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γ-Carotene
Names
	IUPAC name β,ψ-Carotene
	Systematic IUPAC name 2-[(1E,3E,5E,7E,9E,11E,13E,15E,17E,19E)-3,7,12,16,20,24-Hexamethylpentacosa-1,3,5,7,9,11,13,15,17,19,23-undecaen-1-yl]-1,3,3-trimethylcyclohex-1-ene
Identifiers
CAS Number	472-93-5 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:27740;
ChemSpider	4444349;
PubChem CID	5280791;
UNII	DH369M0SOE ;
CompTox Dashboard (EPA)	DTXSID70897429 ;
	InChI InChI=1S/C40H56/c1-32(2)18-13-21-35(5)24-15-26-36(6)25-14-22-33(3)19-11-12-20-34(4)23-16-27-37(7)29-30-39-38(8)28-17-31-40(39,9)10/h11-12,14-16,18-20,22-27,29-30H,13,17,21,28,31H2,1-10H3/b12-11+,22-14+,23-16+,26-15+,30-29+,33-19+,34-20+,35-24+,36-25+,37-27+ Key: HRQKOYFGHJYEFS-BXOLYSJBSA-N; InChI=1/C40H56/c1-32(2)18-13-21-35(5)24-15-26-36(6)25-14-22-33(3)19-11-12-20-34(4)23-16-27-37(7)29-30-39-38(8)28-17-31-40(39,9)10/h11-12,14-16,18-20,22-27,29-30H,13,17,21,28,31H2,1-10H3/b12-11+,22-14+,23-16+,26-15+,30-29+,33-19+,34-20+,35-24+,36-25+,37-27+ Key: HRQKOYFGHJYEFS-BXOLYSJBBV;
	SMILES C(/C1=C(/CCCC1(C)C)C)=C\C(=C\C=C\C(=C\C=C\C=C(\C=C\C=C(\C=C\C=C(/C)CC\C=C(/C)C)C)C)C)C;
Properties
Chemical formula	C40H56
Molar mass	536.888 g·mol−1
Melting point	160 to 162 °C (320 to 324 °F; 433 to 435 K)
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

γ-Carotene (gamma-carotene) is a carotenoid, and is a biosynthetic intermediate for cyclized carotenoid synthesis in plants.^[2] It is formed from cyclization of lycopene by lycopene cyclase epsilon.^[3] 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.^[2] Tentative discovery of γ-carotene in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic.^[2] This is significant for reconstructing past oceanic conditions, but so far γ-carotene has only been potentially identified in the one measured sample.

Background

γ-Carotene is a carotenoid, a class of pigments giving color to photosynthetic organisms. Specifically, γ-carotene may be derived from myxoxanthophyll found in cyanobacteria, Chlorobiaceae, and green non-sulfur bacteria (Chloroflexi).^[4]^[5] However, there are over 600 different carotenoids, each with different structures and formulas thus altering their absorption spectrum.^[6] In particular, Chromatiaceae lie between 1.5 to 24 meters deep into the water column with more than 75% of the microbial blooms occurring above 12 meters deep.^[7] Other carotenoids such as chlorobactane and isorenieratene are also biomarkers for the presence of green non-sulfur bacteria. These carotenoids are indicators of the past aquatic geochemical environment of their source water. In particular, γ-carotene is an indicator of the depth at which oxic conditions move towards anoxic conditions due to its relevance to green and purple sulfur bacteria which occupy the boundary layer.^[7] Green non-sulfur bacteria are known to produce 2,3,6-trimethylaryl isoprenoids which are unambiguous, thus permitting the deduction of past aquatic geochemical environments.^[8] In γ-carotene, the end group of lycopene produces a β-ring via a β-cyclase enzyme. The other end member is attributed to an open-chain ψ-end.^[9]

Preservation

Biomarkers may be defined as the molecular remnants of lipids and other biological makeups. Often, in sedimentary environments, lipids are decomposed into hydrocarbon skeletons where they remain preserved in the geologic record over long timescales.^[10] Specifically, diagnostic biomarkers are used to investigate past paleo-environmental conditions such as salinity, temperature, and oxygen availability. In aquatic environments where green non-sulfur bacteria persist, organic carbon is remineralised into carbon dioxide and water such that 0.1% are deposited into the sedimentary record at the aquatic floor.^[11] Although γ-carotene is not the diagnostic biomarker for green non-sulfur bacteria, as it has only been tentatively discovered in a natural environment, it is considered a biomarker for green and purple non-sulfur bacteria. Unlike β-carotene which occurs across a vast array of lineages in all three domains of life, γ-carotene is constrained to only a very few potential precursors.^[11] Both bacteria present genera of Chromatiaceae containing γ-carotene after diagenesis which has a unique carbon skeleton; therefore, γ-carotene is identifiable through measurement techniques, namely gas chromatography-mass spectrometry. In some cases it is possible to discriminate between different sources of a biomarker using carbon isotopic fractionation techniques.^[11]

Measurement techniques

GC/MS

Gas chromatography-mass spectrometry (GC/MS) is an analytical technique in geochemistry widely employed to identify and quantify organic compounds present in sedimentary rocks. The sample must be extracted from the source rock before the analysis may occur, which is often less than 1% due to the thermal maturity of the source rock. The 1.640 ± 0.003-Gyr-old sample from the Barney Creek Formation underwent an extraction for γ-carotene and subsequent analysis with GC/MS such that there exists a peak at m/z 125 indicating the presence of carotenoid derivatives which elute immediately after β-carotene and γ-carotene.^[6]

