DICARBOXYLIC  ACIDS

Although the dicarboxylic acids do not occur in appreciable amounts as components of animal or vegetal lipids, they are in general important metabolic products of fatty acids since they originate from them by oxidation. Dicarboxylic acids are suitable substrates for preparation of organic acids for the pharmaceutical and food industries.
They have the general type formula

It was shown that all these dicarboxylic acids are formed during the drying process of paint oils and that the determination of these decomposition products may be of value in determining the age of old samples. 

The higher weight dicarboxylic acids (n=10 to 21) are found in different plant lipids, particularly in what was named erroneously Japan wax (triglycerides containing C20, 21, 22 and 23 dicarboxylic acids besides normal fatty acids) from the sumach tree (Rhus sp.). Among them, Thapsic acid (n=14) was isolated from the dried roots of the Mediterranean "deadly carrot", Thapsia garganica (Umbelliferae), but others, as Brassylic acid (n=11), were prepared chemically from different sources.
Brassylic acid can be produced chemically from erucic acid by ozonolysis but also by microorganisms (Candida sp) from tridecane. This diacid is produced on a small commercial scale in Japan for the manufacture of fragrances.
A review on the applications and the industrial biotechnology of these molecules has been released by Kroha K (Inform 2004, 15, 568).

A large survey of the dicarboxylic acids present in Mediterranean nuts revealed unusual components (Dembitsky VM et al., Food Chem 2002, 76, 469). A total of 26 minor acids (from 2 in pecan to 8% in peanut) were determined : 8 species derived from butanedioic acid, likely in relation with photosynthesis, and 18 species with a chain from 5 to 22 carbon atoms. 

Higher weight acids (>C20) are found in suberin present at vegetal surfaces (outer bark, root epidermis).  C16 to C26 a,w-dioic acids are considered as diagnostic for suberin. With C18:1 and C18:2, their content amount from 24 to 45% of whole suberin. They are present at low levels (< 5%) in plant cutin, except in Arabidopsis where their content can be higher than 50% (Pollard Met al., Tr Plant Sci 2008, 13, 236).

The first allenic dicarboxylic acid, named glutinic acid (2,3-pentadienedioic acid) was isolated from Alnus glutinosa (Betulaceae) (Hans EA, Berich Deut Chem Ges 1908, 40, 4760).

It was shown that hyperthermophilic microorganisms specifically contained a large variety of dicarboxylic acids (Carballeira NM et al., J Bacteriol 1997, 179, 2766). This is probably the most important difference between these microorganisms and other marine bacteria. Dioic fatty acids from C16 to C22 were found in an hyperthermophilic archaeon, Pyrococcus furiosus. Short and medium chain (up to 11 carbon atoms) dioic acids have been discovered in Cyanobacteria of the genus Aphanizomenon (Dembitsky VM et al., Biochemistry (Moscow) 2001, 66, 72).

A monounsaturated dicarboxylic acid, traumatic acid, (10E-dodeca-1,12-dicarboxylic acid), was among the first biologically active molecules isolated from plant tissues (English J et al., Science 1939, 90, 329).  That dicarboxylic acid was shown to be a potent wound healing agent in plant that stimulates cell division near a wound site (Farmer EE, Plant Mol Biol 1994, 26, 1423), it derives from 18:2 or 18:3 fatty acid hydroperoxides after convertion into oxo fatty acids.

Traumatic acid

While polyunsaturated fatty acids are unusual in plant cuticles, a diunsaturated dicarboxylic acid has been reported as a component of the surface waxes or polyesters of some plant species. Thus, octadeca-c6,c9-diene-1,18-dioate, a derivative of linoleic acid, is present in Arabidopsis and Brassica napus cuticle (Bonaventure G et al., Plant J 2004, 40, 920).  

Dicarboxylic acids were shown in 1934 to be produced by w-oxidation of fatty acids during their catabolism (Verkade PE et al., Biochem J 1934, 28, 31). Thus, these authors have discovered that these compounds appeared in urine after administration of tricaprin and triundecylin. Although the significance of their biosynthesis remains poorly understood, it was demonstrated that w-oxidation occurs in rat liver but at a low rate, needs oxygen, NADPH and cytochrome P450. It was later shown that this reaction is more important in starving or diabetic animals where 15% of palmitic acid is subjected to w-oxidation and then to b-oxidation (Wada F et al. Biochim Biophys Acta 1977, 487, 261), this generates malonyl-coA which is further used in saturated fatty acid synthesis.

