Cholestane

Last updated
Cholestane
Cholestane.svg
Steroid-nomenclature.svg
IUPAC numbering [1]
Names
IUPAC name
Cholestane
Systematic IUPAC name
(1R,3aS,3bR,9aS,9bS,11aR)-9a,11a-Dimethyl-1-[(2R)-6-methylheptan-2-yl]hexadecahydro-1H-cyclopenta[a]phenanthrene
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.035.496 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
UNII
  • InChI=1S/C27H48/c1-19(2)9-8-10-20(3)23-14-15-24-22-13-12-21-11-6-7-17-26(21,4)25(22)16-18-27(23,24)5/h19-25H,6-18H2,1-5H3/t20-,21?,22+,23-,24+,25+,26+,27-/m1/s1 Yes check.svgY
    Key: XIIAYQZJNBULGD-LDHZKLTISA-N Yes check.svgY
  • InChI=1/C27H48/c1-19(2)9-8-10-20(3)23-14-15-24-22-13-12-21-11-6-7-17-26(21,4)25(22)16-18-27(23,24)5/h19-25H,6-18H2,1-5H3/t20-,21?,22+,23-,24+,25+,26+,27-/m1/s1
    Key: XIIAYQZJNBULGD-LDHZKLTIBN
  • C[C@H](CCCC(C)C)[C@H]1CC[C@@H]2[C@@]1(CC[C@H]3[C@H]2CCC4[C@@]3(CCCC4)C)C
  • C41CCCC[C@@]1([C@@H]3[C@H]([C@@H]2CC[C@@H]([C@@]2(C)CC3)[C@H](C)CCCC(C)C)CC4)C
Properties
C27H48
Molar mass 372.681 g·mol−1
Density 0.911 g/ml
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Cholestane is a saturated tetracyclic triterpene. This 27-carbon biomarker is produced by diagenesis of cholesterol and is one of the most abundant biomarkers in the rock record. [2] Presence of cholestane, its derivatives and related chemical compounds in environmental samples is commonly interpreted as an indicator of animal life and/or traces of O2, as animals are known for exclusively producing cholesterol, and thus has been used to draw evolutionary relationships between ancient organisms of unknown phylogenetic origin and modern metazoan taxa. [3] Cholesterol is made in low abundance by other organisms (e.g., rhodophytes, land plants), but because these other organisms produce a variety of sterols it cannot be used as a conclusive indicator of any one taxon. [4] [5] It is often found in analysis of organic compounds in petroleum.

Contents

Background

Cholestane is a saturated C-27 animal biomarker often found in petroleum deposits. It is a diagenetic product of cholesterol, which is an organic molecule made primarily by animals and make up ~30% of animal cell membranes[ citation needed ]. Cholesterol is responsible for membrane rigidity and fluidity, as well as intracellular transport, cell signaling and nerve conduction. [6] In humans, it is also the precursor for hormones (i.e., estrogen, testosterone). It is synthesized via squalene and naturally assumes a specific stereochemical orientation (3β-ol, 5α (H), 14α (H), 17α (H), 20R). This stereochemical orientation is typically maintained throughout diagenetic processes, but cholestane can be found in the fossil record with many stereochemical configurations.

Biomarker

Cholestane in the fossil record is often interpreted as an indicator (biomarker) of ancient animal life and is often used by geochemists and geobiologists to reconstruct animal evolution (particularly in the Precambrian Earth history; e.g., Ediacaran, [3] Cryogenian and Proterozoic in general [7] [8] ). Molecular oxygen is required to produce cholesterol; [9] thus, the presence of cholestane suggests some trace of oxygen in the paleoenvironment. Cholestane is not exclusively derived from diagenesis of animal-derived steroid molecules; cholestane may also be associated with the presence of e.g., rhodophytes and embryophytes, [10] [5] although the abundance of such non-metaozan cholestane is unknown. Embryophytes generally produce a variety of sterols, which are collectively known as phytosterols, [11] and cholesterol remains a minor component. In contrast, bacteria produce other cyclic triterpenoids such as hopanoids and their diagenetic products hopanes are utilized as bacterial biomarkers.

Natural preservation in fossils

Cholesterol degrades to cholestane by loss of OH functional group and saturation of double bond (indicated in pink). Stereochemistry of the molecule is maintained in this degradation. CholestaneDiagenesis3.png
Cholesterol degrades to cholestane by loss of OH functional group and saturation of double bond (indicated in pink). Stereochemistry of the molecule is maintained in this degradation.

