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'''Cholestane''' is a [[Saturation (chemistry)|saturated]] [[tetracyclic]][[triterpene]]. This carbon-27 biomarker is produced by [[diagenesis]] of [[cholesterol]] and is one of the most abundant [[Biomarker|biomarkers]] in the rock record<ref>{{Cite book|url=http://worldcat.org/oclc/1015511618|title=The biomarker guide.|last=Peters, Kenneth E. (Kenneth Eric), 1950-|date=2007|publisher=Cambridge University Press|isbn=9780521039987|oclc=1015511618}}</ref>. Presence of cholestane in environmental samples are commonly interpreted as an indicator of animal life and/or traces of O<sub>2</sub>, 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<ref name=":0">{{Cite journal|last=Bobrovskiy|first=Ilya|last2=Hope|first2=Janet M.|last3=Ivantsov|first3=Andrey|last4=Nettersheim|first4=Benjamin J.|last5=Hallmann|first5=Christian|last6=Brocks|first6=Jochen J.|date=2018-09-20|title=Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals|url=http://dx.doi.org/10.1126/science.aat7228|journal=Science|volume=361|issue=6408|pages=1246–1249|doi=10.1126/science.aat7228|issn=0036-8075}}</ref>. Cholestane is made in low abundance by other organisms (e.g., [[Red algae|rhodophytes]]). It is often found in analysis of organic compounds in [[petroleum]]. |
'''Cholestane''' is a [[Saturation (chemistry)|saturated]] [[tetracyclic]][[triterpene]]. This carbon-27 biomarker is produced by [[diagenesis]] of [[cholesterol]] and is one of the most abundant [[Biomarker|biomarkers]] in the rock record<ref>{{Cite book|url=http://worldcat.org/oclc/1015511618|title=The biomarker guide.|last=Peters, Kenneth E. (Kenneth Eric), 1950-|date=2007|publisher=Cambridge University Press|isbn=9780521039987|oclc=1015511618}}</ref>. Presence of cholestane in environmental samples are commonly interpreted as an indicator of animal life and/or traces of O<sub>2</sub>, 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<ref name=":0">{{Cite journal|last=Bobrovskiy|first=Ilya|last2=Hope|first2=Janet M.|last3=Ivantsov|first3=Andrey|last4=Nettersheim|first4=Benjamin J.|last5=Hallmann|first5=Christian|last6=Brocks|first6=Jochen J.|date=2018-09-20|title=Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals|url=http://dx.doi.org/10.1126/science.aat7228|journal=Science|volume=361|issue=6408|pages=1246–1249|doi=10.1126/science.aat7228|issn=0036-8075}}</ref>. Cholestane is made in low abundance by other organisms (e.g., [[Red algae|rhodophytes]]), but because these other organisms produce a variety of sterols it cannot be used as a conclusive indicator of any one taxa<ref>{{Cite journal|last=Combaut|first=Georges|last2=Saenger|first2=Peter|date=1984-04|title=Sterols of the amansieae (rhodomelaceae: Rhodophyta)|url=http://dx.doi.org/10.1016/s0031-9422(00)85025-6|journal=Phytochemistry|volume=23|issue=4|pages=781–782|doi=10.1016/s0031-9422(00)85025-6|issn=0031-9422}}</ref>. It is often found in analysis of organic compounds in [[petroleum]]. |
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== Background == |
== Background == |
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Cholestane is a [[Saturation (chemistry)|saturated]] C-27 animal biomarker often found in petroleum deposits. It is a [[Diagenesis|diagenetic]] product of [[cholesterol]], which is an organic molecule made primarily by animals and make up ~30% of animal cell membranes. Cholesterol is responsible for [[membrane]] rigidity and fluidity, as well as [[intracellular transport]], [[cell signaling]] and [[Action potential|nerve conduction]]. In humans, it is also the precursor for [[Hormone|hormones]] (i.e., [[estrogen]], [[testosterone]]). It is synthesized via [[squalene]] and naturally assumes a specific [[Stereochemistry|stereochemical]] orientation (3β-ol, 5α (H), 14α (H), 17α (H), 20R). Maintaining stereochemistry of natural cholesterol is typical through diagenetic processes, but cholestane can be found in the fossil record with many stereochemical configurations. |
