CYANOLIPIDS

STRUCTURES, OCCURRENCE AND ANALYSIS

Cyanolipids are components of the lipids of seeds in the family Sapindaceae mainly, although some are known from the Hippocastaneaceae and Boraginaceae. They are based on a five-carbon backbone that comprises a nitrile moiety together with a methylene group or double bond, and one or two hydroxyl groups. The last are esterified with long-chain fatty acids as illustrated.

Structural formulae of cyanolipids

Thus, type I and II cyanolipids are diesters of 1-cyano-2-hydroxymethylprop-2-en-1-ol and 1-cyano-2-hydroxymethylprop-1-en-3-ol, respectively, while type III and IV cyanolipids are monoesters of 1-cyano-2-hydroxymethylprop-1-ene and 1-cyano-2-methylprop-2-en-1-ol, respectively. Of these, the type I cyanolipid appears to be most abundant, although it is often accompanied by the type II. The type III cyanolipid is only found with type II, while type IV is comparatively rare. It is important to note that type I and II cyanolipids are cyanohydrins with the cyanohydrin hydroxyl group esterified, and that they have a chiral centre. Type II and III cyanolipids are simply α,β-unsaturated nitriles, and they do not have a chiral centre. Also, on hydrolysis by acid or base and even with enzymes, the cyanohydrins that are released from type I and IV cyanolipids decompose spontaneously with the production of hydrogen cyanide, i.e. they are cyanogenic, as illustrated below for a type IV cyanolipid.

Hydrolysis of cyanolipids

Related glycosidic structures occur in the same seeds, i.e. with a glycosidic bond to glucose instead of the ester bond, and the two classes of compound are certainly related biosynthetically.

Cyanolipids occur in seeds together with conventional triacylglycerols, with the two lipid classes often being present in comparable amounts. The fatty acid compositions are distinctive, dependent on the species, but typically comprise relatively high amounts of oleic, cis-vaccenic, eicosanoic, eicos-11-enoic and eicos-13-enoic (paullinic) acids. For example, the fatty acid compositions of the cyanolipids and triacylglycerols of one species from the Sapindaceae are listed in Table 1.

Fatty acid	Cyanolipids	Triacylglycerols

Table 1. Fatty acid compositions of the cyanolipids and triacylglycerols of Paullinia cupana v. sorbilis (wt % of the total) (from Avato et al., cited below).

16:0	2	5
18:0	2	7
9-18:1	7	37
11-18:1	30	21
18:2(n-6)	6	10
20:0	2	4
9-20:1	39	12
11-20:1	7	4

Information on the biosynthesis of the cyanolipids is limited, but it seems clear that leucine is the key precursor. Similarly, there are a variety of suggestions as to the function of cyanolipids in seeds. For example, they may be a source of reduced nitrogen for the developing seedling, e.g. for alkaloid biosynthesis, as they disappear very rapidly on germination. At the same time, the cyanoglycosides increase in concentration. Alternatively, cyanolipids may have a protective role against attack by insects or fungi.

The various types of cyanolipids can be separated from each other by adsorption chromatography methods, especially thin-layer chromatography. More recently high temperature gas chromatography coupled to mass spectrometry has enabled separation and identification of the main molecular species of cyanolipids together with those of the triacylglycerols in seed oils. Nuclear magnetic resonance spectroscopy of the intact lipids is an invaluable guide as to the types of cyanolipids present in oils.

The Lipid Library

CYANOLIPIDS

STRUCTURES, OCCURRENCE AND ANALYSIS

Table 1. Fatty acid compositions of the cyanolipids and triacylglycerols of Paullinia cupana v. sorbilis (wt % of the total) (from Avato et al., cited below).

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