Methylene-interrupted unsaturated acids

Cooper and Anders (20) reported the HPLC analysis of unsaturated C l8 and C20 fatty acids. Since the methylene-interrupted polyunsaturated acids show no specific UV absorption, the 2-naphtacylesters were prepared for UV detection at 254 nm [the column was a 3-ft X 0.07-in.-lD stainless steel tube packed withCORASIL-Ci8, methanol/water(85 15) served as the eluent, and a flow rate of 12 ml/h was obtained at a pressure of 300 psig]. The lower detection limit was 4-90 ng of ester. 

Fatty Acid Composition by GLC (Ce lc-89) measures the fatty acid composition and levels of trans unsaturation and cis, cis methylene-interrupted unsaturation of vegetable oils using capillary gas liquid chromatography. 

Fatty acids with trans or non-methylene-interrupted unsaturation occur naturally or are formed during processing for example, vaccenic acid (18 1 Hr) and the conjugated linoleic acid (CLA) rumenic acid (18 2 9tllc) are found in dairy fats. Hydroxy, epoxy, cyclopropane, cyclopropene acetylenic, and methyl branched fatty acids are known, but only ricinoleic acid (12(/f)-hydroxy-9Z-octadecenoic acid) (2) from castor oil is used for oleochemical production. OUs containing vernolic acid (12(5),13(/ )-epoxy-9Z-octadecenoic acid) (3) have potential for industrial use. 

Also identified are about thirty polyene acids in which unsaturation is not completely methylene-interrupted. These acids are probably derived from the more common monoene and polyene acids by insertion of an additional double bond, most often at A5 (though sometimes A2 or 3). Subsequent chain extension may shift these double bonds to the A7 or A9 positions. Such acids occur in some seed oils, in some micro-organisms and in some marine lipids (especially sponges). This last source, for example, has furnished 26 3 (5,9,19), 28 3 (5,9,19) and 30 3 (5,9,23) which may be produced from hexadec-9-enoic, octadec-9-enoic and hexadec-9-enoic acids respectively (Litchfield et al., 1980). 

The movement of double bonds in long-chain unsaturated acids has been known since Varrentrapp converted oleic acid to palmitic by fusion with alkali, an observation which led to the invalid conclusion that oleic acid was the A2 or A3 acid. Double bond migration unaccompanied by chain fission occurs under milder conditions. The reaction also occurs more easily with methylene-interrupted polyene acids to give products with conjugated unsaturation which are easily recognized by ultraviolet spectroscopy (Section 9.3). 

Sometimes 5,9 unsaturation is replaced by 5,11 unsaturation or the 5,9 system can be elongated to 7,11 etc. See also non-methylene-interrupted polyene acids. 

Poiyunsaturated fatty acids (often abbreviated to PUFA) of animai origin can be subdivided into families according to their derivation from specific biosynthetic precursors, in each instance, the famiiies contain from two up to a maximum of six c/s-doubie bonds, separated by singie methyiene groups (methylene-interrupted unsaturation), and they have the same terminal structure. A list of some of the more important of these acids is contained in Table 2.3. 

There have been few detailed studies of the fatty acid compositions of echinids, but these animals often have high levels of polyunsaturated acids, particularly arachi-donic acid (20 4n-6) and EPA (20 5n-3). They also contain several non-methylene interrupted dienoic acids (NMIDs) in which the double bonds are not separated by a methylene (Joseph, 1989, pp. 90-100), but the most original finding may be that of a series of 2-hydroxy acids with 22, 23 or 24 carbon atoms, saturated and mono-unsaturated (n-9). Table 26.4 shows the major fatty acids (>1%) of Strongylocentrotus droebachiensis (Joseph, 1989) and Tripneustes esculentus (Carballeira, Shalabi, and Reyes, 1994). 

