Phytane

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Phytane
Phytane.svg
Names
IUPAC name
2,6,10,14-Tetramethylhexadecane [1]
Identifiers
3D model (JSmol)
1744639
ChEBI
ChemSpider
ECHA InfoCard 100.010.303 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 211-332-2
MeSH phytane
PubChem CID
UNII
  • InChI=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 X mark.svgN
    Key: GGYKPYDKXLHNTI-UHFFFAOYSA-N X mark.svgN
  • CCC(C)CCCC(C)CCCC(C)CCCC(C)C
Properties
C20H42
Molar mass 282.556 g·mol−1
AppearanceColourless liquid
Odor Odourless
Density 791 mg mL−1 (at 20 °C)
Boiling point 301.41 °C (574.54 °F; 574.56 K)at 100 mPa
Related compounds
Related alkanes
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Phytane is the isoprenoid alkane formed when phytol, a constituent of chlorophyll, loses its hydroxyl group. [2] When phytol loses one carbon atom, it yields pristane. [2] Other sources of phytane and pristane have also been proposed than phytol. [3] [4]

Contents

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 C20H42. [2]

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. [2]

Chemical structure of an archaeol, with two phytanyl groups. Archaeol.png
Chemical structure of an archaeol, with two phytanyl groups.
Chemical structure of a-tocopherol. DL-all-rac-alpha-Tocopherol 100.svg
Chemical structure of α-tocopherol.
Chemical structure of trimethyl 2-methyl-2-(4,8,12-trimethyltridecyl)chroman, a type of MTTCs. Trimethyl 2-methyl-2-(4,8,12-trimethyltridecyl)chroman.jpg
Chemical structure of trimethyl 2-methyl-2-(4,8,12-trimethyltridecyl)chroman, a type of MTTCs.

The substituent of phytane is phytanyl. Phytanyl groups are frequently found in archaeal membrane lipids of methanogenic and halophilic archaea [4] (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 K1). 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. [5] Chlorophyll is one of the most important photosynthetic pigments in plants, algae, and cyanobacteria, and is the most abundant tetrapyrrole in the biosphere. [6] Hydrolysis of chlorophyll a, b, d, and f during diagenesis in marine sediments, or during invertebrate feeding [7] releases phytol, which is then converted to phytane or pristane.

Structure of chlorophyll a, with a side chain containing a phytyl group. Chlorophyll a structure.svg
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). [8] [9] [10]

In addition, pristane can be derived from tocopherols [11] and methyltrimethyltridecylchromans (MTTCs). [12]

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.

Pristane and phytane are formed by diagenesis of phytol under oxic and anoxic conditions, respectively. Phytol diagenesis producing pristane and phytane jpeg.jpg
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. [13] Pristane can be formed in 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. [13] 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. [3] [4]

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. [2]

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. [13] [14] 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,and deltaic sediments under oxic to suboxic conditions usually generate crude oil with Pr/Ph above 3. [15] 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. [16] Additionally, during catagenesis, Pr/Ph tends to increase. [17] This variation may be due to preferential release of sulfur-bound phytols from source rocks during early maturation. [18]

Pristane/nC17 and phytane/nC18 ratios

Pristane/n-heptadecane (Pr/nC17) and phytane/n-octadecane (Ph/C18) 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/nC17< 0.5, while those from inland peat swamp had ratios greater than 1. [19]

The ratios should be used with caution for several reasons. Both Pr/nC17and Ph/nC18 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. [15]

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. [20]

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, δ13C(PDB) of phytane in marine sediments and oils has been used to reconstruct ancient atmospheric CO2levels, which affects the carbon isotopic fractionation associated with photosynthesis, over the past 500 million years. [21] In this study, [21] partial pressure of CO2 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 δ13C values differing by no more than 0.3‰. [22]

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‰. [23] 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 C35 homohopane index (i.e. the abundance of C35 homohopane relative to that of C31-C35 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. [24]

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 Permian Kupferschiefer 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. [25] Therefore, this decrease in Pr/Ph should indicate an increase in salinity, instead of an increase in anoxia.

See also

Related Research Articles

Organic geochemistry is the study of the impacts and processes that organisms have had on the Earth. It is mainly concerned with the composition and mode of origin of organic matter in rocks and in bodies of water. The study of organic geochemistry is traced to the work of Alfred E. Treibs, "the father of organic geochemistry." Treibs first isolated metalloporphyrins from petroleum. This discovery established the biological origin of petroleum, which was previously poorly understood. Metalloporphyrins in general are highly stable organic compounds, and the detailed structures of the extracted derivatives made clear that they originated from chlorophyll.

Biogenic substance Product made by or of life forms

A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of plants or animals. In context of molecular biology, biogenic substances are referred to as biomolecules. They are generally isolated and measured through the use of chromatography and mass spectrometry techniques. Additionally, the transformation and exchange of biogenic substances can by modelled in the environment, particularly their transport in waterways.