Cholestane is a [[Saturation (chemistry)|saturated]] C-27 animal biomarker often found in petroleum deposits. It is a [[Diagenesis|diagenetic]] product of [[cholesterol]], which is an organic molecule made primarily by animals and make up ~30% of animal cell membranes. Cholesterol is responsible for [[membrane]] rigidity and fluidity, as well as [[intracellular transport]], [[cell signaling]] and [[Action potential|nerve conduction]]. In humans, it is also the precursor for [[Hormone|hormones]] (i.e., [[estrogen]], [[testosterone]]). It is synthesized via [[squalene]] and naturally assumes a specific [[Stereochemistry|stereochemical]] orientation (3β-ol, 5α (H), 14α (H), 17α (H), 20R). Maintaining stereochemistry of natural cholesterol is typical through diagenetic processes, but cholestane can be found in the fossil record with many stereochemical configurations. |
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=== Biomarker === |
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Cholestane in the fossil record is often interpreted as an indicator of ancient animal life and are often used by geochemists and geobiologists to reconstruct animal evolution (particularly in very early Earth history; i.e., [[Ediacaran]]<ref name=":0" />, [[Neoproterozoic|neo-Proterozoic]] and [[Proterozoic]]<sup><ref name=":1">{{Cite journal|last=Brocks|first=Jochen J.|last2=Jarrett|first2=Amber J. M.|last3=Sirantoine|first3=Eva|last4=Hallmann|first4=Christian|last5=Hoshino|first5=Yosuke|last6=Liyanage|first6=Tharika|date=2017-08|title=The rise of algae in Cryogenian oceans and the emergence of animals|url=http://dx.doi.org/10.1038/nature23457|journal=Nature|volume=548|issue=7669|pages=578–581|doi=10.1038/nature23457|issn=0028-0836}}</ref><ref>{{Cite journal|last=Summons|first=Roger E|last2=Brassell|first2=Simon C|last3=Eglinton|first3=Geoffrey|last4=Evans|first4=Evan|last5=Horodyski|first5=Robert J|last6=Robinson|first6=Neil|last7=Ward|first7=David M|date=1988-11|title=Distinctive hydrocarbon biomarkers from fossiliferous sediment of the Late Proterozoic Walcott Member, Chuar Group, Grand Canyon, Arizona|url=http://dx.doi.org/10.1016/0016-7037(88)90031-2|journal=Geochimica et Cosmochimica Acta|volume=52|issue=11|pages=2625–2637|doi=10.1016/0016-7037(88)90031-2|issn=0016-7037}}</ref></sup>). Oxygen is required to produce cholesterol; thus, the presence of cholestane suggests some trace of oxygen in the paleoenvironment. However, cholestane is not exclusively derived from diagenesis of animal-derived biomolecules; cholestane has also been associated with the presence of [[Red algae|rhodophytes]]<ref>{{Cite journal|last=Summons|first=Roger E.|last2=Erwin|first2=Douglas H.|date=2018-09-20|title=Chemical clues to the earliest animal fossils|url=http://dx.doi.org/10.1126/science.aau9710|journal=Science|volume=361|issue=6408|pages=1198–1199|doi=10.1126/science.aau9710|issn=0036-8075}}</ref>. |
Cholestane in the fossil record is often interpreted as an indicator of ancient animal life and are often used by geochemists and geobiologists to reconstruct animal evolution (particularly in very early Earth history; i.e., [[Ediacaran]]<ref name=":0" />, [[Neoproterozoic|neo-Proterozoic]] and [[Proterozoic]]<sup><ref name=":1">{{Cite journal|last=Brocks|first=Jochen J.|last2=Jarrett|first2=Amber J. M.|last3=Sirantoine|first3=Eva|last4=Hallmann|first4=Christian|last5=Hoshino|first5=Yosuke|last6=Liyanage|first6=Tharika|date=2017-08|title=The rise of algae in Cryogenian oceans and the emergence of animals|url=http://dx.doi.org/10.1038/nature23457|journal=Nature|volume=548|issue=7669|pages=578–581|doi=10.1038/nature23457|issn=0028-0836}}</ref><ref>{{Cite journal|last=Summons|first=Roger E|last2=Brassell|first2=Simon C|last3=Eglinton|first3=Geoffrey|last4=Evans|first4=Evan|last5=Horodyski|first5=Robert J|last6=Robinson|first6=Neil|last7=Ward|first7=David M|date=1988-11|title=Distinctive hydrocarbon biomarkers from fossiliferous sediment of the Late Proterozoic Walcott Member, Chuar Group, Grand Canyon, Arizona|url=http://dx.doi.org/10.1016/0016-7037(88)90031-2|journal=Geochimica et Cosmochimica Acta|volume=52|issue=11|pages=2625–2637|doi=10.1016/0016-7037(88)90031-2|issn=0016-7037}}</ref></sup>). Oxygen is required to produce cholesterol; thus, the presence of cholestane suggests some trace of oxygen in the paleoenvironment. However, cholestane is not exclusively derived from diagenesis of animal-derived biomolecules; cholestane has also been associated with the presence of [[Red algae|rhodophytes]]<ref>{{Cite journal|last=Summons|first=Roger E.|last2=Erwin|first2=Douglas H.|date=2018-09-20|title=Chemical clues to the earliest animal fossils|url=http://dx.doi.org/10.1126/science.aau9710|journal=Science|volume=361|issue=6408|pages=1198–1199|doi=10.1126/science.aau9710|issn=0036-8075}}</ref>. In contrast, plants and bacteria produce other sterols (e.g., [[Hopane|hopanes]]). |