Smith et al. (1978) have described a procedure for the GLC determination of cis and trans isomers of unsaturated fatty acids in butter after fractionation of the saturated, monoenoic, dienoic, and polyenoic fatty acid methyl esters by argentation TLC. Total trans acids were much higher, as measured by infrared spectrophotometry than by GLC, probably because some of the acids could have two or more of the trans bonds designated as isolated by infrared spectrophotometry. Enzymatic evaluation of methylene-interrupted cis, cis double bonds by lipoxidase resulted in lower values than those obtained by GLC. The authors mention that the lipoxidase method is difficult, requiring considerable skill, and suggest that their method is suitable for the determination of the principal fatty acids in complex food lipids such as bovine milk fat. 

The unsaturated fatty acids may have one double bond (monosatu-rated) or have more than one cis-methylene interrupted double bond (polyunsaturated) as illustrated in Fig. 4.1. 

The predominant fatty acids (FA) present in soybeans are palmitic (16 0), stearic (18 0), oleic (18 1), linoleic (18 2), and linolenic (18 3). Table 7.1 shows the common name, the systematic name, structure, and abbreviation of these FA. Other FA, such as arachidic (20 0) and behenic (22 0) acids, are present in minor quantities (<1%). The FA consist of a hydrocarbon chain with a carboxylic group at position 1 and typically contains an even number of carbon atoms. This molecule also can include points of unsaturation (double bonds). Palmitic acid, with 16 carbons, and stearic acid with 18 carbons, are the typical saturated (no double bonds) FA in soybean oil. Oleic, linoleic, and linolenic acids have 18 carbon atoms, with one, two, and three double bonds, respectively. In all the cases, the geometric configuration of the double bonds in the native oil is as, and when there is more than one double bond, they are methylene interrupted (Fig. 7.1). 

Reviews of fungal fatty acid composition (, 7, ) reveal that their primary constituents are 12- to 20-carbon chain length unbranched compounds, with even-numbered chains predominant. Both saturated and unsaturated compounds occur, with palmltolelc (C-16 l), oleic (C-18 l), llnolelc (C-18 2) and llnolenlc (C-18 3) acids the most common unsaturated moieties ( ). As with most naturally occurring fatty acids (9), monounsaturated compounds usually contain a els oleflnlc bond and polyunsaturated acids have methylene-interrupted els double bonds. Although rare In occurrence and subjected to limited study, branched chains, hydroxy, oxo and epoxy acids are also synthesized ( 5). Lists of the structures of unusual fatty acids which can be employed In structure activity studies are presented In detail elsewhere (9-14). 

This reaction was initially reported by Grundmann in 1936. It is the conversion of acyl chloride into aldehyde with the exact same carbon skeleton via the following consecutive steps a) treatment of acyl chloride with diazomethane to form a ketone, (b) conversion of such a ketone into ketol acetate with acetic acid, (c) reduction of ketol acetate with aluminum isopropylate, and d) hydrolysis and oxidation with lead tetraacetate. This method is especially useful in the preparation of aliphatic aldehydes with methylene-interrupted double bond(s). Although polymers might form in the preparation of highly unsaturated aldehydes during the reduction with aluminum isopropylate, the reduction from lithium aluminum hydride can eliminate such drawbacks. ... 

The acetylenic acids are conveniently divided into those with only one triple bond and those with more than one. In the former group of acids one or more double bonds may also be present and unsaturation frequently appears in the conventional methylene-interrupted pattern. 

Acetonitrile, when used as a reagent for chemical ionization in a bench top ion trap mass spectrometer, has been shown to provide complete information on double bond positions in homoallylic (i.e., methylene-interrupted double bonds) unsaturated fatty acid methyl esters (FAME) (4-6). The m/z 54 ion (l-methyleneimino)-l-ethenylium (MIE) is formed from acetonitrile in the ion trap (7) and covalently adds across C-C double bonds of FAME. Isolation and collisional dissociation (i.e., MS/ MS) of the resulting [M+54] " ion yields fragments that are indicative of the location of the added MIE in the parent, and allows unambiguous assignment of double bond position. As MIE adds to any of the C-C double bonds, more fragments are observed... 

