User:Ssilverman00/isoarborinol

= Isoarborinol = Isoarborinol is a triterpenoid ubiquitously produced by angiosperms and is thus considered a biomarker for higher plants. Though no isoarborinol-producing microbe has been identified, isoarborinol is also considered a possible biomarker for marine bacteria, as its diagenetic end product, arborane, has been found in ancient marine sediments that predate the rise of plants. Importantly, isoarborinol may represent the phylogenetic link between hopanols and sterols.

Background
Isoarborinol is a pentacyclic triterpenoid, a class of 30-carbon isoprenoid compounds commonly found in higher plants. It is primarily a hydrocarbon molecule comprised of four cyclohexane rings, one cyclopentane ring, six methyl groups, one alcohol group and one isobutyl group. It is structurally similar to plant cyclics in the lupenoid series (including lupeol, betulin and lupane), primarily differing in the position of the isobutyl functional group (located on C21 of the cyclopental ring for isoarborinol, and on C19 for the lupenoids). Isoarborinol likely serves as a fluidity-buffering component of biological membranes, similar to sterols and hopanols.

Earth history
The known distribution of isoarborinol in extant organisms is predominantly limited to a few angiosperms (e.g., the Gramineae family ), which led many to view isoarborinol as a biomarker for higher plants. In the 1990s, a series of papers published by Verena Hauke and colleagues presented compelling evidence for the existence of isoarborinol during the Permian and Triassic periods based on detection of arborane (the diagenetic product of isoarborinol) in ancient sediments. These geological periods significantly predate the late-Jurassic first appearance of angiosperms, thus precluding the possibility that isoarborinol was produced by higher plants. Furthermore, the arborane compounds detected had carbon isotopic signatures inconsistent with plant origin. Taken together, these observations led the authors to suggest that isoarborinol had a microbial origin, thus supporting an earlier prediction made based on analysis of the structure of isoarborinol. Additionally, isoarborinol was isolated from lacustrine sediments in which higher plants were not present. The idea that isoarborinol is made by bacteria is widely accepted today. Though no isoarborinol-producing microbe has been found, the marine heterotrophic bacterium Eudoraea adriatica was discovered to make adriaticol and eudoraenol, two isomers of isoarborinol.

Measurement
Common approaches to analyzing organic molecules extracted from ancient sediments include quantifying their abundances and measuring the isotopic compositions of various elements (carbon, nitrogen, sulfur, etc.) within these molecules. Lipids are commonly extracted from sedimentary rocks via solvent extraction, which solubilizes the compounds. Column chromatography is used to partition the lipids into different phases (e.g., saturates, aromatics and polars) based on their polarities. The aromatic fraction (containing arborane and other relevant biomarker compounds) can be analyzed via gas chromatography mass spectrometry (GC/MS), through which compounds elute based on their mass-to-charge ratios (m/z). Arborane can be specifically identified via relative retention times and mass spectra patterns. Finally, the carbon and hydrogen isotopic ratios in arborane can be measured via gas chromatography coupled to isotope ratio mass spectrometry.

Biogeochemical significance
The enzyme responsible for making isoarborinol may represent the evolutionary link between the hopanol-producing enzymes in bacteria and the sterol-producing enzymes in eukaryotes. Whereas hopanoid cyclases fold squalene into a five-membered ring with an all-chair conformation, sterol cyclases fold oxidosqualene into a four-membered ring with a chair-boat-chair conformation. Isoarborinol cyclase uses a combination of these aspects, cyclizing oxidosqualene into a five-membered ring with a chair-boat-chair conformation. Steroid cyclases may have evolved from hopanoid cyclases, and isoarborinol cyclase could represent the enzymatic intermediate of this transition. Alternatively, steroid cyclases and hopanoid cyclases may have diverged from a common ancestor, in which case the phylogenetic significance of isoarborinol cyclase is unclear.