{{short description|Alkane formed when phytol, a constituent of chlorophyll, loses its hydroxyl group}} {{Chembox | Verifiedfields = changed | Watchedfields = changed | verifiedrevid = 464206099 | Name = | ImageFile = Phytane.svg | ImageFile_Ref = {{chemboximage|correct|??}} | ImageName = Skeletal formula of phytane | OtherNames = | IUPACName = 2,6,10,14-Tetramethylhexadecane<ref>{{Cite web|title=phytane - Compound Summary|url=https://pubchem.ncbi.nlm.nih.gov/compound/12523|work=PubChem Compound|publisher=National Center for Biotechnology Information|access-date=14 March 2012|location=USA|date=27 March 2005|at=Identification and Related Records}}</ref> | SystematicName = | Section1 = {{Chembox Identifiers | CASNo = 638-36-8 | CASNo_Ref = {{cascite|correct|CAS}} | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 27UZX1Q8TR | PubChem = 12523 | PubChem1 = 54081983 | PubChem1_Comment = <small>6''R'',10''R''</small> | PubChem2 = 42627075 | PubChem2_Comment = <small>6''R'',10''S'',14''S''</small> | PubChem3 = 446564 | PubChem3_Comment = <small>6''S'',10''S'',14''R''</small> | ChemSpiderID = 12006 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID1 = 393886 | ChemSpiderID1_Ref = {{chemspidercite|changed|chemspider}} | ChemSpiderID1_Comment = <small>6''S'',10''S'',14''R''</small> | EINECS = 211-332-2 | MeSHName = phytane | ChEBI = 48937 | ChEBI_Ref = {{ebicite|correct|EBI}} | Beilstein = 1744639 | SMILES = CCC(C)CCCC(C)CCCC(C)CCCC(C)C | StdInChI = 1S/C20H42/c1-7-18(4)12-9-14-20(6)16-10-15-19(5)13-8-11-17(2)3/h17-20H,7-16H2,1-6H3 | StdInChI_Ref = {{stdinchicite|changed|chemspider}} | StdInChIKey = GGYKPYDKXLHNTI-UHFFFAOYSA-N | StdInChIKey_Ref = {{stdinchicite|changed|chemspider}} }} | Section2 = {{Chembox Properties | C = 20 | H = 42 | Appearance = Colourless liquid | Odor = Odourless | Density = 791 mg mL<sup>−1</sup> (at 20&nbsp;°C) | BoilingPtC = 301.41 | BoilingPt_notes = at 100&nbsp;mPa }} | Section4 = {{Chembox Related | OtherFunction_label = alkanes | OtherFunction = {{Unbulleted list|Pristane|Squalane}} }} | Section5 = | Section6 = }}

'''Phytane''' is the isoprenoid alkane formed when phytol, a chemical substituent of chlorophyll, loses its hydroxyl group.<ref name=":5">{{Cite book|chapter=Organic chemistry|last1=Moldowan|first1=J. M.|last2=Walters|first2=C. C.|date=December 2004|website=The Biomarker Guide|language=en|last3=Peters|first3=K. E.|doi=10.1017/CBO9780511524868.004|title=The Biomarker Guide|pages=18–44|isbn=978-0-511-52486-8}}</ref> When phytol loses one carbon atom, it yields pristane.<ref name=":5" /> Other sources of phytane and pristane have also been proposed than phytol.<ref name=":0" /><ref name=":02" />

Pristane and phytane are common constituents in petroleum and have been used as proxies for depositional redox conditions, as well as for correlating oil and its source rock (i.e. elucidating where oil formed). In environmental studies, pristane and phytane are target compounds for investigating oil spills.

==Chemistry== Phytane is a non-polar organic compound that is a clear and colorless liquid at room temperature. It is a head-to-tail linked regular isoprenoid with chemical formula C<sub>20</sub>H<sub>42</sub>.<ref name=":5" />

Phytane has many structural isomers. Among them, crocetane is a tail-to-tail linked isoprenoid and often co-elutes with phytane during gas chromatography (GC) due to its structural similarity.

