HYDROCARBONS

These compounds form the simplest form of lipids, they contain only carbon and hydrogen. They may be divided into aliphatic hydrocarbons with a carbon chain which may be linear (normal), branched, saturated (alkanes) or unsaturated (alcenes), cyclic hydrocarbons and carotenoids. Several hydrocarbons may be substituted with oxygen-containing groups.
These organic compounds include :

- ALKANES, ALKENES

- CYCLIC HYDROCARBONS

- CAROTENOIDS

- ALKANES - ALKENES

Living organisms, eukaryotic or prokaryotic, contain frequently hydrocarbons which are directly derived from fatty acids. They are known to be present in living matter since 1892 when Shall C (Chem Ber 1892, 25, 1489) identified undecane in ants, and Etard A (C R Acad Sci Paris 1892, 114, 364) identified eicosane in Bryonia dioica. They are distinct from the terpenoid hydrocarbons. They have usually a straight chain of up to about 36 carbon atoms but may also be branched, with one or more methyl groups attached at almost any point of the chain. Usually, the methyl group is near the end of the chain (iso or anteiso). They are either saturated or unsaturated (mono or diunsaturated). In contrast with the diversity of methyl-branched alkanes found in insect species, n-alkanes predominate in plants. Among the least polar components of plant surface lipids hydrocarbons with the odd number carbon chains (C15 up to C33) are predominant.
Many microalgae contain the highly unsaturated alkene n-C21:6 formed by decarboxylation of the 22:6n-3 fatty acid (Lee RF et al., Phytochemistry 1971, 10, 593). A few species also contain the n-C21:5 alkene (Volkman et al., Org Geochem 1994, 21, 407). Several microalgae were shown to contain long-chain unsaturated alkenes from 19 to 38 carbon atoms and one to four double bonds (review in Volkman JK et al., Org Geochem 1998, 29, 1163).

Allenic hydrocarbons, such as 9,10-tricosadiene, 9,10-pentacosadiene, and 9,10-heptacosadiene were isolated from Australian insects (melolonthine scarab beetles) (Dembitsky VM et al., Prog Lipid Res 2007, 46, 328).

Hydrocarbons are found at the outer surface in higher plant leaves, in insects and several marine organism. They are thought to serve as a barrier to water influx in the organism, to act as sex attractants (or anti-aphrodisiacs), to affect the absorption of chemicals and microorganisms. These molecules are found mainly in petroleum. Several hydrocarbons (octane, nonane, dodecane, hexadecane...) belong to aroma compounds which are found in environmental or food systems (see the website Flavornet).

Various hydrocarbons are present in the photosynthetic prokaryotes but in low concentrations. Most species have from 15 to 20 carbon atoms, heptadecane being by far the most predominant in all species. In some species, mono- or di-unsaturated chains (alcenes) were found, in others (Cyanobacteria) methyl-branched alkanes are present.

Hydrocarbons are also formed as products of fatty acid cleavage during peroxidation processes. Alkanes as well as alkenes appear during hydroperoxide decomposition.

The normal paraffins : their general formula is CH₃(CH₂)_nCH₃ (n= 6 - 40 or greater).
Paraffins may have branched chains :

- several methyl groups (multibranched), one for each unit deriving from the isoprene formula : CH₂=C(CH₃)-CH=CH₂. Hydrocarbons formed of isoprene units belong to the large group of terpenes.

The widespread use of pristane as a biological marker is related to its structural similarity to phytol and its apparent stability, in connection with inability of microorganisms to carry out its anaerobic destruction. pristane is present in photosynthetic organisms, it has been detected in bacteria, algae, and higher plants. Marine sources of pristane include zooplankton, lobster, fish, sharks, sperm whale. Fossil fuels such as coal and petroleum contain this compound. The stable structure persists even in Precambrian rocks and perhaps in extraterrestrial meteorites. The coexistence of microfossils with pristane and phytane in Precambrian rocks is significant to the paleobotanist. Despite this inertness, pristane can be utilized as the sole source of carbon and energy for growth of a coryneform soil isolate ( McKenna EJ et al., PNAS 1971, 68, 1552).
Crocetane is formed by four isoprene units arranged symmetrically around a tail-to-tail linkage. It was initially synthesized and named by Karrer et al. (Helv Chim Acta 1930, 13, 707). Crocetane is mainly present in oils and in methane-rich sediments, as it was shown to be produced by microorganisms utilizing methane as their carbon source. 2,6,10,15,19-Pentamethylicosane differs from crocetane by the addition of a single isoprene unit, joined head to tail, at one end of the molecule. This compound is present in methane-rich sediment and is likely produced by anaerobic methanotrophs.
Phytane and biphytane, present in sediments and petroleum, are thought to derive from ether-lipids (archaeol, caldarchaeol) of Archaea, the only organisms known to possess such structures.

