Arabinan-cellulose composite in Opuntia ficus-indica prickly pear spines.
The ultrastructure of the spines decorating the cladodes of the cactus Opuntia ficus-indica was investigated by optical microscopy, scanning and transmission electron microscopy, wide angle X-ray, and solid state 13C NMR analyses. Each spine consisted of a compact parallel arrangement of slender cellulosic fibers (0.4 mm in length and 6-10 microm in diameter) with small lumens. The fibers were disencrusted by alkali and sodium chlorite bleaching, yielding a remarkable arabinan-cellulose (1:1) product. X-ray fiber diagrams of the spines before and after purification confirmed the presence of crystalline cellulose domains with molecular axis parallel to the spine axis. CP-MAS 13C T1 NMR data showed a strong interaction at a nanometric level of a fraction of the arabinan and the cellulose crystalline domains. By sequential hydrothermal extractions, followed by a trifluoroacetic acid treatment, a relatively pure cellulose was isolated while the extracted fibers became fibrillated into slender microfibrils having no more than 4-6 nm diameter. The hydrothermal extract yielded the alpha-L-arabinofuranan consisting of a chain of (1-->5)-linked L-arabinosyl residues with branching either at C-2 or C-3 or at both C-2 and C-3. Taken together, these observations suggest that the bulk of the spine fibers consists of an intimate composite of cellulose microfibrils embedded in an arabinan matrix.
Vignon
M R
MR
Centre de Recherches sur les Macromolécules Végétales (CERMAV-CNRS), Université Joseph Fourier, BP 53, 38041 Grenoble 9, France. vignon@cermav.cnrs.fr
Heux
L
L
Malainine
M-E
ME
Mahrouz
M
M
D001089:Arabinose / Q000031:analogs & derivatives* / Q000737:chemistry; D002482:Cellulose / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004043:Dietary Fiber / Q000032:analysis; D009682:Magnetic Resonance Spectroscopy; D008854:Microscopy, Electron; D029747:Opuntia / Q000737:chemistry*; D010936:Plant Extracts / Q000737:chemistry; D011134:Polysaccharides / Q000737:chemistry*
D004043:Dietary Fiber; D010936:Plant Extracts; D011134:Polysaccharides; C431714:arabinofuranose; C030080:araban; D002482:Cellulose; D001089:Arabinose; C109691:microcrystalline cellulose