Phytane also has many stereoisomers because of its three stereo carbons, C-6, C-10 and C-14. Whereas pristane has two stereo carbons, C-6 and C-10. Direct measurement of these isomers has not been reported using gas chromatography.<ref name=":5" /> thumb|Chemical structure of an archaeol, with two phytanyl groups.|alt= alt=|thumb|Chemical structure of α-tocopherol. thumb|Chemical structure of trimethyl 2-methyl-2-(4,8,12-trimethyltridecyl)chroman, a type of MTTCs.|alt= The substituent of phytane is '''phytanyl'''. Phytanyl groups are frequently found in archaeal membrane lipids of methanogenic and halophilic archaea<ref name=":02">{{Cite journal|last1=Rontani|first1=Jean-François|last2=Bonin|first2=Patricia|date=November 2011|title=Production of pristane and phytane in the marine environment: role of prokaryotes|journal=Research in Microbiology|language=en|volume=162|issue=9|pages=923–933|doi=10.1016/j.resmic.2011.01.012|pmid=21288485|doi-access=free}}</ref> (e.g., in archaeol). '''Phytene''' is the singly unsaturated version of phytane. Phytene is also found as the functional group phytyl in many organic molecules of biological importance such as chlorophyll, tocopherol (vitamin E), and phylloquinone (vitamin K<sub>1</sub>). Phytene's corresponding alcohol is phytol. Geranylgeranene is the fully unsaturated form of phytane, and its corresponding substituent is geranylgeranyl.

== Sources == The major source of phytane and pristane is thought to be chlorophyll.<ref>{{Cite journal|last1=Dean|first1=R. A.|last2=Whitehead|first2=E. V.|date=1961-01-01|title=The occurrence of phytane in petroleum|journal=Tetrahedron Letters|volume=2|issue=21|pages=768–770|doi=10.1016/S0040-4039(01)99264-0|issn=0040-4039}}</ref> Chlorophyll is one of the most important photosynthetic pigments in plants, algae, and cyanobacteria, and is the most abundant tetrapyrrole in the biosphere.<ref>{{cite book|title=Biological Markers in the Sedimentary Record|last1=Baker|first1=E.W.|last2=Louda|first2=J.W.|date=1986|publisher=Elsevier|editor-last=Johns|editor-first=R.B.|pages=125–224|chapter=Porphyrins in the geological record}}</ref> Hydrolysis of chlorophyll ''a'', ''b'', ''d'', and ''f'' during diagenesis in marine sediments, or during invertebrate feeding<ref>{{Cite journal|last1=Blumer|first1=Max|last2=Avigan|first2=Joel|date=1968-05-01|title=On the origin of pristane in marine organisms|url=http://www.jlr.org/content/9/3/350|journal=Journal of Lipid Research|language=en|volume=9|issue=3|pages=350–352|doi=10.1016/S0022-2275(20)43103-7 |issn=0022-2275|pmid=5646185|doi-access=free}}</ref> releases phytol, which is then converted to phytane or pristane. alt=|center|thumb|400x400px|Structure of chlorophyll a, with a side chain containing a phytyl group. Another possible source of phytane and pristane is archaeal ether lipids. Laboratory studies show that thermal maturation of methanogenic archaea generates pristane and phytane from diphytanyl glyceryl ethers (archaeols).<ref name=":2">{{Cite journal|last=Rowland|first=S. J.|date=1990-01-01|title=Production of acyclic isoprenoid hydrocarbons by laboratory maturation of methanogenic bacteria|journal=Organic Geochemistry|volume=15|issue=1|pages=9–16|doi=10.1016/0146-6380(90)90181-X|bibcode=1990OrGeo..15....9R |issn=0146-6380}}</ref><ref>{{Cite journal|last=Navale|first=Vivek|date=1994-06-01|title=Comparative study of low and high temperature hydrous pyrolysis products of monoglycerol diether lipid from archaebacteria|journal=Journal of Analytical and Applied Pyrolysis|volume=29|issue=1|pages=33–43|doi=10.1016/0165-2370(93)00786-M|bibcode=1994JAAP...29...33N |issn=0165-2370}}</ref><ref>{{Cite journal|last1=Pease|first1=T. K.|last2=Van Vleet|first2=E. S.|last3=Barre|first3=J. S.|last4=Dickins|first4=H. D.|date=1998-01-01|title=Simulated degradation of glyceryl ethers by hydrous and flash pyrolysis|journal=Organic Geochemistry|volume=29|issue=4|pages=979–988|doi=10.1016/S0146-6380(98)00047-3|bibcode=1998OrGeo..29..979P |issn=0146-6380}}</ref>

