In 1967, Geoff Eglinton (Glasgow) and Richard Hamilton (Liverpool) published a paper entitled ‘Leaf Epicuticular Waxes‘ (Science, v. 156, p. 1322-1335). In it, they investigated the waxy outer coating of plants (the cuticle). The amount of epicuticular wax in a plant varies greatly with species but serves a similar purpose in all; to preserve the water balance of the plant and to minimise mechanical damage to leaf cells.
This idea was not new and had been suggested nearly one hundred years prior (De Bary, 1871). What Geoff was interested in was the long-chain, molecular constituents of leaf wax. Other academics who worked on exotic compounds like porphyrins used to ask Geoff:
“Why are you people working on n-alkanes?! They don’t do anything!!”
But often the most interesting compounds were the ones nobody looked at. New developments in chromatography and spectroscopy allowed greater separation of complex chemical mixtures and allowed Geoff to identify a homologous series¹ of straight-chained organic compounds in extracts of leaf wax (Fig. 1,2). The n-alkaneshad a strong odd-over-even predominance² whereas the n-alkanols (an alkane with an -OH group) and n-alkanoic acids (an alkane with a -COOH group) had an even-over-odd carbon predominance (Fig 1). These compounds contained 21 and 35 carbons (although larger ones were reported). Terpenoids were also reported and from leaf wax extracts.
The apolar extract (the n-alkanes) was easiest to isolate and was eluted in hexane (an apolar organic solvent) using either alumina or silica column chromatography. The alkanes were further separated using gas-liquid chromatography and identified using mass spectra³ and synthetic n-alkane standards.
What Geoff loved was the idea of using n-alkanes as a chemotaxonomic tool (Fig. 3). For example, does Eucalyptus have a different n-alkane distribution compared to tobacco plant? He liked this idea because n-alkanes were abundant, sampling was simple and they were easy to isolate and analyse. Prior to this, Geoff had examined leaf waxes of a compact group of closely related genera endemic to the Canary Islands (Eglinton et al., 1962). He wanted to use n-alkanes in Tenerife in the same way Darwin used finches in the Galapagos. The approach was not a complete success. For example, n-alkanes could not differentiate between monocotyledons and dicotyledons. But it laid the groundwork for the next 40 years.
The development of compound specific carbon isotope analysis allowed organic geochemists to differentiate between C3, C4 and CAM grasses (e.g. Collister et al., 1994) and gymnosperms vs angiosperms (Schouten et al., 2007). Compound specific deuterium isotope analysis may play a role in the future of biomolecular chemotaxonomy (e.g. Sachse et al., 2006).
For more information, see Eglinton and Hamilton, 1967 (Science, v. 156, p. 1322-1335).
1. a series of compounds with a similar general formula, usually varying by a single parameter such as the length of a carbon chain. 2. The odd-over-even predominance (OEP) is reduced over time. In old, old rocks the OEP is low. In modern samples it is high. 3. n-alkanes are characterised by three small mass fragments (m/z 57, 71, 85) and a molecular ion (e.g. 408, 436, 464).
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