Cholesterol has 256 stereoisomers, but only one of them is formed naturally in production of cholesterol (3β-ol, 5α (H), 14α (H), 17α (H), 20R) and is therefore the primary stereoisomer of interest for cholestane measurements. Deviations from this stereochemistry often reflects diagenesis, thermal maturation and preservation bias.

Diagenesis typically leads to the loss of functional groups and double bonds in organic molecules. For cholestane specifically, diagenesis of cholesterol to cholestane produces a molecule that is fully saturated compared to its steroid counterpart. This process occurs without the loss or gain of carbon atoms and therefore can serve as an indicator of the original steroid produced by the organism in the environment. [12]

Thermal alteration can also cause loss of the alkane side-chain at C17. [13] An experiment demonstrated that over 4 weeks at 300 °C, cholestane underwent 17% decomposition of its alkane side chain. In contrast, the polycyclic structure (C1-17) is very thermally stable. Diagenetic processes can also cause methyl shifts and aromatization.

Stereochemical alteration

Additional diagenetic processes can further alter the cholestane molecule. For instance, cholestane is susceptible to stereochemical shifts over time from its natural isomer. These changes can be the effect of thermal or microbial alteration. Thermal alteration can cause changes in stereochemistry at both the C20 chiral center, as well as the hydrogen atoms. The ratio of R/S stereoisomers is typically reported as a measure of “thermal maturity”. [14] In contrast, conversion of the hydrogen atom at the C5 position from α to β configuration reflects anaerobic microbial activity, [3] and can be understood through isotope labeling experiments on controlled microbe experiments metabolizing the steroid of interest. [15] [16] One study demonstrated that there are two reactions that can produce loss of the cholesterol double bond—(1) direct reduction of double bond or (2) production of ketone prior to reduction of double bond—resulting in distinct isomerization of the hydrogen atom at the C5 position. [15] The C14 and C17α hydrogen atoms are more stable and undergo changes to β configuration in much lower abundances than the 5 hydrogen atom.

Measurement techniques

GC/MS

Cholestane isomers elute at different times in GC/MS/MS experiments in the m/z 372-217 fragment. Figure adapted from Bobrovskiy et al. CholestaneIsomerElutionSpectra.png
Cholestane isomers elute at different times in GC/MS/MS experiments in the m/z 372→217 fragment. Figure adapted from Bobrovskiy et al.

Cholestane can be extracted from samples and measured on the GC/MS to quantify relative abundance to other organic compounds. This measurement is done by extraction of the steranes into a non-polar solvent (e.g., dichloromethane or chloroform) and purified into a “saturates” fraction using silica gel column gas chromatography. Cholestane isomers will elute from the column based on molecular weight and various stereochemistry, which makes traditional mass spectrometry challenging due to close co-elution of isomers. Alternatively, one can measure cholestane using GC/MS/MS experiments which target the m/z fragment 217 (from molecular ion 372). This specific method first looks for the 372 molecular ion of cholestane, and then fragments that molecular ion further to its m/z 217 fragment in order to improve identification of specific isomers.

δ13C isotope ratios

δ13C values of cholestane reflect the carbon isotope composition of the animals that created the original cholesterol molecules. Animal carbon isotope composition is typically understood to be a function of their diet; [17] therefore, carbon isotope composition of cholestane would reflect this original diet value as well. δ13C values can be measured using a gas chromatograph coupled to an IRMS.

More generally, steranes can be used as an indicator of environmental shifts. A study has presented δ13C values of steranes versus hopanes and used it to propose changes in the photic zone over the course of the Miocene, as changes in the isotope value must be either a result of dissolved inorganic carbon within the water or biological isotope fractionation. [14]

Case studies

Early life biomarkers

Dickinsonia fossil was proved to be ancient animal via cholestane biomarker identification. Dickinsonia tenuis.jpg
Dickinsonia fossil was proved to be ancient animal via cholestane biomarker identification.

Presence of cholestane does not necessarily indicate presence of animals, but is often used in conjunction with other biomarkers to note the rise of distinct taxa in the fossil record; with regard to this, a study measured relative abundance in cholestane versus other triterpenoid biomarkers to demonstrate the rise of algae during the Neoproterozoic. [7] [18]

Tracing the actual origins of cholestane within the fossil record is challenging, as most of the rocks from that time period are heavily metamorphosed and thus potential biomarkers are thermally altered.[ citation needed ] A study linked the source of cholestane to a specific Ediacaran fossil (Dickinsonia) to provide constraints to the taxonomic classification of Ediacaran biota as evolutionary preludes to metazoan life. [3] Cholestane is not a specific marker for animals though and is found in most eukaryotic lineages.

