Chemical and physical analyses of wax ester properties

Wax esters are major constituents of the surface lipids in many terrestrial arthropods, but their study is complicated by their diversity. We developed a procedure for quantifying isomers in mixtures of straight-chain saturated and unsaturated wax esters having the same molecular weights, using single-ion monitoring of the total ion current data from gas chromatography-mass spectrometry. We examined the biological consequences of structural differences by measuring the melting temperatures, T(m), of >60 synthetic wax esters, containing 26–48 carbon atoms. Compounds containing saturated alcohol and acid moieties melted at 38–73°C. The main factor affecting T(m) was the total chain length of the wax ester, but the placement of the ester bond also affected T(m). Insertion of a double bond into either the alcohol or acid moiety decreased T(m) by ∼30°C. Simple mixtures of wax esters with n-alkanes melted several °C lower than predicted from the melting points of the component lipids. Our results indicate that the wax esters of primary alcohols that are most typically found on the cuticle of terrestrial arthropods occur in a solid state under physiological conditions, thereby conferring greater waterproofing. Wax esters of secondary alcohols, which occur on melanopline grasshoppers, melted >60°C below primary esters of the same molecular weight and reduced T(m) of the total surface lipids to environmental values.