Hydrocarbon Structure

Cuticular hydrocarbons are compounds consisting only of carbon and hydrogen atoms, and all have the same basic structure consisting of a long carbon chain, which in social insects appears to be between 19 and 35 carbon atoms long. However, the apparent abundance of C19-C35 cuticular hydrocarbons may be a reflection of the limitations of the detection techniques, since new high temperature GC columns (Akino 2006) and MALDI-MS techniques (Cvacka et al. 2006) are revealing cuticular hydrocarbons with chain lengths of up to 70 carbon atoms. Of particular interest is the discovery that Formica truncorum, and probably all other wood-ants (Formica s.str.), have 56% of their cuticular hydrocarbons with chain-lengths >C34 (Akino 2006).

Hydrocarbon chains occur in their saturated or unsaturated form and may have one or more methyl groups (CH3) attached (Fig. 10.1). In the saturated form (alkanes or

Linear alkane: tricosane

Z-Alkene: (Z9)-tricosene alkadiene: (Z6,Z9)-tricosadiene alkadiene: (Z6,Z9)-tricosadiene

Methyl branched alkane: 7-Methyltricosane Fig. 10.1 Structures of some of the main classes of hydrocarbons

Methyl branched alkane: 7-Methyltricosane Fig. 10.1 Structures of some of the main classes of hydrocarbons sometimes referred to as paraffins) all of the carbon atoms are joined by single bonds, while the unsaturated compounds (olefins) have either one (alkenes or monoenes), two (alkadienes or dienes) or three (alkatrienes or trienes) double bonds at various positions along the chain. In addition, olefins can take the form of one of two isomeric forms between carbon atoms at the double bond in the hydrocarbon chain. These are referred to as cis-alkenes (or Z-alkenes) and trans-alkenes (or ¿-alkenes), however, all known insect cuticular alkenes have the 'Z configuration.

The chain length, in addition to the presence and position of methyl groups and/ or double bonds, has a large impact on the physical properties of the compound, such as structure and volatility (Gibbs 1995), which in turn underlies its suitability for any particular function. The shape of saturated hydrocarbons (alkanes) allows close packing of the molecules and they are therefore ideally suited to function as waterproofing molecules. Species like Drosophila pseudoobscura and D. mojavensis, appear to be incapable of producing alkanes in reasonable quantities and therefore suffer from high cuticular permeability and are susceptible to desiccation stress (Blomquist et al. 1985; Toolson et al. 1990). Also, as chain-length increases, alkanes become less volatile and are better at producing films with low water permeability. Therefore, a C33 alkane is better at reducing trans-cuticular water flux than C23 alkane. This is because of the stabilizing effect of weak intermolecular forces (van der Waals forces), which increase in strength as the molecular size increases. It is these forces that are disrupted by methyl-branches and double bonds consequently lowering the melting point (Gibbs 1998). For example, alkanes larger than C18 are wax-like solids at room temperature; however, the introduction of double bonds (Toolson and Kuper-Simbron 1989; Gibbs and Pomonis 1995) or methyl groups (Morgan 2004) into the molecule will drastically decrease their melting points. It can be assumed that boiling point and thus volatility is affected in a similar way but as yet little is known about the boiling points of alkenes and methyl branched hydrocarbons. The rich mixes of different groups of hydrocarbons e.g. alkanes and alkenes, ensures that the melting temperature range is low and broad due to intermolecular interactions (Morgan 2004), which is needed to regulate the permeability of the cuticle in a highly variable terrestrial environment. Gibbs (1998) suggested that the cuticular lipid layer is a mixture of microscopic solid areas (alkanes) and liquid areas (alkenes). The role of melting point in determining the action of compounds has already been suggested for ant repellents used by wasps, where it appears that its state (liquid or solid) is crucial to its function (Dani et al. 2003). The state of cuticular hydrocarbons will also affect their mobility and hence how accessible they are to chemoreceptors in insects.

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