DIACYLGLYCEROLS

These lipids (known also as diglycerides) are fatty acid diesters of glycerol and occur in two isomeric forms:

R and R' are saturated or unsaturated hydrocarbon chains.
Diacylglycerols are generated mainly during digestion in the stomach and in the duodenal part of the intestine. In man, the lingual lipase preferentially hydrolyzes the ester bond in the sn-3 position of the triacylglycerols, thus generating sn-1,2-diacylglycerols. These molecules are isomerized in acidic solution with generation of sn-1,3-diacylglycerols which again are hydrolyzed in monoacylglycerols commonly found in the stomach content. In the duodenum as in the test tube, the lipase hydrolyzes easily the triacylglycerol ester bonds at the sn-1 and sn-3 positions giving a mixture of sn-2,3 and sn-1,2-diacylglycerols which again undergo chemical isomerization. In vitro, the isomerization is subject to both acid and base catalysis, and is particularly troublesome in the preparation of 1,2-diacylglycerols.
This process is used to do stereospecific analyses of triacylglycerols. More accurately, a non specific procedure is now used to generate diacylglycerols: the reaction with the Grignard reagent ethyl magnesium bromide which give all the possible combinations of diacylglycerols.
The abiotic synthesis of diacylglycerols has been shown to be possible in the laboratory under simulated hydrothermal conditions (Rushdi AI et al., Orig Life Evol Biosph 2006, 36, 93). These results indicate that condensation reactions under these primitive physical conditions may be the source of compounds for abiotic membranes which are candidates for micelle/vesicle formation at the beginning of life.

In cell biology, diacylglycerols are generated from phospholipids, the acyl chains being in the sn-1 and 2 positions. In lipid extracts, sn-1,3-diacylglycerols are always present, even in lower amounts than the other form, because of an artefactual isomerisation during isolation and purification. In cell extracts, diacylglycerol molecules have generally two different fatty acid chains, the most unsaturated being found at the sn-2 position.
Diacylglycerols are important intermediates in the biosynthesis of triacylglycerols and phospholipids and play a fundamental role in the cellular signaling. They are able to activate cellular mechanisms directly via protein activation or indirectly via the liberation of fatty acids which may be metabolized in agonist molecules (eicosanoids). The most famous molecular species of diacylglycerol generated by phospholipase C from phosphatidyl inositol is 1-stearoyl-2-arachidonoyl-sn-glycerol. Diacylglycerols are recognized by a well conserved protein motif, the C1 domain. It was observed for the first time in protein kinases C but is present also in many other enzyme proteins. Furthermore, diacylglycerols are able to modulate the biophysical properties of biomembranes (Gomez- Fernandez JC et al., Chem Phys Lipids 2007, 148, 1).

In bacteria, diacylglycerol is found as a component of the Braun's lipoprotein which is linked to a large peptidoglycan (murein lipoprotein) and forms the cell envelope of Gram-negative and positive bacteria (review in Hayashi S et al., J Bioenerg Biomemb 1990, 22, 451). The murein lipoprotein of Escherichia coli was discovered by Braun V et al. (Eur J Biochem 1969, 10, 426). They elucidate the structure of the novel lipo-amino acid at the amino terminus of the protein as N-acyl diglyceride-cysteine, i.e., glycerylcysteine containing two ester-linked fatty acids and one amide-linked fatty acid (Hantke K et al., Eur J Biochem 1973, 34, 284). The ester-linked fatty acids are similar to those present in bacteria phospholipids, mainly palmitic acid and cyclopropane fatty acids. The biosynthesis of Braun's lipoprotein begins with the addition of glycerol, from phosphatidylglycerol, to a cystein residue of a prolipoprotein, followed by the acylation of that glycerol. A cleavage by a peptidase generates a free-form lipoprotein (58 amino acids in E. coli) which will be finally linked to a peptidoglycan.

Mature Braun's lipoprotein

It has been shown that Braun's lipoprotein was a fully active endotoxic agonist for endothelial cells (Neilsen PO et al., J Immunol 2001, 167, 5231). 