It was proposed recently that dicarboxylic acids are alternate lipid substrates in parenteral nutrition (Greco AV et al., Clin Nutr 1995, 14, 143). Basically, they are water soluble, undergo b-oxidation, do not induce ketogenesis but rather promote gluconeogenesis. They could represent an immediately available form of energy. Thus, inorganic salts of sebacic (C10)  and dodecanedioic (C12) acids were firstly proposed, but now, triglycerides containing these fatty acids are under investigation (Capristo E et al. Clin Chim Acta 1999, 289, 11). Treatment of rats with derivatives of  C16 dioic acid have shown that this compound markedly improved lipid metabolism (Russell JC et al., Arterioscl Thromb 1991, 11, 602) and inhibited the development of advanced cardiovascular disease (Russell JC et al., Arteriocl Thromb Vasc Biol 1995, 15, 918).

It must be recalled that the determination of the dicarboxylic acids generated by permanganate-periodate oxidation of monoenoic fatty acids was useful to study the position of the double bond in the carbon chain (Longmuir KJ et al., Anal Biochem 1987, 167, 213).

Branched-chain diacids

Long-chain dicarboxylic acids containing vicinal dimethyl branching near the centre of the carbon chain have been discovered in the genus Butyrivibrio, bacteria which participate in the digestion of cellulose in the rumen (Klein RA et al., Biochem J 1979, 183, 691). These fatty acids, named diabolic acids, have a chain length depending on the fatty acid used in the culture medium. The most abundant diabolic acid in Butyrivibrio had a 32-carbon chain length.

Diabolic acid (15,16-dimethyltriacontanedioic acid)

These diacids were also detected in he core lipids of the genus Thermotoga of the order Thermotogales, bacteria living in solfatara springs, deep-sea marine hydrothermal systems and high-temperature marine and continental oil fields (Huber R et al., Arch Microbiol 1986, 144, 324). It was shown that about 10% of their lipid fraction were symmetrical C30 to C34 diabolic acids. The C30 (13,14-dimethyloctacosanedioic acid) and C32 (15,16-dimethyltriacontanedioic acid) diabolic acids have been described in Thermotoga maritima (Caballeira NM et al., J Bacteriol 1997, 179, 2766).
Some parent C29 to C32 diacids but with methyl groups on the carbons C-13 and C-16 have been isolated and characterized from the lipids of thermophilic anaerobic eubacterium Themanaerobacter ethanolicus (Jung S et al., J Lipid Res 1994, 35, 1057). The most abundant diacid was the C30 a,w-13,16-dimethyloctacosanedioic acid.    

Biphytanic diacids are present in geological sediments and are considered as tracers of past anaerobic oxidation of methane (Birgel D et al., Org Geochem 2008, 39, 152). Several forms without or with one or two pentacyclic rings have been detected in Cenozoic seep limestones. These lipids may be unrecognized metabolites from Archaea.

Crocetin is the core compound of crocins (crocetin glycosides) which are the main red pigments of the stigmas of saffron (Crocus sativus) and the fruits of gardenia (Gardenia jasminoides). Crocetin is a 20-carbon chain dicarboxylic acid which is a diterpenenoid and can be considered as a carotenoid. It was the first plant carotenoid to be recognized as early as 1818 while the history of saffron cultivation reaches back more than 3,000 years.

Crocetin

The major active ingredient of saffron is the yellow pigment crocin 2 (three other derivatives with different glycosylations are known) containing a gentiobiose (disaccharide) group at each end of the molecule.

Crocin

A simple and speccific HPLC-UV method has been developed to quantify the five major biologically active ingredients of saffron, namely the four crocins and crocetin (Li N et al., J Chromatogr A 1999, 849, 349).

2 - Alkylitaconates

Several dicarboxylic acids having an alkyl side chain and an itaconate core have been isolated from lichens and fungi, itaconic acid (methylenesuccinic acid) being a metabolite produced by filamentous fungi.