See also

Related Research Articles

<span class="mw-page-title-main">Steroid</span> Polycyclic organic compound having sterane as a core structure

A steroid is an organic compound with four fused rings arranged in a specific molecular configuration.

<span class="mw-page-title-main">Sterol</span> Chemical compound

Sterol is an organic compound with formula C
17H
28O, whose molecule is derived from that of gonane by replacement of a hydrogen atom on C3 position by a hydroxyl group. It is therefore an alcohol of gonane. More generally, any compounds that contain the gonane structure, additional functional groups, and/or modified ring systems derived from gonane are called steroids. Therefore, sterols are a subgroup of the steroids. They occur naturally in most eukaryotes, including plants, animals, and fungi, and can also be produced by some bacteria. The most familiar type of animal sterol is cholesterol, which is vital to cell membrane structure, and functions as a precursor to fat-soluble vitamins and steroid hormones.

<span class="mw-page-title-main">Sterane</span> Class of tetracyclic compounds derived from steroids

Steranes constitute a class of tetracyclic triterpanes derived from steroids or sterols via diagenetic and catagenetic degradation, such as hydrogenation. They are found in sediments and sedimentary rocks in nature. Steranes are derivatives of gonane, the steroid nucleus which is also called "cyclopentanoperhydrophenanthrene". They have an androstane skeleton with a side chain at the C-17 carbon. The sterane structure constitutes the core of all sterols. Steranes are widely used as biomarkers for the presence of eukaryotes in past ecosystems because steroids are nearly exclusively produced by eukaryotes. In particular, cholestanes are diagenetic products of cholesterol in animals, while stigmastanes are diagenetic products of stigmasterols in algae and land plants. However, some bacteria are now known to produce sterols and it is inferred that the ultimate origin of sterol biosynthesis is in bacteria. Sterols are produced via protosterols that are direct cyclization compounds of squalene by the catalysis of oxidosqualene cyclase. All known sterols in eukaryotes are enzymatically extensively modified from protosterols, while organisms that only produce protosterols are not known. The oldest record of modified steranes are in sedimentary rocks deposited ca. 720–820 million years ago. In contrast, diagenetic products of protosterols are widely distributed in older Proterozoic rocks and imply the presence of extinct proto-eukaryotes and/or sterol-producing bacteria before the evolution of crown-group eukaryotes.

<span class="mw-page-title-main">Carbon-to-nitrogen ratio</span>

A carbon-to-nitrogen ratio is a ratio of the mass of carbon to the mass of nitrogen in organic residues. It can, amongst other things, be used in analysing sediments and soil including soil organic matter and soil amendments such as compost.

Phytane is the isoprenoid alkane formed when phytol, a chemical substituent of chlorophyll, loses its hydroxyl group. When phytol loses one carbon atom, it yields pristane. Other sources of phytane and pristane have also been proposed than phytol.

γ-Carotene (gamma-carotene) is a carotenoid, and is a biosynthetic intermediate for cyclized carotenoid synthesis in plants. It is formed from cyclization of lycopene by lycopene cyclase epsilon. Along with several other carotenoids, γ-carotene is a vitamer of vitamin A in herbivores and omnivores. Carotenoids with a cyclized, beta-ionone ring can be converted to vitamin A, also known as retinol, by the enzyme beta-carotene 15,15'-dioxygenase; however, the bioconversion of γ-carotene to retinol has not been well-characterized. γ-Carotene has tentatively been identified as a biomarker for green and purple sulfur bacteria in a sample from the 1.640 ± 0.003-Gyr-old Barney Creek Formation in Northern Australia which comprises marine sediments. Tentative discovery of γ-carotene in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic. This is significant for reconstructing past oceanic conditions, but so far γ-carotene has only been potentially identified in the one measured sample.

<span class="mw-page-title-main">Isorenieratene</span> Chemical compound

Isorenieratene /ˌaɪsoʊrəˈnɪərətiːn/ is a carotenoid light harvesting pigment produced exclusively by the genus Chlorobium. Chlorobium are the brown-colored strains of the family of green sulfur bacteria (Chlorobiaceae). Green sulfur bacteria are anaerobic photoautotrophic organisms meaning they perform photosynthesis in the absence of oxygen using hydrogen sulfide in the following reaction:

<span class="mw-page-title-main">Abietane</span> Chemical compound

Abietane is a diterpene that forms the structural basis for a variety of natural chemical compounds such as abietic acid, carnosic acid, and ferruginol which are collectively known as abietanes or abietane diterpenes.

