Fatty acids cyclopropene

Traditional procedures for quantification of total cyclopropene acid content have been reviewed by Christie (1970) and include titration with hydro-bromic acid and GC of the methyl mercaptan derivatives, products from reaction with silver nitrate/methanol and from methanethiol addition. However, a colorimetric test involving reaction with sulphur/carbon disulphide (the Halphen test) was found to be one of the most reliable methods for determining the small levels of cyclopropene acids present in oils containing cottonseed oil (Coleman, 1970). Another approach is GC of the relatively stable cyclopropane acids after hydrogenation with hydrazine (Conway, Ratnayake and Ackman, 1985) 

A popular method is to analyse the total fatty acid methyl esters by CjC after reaction with silver nitrate/anhydrous methanol for 2 h at 30°C (Bianchini, Ralaimanarivo and Gaydou, 1981 Eisele et aL, 1974 Gaydou, Bianchini and Ralaimanarivo, 1983 Ralaimanarivo, Gaydou and Bianchini, 1982). Most fatty esters remain unchanged, but cyclopropene esters are converted to later-eluting methoxy ether derivatives and small amounts of ketone derivatives. Two partially resolved peaks, those from sterculic acid eluting later, are observed for each type of derivative from each cyclopropene acid and can be quantified and used to determine the proportion of sterculic and malvalic acids in the untreated oil. Verification of the identities of the acids can be determined from GC-MS of the methyl esters of the products (Ahmad et al., 1979 Eisele et ai, 1974) but prominent allylic ions in the mass spectra of the DMOX derivatives of the methoxy ethers are more readily interpretable to reveal the positions of the cyclopropene rings (Spitzer, 1995). 

In the NMR spectrum of the total fatty acid methyl esters from seed oils, a specific signal at 0.76 ppm for the cyclopropene methylene protons has been used, relative to the protons of either the terminal methyl or methyl ester, to give quantification of total cyclopropene acids down to about the 1% level (Boudreaux, Bailey and Tripp, 1972 Pawlowski, Nixon and Sinn-huber, 1972). However, because of incomplete resolution of signals, quantitation becomes increasingly problematic as the proportion of cyclopropene acids decreases. 

In the infra-red (IR) spectrum a band at 1010 cm characteristic of the cyclopropene system has been used to determine high levels of cyclopropene acids (Bailey, Boudreaux and Skau, 1965). There is another specific band at 1870 cm which is weak in the IR spectrum but intense in the Raman spectrum (Kint et aL, 1981). Using peak height ratios of this band relative to that at 1745 cm (for triacylglycerol carbonyls), quantification of cyclopropene acids down to 0.03% was possible. The method may have potential for detecting vegetable oil contamination with, for example, cottonseed oil, but information is needed on its accuracy and precision. 

HPLC methods for quantification do not suffer from the same problems as GC methods, and cyclopropene acids can be analysed directly as methyl 

---

Cyclopropenoid Fatty Acids. Cotton, and other plants in the Malvaceae family, contain a pair of unique cyclopropene fatty acids (CPFA). These two fatty acids, sterculic and malvalic acid, are generally referred to collectively as cyclopropenoid fatty acids. Sterculic acid is the most active of the two fatty acids whose general action is to inhibit the desaturation of stearic to oleic fatty acid in the animal body with a resultant alteration in membrane permeability or an increase in the melting point of fats. 

Whilst the potential for synthesis of cyclopropene under volcanic conditions on primeval earth is recognized , the extent to which the ring system occurs in nature is known for only four classes of compounds, one of which involves cyclopropenone derivatives. Plants of the order Malvales contain varying amounts of the biologically active and homologous cyclopropene fatty acids sterculic acid (2) and malvalic acid (3). The... 

Cyclopropene fatty acids are converted into cyclopropanes by an enzyme in germinating cotton seed, ° while nitrogenase enzymes convert cyclopropene itself into cyclopropane and propane. 

The synthesis of a novel cyclopropyl analog of arachidonic acid [7] via a convergent synthesis that employed methyl (lR,25)-2-formylcyclopropane-carboxylate in conjunction with the ylide from (3Z,6Z)-pentadeca-3,6-dienyl(triphenyl)-phosphonium iodide was reported (62). A new approach to cyclopropene fatty acids has been developed for the synthesis of methyl sterculate [8] and methyl 2-hydroxy-sterculate this involves the 1,2-deiodination of 1,2-diiodocyclopropanes with butyllithium at low temperature (63). The synthesis of deuterated cyclopropene fatty esters structurally related to palmitic and myristic acids has been reported (64). 

Baird, M.S., and B. Grehan, A New Approach to Cyclopropene Fatty Acids Involving 1.2-Deiodination, J. Chem. Soc. Perkin Trans. I, 1547-1548 (1993). 