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== Preservation == |
== Preservation == |
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⚫ | [[File:CholestaneDiagenesis3.png|thumb|''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.'']] |
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Cholesterol has 256 [[Stereoisomerism|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 [[Taphonomy|preservation bias]]. |
Cholesterol has 256 [[Stereoisomerism|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 [[Taphonomy|preservation bias]]. |
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[[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<ref>{{Cite journal|last=Grantham|first=P.J.|last2=Wakefield|first2=L.L.|date=1988-01|title=Variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time|url=http://dx.doi.org/10.1016/0146-6380(88)90115-5|journal=Organic Geochemistry|volume=12|issue=1|pages=61–73|doi=10.1016/0146-6380(88)90115-5|issn=0146-6380}}</ref>. |
[[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<ref>{{Cite journal|last=Grantham|first=P.J.|last2=Wakefield|first2=L.L.|date=1988-01|title=Variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time|url=http://dx.doi.org/10.1016/0146-6380(88)90115-5|journal=Organic Geochemistry|volume=12|issue=1|pages=61–73|doi=10.1016/0146-6380(88)90115-5|issn=0146-6380}}</ref>. |
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⚫ | [[File:CholestaneDiagenesis3.png|thumb|''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.'']] |
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⚫ | 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 C<sub>20</sub> chiral center, as well as the hydrogen atoms. The ratio of R/S stereoisomers is typically reported as a measure of “thermal maturity”<ref>{{Cite journal|last=Schoell|first=M.|last2=Schouten|first2=S.|last3=Damste|first3=J. S. S.|last4=de Leeuw|first4=J. W.|last5=Summons|first5=R. E.|date=1994-02-25|title=A Molecular Organic Carbon Isotope Record of Miocene Climate Changes|url=http://dx.doi.org/10.1126/science.263.5150.1122|journal=Science|volume=263|issue=5150|pages=1122–1125|doi=10.1126/science.263.5150.1122|issn=0036-8075}}</ref>. In contrast, conversion of 5α → β configuration reflects anaerobic microbial activity<ref name=":0" />, and can be understood through isotope labeling experiments on controlled microbe experiments metabolizing the steroid of interest<ref name=":2">{{Cite journal|last=Mermoud|first=F.|last2=Wünsche|first2=L.|last3=Clerc|first3=O.|last4=Gülaçar|first4=F.O.|last5=Buchs|first5=A.|date=1984-01|title=Steroidal ketones in the early diagenetic transformations of Δ5 sterols in different types of sediments|url=http://dx.doi.org/10.1016/0146-6380(84)90023-8|journal=Organic Geochemistry|volume=6|pages=25–29|doi=10.1016/0146-6380(84)90023-8|issn=0146-6380}}</ref><ref>{{Cite journal|last=Taylor|first=Craig D.|last2=Smith|first2=Steven O.|last3=Gagosian|first3=Robert B.|date=1981-11|title=Use of microbial enrichments for the study of the anaerobic degradation of cholesterol|url=http://dx.doi.org/10.1016/0016-7037(81)90068-5|journal=Geochimica et Cosmochimica Acta|volume=45|issue=11|pages=2161–2168|doi=10.1016/0016-7037(81)90068-5|issn=0016-7037}}</ref>. A previous 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 C<sub>5</sub> isomers<ref name=":2" />. The 14 and 17α hydrogen sites are more stable and undergo changes to β configuration in much lower abundances than the 5 hydrogen site. |