Phytol is an acyclic hydrogenated diterpene alcohol that can be used as a precursor for the manufacture of synthetic forms of vitamin E and vitamin K1. In ruminants, the gut fermentation of ingested plant materials liberates phytol, a constituent of chlorophyll, which is then converted to phytanic acid and stored in fats. In shark liver it yields pristane.

Pristane is a natural saturated terpenoid alkane obtained primarily from shark liver oil, from which its name is derived. It is also found in the stomach oil of birds in the order Procellariiformes and in mineral oil and some foods. Pristane and phytane are used in the fields of geology and environmental science as biomarkers to characterize origins and evolution of petroleum hydrocarbons and coal.

Paleolimnology Scientific study of ancient lakes and streams

Paleolimnology is a scientific sub-discipline closely related to both limnology and paleoecology. Paleolimnological studies focus on reconstructing the past environments of inland waters using the geologic record, especially with regard to events such as climatic change, eutrophication, acidification, and internal ontogenic processes.

Cholestane Chemical compound

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. 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. 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. It is often found in analysis of organic compounds in petroleum.

Carbon-to-nitrogen ratio

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.

Archaeol is one of the main core membrane lipids of archaea, one of the three domains of life. One of the key features that distinguishes archaea from bacteria and eukarya is their membrane lipids, where archaeol plays an important role. Because of this, archaeol is also broadly used as a biomarker for ancient archaea, especially methanogen, activity.

Abietane 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.

Taraxerol Chemical compound

Taraxerol is a naturally-occurring pentacyclic triterpenoid. It exists in various higher plants, including Taraxacum officinale (Asteraceae), Alnus glutinosa (Betulaceae), Litsea dealbata (Lauraceae), Skimmia spp. (Rutaceae), Dorstenia spp. (Moraceae), Maytenus spp. (Celastraceae), and Alchornea latifolia (Euphobiaceae). Taraxerol was named "alnulin" when it was first isolated in 1923 from the bark of the grey alder by Zellner and Röglsperger. It also had the name "skimmiol" when Takeda and Yosiki isolated it from Skimmia (Rutaceae). A large number of medicinal plants are known to have this compound in their leaves, roots or seed oil.

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.

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.

Dinosterane Chemical compound

Dinosterane is a steroidal alkane, also known as 4α,23,24-trimethylcholestane. It is used in geochemistry as a biomarker, interpreted as an indication of dinoflagellate presence due to its derivative dinosterol high occurrence in extant dinoflagellate species and its rarity in other taxa, although it has been shown to be produced by a single species of marine diatom as well.

27-Norcholestane Chemical compound

27-Norcholestane, is a chemical compound with formula C
26H
46, that is a steroid derivative. 27-Norcholestane 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.

Arborane is a class of pentacyclic triterpene consisting of organic compounds with four 6-membered rings and one 5-membered ring. Arboranes are thought to be derived from arborinols, a class of natural cyclic triterpenoids typically produced by flowering plants. Thus arboranes are used as a biomarker for angiosperms and cordaites. Arborane is a stereoisomer of a compound called fernane, the diagenetic product of fernene and fernenol. Because aborinol and fernenol have different biological sources, the ratio of arborane/fernane in a sample can be used to reconstruct a record for the relative abundances of different plants.

Highly branched isoprenoids (HBIs) are long-chain alkenes produced by a small number of marine diatoms. There are a variety of highly branched isoprenoid structures, but C25 Highly branched isoprenoids containing 1 to 3 double bonds are the most common in the sedimentary record. Highly branched isoprenoids have been used as environmental proxies for sea ice conditions in the Arctic and Antarctic throughout the Holocene, and more recently, are being used to reconstruct more ancient ice records.

Lycopane 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.

Glycerol dialkyl glycerol tetraether lipids (GDGTs) are a class of membrane lipids synthesized by archaea and some bacteria, making them useful biomarkers for these organisms in the geological record. Their presence, structure, and relative abundances in natural materials can be useful as proxies for temperature, terrestrial organic matter input, and soil pH for past periods in Earth history. Some structural forms of GDGT form the basis for the TEX86 paleothermometer. Isoprenoid GDGTs, now known to be synthesized by many archaeal classes, were first discovered in extremophilic archaea cultures. Branched GDGTs, likely synthesized by acidobacteriota, were first discovered in a natural Dutch peat sample in 2000.

Xylitol pentacetate Chemical compound

Xylitol pentacetate is an organic compound with the formula C15H22O10. It is an acetylated sugar alcohol that is used as an intermediary in the production of xylitol pentanitrate. It is also commonly made to isolate and identify xylitol from complex organic mixtures.

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.

