Phytane
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IUPAC name
2,6,10,14-Tetramethylhexadecane[1]
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Identifiers | |
3D model (
JSmol ) |
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1744639 | |
ChEBI | |
ChemSpider | |
ECHA InfoCard
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100.010.303 |
EC Number |
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MeSH | phytane |
PubChem CID
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UNII | |
CompTox Dashboard (EPA)
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Properties | |
C20H42 | |
Molar mass | 282.556 g·mol−1 |
Appearance | Colourless 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
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Phytane is the
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]
The
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.
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
Studies suggested that pristane and phytane are formed via diagenesis of phytol under different redox conditions.[13] 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.[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
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 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
See also
References
- ^ "phytane - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 27 March 2005. Identification and Related Records. Retrieved 14 March 2012.
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- ^ PMID 21288485.
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- ^ 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.
- PMID 5646185.
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- S2CID 4329068.
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- ^ S2CID 128737515.
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- ^ ISBN 9781107326040
- ^ 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.
- ^ VOLKMAN, J. K. (1986). "Acyclic isoprenoids as biological markers". Biological Markers in the Sedimentary Record.: 1817–1828.
- ISSN 0016-7037.
- ^ Lijmbach, W. M. (1975-01-01). "SP (1) On the Origin of Petroleum". World Petroleum Congress.
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- ^ PMID 30498776.
- PMID 11540919.
- ISSN 0146-6380.
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