<span class="mw-page-title-main">Dinosterol</span> Chemical compound

Dinosterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol) is a 4α-methyl sterol that is produced by several genera of dinoflagellates and is rarely found in other classes of protists. The steroidal alkane, dinosterane, is the 'molecular fossil' of dinosterol, meaning that dinosterane has the same carbon skeleton as dinosterol, but lacks dinosterol's hydroxyl group and olefin functionality. As such, dinosterane is often used as a biomarker to identify the presence of dinoflagelletes in sediments.

Hydrogen isotope biogeochemistry is the scientific study of biological, geological, and chemical processes in the environment using the distribution and relative abundance of hydrogen isotopes. There are two stable isotopes of hydrogen, protium 1H and deuterium 2H, which vary in relative abundance on the order of hundreds of permil. The ratio between these two species can be considered the hydrogen isotopic fingerprint of a substance. Understanding isotopic fingerprints and the sources of fractionation that lead to variation between them can be applied to address a diverse array of questions ranging from ecology and hydrology to geochemistry and paleoclimate reconstructions. Since specialized techniques are required to measure natural hydrogen isotope abundance ratios, the field of hydrogen isotope biogeochemistry provides uniquely specialized tools to more traditional fields like ecology and geochemistry.

24-isopropyl cholestane is an organic molecule produced by specific sponges, protists and marine algae. The identification of this molecule at high abundances in Neoproterozoic rocks has been interpreted to reflect the presence of multicellular life prior to the rapid diversification and radiation of life during the Cambrian explosion. In this transitional period at the start of the Phanerozoic, single-celled organisms evolved to produce many of the evolutionary lineages present on Earth today. Interpreting 24-isopropyl cholestane in ancient rocks as indicating the presence of sponges before this rapid diversification event alters the traditional understanding of the evolution of multicellular life and the coupling of biology to changes in end-Neoproterozoic climate. However, there are several arguments against causally linking 24-isopropyl cholestane to sponges based on considerations of marine algae and the potential alteration of organic molecules over geologic time. In particular the discovery of 24-isopropyl cholestane in rhizarian protists implies that this biomarker cannot be used on its own to trace sponges. Interpreting the presence of 24-isopropyl cholestane in the context of changingglobal biogeochemical cycles at the Proterozoic-Phanerozoic transition remains an area of active research.

Crocetane, or 2,6,11,15-tetramethylhexadecane, is an isoprenoid hydrocarbon compound. Unlike its isomer phytane, crocetane has a tail-to-tail linked isoprenoid skeleton. Crocetane has been detected in modern sediments and geological records as a biomarker, often associated with anaerobic methane oxidation.

Okenane, the diagenetic end product of okenone, is a biomarker for Chromatiaceae, the purple sulfur bacteria. These anoxygenic phototrophs use light for energy and sulfide as their electron donor and sulfur source. Discovery of okenane in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic. This is potentially tremendously important for reconstructing past oceanic conditions, but so far okenane has only been identified in one Paleoproterozoic rock sample from Northern Australia.

24-<i>n</i>-Propylcholestane Chemical compound

24-n-Propylcholestane is a sterane biomarker molecule often found in marine source rocks. It is a C30 molecule, meaning that it is composed of thirty carbon atoms, and is one of the leading ways to distinguish a marine source rock from a terrigenous sample. It is composed of three six-carbon rings and one five-carbon ring, with two methyl groups and one eleven carbon side chain. 24-n-Propylcholestane has a molar mass of 414.76 g/mol.

<span class="mw-page-title-main">24-Norcholestane</span> Chemical compound

24-Norcholestane, a steroid derivative, is used as a biomarker to constrain the source age of sediments and petroleum through the ratio between 24-norcholestane and 27-norcholestane, especially when used with other age diagnostic biomarkers, like oleanane. While the origins of this compound are still unknown, it is thought that they are derived from diatoms due to their identification in diatom rich sediments and environments. In addition, it was found that 24-norcholestane levels increased in correlation with diatom evolution. Another possible source of 24-norcholestane is from dinoflagellates, albeit to a much lower extent.