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Thermal alteration can also cause loss of the [[alkane]] side-chain through β-fission that cleaves the carbon-carbon double bond<ref>{{Cite journal|last=Mango|first=Frank D.|date=1990-01|title=The origin of light cycloalkanes in petroleum|url=http://dx.doi.org/10.1016/0016-7037(90)90191-m|journal=Geochimica et Cosmochimica Acta|volume=54|issue=1|pages=23–27|doi=10.1016/0016-7037(90)90191-m|issn=0016-7037}}</ref>. A previous experiment demonstrated that over 4 weeks at 300°C, cholestane underwent 17% decomposition of its alkane side chain. In contrast, the [[Polycyclic aromatic hydrocarbon|polycyclic]] structure is very thermally stable. Diagenetic processes can also cause methyl shifts and [[aromatization]]. |
Thermal alteration can also cause loss of the [[alkane]] side-chain through β-fission that cleaves the carbon-carbon double bond<ref>{{Cite journal|last=Mango|first=Frank D.|date=1990-01|title=The origin of light cycloalkanes in petroleum|url=http://dx.doi.org/10.1016/0016-7037(90)90191-m|journal=Geochimica et Cosmochimica Acta|volume=54|issue=1|pages=23–27|doi=10.1016/0016-7037(90)90191-m|issn=0016-7037}}</ref>. A previous experiment demonstrated that over 4 weeks at 300°C, cholestane underwent 17% decomposition of its alkane side chain. In contrast, the [[Polycyclic aromatic hydrocarbon|polycyclic]] structure is very thermally stable. Diagenetic processes can also cause methyl shifts and [[aromatization]]. |
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[[File:CholestaneIsomerElutionSpectra.png|thumb|Cholestane isomers elute at different times in GC/MS/MS experiments in the ''m/z'' 372→217 fragment. Figure adapted from Bobrovskiy et al.<ref name=":0" />]] |
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=== Stereochemical alteration === |
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⚫ | 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 C<sub>20</sub> chiral center, as well as the hydrogen atoms. The ratio of R/S stereoisomers is typically reported as a measure of “thermal maturity”<ref>{{Cite journal|last=Schoell|first=M.|last2=Schouten|first2=S.|last3=Damste|first3=J. S. S.|last4=de Leeuw|first4=J. W.|last5=Summons|first5=R. E.|date=1994-02-25|title=A Molecular Organic Carbon Isotope Record of Miocene Climate Changes|url=http://dx.doi.org/10.1126/science.263.5150.1122|journal=Science|volume=263|issue=5150|pages=1122–1125|doi=10.1126/science.263.5150.1122|issn=0036-8075}}</ref>. In contrast, conversion of 5α → β configuration reflects anaerobic microbial activity<ref name=":0" />, and can be understood through isotope labeling experiments on controlled microbe experiments metabolizing the steroid of interest<ref name=":2">{{Cite journal|last=Mermoud|first=F.|last2=Wünsche|first2=L.|last3=Clerc|first3=O.|last4=Gülaçar|first4=F.O.|last5=Buchs|first5=A.|date=1984-01|title=Steroidal ketones in the early diagenetic transformations of Δ5 sterols in different types of sediments|url=http://dx.doi.org/10.1016/0146-6380(84)90023-8|journal=Organic Geochemistry|volume=6|pages=25–29|doi=10.1016/0146-6380(84)90023-8|issn=0146-6380}}</ref><ref>{{Cite journal|last=Taylor|first=Craig D.|last2=Smith|first2=Steven O.|last3=Gagosian|first3=Robert B.|date=1981-11|title=Use of microbial enrichments for the study of the anaerobic degradation of cholesterol|url=http://dx.doi.org/10.1016/0016-7037(81)90068-5|journal=Geochimica et Cosmochimica Acta|volume=45|issue=11|pages=2161–2168|doi=10.1016/0016-7037(81)90068-5|issn=0016-7037}}</ref>. A previous 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 C<sub>5</sub> isomers<ref name=":2" />. The 14 and 17α hydrogen sites are more stable and undergo changes to β configuration in much lower abundances than the 5 hydrogen site. |
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== Measurement techniques == |
== Measurement techniques == |
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=== GC/MS === |
=== GC/MS === |
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Cholestane can be extracted from samples and measured on the [[Gas chromatography–mass spectrometry|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 “[[Saturated and unsaturated compounds|saturates]]” fraction using silica gas chromatography. Cholestane [[Isomer|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) and improves identification of specific isomers. |
