The fruit of Margaritaria nobilis exhibit brilliant blue structural colouration, caused by the arrangement of biopolymers in its endocarp secondary cell wall. In this cell wall, together with xylan and lignin, cellulose microfibrils are assembled in a helicoidal architecture, lying parallel in layers that are stacked up with a small rotation angle between them, giving rise to a cholesteric-like arrangement. In this work, we examined the morphology and crystallinity of the cellulose microfibrils and the molecular structure of xylan. After an extensive series of chemical purifications, we discovered the coexistence of two distinct populations exceptionally short cellulose microfibrils: of the order of less than 1 micrometre. Such fibrils have different width and length distributions. Furthermore, we elucidated the molecular structure of xylan found in an intimate composite with such cellulose microfibrils via biochemical methods like enzyme digestions and gel electrophoresis. We discovered that xylan has a very regular structure, no arabinose decoration, acetyl groups on every other xylose unit, and 5 % of decoration with glucuronic acid. Combining such information, we speculate on the role of the interactions and assembly of secondary cell wall biopolymers.

Structural colour from a helicoidal cellulose microfibrils-xylan architecture in the cell wall of Margaritaria nobilis

2019 Spring National Meeting

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