Among these compounds, several analogues, called chaetomellic acids with different chain lengths and degrees of unsaturation have been isolated from various species of the lichen Chaetomella (two of them are shown below).

These molecules were shown to be valuable as basis for the development of anticancer drugs due to their strong farnesyltransferase inhibitory effects (Singh SB et al., Bioorg Med Chem 2000, 8, 571).

In 1999, a series of new fungal alkyl- and alkenyl-itaconates, ceriporic acids, were found in cultures of a selective lignin-degrading fungus, Ceriporiopsis subvermispora (Enoki M et al., Chem Lett 2000, 54-55, Amirta R et al., Chem Phys Lipids 2003, 126, 121). Two of them are shown below.

FATTY ACID CARBONATES

Carbonates (esters of carbonic acid, H₂CO₃) are well known to chemists as they represent an important class of organic compounds and among them oleochemical carbonates have interesting characteristics which make them candidates for many industrial applications. 
The most common carbonates have the following structure : RO-CO-OR
R is a linear chain with 8 to 18 carbon atoms, saturated or with one double bond (dioleyl carbonate), or a branched chain (ethylhexyl, butyloctyl or hexyldecyl).
They are miscible in organic solvents but insoluble in water. Unsaturation or branching on the alkyl chain lowers their melting point (Kenar JA, Inform 2004, 15, 580). 
The condensation of phosgene (ClCOCl) with an alcohol appears the most commonly used procedure to synthesize oleochemical carbonates.
The polar nature of the carbonate moiety enables it to adhere strongly to metal surfaces. Thus, they are used as lubricant components which have a protective property for metal corrosion. Some C8 to C18 carbonates have been exploited in personal-care products (sunscreen, cosmetics), dioctyl carbonate being also used as emollient or solvent in UV-filter solutions.
Extraction of metal ions (gold, silver, platinum) is improved by the use of the chelating properties of oleochemical carbonates when mixed with the metal-containing aqueous phase. Future developments will ensure a growing interest in these molecules. 

PHENYLALKANOIC ACIDS

Short chain w-phenylalkanoic acids have long been known to occur in natural products. Phenylacetic, 3-phenylpropanoic and 3-phenylpropenoic (cinnamic) acids were found in propolis, mammalian exocrine secretions or plant fragrances.
During a systematic study of the lipids from seeds of the plant Araceae, Schmid PC et al. (Phytochemistry 1997, 45, 1173) discovered the presence of 13-phenyltridecanoic acid as a major component (5-16% of total fatty acids). Other similar compounds but with 11 and 15 carbon chain lengths and saturated or unsaturated were shown to be also present but in lower amounts. At the same time, the even carbon chain w-phenylalkanoic acids of C10 up to C16 were discovered in halophilic bacteria (Caballeira NM et al., Lipids 1997, 32, 1271).

w-phenylalkanoic acid (x = 1 to 17)

Later, an exhaustive study of 17 genus of the subfamily Aroideae of Araceae revealed the presence of three major acids, 11-phenylundecanoic acid, 13-phenyltridecanoic acid and 15-phenylpentadecanoic acid in seed lipids (Meija J et al., Phytochemistry 2004, 65, 2229).  Other odd carbon number acids from C7 to C23 were detected but in trace amounts. Similarly, two series of homologous odd carbon number monounsaturated w-phenylalkanoic acids were found. 
Thus, it can be stated that all odd carbon chain w-phenylalkanoic acids from C1 through C23 have been found in nature. Furthermore, even carbon chain w-phenylalkanoic acids from C10 through C16 were also detected.

Substituted phenylalkenoic acids are periodically encountered in nature. As an example, rubrenoic acids were purified from Alteromonas rubra, compounds which showed bronchodilatatoric properties (Holland GS et al., Chem Ind 1984, 850).

Methyl phenylalkenoic acids (5 carbon chain) have been described from a terrestrial Streptomycete (Mukku VJ et al., Z Naturforsch 2002, 57b, 335).
Serpentene, a similar polyunsaturated phenylalkenoic acid, is also produced by Streptomyces and was shown to have some antibacterial properties.