[[File:CholestaneIsomerElutionSpectra.png|thumb|Cholestane isomers elute at different times in GC/MS/MS experiments in the ''m/z'' 372→217 fragment. Figure adapted from Bobrovskiy et al.<ref name=":0" />]]Cholestane can be extracted from samples and measured on the [[Gas chromatography–mass spectrometry|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 “[[Saturated and unsaturated compounds|saturates]]” fraction using silica gel column gas chromatography. Cholestane [[Isomer|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) and improves identification of specific isomers. |
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=== δ<sup>13</sup>C isotope ratios === |
=== δ<sup>13</sup>C isotope ratios === |
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[[Isotopes of carbon|δ<sup>13</sup>C]] 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<ref>{{Citation|last=Hayes|first=John M.|title=3. Fractionation of Carbon and Hydrogen Isotopes in Biosynthetic Processes|date=2001-12-31|url=http://dx.doi.org/10.1515/9781501508745-006|work=Stable Isotope Geochemistry|pages=225–278|publisher=De Gruyter|isbn=9781501508745|access-date=2019-05-28}}</ref>; therefore, carbon isotope composition of cholestane would reflect this original diet value as well. More generally, steranes can be used as an indicator of environmental shifts. A previous study has presented δ<sup>13</sup>C values of steranes versus [[Hopane|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]]<ref>{{Cite journal|last=Schoell|first=M.|last2=Schouten|first2=S.|last3=Damste|first3=J. S. S.|last4=de Leeuw|first4=J. W.|last5=Summons|first5=R. E.|date=1994-02-25|title=A Molecular Organic Carbon Isotope Record of Miocene Climate Changes|url=http://dx.doi.org/10.1126/science.263.5150.1122|journal=Science|volume=263|issue=5150|pages=1122–1125|doi=10.1126/science.263.5150.1122|issn=0036-8075}}</ref>. |
[[Isotopes of carbon|δ<sup>13</sup>C]] 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<ref>{{Citation|last=Hayes|first=John M.|title=3. Fractionation of Carbon and Hydrogen Isotopes in Biosynthetic Processes|date=2001-12-31|url=http://dx.doi.org/10.1515/9781501508745-006|work=Stable Isotope Geochemistry|pages=225–278|publisher=De Gruyter|isbn=9781501508745|access-date=2019-05-28}}</ref>; therefore, carbon isotope composition of cholestane would reflect this original diet value as well. δ<sup>13</sup>C values can be measured using a gas chromatograph coupled to an Elemental Analyzer (EA). |
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More generally, steranes can be used as an indicator of environmental shifts. A previous study has presented δ<sup>13</sup>C values of steranes versus [[Hopane|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]]<ref>{{Cite journal|last=Schoell|first=M.|last2=Schouten|first2=S.|last3=Damste|first3=J. S. S.|last4=de Leeuw|first4=J. W.|last5=Summons|first5=R. E.|date=1994-02-25|title=A Molecular Organic Carbon Isotope Record of Miocene Climate Changes|url=http://dx.doi.org/10.1126/science.263.5150.1122|journal=Science|volume=263|issue=5150|pages=1122–1125|doi=10.1126/science.263.5150.1122|issn=0036-8075}}</ref>. |
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== Case studies == |
== Case studies == |
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=== Early life biomarkers === |
=== Early life biomarkers === |
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[[File:Dickinsonia tenuis.jpg|thumb|''Dickinsonia'' fossil was proved to be ancient animal via cholestane biomarker identification.]] |
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Presence of cholestane not only indicates presence of animals, but is often used in conjunction with other biomarkers to note the rise of distinct taxa in the fossil record. A previous study measured relative abundance in cholestane versus other [[Triterpene|triterpenoid]] biomarkers to demonstrate the rise of algae during the [[Neoproterozoic]]<ref name=":1" />. |
Presence of cholestane not only indicates presence of animals, but is often used in conjunction with other biomarkers to note the rise of distinct taxa in the fossil record. A previous study measured relative abundance in cholestane versus other [[Triterpene|triterpenoid]] biomarkers to demonstrate the rise of algae during the [[Neoproterozoic]]<ref name=":1" />. |
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![]() | |
