, Volume 195, Issue 2, pp 257-270

Epicuticular crystals of nonacosan-10-ol: In-vitro reconstitution and factors influencing crystal habits

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Abstract

The primary aerial surfaces of plant species from many families (e.g. Pinaceae, Liliaceae, Ranunculaceae, Papaveraceae) are covered by epicuticular tubules 5–20 μm long and 0.5 μn in diameter. The composition, mechanism of growth and molecular structure of this type of epicuticular aggregates have been studied. Pure nonacosan-10-ol extracted from Picea pungens needle surfaces formed, in vitro, tubular crystals like those occurring in vivo. This crystal habit was obtained irrespective of the type of solvent or substratum, if the solvent was evaporated within minutes. This shows that tubules of nonacosan-10-ol are formed in the kinetic regime of crystallization (limited by the diffusion of molecules from the solution to the crystal surface). Slow evaporation of the solvent or crystallization from the melt resulted in rhombic scales. These planar crystals represent the thermodynamic, stable modification of native nonacosan-10-ol. Homologous impurities in natural nonacosan-10-ol (3–14%) had no effect on the formation of the tubules. However, racemic nonacosan-10-ol invariably crystallized in scales. The phase behaviour of mixtures of natural nonacosan-10-ol and its synthetic racemate as well as synthetic (S)-nonacosan-10-ol provided evidence for the presence of the pure (S)-enantiomer on plant surfaces. The findings are discussed in terms of the mechanisms leading to epicuticular tubules consisting of nonacosan-10-ol and their molecular structure. Crystal structures for the pure enantiomer and the racemate of nonacosan-10-ol are proposed. It is concluded that the principles responsible for the formation of tubules are both the special molecular geometry of the naturally occurring (S)-nonacosan-10-ol and the mobility barrier of the plant cuticle. Further specific biological processes are not necessary for the formation of (S)-nonacosan-10-ol tubules. The alterations of epicuticular structures during ageing or the impact of pollutants are explained as spontaneous transitions between two crystal modifications of (S)-nonacosan-10-ol.

The authors are indebted to Prof. W. Barthlott and Dr. C. Neinhuis (Botanisches Institut, Universität Bonn, Germany) for performing major parts of the scanning electron microscopy, to Prof. H.-J. Fuhrhop (Institut für Organische Chemie, Freie Universität Berlin, Germany) for a sample of (S)-nonacosan-10-ol, to Dr. G. Jackson (Exxon Fuels, Abbingdon, UK), Dr. C. Mioskowski and A. Seyer (Chimie Bio-Organique, Université Louis Pasteur, Strasbourg, France) for synthesising racemic nonacosan-10-ol and to Prof. M.H. Zenk (Lehrstuhl für Pharmazeutische Biologie, Universität München, Germany) for supplying plant material. Valuable suggestions and a critical review of the manuscript by Prof. J. Schönherr, (Institut für Obstbau und Baumschule, Universität Hannover, Germany) are gratefully acknowledged. This work was supported by the Deutsche Forschungsgemeinschaft.