation processes can increase the concentration of oleananes in the mature sediment.[4] 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). That being said, it appears 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.[4]
Oleanoid Series
[edit]Oleananes are part of a larger series of triterpenoids known as the oleanoid series. This series still consists of pentacyclic triterpenoids where all of the rings are six-membered; however, other members of the series differ form oleanane in terms of the functional groups present, placement of double bonds and orientation of methyl groups. Other members of the oleanoid series include taxerol and beta-amyrin, both of which are also common in woody angiosperms.[1]
References
[edit]- ^ a b c d "killopsiog". sites.google.com. Retrieved 2019-05-15.
- ^ "ScienceDirect". www.sciencedirect.com. doi:10.1016/s0031-9422(00)83085-x. Retrieved 2019-05-15.
- ^ a b c Taylor, David Winship; Peakman, Torren M.; Hickey, Leo J.; Fago, Frederick J.; Huizinga, Bradley J.; Dahl, Jeremy; Moldowan, J. Michael (1994-08-05). "The Molecular Fossil Record of Oleanane and Its Relation to Angiosperms". Science. 265 (5173): 768–771. doi:10.1126/science.265.5173.768. ISSN 0036-8075. PMID 17736275.
- ^ a b "Oleananes in oils and sediments: Evidence of marine influence during early diagenesis? | Request PDF". ResearchGate. Retrieved 2019-05-15.
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