![]() IUPAC numbering[1] | |
Names | |
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IUPAC name
(8R,9S,10S,13R,14S,17R)-10,13-Dimethyl-17-[(2R)-6-methylheptan-2-yl]-2,3,4,5,6,7,8,9,11,12,14,15,16,17-tetradecahydro-1H-cyclopenta[a]phenanthrene | |
Identifiers | |
![]() ![]() | |
3D model (JSmol) |
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ChEBI | |
ChemSpider |
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ECHA InfoCard | 100.035.496 ![]() |
PubChem CID |
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CompTox Dashboard (EPA) |
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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). |
Cholestane is a saturated tetracyclictriterpene. This carbon-27 biomarker is produced by diagenesisofcholesterol and is one of the most abundant biomarkers in the rock record[2]. Presence of cholestane in environmental samples are 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]. Cholestane is made in low abundance by other organisms (e.g., rhodophytes), but because these other organisms produce a variety of sterols it cannot be used as a conclusive indicator of any one taxa[4]. It is often found in analysis of organic compounds in petroleum.
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. Cholesterol is responsible for membrane rigidity and fluidity, as well as intracellular transport, cell signaling and nerve conduction. 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). Maintaining stereochemistry of natural cholesterol is typical through diagenetic processes, but cholestane can be found in the fossil record with many stereochemical configurations.
Cholestane in the fossil record is often interpreted as an indicator of ancient animal life and are often used by geochemists and geobiologists to reconstruct animal evolution (particularly in very early Earth history; i.e., Ediacaran[3], neo-Proterozoic and Proterozoic[5][6]). Oxygen is required to produce cholesterol; thus, the presence of cholestane suggests some trace of oxygen in the paleoenvironment. However, cholestane is not exclusively derived from diagenesis of animal-derived biomolecules; cholestane has also been associated with the presence of rhodophytes[7]. In contrast, plants and bacteria produce other sterols (e.g., hopanes).
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[8].
Thermal alteration can also cause loss of the alkane side-chain through β-fission that cleaves the carbon-carbon double bond[9]. A previous experiment demonstrated that over 4 weeks at 300°C, cholestane underwent 17% decomposition of its alkane side chain. In contrast, the polycyclic structure is very thermally stable. Diagenetic processes can also cause methyl shifts and aromatization.
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”[10]. In contrast, conversion of 5α → β configuration reflects anaerobic microbial activity[3], and can be understood through isotope labeling experiments on controlled microbe experiments metabolizing the steroid of interest[11][12]. A previous 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 C5 isomers[11]. The 14 and 17α hydrogen sites are more stable and undergo changes to β configuration in much lower abundances than the 5 hydrogen site.
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., dichloromethaneorchloroform) 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) and improves identification of specific isomers.
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[13]; 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 Elemental Analyzer (EA).
More generally, steranes can be used as an indicator of environmental shifts. A previous 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].
Presence of cholestane not only indicates presence of animals, but is often used in conjunction with other biomarkers to note the rise of distinct taxa in the fossil record. A previous study measured relative abundance in cholestane versus other triterpenoid biomarkers to demonstrate the rise of algae during the Neoproterozoic[5].
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. A previous study tried to constrain 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].
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