Structural analysis of wheat wax (Triticum aestivum, c.v. ‘Naturastar’ L.): from the molecular level to three dimensional crystals

Abstract

In order to elucidate the self assembly process of plant epicuticular waxes, and the molecular arrangement within the crystals, re-crystallisation of wax platelets was studied on biological and non-biological surfaces. Wax platelets were extracted from the leaf blades of wheat (Triticum aestivum L., c.v. ‘Naturastar’, Poaceae). Waxes were analysed by gas chromatography (GC) and mass spectrometry (MS). Octacosan-1-ol was found to be the most abundant chemical component of the wax mixture (66 m%) and also the determining compound for the shape of the wax platelets. The electron diffraction pattern showed that both the wax mixture and pure octacosan-1-ol are crystalline. The re-crystallisation of the natural wax mixture and the pure octacosan-1-ol were studied by scanning tunnelling microscopy (STM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Crystallisation of wheat waxes and pure octacosano-1-ol on the non polar highly ordered pyrolytic graphite (HOPG) led to the formation of platelet structures similar to those found on the plant surface. In contrast, irregular wax morphologies and flat lying plates were formed on glass, silicon, salt crystals (NaCl) and mica surfaces. Movement of wheat wax through isolated Convallaria majalis cuticles led to typical wax platelets of wheat, arranged in the complex patterns typical for C. majalis. STM of pure octacosan-1-ol monolayers on HOPG showed that the arrangement of the molecules strictly followed the hexagonal structure of the substrate crystal. Re-crystallisation of wheat waxes on non-polar crystalline HOPG substrate showed that technical surfaces could be used to generate microstructured, biomimetic surfaces. AFM and SEM studies proved that a template effect of the substrate determined the orientation of the re-grown crystals. These effects of the structure and polarity of the substrate on the morphology of the epicuticular waxes are relevant for understanding interactions between biological as well as technical surfaces and waxes.

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Abbreviations

STM:

Scanning tunnelling microscopy

AFM:

Atomic force microscopy

SEM:

Scanning electron microscopy

TEM:

Transmission electron microscopy

TMS:

Trimethylsilyl

HOPG:

Highly ordered pyrolytic graphite

GC:

Gas chromatography

MS:

Mass spectrometry

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Acknowledgements

We thank the Deutsche Forschungs Gemeinschaft, the Scientific Research-Flanders and the University of Bonn for the financial support of our research and the Physikalisch-Technische Bundesanstalt Braunschweig (Germany) for providing the standard normal for the AFM calibration, L. Schreiber and A. Dommisse (University of Bonn) for assistance in wax analysis, H.J. Ensikat for TEM analysis, the Institute for Organic Agriculture (University of Bonn) for providing the wheat seeds, and the reviewers for their helpful comments.

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Koch, K., Barthlott, W., Koch, S. et al. Structural analysis of wheat wax (Triticum aestivum, c.v. ‘Naturastar’ L.): from the molecular level to three dimensional crystals. Planta 223, 258–270 (2006). https://doi.org/10.1007/s00425-005-0081-3

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Keywords

  • Biomimetic surfaces
  • Epicuticular waxes
  • Epitaxial growth
  • Octacosan-1-ol
  • Self-assembly
  • Triticum
  • Wax crystals