In addition, pristane can be derived from tocopherols<ref name=":3">{{Cite journal|last1=Brassell|first1=S. C.|last2=P. A. Schenck|last3=de Leeuw|first3=J. W.|last4=Goossens|first4=H.|date=November 1984|title=Tocopherols as likely precursors of pristane in ancient sediments and crude oils|journal=Nature|language=en|volume=312|issue=5993|pages=440–442|doi=10.1038/312440a0|issn=1476-4687|bibcode=1984Natur.312..440G|s2cid=4329068}}</ref> and methyltrimethyltridecylchromans (MTTCs).<ref name=":4">{{Cite journal|last1=Li|first1=Maowen|last2=Larter|first2=Steve R.|last3=Taylor|first3=Paul|last4=Jones|first4=D. Martin|last5=Bowler|first5=Bernard|last6=Bjorøy|first6=Malvin|date=1995-02-01|title=Biomarkers or not biomarkers? A new hypothesis for the origin of pristane involving derivation from methyltrimethyltridecylchromans (MTTCs) formed during diagenesis from chlorophyll and alkylphenols|journal=Organic Geochemistry|volume=23|issue=2|pages=159–167|doi=10.1016/0146-6380(94)00112-E|bibcode=1995OrGeo..23..159L |issn=0146-6380}}</ref>

== Preservation == In suitable environments, biomolecules like chlorophyll can be transformed and preserved in recognizable forms as biomarkers. Conversion during diagenesis often causes the chemical loss of functional groups like double bonds and hydroxyl groups. [[File:Phytol diagenesis producing pristane and phytane jpeg.jpg|alt=|center|thumb|700x700px|Pristane and phytane are formed by diagenesis of phytol under oxic and anoxic conditions, respectively.]] Studies suggested that pristane and phytane are formed via diagenesis of phytol under different redox conditions.<ref name=":1" /> Pristane can be formed in oxic (oxidizing) conditions by phytol oxidation to phytenic acid, which may then undergo decarboxylation to pristene, before finally being reduced to pristane. In contrast, phytane is likely from reduction and dehydration of phytol (via dihydrophytol or phytene) under relatively anoxic conditions.<ref name=":1">{{Cite journal|last1=Eglinton|first1=G.|last2=S. C. Brassell|last3=Simoneit|first3=B. R. T.|last4=Didyk|first4=B. M.|date=March 1978|title=Organic geochemical indicators of palaeoenvironmental conditions of sedimentation|journal=Nature|language=en|volume=272|issue=5650|pages=216–222|doi=10.1038/272216a0|issn=1476-4687|bibcode=1978Natur.272..216D|s2cid=128737515}}</ref> However, various biotic and abiotic processes may control the diagenesis of chlorophyll and phytol, and the exact reactions are more complicated and not strictly-correlated to redox conditions.<ref name=":0">{{Cite journal|last1=Rontani|first1=Jean-François|last2=Volkman|first2=John K.|date=2003-01-01|title=Phytol degradation products as biogeochemical tracers in aquatic environments|journal=Organic Geochemistry|volume=34|issue=1|pages=1–35|doi=10.1016/S0146-6380(02)00185-7|bibcode=2003OrGeo..34....1R |issn=0146-6380}}</ref><ref name=":02" />

In thermally immature sediments, pristane and phytane has a configuration dominated by 6R,10S stereochemistry (equivalent to 6S, 10R), which is inherited from C-7 and C-11 in phytol. During thermal maturation, isomerization at C-6 and C-10 leads to a mixture of 6R, 10S, 6S, 10S, and 6R, 10R.<ref name=":5" />

== Geochemical parameters ==

=== Pristane/Phytane ratio === Pristane/phytane (Pr/Ph) is the ratio of abundances of pristane and phytane. It is a proxy for redox conditions in the depositional environments. The Pr/Ph index is based on the assumption that pristane is formed from phytol by an oxidative pathway, while phytane is generated through various reductive pathways.<ref name=":1" /><ref>{{Cite journal|last1=D. M. McKIRDY|last2=Powell|first2=T. G.|date=May 1973|title=Relationship between Ratio of Pristane to Phytane, Crude Oil Composition and Geological Environment in Australia|journal=Nature Physical Science|language=en|volume=243|issue=124|pages=37–39|doi=10.1038/physci243037a0|issn=2058-1106|bibcode=1973NPhS..243...37P}}</ref> In non-biodegraded crude oil, Pr/Ph less than 0.8 indicates saline to hypersaline conditions associated with evaporite and carbonate deposition, whereas organic-lean terrigenous, fluvial,<nowiki/>and deltaic sediments under oxic to suboxic conditions usually generate crude oil with Pr/Ph above 3.<ref name=":6">{{Citation|last1=Peters|first1=K. E.|chapter=Source- and age-related biomarker parameters|pages=483–607|publisher=Cambridge University Press|doi=10.1017/cbo9781107326040.004|isbn=978-1-107-32604-0|last2=Walters|first2=C. C.|last3=Moldowan|first3=J. M.|title=The Biomarker Guide|year=2004}}</ref> Pr/Ph is commonly applied because pristane and phytane are measured easily using gas chromatography.