Diacylglycerols, which are present at low leveis in edible oils (1-6 %), have been used by the food industry as emulsifiers for some time. However, the Kao Corporation introduced a DAG cooking oil in 1999 that contains more than 80% DAG. The DAG in these oils have fatty acids mainly in the 1,3 configuration and are the main components of new cooking oils (ex: Econa, Kao corp., Enova). These oils are proposed to slow the increase of postprandial blood triglycerides to help prevent the accumulation of body fat and high blood cholesterol levels by increasing energy expenditure (review in Yanai H et al., Nutr J 2007, 6:43). In 2000, the FDA granted Kao generally recognized as safe (GRAS) status for their product. Enova has been also ruled safe for human consumption by the Commission of the European Communities in 2006. In the United States, Enova oil is commercially available and may be used in home cooking oil and vegetable oil spreads. Studies indicate that body weight and body fat losses may occur when consuming 10 to 25 grams of Enova oil per day in place of conventional oil. A review of the nutritional characteristics of DAG oil has been released by Flickinger BD (Flickinger BD et al., Lipids 2003, 38, 129).

It is well known since 1978 that when foodstuffs are heated, incorporated fats generate glycerol, various acylglycerols and with chloride ions they form mainly 3-chloropropane-1,2-diol esters (Svejkovska B et al., Czech J Food Sci 2004, 22, 190). These compounds are similar to diacylglycerols but with a chlorine atom at the carbon-3 position.

Studies have shown that free 3-chloropropane-1,2-diol (3-MCPD or chlorinated glycerol) is present at only very low levels in most foodstuffs (10-80 mg/kg), while the major part is ester-linked with two fatty acids (up to 6 mg/kg). In refined fats and oils, concentrations of these contaminants can be higher (up to 20 mg/kg) since they are formed during the deodorization step. Based on several toxicological studies, the European Commission has adopted a regulatory limit of 20 m/kg for 3-MCPD in some foodstuffs. At present, no information are given on the toxicity of the fatty esters (Weisshaar R, Eur J Lipid Sci Technol 2008, 110, 671).

Cyclic glycerol esters (tethered lipids) have been isolated from the fruits of the Mediterranean plant Thapsia garganica (Apiaceae) (Liu H et al., J Nat Prod 2004, 67,1439). These diacylglycerols have two of the oxygens of glycerol included in a macrocyclic ring "macrocyclic lipids", the cyclic chain having 18 carbon atoms.

These tethered lipids are unprecedented in the plant kingdom.

Several terpenoic diacylglycerols have been isolated from nudibranch mollusks. Thus, in Archidoris montereyensis, diacylglycerols with the structure of the terpenoic monoglycerides, also described in these animals, but with acetylation of one of the free hydroxyl groups of the glycerol molecule (Gustafson K et al., Tetrahedron 1985, 41, 1101) . Similar acetylated compounds, but with the double bond confined in the B-ring between the carbon 5 and 6, were isolated in Archidoris verrucosa (Cimino G et al., Tetrahedron 1988, 44, 2301). These compounds, which were named verrucosin A and B, may contribute to the survival of the nudibranch in predator-rich areas as they are highly ichthyotoxic, others having feeding-deterrent properties. 

Verrucosin A (R₁=H, R₂=COCH₃) and B (R₁=COCH₃, R₂=H)

An unusual diacylglycerol, archidorin, has been isolated from the mantle of Archidoris tuberculata (Cimino G et al., J Nat Prod 1993, 56, 1642). Glycerol is esterified with tiglic acid (2-methylbut-2-enoic acid) and a diterpenoid clerodane acid at positions sn-1 and sn-2, respectively. Further studies on the Antarctic nudibranch (Austrodoris kerguelenensis) have characterized similar structures but with a diterpenoid moiety (of the clerodane or halimane type) at C-2 of glycerol and linked to an acetyl group at C-1 or C-3 (Gavagnin M et al., Tetrahedron 2003, 59, 5579).

In vegetals, phenolic diacylglycerols have been described. Thus, 1,3-diferulylglycerols have been described in the fruits of Aegilops, a Gramineae which is considered as the ancestors of the cultivated wheat (Cooper R et al., Phytochemistry 1978, 17, 1673). In one form, the two ferulic groups are methylated (see figure below), in the other, only one ferulic group is methylated.