References

  1. ↑ "phytane - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 27 March 2005. Identification and Related Records. Retrieved 14 March 2012.
  2. 1 2 3 4 5 Moldowan, J. M.; Walters, C. C.; Peters, K. E. (December 2004). "Organic chemistry". The Biomarker Guide. The Biomarker Guide. pp. 18–44. doi:10.1017/CBO9780511524868.004. ISBN   9780511524868.
  3. 1 2 Rontani, Jean-François; Volkman, John K. (2003-01-01). "Phytol degradation products as biogeochemical tracers in aquatic environments". Organic Geochemistry. 34 (1): 1–35. doi:10.1016/S0146-6380(02)00185-7. ISSN   0146-6380.
  4. 1 2 3 Rontani, Jean-François; Bonin, Patricia (November 2011). "Production of pristane and phytane in the marine environment: role of prokaryotes". Research in Microbiology. 162 (9): 923–933. doi:10.1016/j.resmic.2011.01.012. PMID   21288485.
  5. ↑ Dean, R. A.; Whitehead, E. V. (1961-01-01). "The occurrence of phytane in petroleum". Tetrahedron Letters. 2 (21): 768–770. doi:10.1016/S0040-4039(01)99264-0. ISSN   0040-4039.
  6. ↑ Baker, E.W.; Louda, J.W. (1986). "Porphyrins in the geological record". In Johns, R.B. (ed.). Biological Markers in the Sedimentary Record. Elsevier. pp. 125–224.
  7. ↑ Blumer, Max; Avigan, Joel (1968-05-01). "On the origin of pristane in marine organisms". Journal of Lipid Research. 9 (3): 350–352. doi:10.1016/S0022-2275(20)43103-7. ISSN   0022-2275. PMID   5646185.
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  9. ↑ Navale, Vivek (1994-06-01). "Comparative study of low and high temperature hydrous pyrolysis products of monoglycerol diether lipid from archaebacteria". Journal of Analytical and Applied Pyrolysis. 29 (1): 33–43. doi:10.1016/0165-2370(93)00786-M. ISSN   0165-2370.
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  14. ↑ D. M. McKIRDY; Powell, T. G. (May 1973). "Relationship between Ratio of Pristane to Phytane, Crude Oil Composition and Geological Environment in Australia". Nature Physical Science. 243 (124): 37–39. Bibcode:1973NPhS..243...37P. doi:10.1038/physci243037a0. ISSN   2058-1106.
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  16. ↑ Schenck, P. A.; Lange, F. de; Boon, J. J.; Rijpstra, C.; Irene, W.; Leeuw, J. W. de (1977). "relationship between lipids from Fontinalis antipyretica, its detritus and the underlying sediment: the fate of waxesters and sterolesters". Interactions Between Sediments and Fresh Water; Proceedings of an International Symposium.
  17. ↑ VOLKMAN, J. K. (1986). "Acyclic isoprenoids as biological markers". Biological Markers in the Sedimentary Record.: 1817–1828.
  18. ↑ De Graaf, Wim; Damsté, Jaap S. Sinninghe; de Leeuw, Jan W (1992-12-01). "Laboratory simulation of natural sulphurization: I. Formation of monomeric and oligomeric isoprenoid polysulphides by low-temperature reactions of inorganic polysulphides with phytol and phytadienes". Geochimica et Cosmochimica Acta. 56 (12): 4321–4328. Bibcode:1992GeCoA..56.4321D. doi:10.1016/0016-7037(92)90275-N. ISSN   0016-7037.
  19. ↑ Lijmbach, W. M. (1975-01-01). "SP (1) On the Origin of Petroleum". World Petroleum Congress.{{cite journal}}: Cite journal requires |journal= (help)
  20. ↑ Peters, K. E.; Walters, C. C.; Moldowan, J. M. (2004), "Biodegradation parameters", The Biomarker Guide, Cambridge University Press, pp. 645–708, doi:10.1017/cbo9781107326040.007, ISBN   9781107326040
  21. 1 2 Damsté, Jaap S. Sinninghe; Schouten, Stefan; Blais, Brian; Weijers, Johan W. H.; Witkowski, Caitlyn R. (2018-11-01). "Molecular fossils from phytoplankton reveal secular Pco2 trend over the Phanerozoic". Science Advances. 4 (11): eaat4556. Bibcode:2018SciA....4.4556W. doi:10.1126/sciadv.aat4556. ISSN   2375-2548. PMC   6261654 . PMID   30498776.
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  23. ↑ Sessions, Alex L.; Burgoyne, Thomas W.; Schimmelmann, Arndt; Hayes, John M. (1999-09-01). "Fractionation of hydrogen isotopes in lipid biosynthesis". Organic Geochemistry. 30 (9): 1193–1200. doi:10.1016/S0146-6380(99)00094-7. ISSN   0146-6380.
  24. ↑ K. E. Peters (2), M. E. Clark (3) (1995). "Recognition of an Infracambrian Source Rock Based on Biomarkers in the Baghewala-1 Oil, India". AAPG Bulletin. 79 (10). doi:10.1306/7834da12-1721-11d7-8645000102c1865d. ISSN   0149-1423.
  25. ↑ Schwark, L; Vliex, M; Schaeffer, P (1998-12-01). "Geochemical characterization of Malm Zeta laminated carbonates from the Franconian Alb, SW-Germany (II)". Organic Geochemistry. 29 (8): 1921–1952. doi:10.1016/S0146-6380(98)00192-2. ISSN   0146-6380.
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