Several serpentene-like compounds have also been isolated from the same bacterial source (Wenzel SC et al., J Nat Prod 2004, 67, 1631).

Several bicyclic derivatives of linolenic acid were shown to be generated by alkali isomerization  (Matikainen J et al., Tetrahedron Lett 2003, 59, 567).

Bicyclic hexahydroindenoic acid

Some others (alkyl-phenyl)-alkanoic acids) are formed when linolenic acid is warmed at 260–270°C (Hase A et al., JAOCS 1978, 55, 407).

Several forms with 16, 18 and 20 carbon atoms were identified in archaeological pottery vessels and were presumed to have been generated during heating triunsaturated fatty acids. They were used as biomarkers to trace the ancient processing of marine animal in these vessels (Hansel FA et al., Tetrahedron Lett 2004, 45, 2999).

FATTY ACYL-CoA ESTERS 

These fatty acid derivatives may be considered as complex lipids since they are formed of one fatty acid, a 3'-phospho-AMP linked to phosphorylated pantothenic acid (vitamin F) and cysteamine. However, to simplify the nomenclature and taking into account their metabolism, we classify them within the big group of the fatty acids and their simple derivatives rather than within the complex and phosphorylated lipids. 
Long-chain acyl-CoA esters are substrates for a number of important enzymatic reactions and play a central role in the regulation of metabolism as allosteric regulators of several enzymes. To participate in specific metabolic processes, fatty acids must first be activated by being joined in thioester linkage (R-CO-SCoA) to the -SH group of coenzyme A. The thioester bond is a high energy bond. 

R = fatty carbon chain

The activation reaction normally occurs in the endoplasmic reticulum or the outer mitochondrial membrane. This is an ATP-requiring reaction (fatty acyl-CoA synthase), yielding AMP and pyrophosphate (PP_i). Different enzymes are specific for fatty acids of different chain length. 
Then, the acyl CoA esters are transported in mitochondria. They are converted to fatty acyl carnitine by carnitine acyl transferase I, an enzyme of the inner leaflet of the outer mitochondrial membrane. Fatty acyl carnitine is then transported by an antiport in exchange for free carnitine to the inner surface of the inner mitochondrial membrane. There carnitine acyl transferase II reverses the process, producing fatty acyl-CoA and carnitine. This shuttle mechanism is required only for longer chain fatty acids. 
Once inside the mitochondrial matrix, the fatty acyl-CoA derivatives are degraded by a series of reactions that release acetyl-CoA and leads to the production of NADH and FADH2. There are four steps in fatty acid oxidation pathway; oxidation, hydration, oxidation, and thiolysis. It requires 7 rounds of this pathway to degrade palmitate (a C16 fatty acid).
A graphic chart of these important metabolic steps may be found in the BioCarta web site.

DIVINYL ETHER FATTY ACIDS

Fatty acid hydroperoxides generated by plant lipoxygenases from linoleic and linolenic acids are known to serve as substrates  for a divinyl ether synthase which produces divinyl ether fatty acids.
The discovery of that class of compounds dates back to 1972, when Galliard T et al. described the structures of two ether C18 fatty acids generated by homogenates of the potato tuber (Galliard T et al., Biochem J 1972, 129, 743). These compounds, named colneleic acid (from linoleic acid) and colnelenic acid (from linolenic acid), could be also produced in potato leaves and tomato roots by rearrangement of 9-hydroperoxides.

Isomers of colneleic and colnelenic acids were isolated from homogenates of leaves of Clematis vitalba (Ranunculaceae) (Hamberg M, Lipids 2004, 39, 565). 

Similarly, 13-lipoxygenase-generated hydroperoxides serve as precursor of other divinyl ether fatty acids which are produced in bulbs of garlic (Grechkin AN et al., FEBS Lett 1995, 371, 159) or Ranunculus leaves (Hamberg M, Lipids 1998, 33, 1061). These compounds were named etheroleic and etherolenic acids.

As infection of potato leaves leads to increased levels of divinyl ether synthase, it was suggested that this pathway could be of importance in the defense of plants against attacking pathogens (Göbel C et al., Biochim Biophys Acta 2002, 1584, 55).