However, the index should be used with caution, as pristane and phytane may not result from degradation of the same precursor (see *Source*). Also, pristane, but not phytane, can be produced in reducing environments by clay-catalysed degradation of phytol and subsequent reduction.<ref>{{Cite journal|last1=Schenck|first1=P. A.|last2=Lange|first2=F. de|last3=Boon|first3=J. J.|last4=Rijpstra|first4=C.|last5=Irene|first5=W.|last6=Leeuw|first6=J. W. de|date=1977|title=relationship between lipids from Fontinalis antipyretica, its detritus and the underlying sediment: the fate of waxesters and sterolesters|url=http://agris.fao.org/agris-search/search.do?recordID=US201302970277|journal=Interactions Between Sediments and Fresh Water; Proceedings of an International Symposium|language=English}}</ref> Additionally, during catagenesis, Pr/Ph tends to increase.<ref>{{Cite journal|last=VOLKMAN|first=J. K.|date=1986|title=Acyclic isoprenoids as biological markers.|url=https://ci.nii.ac.jp/naid/10007187788/|journal=Biological Markers in the Sedimentary Record.|pages=1817–1828}}</ref> This variation may be due to preferential release of sulfur-bound phytols from source rocks during early maturation.<ref>{{Cite journal|last1=De Graaf|first1=Wim|last2=Damsté|first2=Jaap S. Sinninghe|last3=de Leeuw|first3=Jan W|date=1992-12-01|title=Laboratory simulation of natural sulphurization: I. Formation of monomeric and oligomeric isoprenoid polysulphides by low-temperature reactions of inorganic polysulphides with phytol and phytadienes|journal=Geochimica et Cosmochimica Acta|volume=56|issue=12|pages=4321–4328|doi=10.1016/0016-7037(92)90275-N|issn=0016-7037|bibcode=1992GeCoA..56.4321D}}</ref>

=== Pristane/nC<sub>17</sub> and phytane/nC<sub>18</sub> ratios === Pristane/n-heptadecane (Pr/nC<sub>17</sub>) and phytane/n-octadecane (Ph/C<sub>18</sub>) are sometimes used to correlate oil and its source rock (i.e. to elucidate where oil formed). Oils from rocks deposited under open-ocean conditions showed Pr/nC<sub>17</sub>< 0.5, while those from inland peat swamp had ratios greater than 1.<ref>{{Cite journal|last=Lijmbach|first=W. M.|date=1975-01-01|title=SP (1) On the Origin of Petroleum|url=https://www.onepetro.org/conference-paper/WPC-16134|language=english|publisher=World Petroleum Congress}}</ref>

The ratios should be used with caution for several reasons. Both Pr/nC<sub>17</sub>and Ph/nC<sub>18</sub> decrease with thermal maturity of petroleum because isoprenoids are less thermally stable than linear alkanes. In contrast, biodegradation increases these ratios because aerobic bacteria generally attack linear alkanes before the isoprenoids. Therefore, biodegraded oil is similar to low-maturity non-degraded oil in the sense of exhibiting low abundance of n-alkanes relative to pristane and phytane.<ref name=":6" />

=== Biodegradation scale === Pristane and phytane are more resistant to biodegradation than n-alkanes, but less so than steranes and hopanes. The substantial depletion and complete elimination of pristane and phytane correspond to a Biomarker Biodegradation Scale of 3 and 4, respectively.<ref>{{Citation|last1=Peters|first1=K. E.|chapter=Biodegradation parameters|pages=645–708|publisher=Cambridge University Press|doi=10.1017/cbo9781107326040.007|isbn=978-1-107-32604-0|last2=Walters|first2=C. C.|last3=Moldowan|first3=J. M.|title=The Biomarker Guide|year=2004}}</ref>