<span class="mw-page-title-main">Tetrahymanol</span> Chemical compound

Tetrahymanol is a gammacerane-type membrane lipid first found in the marine ciliate Tetrahymena pyriformis. It was later found in other ciliates, fungi, ferns, and bacteria. After being deposited in sediments that compress into sedimentary rocks over millions of years, tetrahymanol is dehydroxylated into gammacerane. Gammacerane has been interpreted as a proxy for ancient water column stratification.

<span class="mw-page-title-main">Lycopane</span> Chemical compound

Lycopane (C40H82; 2,6,10,14,19,23,27,31-octamethyldotriacontane), a 40 carbon alkane isoprenoid, is a widely present biomarker that is often found in anoxic settings. It has been identified in anoxically deposited lacustrine sediments (such as the Messel formation and the Condor oil shale deposit). It has been found in sulfidic and anoxic hypersaline environments (such as the Sdom Formation). It has been widely identified in modern marine sediments, including the Peru upwelling zone, the Black Sea, and the Cariaco Trench. It has been found only rarely in crude oils.

Elizabeth A. Canuel is a chemical oceanographer known for her work on organic carbon cycling in aquatic environments. She is the Chancellor Professor of Marine Science at the College of William & Mary and is an elected fellow of the Geochemical Society and the European Association of Geochemistry.

Biphytane (or bisphytane) is a C40 isoprenoid produced from glycerol dialkyl glycerol tetraether (GDGT) degradation. As a common lipid membrane component, biphytane is widely used as a biomarker for archaea. In particular, given its association with sites of active anaerobic oxidation of methane (AOM), it is considered a biomarker of methanotrophic archaea. It has been found in both marine and terrestrial environments.

<span class="mw-page-title-main">Martin Schoell</span> German geochemist

Martin Schoell is a German geochemist. His research focuses on using stable isotopes to characterize the geochemistry of petroleum. Schoell is known for his work regarding CO2, sedimentary rocks, methane, natural gas, carbon isotopes, and acetate fermentation and how these factors enable identification of the origins of greenhouse gasses. Schoell was the founder, CEO and president of Gas Consult International, Inc., a private natural gas consulting firm, from 2001 to 2015. Schoell was awarded the Alfred Treibs Award by the Geochemical Society in 2008.