Certain specific lipids have excited interest recently because of worries about their actual or potentially harmful effects. Examples of these have been saturated fatty acids (Kunau and Holman, 1977), cholesterol (McGandy and Hegsted, 1975 Sabine 1977) and erudc acid from some rapeseeds (Vies, 1975). Cyclopropene fatty acids, which occur in some seed oils, are inhibitors of fatty acid desaturation (cf. Section 11.1.3). 

Cyclopropenoid fatty acids (Fig. 2.17) inhibit fatty acid desaturation in several species, including chicken, rat, pig, cow, and trout, causing a rise in the stearate to oleate ratio. This may have an effect on the composition and function of membranes as variation in lipid composition is known to alter permeability of membrane systems. Numerous types of deleterious effects from consumption of seed oils with cyclopropenoid fatty acids have been observed (Seigler, 1979). A cyclopropene fatty acid from Sterculia foetida oil was lethal to the larvae of Callosobruchus maculatus at 0.1% in artificial diets (Janzen et al., 1977). The most important edible oil with these compounds is cottonseed oil from Gos-sypium hirsutum (Malvaceae). Much of the cyclopropenoid... 

Figure 5.7 Reversed-phase high-pressure liquid chromatogram of phenacyl ester derivatives of the fatty acids from cottonseed cotyledons and embryos. CIS (50 nun x 4.5 nun, 3 pm particle size) and C8 (150 nun x 4.5 nun, 5 pm) were used in tandem. The mobile phase was composed of acetonitrile and water, and the flow rate was 1.5 ml min . An initial mixture of acetonitrile/ water (80 20 vol./vol.) was maintained for 25 min, followed by a gradient to acetonitrile/water (85 15) for a further 10 min and then to 100% acetonitrile for a final 10 min. A variable-wavelength detector was employed. Redrawn from Wood, R., Comparison of the cyclopropene fatty acid content of cottonseed varieties, glanded and glandless seeds, and various seed structures, Biochem. Arch., 2, 73-80, 1986.

Christie, W. W. (1970) Cyclopropane and cyclopropene fatty acids, in Topics in Lipid Chemistry, Vol. 1 (ed. F. D. Gunstone), Logos Press, London, pp. 1-49. 

Pasha, M. K. and Ahmad, F. (1992) Analysis of triacylglycerols containing cyclopropene fatty acids in Sterculia foetida (Linn.) seed lipids. J. Agric. Food Chem., 40, 626-9. 

Schuch, R., Ahmad, F. and Mukherjee, K. D. (1986) Composition of triacylglycerols containing cyclopropene fatty acids in seed lipids of Munguba Bombax munguba Mart.). J. Am. Oil Chem. Soc., 63 (6), 778-83. 

Wood, R. (1986a) High-performance liquid chromatography analysis of cyclopropene fatty acids. Biochem. Arch., 2, 63-71. 

A key question is "Will chemical modification of the double bond of cyclopropene fatty acids by thiols or other nucleophiles prevent cancer promotion "... 

Berry, S.K. (1980). Cyclopropene fatty acids in Gnetum. qenom. seeds and leaves. 3. Sci. Food Agric.. 31, 657-662. 

The seed on a wet basis contained 0.5% fat. The fatty acid composition of the seed oil is given in Table 5.13. The seed oil contained an undesirable amount of cyclopropene fatty acids which are not completely removed by cooking. These compounds have been reported to produce skin irritation and shortness of breath. It has been suggested that these seed should not be consumed in large quantities (Berry, 1980). 

Table 9.4 Effect of A9- and A12-cyclopropene fatty acids on the fatty acyl composition of Rhodosporidium toruloides (from Moreton, 1988a,b)...

Moreton, R.S. (1985) Modification of fatty add composition of lipid accumulating yeasts with cyclopropene fatty acid desaturase inhibitors. Appl. Microbiol. Biotechnol. 33,41-45. Moreton, R.S. (1988a) Single Cell Oil, Longman, Harlow. 

Methyl esters of cyclopropene fatty acids are less easily analysed by gas chromatography as they tend to decompose or rearrange on GC columns to give... 

Most workers have preferred to prepare stable derivatives prior to analysis. For example, cyclopropene fatty acids can be subjected to hydrogenation, or reaction with silver nitrate [449] or methanethiol [746]. Silver nitrate in anhydrous methanol reacts with cyclopropene rings in about 2 hours at 30 C to form predominantly methoxy ether but with some enonic derivatives, which appear as twin peaks (because of reaction on either side of the ring) on analysis by GC [99,241,281]. An application of this procedure to the analysis of kapok seed oil is illustrated in Figure 5.14. Alternatively, a brief reaction with hydrazine will selectively reduce the cyclopropene compounds to the more stable cyclopropanes by examination by GC before and after the reaction, the small amounts of natural cyclopropane components can also be identified [194]. 

Seed oils containing cyclopropene fatty acids have been successfully analysed by means of HPLC in the reversed-phase mode (reviewed elsewhere [168]) and this may now be the method of choice. 

---