Journal
CARBOHYDRATE RESEARCH
Volume 339, Issue 1, Pages 123-131Publisher
ELSEVIER SCI LTD
DOI: 10.1016/j.carres.2003.09.023
Keywords
cactus spine; Opuntia ficus-indica; microscopy; ultrastructure; cellulose; arabinan; C-13 CP-MAS NMR
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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 C-13 NMR analyses. Each spine consisted of a compact parallel arrangement of slender cellulosic fibers (0.4 mm. in length and 6-10 mum 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 C-13 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. (C) 2003 Elsevier Ltd. All rights reserved.
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