References

  1. ↑ The Nomenclature of Steroids Archived 2011-05-14 at the Wayback Machine , IUPAC
  2. ↑ Peters, Kenneth E. (Kenneth Eric), 1950- (2007). The biomarker guide. Cambridge University Press. ISBN   9780521039987. OCLC   1015511618.{{cite book}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  3. 1 2 3 4 5 Bobrovskiy, Ilya; Hope, Janet M.; Ivantsov, Andrey; Nettersheim, Benjamin J.; Hallmann, Christian; Brocks, Jochen J. (2018-09-20). "Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals". Science. 361 (6408): 1246–1249. Bibcode:2018Sci...361.1246B. doi: 10.1126/science.aat7228 . hdl: 1885/230014 . ISSN   0036-8075. PMID   30237355.
  4. ↑ Combaut, Georges; Saenger, Peter (April 1984). "Sterols of the amansieae (rhodomelaceae: Rhodophyta)". Phytochemistry. 23 (4): 781–782. Bibcode:1984PChem..23..781C. doi:10.1016/s0031-9422(00)85025-6. ISSN   0031-9422.
  5. 1 2 Sonawane, Prashant D.; Pollier, Jacob; Panda, Sayantan; Szymanski, Jedrzej; Massalha, Hassan; Yona, Meital; Unger, Tamar; Malitsky, Sergey; Arendt, Philipp; Pauwels, Laurens; Almekias-Siegl, Efrat (2016-12-22). "Plant cholesterol biosynthetic pathway overlaps with phytosterol metabolism". Nature Plants. 3 (1): 16205. doi:10.1038/nplants.2016.205. ISSN   2055-0278. PMID   28005066. S2CID   5518449.
  6. ↑ Life : the science of biology. Sadava, David E. (9th ed.). Sunderland, Mass.: Sinauer Associates. 2011. ISBN   978-1429219624. OCLC   368046231.{{cite book}}: CS1 maint: others (link)
  7. 1 2 Brocks, Jochen J.; Jarrett, Amber J. M.; Sirantoine, Eva; Hallmann, Christian; Hoshino, Yosuke; Liyanage, Tharika (August 2017). "The rise of algae in Cryogenian oceans and the emergence of animals". Nature. 548 (7669): 578–581. Bibcode:2017Natur.548..578B. doi:10.1038/nature23457. ISSN   0028-0836. PMID   28813409. S2CID   205258987.
  8. ↑ Summons, Roger E; Brassell, Simon C; Eglinton, Geoffrey; Evans, Evan; Horodyski, Robert J; Robinson, Neil; Ward, David M (November 1988). "Distinctive hydrocarbon biomarkers from fossiliferous sediment of the Late Proterozoic Walcott Member, Chuar Group, Grand Canyon, Arizona". Geochimica et Cosmochimica Acta. 52 (11): 2625–2637. Bibcode:1988GeCoA..52.2625S. doi:10.1016/0016-7037(88)90031-2. ISSN   0016-7037.
  9. ↑ Mehta, Sweety (2013-09-17). "Biosynthesis and Regulation of Cholesterol (with Animation) | Animations". PharmaXChange.info. Retrieved 2019-06-04.
  10. ↑ Summons, Roger E.; Erwin, Douglas H. (2018-09-20). "Chemical clues to the earliest animal fossils". Science. 361 (6408): 1198–1199. Bibcode:2018Sci...361.1198S. doi:10.1126/science.aau9710. hdl: 1721.1/134716.2 . ISSN   0036-8075. PMID   30237342. S2CID   52306517.
  11. ↑ Darnet, Sylvain; Blary, Aurélien; Chevalier, Quentin; Schaller, Hubert (2021). "Phytosterol Profiles, Genomes and Enzymes – An Overview". Frontiers in Plant Science. 12: 878. doi: 10.3389/fpls.2021.665206 . ISSN   1664-462X. PMC   8172173 . PMID   34093623.
  12. ↑ Grantham, P.J.; Wakefield, L.L. (January 1988). "Variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time". Organic Geochemistry. 12 (1): 61–73. Bibcode:1988OrGeo..12...61G. doi:10.1016/0146-6380(88)90115-5. ISSN   0146-6380.
  13. ↑ Mango, Frank D. (January 1990). "The origin of light cycloalkanes in petroleum". Geochimica et Cosmochimica Acta. 54 (1): 23–27. Bibcode:1990GeCoA..54...23M. doi:10.1016/0016-7037(90)90191-m. ISSN   0016-7037.
  14. 1 2 Schoell, M.; Schouten, S.; Damste, J. S. S.; de Leeuw, J. W.; Summons, R. E. (1994-02-25). "A Molecular Organic Carbon Isotope Record of Miocene Climate Changes". Science. 263 (5150): 1122–1125. Bibcode:1994Sci...263.1122S. doi:10.1126/science.263.5150.1122. hdl: 1874/4185 . ISSN   0036-8075. PMID   17831625. S2CID   40960698.
  15. 1 2 Mermoud, F.; Wünsche, L.; Clerc, O.; Gülaçar, F.O.; Buchs, A. (January 1984). "Steroidal ketones in the early diagenetic transformations of Δ5 sterols in different types of sediments". Organic Geochemistry. 6: 25–29. doi:10.1016/0146-6380(84)90023-8. ISSN   0146-6380.
  16. ↑ Taylor, Craig D.; Smith, Steven O.; Gagosian, Robert B. (November 1981). "Use of microbial enrichments for the study of the anaerobic degradation of cholesterol". Geochimica et Cosmochimica Acta. 45 (11): 2161–2168. Bibcode:1981GeCoA..45.2161T. doi:10.1016/0016-7037(81)90068-5. ISSN   0016-7037.
  17. ↑ Hayes, John M. (2001-12-31), "3. Fractionation of Carbon and Hydrogen Isotopes in Biosynthetic Processes", Stable Isotope Geochemistry, De Gruyter, pp. 225–278, doi:10.1515/9781501508745-006, ISBN   9781501508745
  18. ↑ Hoshino, Yosuke; Poshibaeva, Aleksandra; Meredith, William; Snape, Colin; Poshibaev, Vladimir; Versteegh, Gerard J. M.; Kuznetsov, Nikolay; Leider, Arne; van Maldegem, Lennart; Neumann, Mareike; Naeher, Sebastian (2017). "Cryogenian evolution of stigmasteroid biosynthesis". Science Advances. 3 (9): e1700887. Bibcode:2017SciA....3E0887H. doi:10.1126/sciadv.1700887. PMC   5606710 . PMID   28948220.
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