The separation of straight chain and branched chain alkanes is efficient on a micro scale using the urea complex formation as described for fatty acids (Xu S et al., Org Geochem 2005, 36, 1334). That efficient method is based on the urea inclusion directly on the TLC plates and successive elutions with two solvents to separate straight and branched alkanes.

One important member of isoprenoid polyenes is squalene (C₃₀H₅₀) which is a metabolic precursor of sterols and steroids and classified into the triterpenoids. It is also a component of sebaceous lipids (12-15% of sebum weight) found on human skin. It consists of 6 isoprene units and contains 6 trans double bonds. It was discovered in 1906 in shark oil (Tsujimoto M, J Soc Chem Ind Jpn, 1906, 9, 953). It was suggested that squalene and its peroxidized derivatives (6 are possible) occurring by UV irradiation have an important role in the occurrence of sunburn, and/or protection from sunburn skin damage (Ohsawa K et al., J Toxicol Sci 1984, 9, 151). Furthermore, it has been suggested that squalene peroxides may play an important part in the pathology of acne, pityriasis versicolor, and skin aging. There is some evidence that squalene reduces colon cancer (Rao et al., Carcinogenesis 1998, 19, 287) and skin cancer (Owen R et al., Food Chem Toxicol 2000, 38, 647).

Presqualene diphosphate

Resveratrol

Several forms of phenanthrenes are present in in higher plants, mainly in Orchidaceae family but also in Dioscoreaceae, Combretaceae, Euphorbiaceae, and Hepaticae. The original phenanthrene molecule may be substituted in various positions by hydroxyl, methoxyl, methyl and/or prenyl groups. Most of them are monomeric but some dimeric and trimeric forms were described. As an example, the compound below (denthyrsinin) is reported in the orchid species Cymbidium pendulum, Dendrobium spp, Eulophia nuda, Nidema boothii, Scaphyglottis livida, Thunia alba. That compound, as others, displayed potent cytotoxic activities (review in Kovacs et al., Phytochemistry 2008, 69, 1084).

Denthyrsinin

-- CAROTENOIDS

The nature of these compounds was discovered during the 19th century. In 1831, Wachen roder H. proposed the term "carotene" for the hydrocarbon pigment he had cristallized from carrot roots. Berzelius J. called the more polar yellow pigments extracted from autumn leaves "xanthophylls" and Tswett M., who separated many pigments by column chromatography, called the whole group "carotenoids".

Among this important group, the numerous compounds consist of C40 chains (tetraterpenes) with conjugated double bonds, they show strong light absorption and often are brightly colored (red, orange). They occur as pigments in bacteria, algae and higher plants. Carotenoids perform three major functions in plants : accessory pigments for light harvesting, prevention of photooxidative damage and pigmentation attracting insects.
The hydrocarbon carotenoids are known as carotenes, while oxygenated derivatives of these hydrocarbons are known as xanthophylls.
Carotenoids are important components of the light harvesting in plants, expanding the absorption spectra of photosynthesis. The major carotenoids in this context are lutein, violaxanthin and neoxanthin. Additionally, there is considerable evidence which indicates a photoprotective role of xanthophylls preventing damage by dissipating excess light. In mammals, carotenoids exhibit immunomodulatory actions, likely related to their anticarcinogenic effects. b-Carotene was thus shown to enhance cell-mediated immune responses (Hughes DA, Proc Nutr Soc 1999, 58, 713).

Carotenoids consist of eight isoprenoid units joined in such a manner that the arrangement of isoprenoid units is reversed at the center of the molecule so that the two central methyl groups are in a 1,6-position relationship and the remaining non-terminal methyl groups are in a 1,5-position relationship. They are, by far the predominant class of tetraterpenes. They may be also classified in the terpenoids.
Carotenoids can be considered derivatives of lycopene, found in tomatoes, fruits and flowers. Its long straight chain is highly unsaturated and composed of two identical units joined by a double bond between carbon 15 and 15'. Each of these 20 carbon units may be considered to be derived from 4 isoprene units.

