Cuticular Hydrocarbons

The outer covering of insects consists of a layer of water repellent lipids, frequently made up of alkanes, methyl-branched alkanes and alkenes. This lipid layer is important to prevent dehydration and to repel rain; and in social insects (bees, wasps, ants and termites), the mixture is characteristic of the group, and the available evidence suggests the mixture helps individuals to distinguish between nestmates and individuals from another colony. Cuticular hydrocarbons have from 17 to 49 carbon atoms in the chain, and they have been shown to be derived from fatty acids through chain lengthening and decarboxylation. The carbon chain is extended with more acetate groups (converted to malonate for the synthesis) to make straight chain hydrocarbons (Figure 3.15). The long chain acid is then reduced to an aldehyde, and, with the aid of oxygen and a cytochrome P450 the carbonyl group is lost as C02. As the fatty acids have an even number of carbon atoms, the hydrocarbons have an odd number of carbon atoms. It has been shown by labelling that the hydrogen of aldehyde becomes attached to the end of the hydrocarbon chain. The cuticular hydrocarbons appear to be synthesized in the oeno-cyte cells of the epidermis or the fat body, and if the latter, are transported through the haemolymph (insect blood) by a protein called lipophorin.

Ci8 stearic acid

J 4 x ch3cooh | reduction, NADPH

| decarboxylation, cytochrome P450, NADPH, 02 pentacosane

C26 hexacosanoic acid hexacosanal

Figure 3.15 Outline of the synthesis of a C25 hydrocarbon by chain-lengthening from stearic acid and decarboxylation. The dot on the hydrogen atom indicates that a labelled hydrogen atom in this position is retained on the terminal carbon atom of the alkane

Methyl-branched hydrocarbons are produced through the intervention of propionic acid (converted to methylmalonate) as shown in Figure 3.16. The methyl-branched fatty acids and the microsomal synthetase for making them have been isolated. The system is capable of selecting between malonyl CoA and methylmalonyl CoA and adding the correct intermediate at each step in the chain lengthening. While common fatty acids have no chiral centres, introduction of a propionate group creates a chiral centre, and branched hydrocarbons are chiral, although not much is known about their chirality as yet.

Beyond hexadecane (m.p. 18 °C), all straight-chain alkanes are waxlike solids. Pentacosane melts at 50 °C, triacontane at 60 °C and tetra-contane (C40H82) at 80 °C. To keep the cuticular-hydrocarbon surface soft and flexible, where more long-chain alkanes are present in the cuticle, more alkenes and methyl-branched alkanes are added. Introducing an internal double bond reduces the melting point -50 °C compared with

C-18 stearic acid

C-18 stearic acid

3-methyltricosane

Figure 3.16 An illustration of the synthesis of a methyl-branched hydrocarbon. Conversion of the acid to hydrocarbon occurs as a concerted step. Note that a chiral centre is introduced

3-methyltricosane

Figure 3.16 An illustration of the synthesis of a methyl-branched hydrocarbon. Conversion of the acid to hydrocarbon occurs as a concerted step. Note that a chiral centre is introduced the alkane, and a methyl branch (depending upon where in the chain) reduces melting point by -30 °C. The melting temperature range of the complex mixture on most insect cuticle will be low and very broad.

Decarboxylation of isoalkanoic and anteisoalkanoic acids (Figure 3.12), with or without chain lengthening, gives 2-methylalkanes and 3-methylalkanes respectively. But notice also that 3-methylalkanes can be formed in two possible ways (Figure 3.17), although the second alternative is more likely. Only labelling experiments will permit a decision between the two alternatives.

Pupae of a number of lepidopterans make very long-chain methyl-branched alcohols and acetates, like those shown for the southern army worm Spodoptera eridania in Figure 3.18. From studies with

a 3-methylalkane

Figure 3.17 The formation of 2-methylalkanes and 3-methylalkanes. The latter can be formed in two alternative ways, starting from an anteiso-acid, or from a straight chain acid by insertion of a propionic acid unit later n-2

a 3-methylalkane

Figure 3.17 The formation of 2-methylalkanes and 3-methylalkanes. The latter can be formed in two alternative ways, starting from an anteiso-acid, or from a straight chain acid by insertion of a propionic acid unit later

24,28-dimethyloctatriacontanol

24,28-dimethyloctatriacontanol

26,30-dimethyldotetracontanol

Figure 3.18

Very long chain alcohols synthesized by the pupae o/Spodoptera eridania. Such compounds appear to be characteristic of lepidopteran pupae incorporation of [3H]acetate and [l-14C]propionate it is concluded these are produced from the alkyl end and terminate at the alcohol end, so the methyl branches are added early in the synthesis.

Terminal double bonds are not usually found in cuticle alkenes but do occur in other insect secretions. W. Boland's group have shown, by deuterium labelling in several places in the chain that terminal alkenes are formed by an anti elimination of the carboxyl group and the pro-S hydrogen (see Figure 2.25) on the second carbon atom of a fatty acid, which is held firmly in one configuration on the enzyme (Figure 3.19). The same mechanism applies to this reaction in plants.

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