1,3-Diferulylglycerol

That compound and other phenolic glycerides have been isolated from bulbs of Lilium auratum (Shimomura H et al., Phytochemistry 1987, 26, 844). These compounds were identified as 1,2-diferuloylglycerol, 1-feruloyl-2-coumaroylglycerol, 1-coumaroyl-2-feruloylglycerol. These diglycerides have an original structure, with the aromatic acid moieties located at positions C-1 and C-2.

Ferulyl monooleine, a useful sunscreen ingredient, has been produced by lipase-catalyzed transesterification of ethyl ferulate with trioleine (Compton DL et al., JAOCS 2000, 77, 513).

DIALKYL GLYCEROLS

While eubacteria, plants and animals contain largely diacylglycerol-derived lipids, unusual dialkylglycerols are found in the membranes of many thermophilic bacteria. 
Non-isoprenoid dialkyl glycerol tetraether have been detected in peats but their biological origin is as yet unknown (JS Sinninge Damste et al., 2000, Chem Commun 1683). Similar structures have been described in the genus Thermotoga, thermophillic bacteria of the order Thermotogales (Sinninghe Damste JS et al., Arch Microbiol 2007, 188, 629). These bacteria are commonly found in solfatara springs and in deep-sea marine hydrothermal systems. These lipids consist of two glycerol units connected by two (C32) dimethyl chains derived from diabolic acid (15,16-dimethyltriacontanedioic acid), this acid being also present in these bacteria.

Dialkyl glycerol tetraether

Other parent compounds are also present in Thermotogales, they have one to three ester bonds instead of ether bonds. As these bacteria appeared early during the evolution of life on Earth, it may be hypothesized that ether and ester glycerol membrane lipids developed relatively early during evolution.
It must be emphasized that the stereochemistry of the glycerol backbone is 1,2-di-O-alkyl-sn-glycerol.

More branched lipids have been described in Archaea (previously Archaebacteria), organisms living in extremely halophilic conditions or in hot and acid springs in deep oceanic waters (Kates M Prog Chem Fats Lipids 1978, 15, 301; Kates M et al., The biochemistry of archaea, Elsevier 1993). These lipids consist of derivatives of a C20,C20-isopranyl glycerol diether (diphytanylglycerol diether) and its dimer (dibiphytanyldiglycerol tetraether). The O-alkyl chains have a branched structure deriving from phytanic acid. In contrast with the structures observed in bacteria, the stereochemistry of the glycerol backbone in Archaea is 2,3-di-O-alkyl-sn-glycerol.
The simplest structure (diphytanylglycerol diether) is now called archaeol. This molecule structure, discovered in extremely halophilic bacteria (Kates M, Prog Chem Fats other Lipids 1978, 15, 301), is found either free or in phospholipid-bound forms.

The more complex tetraether structures (dibiphytanyldiglycerol tetraether) known as caldarchaeol, have been found in membranes from thermoacidophiles and methanogenic bacteria (Langworthy TA, Biochim Biophys Acta 1977, 487, 37).

This structure may also contain one to four cyclopentane rings in each of the C40 biphytanyl groups. These tetraethers (crenarchaeols) are present in marine crenarchaeota, an ubiquitous and abundant component of plankton. It has been shown that the number of cyclopentane rings is linearly correlated to the water temperature (Schouten S et al., Earth Planet Sci Lett 2002, 204, 265).. 
It is noticeable that extreme halophiles contain only archaeol-derived lipids, the methanogens bacteria contain both archaeol- and caldarchaeol-derived lipids, and the thermoacidophiles contain largely caldarchaeol-derived lipids.
Unsaturated terpenoid chains were found in Thermococcus hydrothermalis, an Archaea living in deep-sea hydrothermal vents (Lattuati A et al., Lipids 1998, 33, 319).

These lipids have been discovered in peat bogs and soils and a survey of their distribution in various locations has shown that they can be used in palaeoenvironmental studies to estimate annual mean air temperature and soil pH (Weijers J et al., Geochem Cosmochim Acta 2007, 71, 703).