Phytane is the

isoprenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpene ...

alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in wh ...

formed when phytol, a constituent of

chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...

, loses its

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...

group. When phytol loses one carbon atom, it yields pristane. Other sources of phytane and pristane have also been proposed than phytol. Pristane and phytane are common constituents in

petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...

and have been used as proxies for depositional

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

conditions, as well as for correlating oil and its

source rock In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been depo ...

(i.e. elucidating where oil formed). In environmental studies, pristane and phytane are target compounds for investigating

oil spill An oil spill is the release of a liquid petroleum hydrocarbon into the environment, especially the marine ecosystem, due to human activity, and is a form of pollution. The term is usually given to marine oil spills, where oil is released into t ...

Chemistry

Phytane is a

non-polar In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end. Polar molecules must contain one or more polar ...

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...

that is a clear and colorless liquid at room temperature. It is a head-to-tail linked regular

with chemical formula C₂₀H₄₂. Phytane has many

structural isomers In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term meta ...

. Among them, crocetane is a tail-to-tail linked

and often co-elutes with phytane during

gas chromatography Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substanc ...

(GC) due to its structural similarity. Phytane also has many

stereoisomers In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms ...

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. Archaeol

Trimethyl 2-methyl-2-(4,8,12-trimethyltridecyl)chroman

The

substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...

of phytane is phytanyl. Phytanyl groups are frequently found in archaeal membrane lipids of

methanogen Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain Archaea. All known methanogens are members of the archaeal phylum Euryarchaeota. Methanogens are c ...

ic and halophilic archaea (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

tocopherol Tocopherols (; TCP) are a class of organic chemical compounds (more precisely, various methylated phenols), many of which have vitamin E activity. Because the vitamin activity was first identified in 1936 from a dietary fertility factor in rat ...

(vitamin E), and

phylloquinone Phytomenadione, also known as vitamin K1 or phylloquinone, is a vitamin found in food and used as a dietary supplement. It is on the World Health Organization's List of Essential Medicines. As a supplement it is used to treat certain bleeding ...

(vitamin K₁). Phytene's corresponding alcohol is phytol. Geranylgeranene is the fully unsaturated form of phytane, and its corresponding

is geranylgeranyl.

Sources

The major source of phytane and pristane is thought to be

. Chlorophyll is one of the most important

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...

s in plants,

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...

, and

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

, and is the most abundant tetrapyrrole in the biosphere. Hydrolysis of chlorophyll ''a'', ''b'', ''d'', and ''f'' during diagenesis in marine sediments, or during

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...

feeding releases phytol, which is then converted to phytane or pristane. Chlorophyll a structure

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). In addition, pristane can be derived from

s and methyltrimethyltridecylchromans (MTTCs).

Preservation

In suitable environments, biomolecules like chlorophyll can be transformed and preserved in recognizable forms as

biomarkers In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, pa ...

. Conversion during

diagenesis Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...

often causes the chemical loss of functional groups like

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

s and hydroxyl groups. Phytol diagenesis producing pristane and phytane jpeg

Studies suggested that pristane and phytane are formed via diagenesis of phytol under different redox conditions. 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. 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. 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.

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. In non- biodegraded crude oil, Pr/Ph less than 0.8 indicates saline to

hypersaline A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride, brines, and other salts, with saline levels surpassing that of ocean water (3.5%, i.e. ). Specific microbial species can thrive in hi ...

conditions associated with evaporite and carbonate deposition, whereas organic-lean terrigenous,

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

,and deltaic sediments under oxic to suboxic conditions usually generate

crude oil Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...

with Pr/Ph above 3. 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 Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...

-catalysed degradation of phytol and subsequent reduction. Additionally, during catagenesis, Pr/Ph tends to increase. This variation may be due to preferential release of sulfur-bound phytols from source rocks during early maturation.

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

(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. The ratios should be used with caution for several reasons. Both Pr/nC₁₇and Ph/nC₁₈ decrease with thermal maturity of petroleum because

isoprenoids The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpenes" ...

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.

Biodegradation scale

Pristane and phytane are more resistant to

biodegradation Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegra ...

than n-alkanes, but less so than

sterane Sterane (cyclopentanoperhydrophenanthrenes) compounds are a class of tetracyclic compounds derived from steroids or sterols via diagenetic and catagenetic degradation and saturation. Steranes have an androstane skeleton with a side chain at carb ...

s and hopanes. The substantial depletion and complete elimination of pristane and phytane correspond to a Biomarker Biodegradation Scale of 3 and 4, respectively.

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. In this study, 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‰.

Hydrogen isotopes

Hydrogen isotope composition of phytol in marine

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...

and

starts out as highly depleted, with δD (VSMOW) ranging from -360 to -280‰. 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. However, drawing conclusion on the oxic state of depositional environments only from Pr/Ph ratio can be misleading because

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

often controls the Pr/Ph in

environments. In another example, the decrease in Pr/Ph during deposition of the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

Kupferschiefer The Kupferschiefer ( German for Copper Shale, also called Copper Slate) or Kupfermergel (Copper Marl), (T1 or Z1) is an extensive and remarkable sedimentary unit in Central Europe. The relatively monotonous succession is typically and maximum t ...

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. Therefore, this decrease in Pr/Ph should indicate an increase in salinity, instead of an increase in anoxia.

References

{{Reflist Alkanes Diterpenes