Structural colour is caused by the architecture of materials at the nanoscale, interfering with visible light, and giving rise to some of the most spectacular colourations in nature. A beautiful example is the fruit of Margaritaria nobilis, where the arrangement of cellulose fibres in the plant cell wall of the endocarp gives the fruit a brilliant blue appearance. In an approximation, the cellulose fibres lie parallel in layers, and the layers are stacked up with a small rotation angle, thus building up a helicoidal architecture, which selectively reflects left-handed circularly polarised light, mainly in the blue region. In order to unravel how nature makes up this fascinating architecture, it is key to understand the main building blocks of this specific plant cell wall. We therefore isolated the cellulose fibres and characterised their morphology via electron and atomic force microscopy. Crystallinity of the cellulose fibres was assessed by NMR and XRD measurements and was found to be higher than wood cellulose. Small angle X-ray scattering experiments were carried out on the native endocarp to estimate the fibre dimensions and spacings. Additionally to cellulose, another major component is xylan, a type of hemicellulose. Via enzyme digestions, gel electrophoresis and mass spectrometry, it was established that the xylan in the Margaritaria nobilis endocarp has a very regular acetylation pattern, an acetyl group is attached on every other xylose unit. Other decorations are sparse, no arabinose decoration is found, and only a small amount of glucuronic acid, which seems to be attached in clusters. Furthermore, computational modelling is in progress to understand the influence of xylan on the helicoidal arrangement of the cellulose fibres. We hope to better understand how nature produces such intricate nanostructures and to get inspiration for synthesising novel optical materials.

Structural colour from a helicoidal cellulose fibres architecture in the cell wall of Margaritaria nobilis

255th National Meeting (2018)

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