Carotenoids may be acyclic (seco-carotenoids) or cyclic (mono- or bi-, alicyclic or aryl). Oxyfunctionalization of various carotenoids leads to a large number of xanthophylls in which the function may be a hydroxyl, methoxyl, carbonyl, oxo, formyl or epoxy group.

Only some of the most common carotenes and xanthophylls are given below:

Among the represented molecules, b-carotene is probably the most important as a precursor of vitamin A by central cleavage into retinol and some retinoic acid (b-carotene provides about 40% of human dietary retinol equivalents).

In human serum several carotenes and xanthophylls have been detected. If a- , b-carotene and lycopene are frequently quoted in specialized papers, some others are now determined with precise HPLC methods (lutein, zeaxanthin, cantaxanthin and b-cryptoxanthin). These compounds originate from ingested fruit, green leaves, berries and yellow corn.

Two cycloaddition products of trans-violaxanthin with a-tocopherol have been isolated from seeds of Pittosporum tobira and their structures elucidated (Fujiwara Y et al., Tetrahedron Lett 2001, 42, 2693). These C69 carotenoids were named pittosporumxanthins. Their global structure is given below.

Pittosporumxanthin

Further investigations have revealed the existence of six other forms based on addition of a-tocopherol with antheraxanthin, neoxanthin or violaxanthin (Maoka T et al., J Nat Prod 2008, 71, 622).

Fucoxanthin, as an allenic carotenoid with a 5,6-monoepoxide group, is one of the most abundant carotenoid in brown algae and diatoms. This compound contributes more than 10% of the estimated total production of carotenoids in nature. Fucoxanthin was first isolated by Willstätter R et al. (Ann 1914, 404, 237).

This carotenoid has shown interesting biological properties, such as anticarcinogenetic effects, anti-inflammatory effects and radical scavenging activity. Furthermore, it has been shown that fucoxanthin has an anti-obesity effect in connection with the expression of the uncoupling protein UCP1 in white adipose tissue (Maeda H et al., Biochem Biophys Res Comm 2005, 332, 392). Its ability to enhance the docosahexaenoic acid concentration in the liver of treated obese mice remains to be examined more thoroughly (Tsukui T et al., J Agric Food Chem 2007, 55, 5025).

Neoxanthin is another common allenic carotenoid, widely distributed in higher plants and algae, which was first isolated from the green leaves of barley in 1938 by Strain HH. 

Neoxanthin is thought to be a part of the light harvesting complexes and as a precursor to the plant growth hormone abscisic acid.

More than 40 allenic carotenoids have been described in vegetals and accumulated in animals (Dembitsky VM et al., Prog Lipid Res 2007, 46, 328).

 That new molecule was shown to be strongly cytotoxic toward cultured human colon tumor cells.

Although green leaves contain unesterified hydroxy carotenoids, most carotenoids in ripe fruit are esterified with fatty acids. However, those of a few fruits, particularly those that remain green when ripe, such as kiwi, undergo limited or no esterification. Carotenoid mono- and diesters were identified in mandarin essential oil (Giuffrida D et al. Flavour Fragr J 2006, 21, 319), in Haematococcus pluvialis (Breithaupt DE et al., J Agric Food Chem 2004, 52, 3870) and in the Antarctic krill Euphausia superba (Takaichi S et al. Comp Biochem Physiol B 2003, 136, 317).
Carotenoids fatty acid esters were also descried in the carapace of the spiny lobster Panulirus japonicus (Maoka T et al., J Oleo Sci 2008, 57, 145). Astaxanthin, adonixanthin, and pectenolone were esterified with fatty acids with a large range of chain length and unsaturation.
Several esterified seco-carotenoids, the tobiraxanthins, have been isolated from the seeds of Pittosporum tobira (Fujiwara Y et al., Tetrahedron Lett 2002, 43, 4385). Two fatty acid molecules (lauric or myristic acid) acylated the carotenoid part. One of these compounds is shown below (Tobiraxanthin A1).

Apocarotenoids are carotenoid derivatives formed by the removal of fragments of the carbon backbone from either or both ends of a C40 precursor such as lycopene or beta,beta-carotene. These modification originate in the oxidative degradation at the level of the terminal rings. They may be the result of nonspecific mechanisms (lipoxygenase, photo-oxidation) as well as of specific mechanisms (dioxygenases). They have significant roles as developmental and environmental response signals. They also make important contributions to flavor and nutritional quality of foods (fruits, tea, wine) and tobacco.