Although the number of fatty acids detected in plant tissues approaches 300, most of them only occur in a few plant species (Hitchcock and Nichols, 1971). The major fatty acids are all saturated or unsaturated monocarboxylic acids with an unbranched even-numbered carbon chain. The saturated fatty acids, lauric (dodecanoic), myristic (tetradecanoic), palmitic (hexadeca-noic), and stearic (octadecanoic), and the unsaturated fatty acids, oleic (cis-9-octadecenoic), linoleic (c/5 -9,cw-12-octadecadienoic), and linolenic (all-cij-9,12,15-octadecatrienoic (Table I), together account for almost all of the fatty acid content of higher plants. For example, about 94% of the total fatty acids of commercial oils and 89-97% of leaf fatty acids consist of these seven structures alone. It will be noted that the unsaturated acids all contain a cis-9 double bond and that the polyunsaturated acids contain a methylene-interrupted structure. The four saturated fatty acids differ from each other by two carbons. These structural relationships are due to the principal pathways of fatty acid biosynthesis in plants (see Stumpf, this volume. Chapter 7). 

It is probable that our ancestors of several million years ago developed the characteristics leading to our modem biochemistry by eating animal fats (Crawford and Marsh, 1989 Sinclair and O Dea, 1990 O Dea, 1991). At first glance this should simplify discussion of animal fats, as shown by the basic fatty acids of Table 10.1. A popular shorthand notation is used to indicate the stmctures of common fatty acids. In the format x yn-z, x is the chain length or number of carbons in the chain, y is the number of methylene-interrupted cis ethylenic bonds and z is the inclusive number of carbon atoms from the terminal methyl group to the center of the nearest bond. As few as six fatty acids appear to adequately describe animal depot fats. Those fats listed are dominated by two fatty acids, 16 0 (palmitic) and 18 1 (oleic) add. Although tropical seed oils may be rich in C12-C18 saturated fatty adds (Elson, 1992), temperate oilseeds are rich in oleic acid and tend to include quantities of two fatty acids more unsaturated than oleic, especially 18 2n-6 (linoleic), and sometimes 18 3n-3 (linolenic). Even the original rapeseed (Brassica sp.) oil, with up to 50% of 22 ln-9 (emdc) acid usually had approximately 20% 18 2/1-6 and 10% 18 3/i-3 adds (Ackman 1983, 1990). 

Throughout this book, an abbreviated nomenclature is used to designate the structures of fatty acids. For example, y-linolenic acid (GLA), the commonly used term for all-cw-6,9,12-octadecatrienoic acid, can also be designated as 18 3n-6. This indicates a fatty acid with 18 carbon atoms and 3 double bonds, with the first double bond being found on the 6th carbon from the methyl group. It is understood, in the absence of other indicators, that all the double bonds of all fatty acids have the cis configuration and are methylene-interrupted i.e. the unsaturated centres are separated from each other by one CH group ... 

The essential fatty acids (EFA) are long-chain unsaturated lipids so named as they are essential to the diet of mammals and cannot be synthesized de novo as are all other lipids. There are two major EFAs, both of co-6 configuration (denoting the position of the first methylene-interrupted double bond, numbered from the methyl end of the chain) and their structures are shown in Figure 2.1. Linoleic acid is the most common EFA, and arachidonic acid is a chain elongation product of it. 

Fatty acid desaturation, a second major source of variation in the phylogenetic distribution of fatty acids, has been reviewed in detail (294-302). Some bacteria have a unique anaerobic system for production of monounsaturated fatty acids. This mechanism is involved in elongation of medium-chain length c/.r-3-unsaturated fatty acyl intermediates, and functions via P,y-dehydration of P-OH intermediates. It should be noted that this process cannot generate methylene-interrupted polyunsaturated fatty acids. 

Lipoxidase, which catalyzes aerobic formation of hydroperoxides from unsaturated fatty acids, occurs widely among plants, and appears to exist in animals as well (see reviews of Holman and Bergstrom (3G3), Singer and Kearney (668), and Bergstrom and Holman (56,57)). The enzyme attacks methylene-interrupted multiply unsaturated systems in which the double bonds have the m-configuration (A) (363). 

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