Carbon Isotope Ratios

Additional analysis of γ-carotene can be accomplished through the use of an isotope ratio mass spectrometer. Chromatiaceae is generally found to be depleted in δ¹³C as where Chlorobiaceae are enriched in δ¹³C in comparison to typical oxygenic bacteria by 7-8 ppm respectively.^[12] The results from isotope ratio mass spectroscopy and GC/MS can accurately discriminate the presence of γ-carotene in an extraction from a sedimentary sample. The identification of γ-carotene through these methods would provide a compelling indication of a past euxinic environment, where water columns were anoxic and sulfidic.^[2]

References

^ Ruegg, R.; Schwieter, U.; Ryser, G.; Schudel, P.; Isler, O. (1961). "Synthesen in der Carotinoid-Reihe. 17. Mittelung. γ-Carotin sowie d,l-α- und β-Carotin aus Dehydro-β-apo-12′-carotinal(C25)". Helvetica Chimica Acta. 44 (4): 985–93. doi:10.1002/hlca.19610440414.
^ ^a ^b ^c ^d Brocks, Jochen J.; Love, Gordon D.; Summons, Roger E.; Knoll, Andrew H.; Logan, Graham A.; Bowden, Stephen A. (October 2005). "Biomarker evidence for green and purple sulphur bacteria in a stratified Palaeoproterozoic sea". Nature. 437 (7060): 866–870. Bibcode:2005Natur.437..866B. doi:10.1038/nature04068. ISSN 1476-4687. PMID 16208367. S2CID 4427285.
^ Schaeffer, Philippe; Adam, Pierre; Wehrung, Patrick; Albrecht, Pierre (1997-12-01). "Novel aromatic carotenoid derivatives from sulfur photosynthetic bacteria in sediments". Tetrahedron Letters. 38 (48): 8413–8416. doi:10.1016/S0040-4039(97)10235-0. ISSN 0040-4039.
^ Palmisano, Anna C.; Cronin, Sonja E.; Des Marais, David J. (1988-08-01). "Analysis of lipophilic pigments from a phototrophic microbial mat community by high performance liquid chromatography". Journal of Microbiological Methods. 8 (4): 209–217. doi:10.1016/0167-7012(88)90003-6. ISSN 0167-7012. PMID 11539747.
^ Vogl, Kajetan; Glaeser, Jens; Pfannes, Kristina R.; Wanner, Gerhard; Overmann, Jörg (2006-06-01). "Chlorobium chlorochromatii sp. nov., a symbiotic green sulfur bacterium isolated from the phototrophic consortium "Chlorochromatium aggregatum"". Archives of Microbiology. 185 (5): 363–372. doi:10.1007/s00203-006-0102-z. ISSN 1432-072X. PMID 16555074. S2CID 25708770.
^ ^a ^b Brocks, Jochen J.; Schaeffer, Philippe (2008-03-01). "Okenane, a biomarker for purple sulfur bacteria (Chromatiaceae), and other new carotenoid derivatives from the 1640Ma Barney Creek Formation". Geochimica et Cosmochimica Acta. 72 (5): 1396–1414. Bibcode:2008GeCoA..72.1396B. doi:10.1016/j.gca.2007.12.006. ISSN 0016-7037.
^ ^a ^b Van Gemerden, Hans; Mas, Jordi (1995), Blankenship, Robert E.; Madigan, Michael T.; Bauer, Carl E. (eds.), "Ecology of Phototrophic Sulfur Bacteria", Anoxygenic Photosynthetic Bacteria, Advances in Photosynthesis and Respiration, Dordrecht: Springer Netherlands, pp. 49–85, doi:10.1007/0-306-47954-0_4, ISBN 978-0-306-47954-0
^ Summons, R. E.; Powell, T. G. (1987-03-01). "Identification of aryl isoprenoids in source rocks and crude oils: Biological markers for the green sulphur bacteria". Geochimica et Cosmochimica Acta. 51 (3): 557–566. Bibcode:1987GeCoA..51..557S. doi:10.1016/0016-7037(87)90069-X. ISSN 0016-7037.
^ Vogl, K.; Bryant, D. A. (May 2012). "Biosynthesis of the biomarker okenone: χ-ring formation: Biosynthesis of the biomarker okenone". Geobiology. 10 (3): 205–215. doi:10.1111/j.1472-4669.2011.00297.x. PMID 22070388. S2CID 205627793.
^ Brocks, Jochen J.; Grice, Kliti (2011), "Biomarkers (Molecular Fossils)", in Reitner, Joachim; Thiel, Volker (eds.), Encyclopedia of Geobiology, Encyclopedia of Earth Sciences Series, Dordrecht: Springer Netherlands, pp. 147–167, doi:10.1007/978-1-4020-9212-1_30, ISBN 978-1-4020-9212-1
^ ^a ^b ^c CB, Gregor (1988). Biogeochemical Cycles of Carbon and Sulfur. John Wiley & Sons. pp. 105–174.
^ Summons, Roger E.; Powell, Trevor G. (February 1986). "Chlorobiaceae in Palaeozoic seas revealed by biological markers, isotopes and geology". Nature. 319 (6056): 763–765. Bibcode:1986Natur.319..763S. doi:10.1038/319763a0. ISSN 1476-4687. S2CID 4368960.