Two well-known apocarotenoids are bixin and crocetin which have economic importance as pigments and aroma in foods.

 Bixin (cis- or trans-bixin) is one of several highly colored molecules extracted from annatto seed coats (Bixa orellana) and used as a food coloring and cosmetic compound since pre-Colombian times. It was determined that lycopene is the precursor of bixin (Bouvier F et al., Science 2003, 300, 2089). Crocetin, occurring as various glycosyl esters, is the coloring principle of saffron, pollen harvested from Crocus sativus. This apocarotenoid originates in the rupture of zeaxanthin (Bouvier F et al., Plant Cell 2003, 15, 47). Two other C26 and C30 apocarotenoids have been characterized in the seeds of Ditaxis heterantha (a native Euphorbiaceae of Mexico), heteranthin and ditaxin (Mendez-Robles MD et al., J Nat Prod 2006, 69, 1140). The presence of these products explains the use of the Ditaxis seeds as a natural food flavor and coloring. 

There is an enormous interest in apocarotenoids in detergent, food and perfume industry (Carotenoid-derived aroma compounds. Winterhalter P and Rouseff RL Eds, ACS Symp Series, v.802, 2002). 
It was shown that several volatile products are present after enzymatic degradation. Among them, the so-called norisoprenoids, the C13-cleavage products of carotenoids, have extremely low detection threshold values (some ng per liter of water for ionone and damascone). 

A peroxidase from edible fungi was shown to be a key enzyme able to degrade carotenoids into important flavor compounds (ionone, cyclocitral, terpineol....) (Zorn H et al., Biol Chem 2003, 384, 1049). A review of the oxidative remodeling of  plastid carotenoids initiated by specific dioxygenases has been released by Camara B et al (Arch Biochem Biophys 2004, 430, 16). 
Furthermore, apocarotenoids act as visual or volatile signals to attract pollinating agents, and are also key players in plant defense. Studies have shown that the loss of cleavage enzymes induces the development of axillary branches, indicating that apocarotenoids convey signals that regulate plant architecture (Bouvier F et al., Tr Plant Sci 2005, 10, 187).

It is generally accepted that oxidation of carotenoids begins with epoxidation and cleavage to apocarotenals prior a transformation into other derivatives. Several epoxycarotenoids and apocarotenals were observed in experimental oxidation models but also some were identified in processed foods (Rodriguez EB et al., Food Chem 2007, 101, 563). The two compounds shown below were detected in fruit extracts :

Another apocarotenal, trans-b-apo-8'-carotenal, is found in spinach and citrus fruits. It has a low pro-vitamin A activity and is used in pharmaceuticals and cosmetic products. This apocarotenal is also used as an additive (E160e) legalized by the European commission for human food.

Carotenoid glycosides

Carotenoids may be linked to a sugar by a glycosidic link or an ester link. Such compounds are known for plant and bacteria. As an example, a glucoside of rhodopsin, found in the photosynthetic part of Rhodopseudomonas acidophila,  is shown below.

Salinixanthin is a glycosylated and acylated carotenoid associated with the protein xanthorodopsine which was isolated from the extremely halophilic eubacterium Salinibacter ruber living in salt pond in Spain. This carotenoid has a keto group in the ring, a glycoside group and an acyl tail, probably immersed in the membrane (Balashov SP et al., Cell Mol Life Sci 2007, 64, 2323).

Astaxanthin diglucoside diesters have been determined in lipid extracts of the snow alga (Chlamydomonas nivalis) (Rezanka T et al., Phytochemistry 2008, 69, 479). The C-3 hydroxyl group of astaxanthin is glucosylated and the C-6 hydroxyl group of the glucosyl moiety is esterified with specific fatty acids. Among these fatty acids (14 species were detected), the most abundant were 16:3, 16:4, 18:1, 18:4 and 18:5. 

Carotenoids are said to have antioxidant properties, as tocopherols, and thus may prevent the oxidation of the lipid moieties of LDL (low density lipoproteins) which renders these lipoproteins atherogenic.

To know more about their antioxidant properties, consult the VERIS site

The web site of the International Carotenoid Society may be consulted for further information on carotenoids.

A very informative guide to carotenoid analysis in foods has been released by Rodriguez-Amaya DB.