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 :
- CYCLIC HYDROCARBONS
- CAROTENOIDS
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.
Among hydrocarbons, a variety of forms are described (saturated or unsaturated):
The normal paraffins : their general formula is CH3(CH2)nCH3 (n= 6 - 40
or greater).
Paraffins may have branched chains :
- one methyl group (monobranched), iso-branched hydrocarbons (methyl group on the second carbon) or anteiso-branched hydrocarbons (methyl group on the third carbon)
(as below)
- 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.
This chain type is frequently found in several lipid forms, either isolated or combined
with other chemical structures. A series of long-chain methylated alkanes (more than 23
carbon atoms), saturated or with one double bond, were identified in settling
particles and surface sediments from Japanese lakes and were shown to be
produced by planktonic bacteria being thus useful molecular markers (Fukushima
K et al., Org Geochem 2005, 36, 311).
It must be noticed that highly branched and unsaturated (2-5 double bonds)
isoprenoids are widespread components in marine sediments (review by Rowland
SJ et al., Marine Envir Res 1990, 30, 191; Belt ST et al.,
Geochim Cosmochim Acta 2000, 64, 3839). The identification of C25 and even
of C30 highly branched isoprenoid alkenes in diatoms (Johns L et al., Org
Geochem 1999, 30, 1471) have clearly established that they are the source of
these compounds found in sediments.
Among the saturated isoprenoids found in geological sediments and oils, the most
frequent are pristane (2,6,10,14-tetramethylpentadecane) and phytane
(2,6,10,15,19,23-hexamethyltetracosane). Both compounds can be generated
diagenetically from the phytol side chain of chlorophyll. Pristane may also
derive from the side chain of tocopherols while phytane is also generated by
Archaea.
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).
It must be mentioned that if squalene is found in large quantities (from
0.2 to 0.9 g per Kg) in some fish liver oils (shark), it is also found in olive oil
(its content may vary from
1 to 40 mg per Kg) where it is used to detect any adulteration. Other vegetable oils
contain only traces of this compound (from 0.02 up to 0.3 mg per Kg). Squalene
was also found in the epicuticular wax of fruit (grapefruit) and in the
hydrocarbon fraction of wheat.
It has been shown that presqualene diphosphate, intermediate between farnesyl
diphosphate and squalene, carries biological activity in human neutrophiles and
serves as a negative intracellular signal preventing superoxide anion generation
(Levy
BD et al., Nature 1997, 389, 985). An inhibition of phosphatidylinositol
3-kinase in the same cells was also demonstrated (Bonnans
C et al., J Exp Med 2006, 17, 203, 857). During that signaling step,
squalene diphosphate is transformed into the inactive monophosphate species (Fukunaga
K et al., J Biol Chem 2006, 281, 9490).

Presqualene diphosphate
Several branched-alkylbenzenes have been described in Archaebacteria such
as Thermoplasma and Sulfolobus. They have mostly two
methyl groups branched on a saturated chain of 9 to 12 carbon atoms. One of them
is shown below.

After the first hypothesis of a communication system (chemotaxis)
by Thuret MG for the fertilization of brown algae, 117 years were needed to know
the structure of the first algal pheromone (Müller DG et al., Science 1971,
171, 815). This compound, ectocarpene, is an unsaturated hexacyclic
hydrocarbon.

Thereafter, several
parent molecules (linear, tri-, penta- or hexacyclic) with different degrees of
unsaturation and chain lengths have been described in various algal species (Pohnert
G et al., Nat Prod Rep 2002, 19, 108). The C11-hydrocarbon ectocarpene was
also detected as component in the odor of ripening mango (Berger RG et al., J
Agric Food Chem 1985, 33, 232) but was also shown to be a metabolite in
leaves of the Asteraceae Senecio isatideus (Bohlmann F et al.,
Phytochemistry 1979, 18, 79). Several biosynthetic studies suggest
unsaturated fatty acids as precursors of these pheromone hydrocarbons (Pohnert
G et al., Nat Prod Rep 2002, 19, 108).
Among the polycyclic hydrocarbons, few groups are described in plants.
The stilbenes, sometimes classed into the polyphenol group, are present in several
vegetal sources. Several forms have been described, differentiated by various
substitutions including a glucidic group (glucosides). Resveratrol
(3,4',5-trihydroxystilbene) is the most studied because of its presence in
grapes and wine and its numerous pharmacological properties (anti-cancer,
antiviral, neuroprotective, anti-aging, and anti-inflammatory).

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
Many plants producing phenanthrenes are used in traditional
medicine, likely in connection with the cytotoxicity, anti-microbial,
spasmolytic, anti-allergic, anti-inflammatory activities of the natural
phenanthrenes present in these plants.
-- 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.

Xanthophylls
Lutein has its maximum absorption at 450 nm, cryptoxanthin at 453 nm and
zeaxanthin at 454 nm.
Lutein and zeaxanthin are the only xanthophylls found in human serum that are
present in retina, macula (the central region of the eye (they are frequently
referred to as macular pigment), and lenses. Thus, they are at
the origin of the name of the central part of the retina, macula lutea
(yellow spot).
Several nutritional investigations have suggested that lutein supplements
improved visual function and optical density in aged people (Olmedilla B et
al., Nutrition 2003, 19, 21).
Ketocarotenoids belong to the xanthophyll group and are quite unique in nature.
Among these compounds, echinenone and canthaxantin are abundant in cyanobacteria
are characteristic of cyanobacteria (Takaichi S et al., Cell Mol Life Sci
2007, 64, 2607).
Violaxanthin is an epoxidized derivative of antheraxanthin (hydroxylated
cryptoxanthin) and forms with zeaxanthin the xanthophyll cycle that is
said to protect the photosynthetic system of plants against damage by excess
light.
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).
An unusual carotenoid ester was identified in fresh mango lipids
as violaxanthin dibutyrate (Pott I et al., Phytochemistry 2003, 64, 825).
Another unusual acetylenic carotenoid was discovered and identified in a marine
sponge (Prianos osiros) (Rogers EW et al., J Nat Prod 2005, 68, 450).
That new molecule was shown to be strongly cytotoxic toward cultured human colon
tumor cells.
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).
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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.