=== Compound specific isotope analyses ===

==== Carbon isotopes ==== The carbon isotopic composition of pristane and phytane generally reflects the kinetic isotope fractionation that occurs during photosynthesis. For example, δ<sup>13</sup>C(PDB) of phytane in marine sediments and oils has been used to reconstruct ancient atmospheric CO<sub>2</sub>levels, which affects the carbon isotopic fractionation associated with photosynthesis, over the past 500 million years.<ref name=":7">{{Cite journal|last1=Damsté|first1=Jaap S. Sinninghe|last2=Schouten|first2=Stefan|last3=Blais|first3=Brian|last4=Weijers|first4=Johan W. H.|last5=Witkowski|first5=Caitlyn R.|date=2018-11-01|title=Molecular fossils from phytoplankton reveal secular Pco2 trend over the Phanerozoic|journal=Science Advances|language=en|volume=4|issue=11|article-number=eaat4556|doi=10.1126/sciadv.aat4556|pmid=30498776|pmc=6261654|issn=2375-2548|bibcode=2018SciA....4.4556W}}</ref> In this study,<ref name=":7" /> partial pressure of CO<sub>2</sub> reached more than 1000 ppm at maxima compared to 410 ppm today.

Carbon isotope compositions of pristane and phytane in crude oil can also help to constrain their source. Pristane and phytane from a common precursor should have δ<sup>13</sup>C values differing by no more than 0.3‰.<ref>{{Cite journal|last1=Hayes|first1=J. M.|last2=Freeman|first2=Katherine H.|last3=Popp|first3=Brian N.|last4=Hoham|first4=Christopher H.|date=1990-01-01|title=Compound-specific isotopic analyses: A novel tool for reconstruction of ancient biogeochemical processes|journal=Organic Geochemistry|series=Proceedings of the 14th International Meeting on Organic Geochemistry|volume=16|issue=4|pages=1115–1128|doi=10.1016/0146-6380(90)90147-R|pmid=11540919|bibcode=1990OrGeo..16.1115H |issn=0146-6380}}</ref>

==== Hydrogen isotopes ==== Hydrogen isotope composition of phytol in marine phytoplankton and algae starts out as highly depleted, with δD (VSMOW) ranging from -360 to -280‰.<ref>{{Cite journal|last1=Sessions|first1=Alex L.|last2=Burgoyne|first2=Thomas W.|last3=Schimmelmann|first3=Arndt|last4=Hayes|first4=John M.|date=1999-09-01|title=Fractionation of hydrogen isotopes in lipid biosynthesis|journal=Organic Geochemistry|volume=30|issue=9|pages=1193–1200|doi=10.1016/S0146-6380(99)00094-7|bibcode=1999OrGeo..30.1193S |issn=0146-6380}}</ref> Thermal maturation preferentially releases light isotopes, causing and pristane and phytane to become progressively heavier with maturation.

== Case study: limitation of Pr/Ph as a redox indicator == Inferences from Pr/Ph on the redox potential of source sediments should always be supported by other geochemical and geological data, such as sulfur content or the C<sub>35</sub> homohopane index (i.e. the abundance of C<sub>35</sub> homohopane relative to that of C<sub>31</sub>-C<sub>35</sub> homohopanes). For example, the Baghewala-1 oil from India has low Pr/Ph (0.9), high sulfur (1.2 wt.%) and high C35 homohopane index, which are consistent with anoxia during deposition of the source rock.<ref>{{Cite journal|last=K. E. Peters (2), M. E. Clark (3)|date=1995|title=Recognition of an Infracambrian Source Rock Based on Biomarkers in the Baghewala-1 Oil, India|journal=AAPG Bulletin|language=en-US|volume=79|issue=10|doi=10.1306/7834da12-1721-11d7-8645000102c1865d|issn=0149-1423}}</ref>

However, drawing conclusion on the oxic state of depositional environments only from Pr/Ph ratio can be misleading because salinity often controls the Pr/Ph in hypersaline environments. In another example, the decrease in Pr/Ph during deposition of the PermianKupferschiefer sequence in Germany is in coincidence with an increase in trimethylated 2-methyl-2-(4,8,12-trimethyltridecyl)chromans, an aromatic compound believed to be markers of salinity.<ref>{{Cite journal|last1=Schwark|first1=L|last2=Vliex|first2=M|last3=Schaeffer|first3=P|date=1998-12-01|title=Geochemical characterization of Malm Zeta laminated carbonates from the Franconian Alb, SW-Germany (II)|journal=Organic Geochemistry|volume=29|issue=8|pages=1921–1952|doi=10.1016/S0146-6380(98)00192-2|bibcode=1998OrGeo..29.1921S|issn=0146-6380}}</ref> Therefore, this decrease in Pr/Ph should indicate an increase in salinity, instead of an increase in anoxia.

==See also== * Phytol * Pristane * Biomarker * Crocetane * Archaeol * Tocopherols * Sterane * Hopane

==References== {{Reflist}}

Category:Alkanes Category:Diterpenes