Arabinan polymer

T. Yoshida, M. Kida, and T. Uryu, Selective ring-opening polymerization of di-O-fert-butyldimethylsilylated and di-O-p-bromobenzylated 1,4-anhydro-u-L-arabinopyranoses and structural analysis of free arabinans, Polym. J. (Tokyo), 19 (1987) 923-931. 

Arabinan. This highly soluble polymer is found in the extracts of many fmits and seeds, in the boiling water extracts of pine wood (127), in the extracts of marshmallow roots (A/t/jaea officina/is) (128), and aspen (63) and willow (Sa/ix a/ba F) (129) bark. Because arabinan can be isolated from mildly degraded pectin fractions, it is often difficult to determine whether it is a hemiceUulose or a labile fragment of a larger polysaccharide and/or lignin complex. Arabinans have a complex stmcture composed almost entirely of 5-linked a-L-arabinofuranosyl units with similar residues linked to them at C-2 and/or C-3 and is soluble in 70% aqueous methanol solution. 

A cell-wall polysaccharide fraction remaining after treatment with a "classical" pectinase is termed rhamnogalacturonan (RG) or modified hairy regions (MHR). It is characterized by a highly branched ihamnogalacturonan-polymer with some arabinan side chains. 

Trichoderma (9-10). Much less is known about the concurrent production of the enzymes which cleave substituent groups of the xylan polymer. The presence of acetyl xylan esterases (11,12) and a-glucuronidases (13-15) in xylanolytic enzyme systems has only recently been pointed out. Although a-arabinosidases have mainly been studied as arabinan-degrading enzymes (16), they have also been shown to release arabinose from xylans (17). 

Zykwinska, A., Rondeau-Mouro, C., Gamier, C., Thibault, J. -F., Ralet, M. -C. (2006). Alkaline extractability of pectic arabinan and galactan and their mobility in sugar beet and potato cell walls. Carbohydr. Polym., 65, 510-520. 

The first polymerizations reported by Kops and Schuerch147 were those of l,4-anhydro-2,3,6-tri-0-methyl-/3-D-galactopyranose and 1,4-anhydro-2,3-di-0-methyl-a -L-arabinopyranose. The latter compound was slightly contaminated with l,4-anhydro-2,3-di-0-methyl-a-D-xy-lopyranose, but the course of the polymerization could nevertheless be monitored reasonably accurately. For the most part, the polymerizations were conducted at 10% concentration (g/mL) in dichloro-methane, or aromatic hydrocarbons, with 1-5 mol% of phosphorus pentafluoride, or boron trifluoride etherate. At low temperature (—78 to —97°), the d.p. of both polymers produced was —90 at increasing temperatures of polymerization, termination processes became more severe, and the d.p. lower. Usually, the reaction times were long (perhaps unnecessarily so), and the conversions were 50 to 90%. The specific rotations of the D-galactans prepared at —28 and —90° differ by only —10° ( — 85 to — 95°), but those of the L-arabinans varied from + 6... 

Cell walls of mycobacteria are composed of a peptidoglycan with covalently attached galactan chains. Branched chains of arabinan, a polymer of the furanose ring form of arabinose with covalently attached mycolic acids, are glycosidically linked to the galactan.327 Shorter glycopeptidolipids, containing... 

The arabinans are highly branched polymers of a-L-arabinofuranosyl residues having a-(l - 3) and a-( 1 - 5) linkages. Beet arabinan was hydrolyzed to the extent of only 3% by endo-a-L-arabinofurananase,142 in agreement with the highly branched structure. On treatment with a-L-arabinofuranosidase,72 a polymer of a-( 1 - 5)-linked L-arabinose could be precipitated from solution. This was hydrolyzed by endo-a-L-arabinofurananase to the extent of 23%, with release of a series of L-arabino-oligosaccharides initially and, on extended incubation, of L-arabinose and (l- 5)-a-L-arabinobiose,142 providing further evidence for the structure of the arabinan substrate. Partially debranched arabinan was hydrolyzed by endo-a-L-arabinofurananase at 16 times the rate for native arabinan.143... 

Oosterveld, A., Pol, I.E., Beldman, G. and Voragen, A.G.J. 2001. Isolation of feruloylated arabinans and rhamnogalacturonans from sugar beet pulp and their gel forming ability by oxidative cross-linking, Carbohydr. Polym., 44 9-17. 

The )8-(1 2)- and a-(1 5)-linked arabinose residues are incorporated into the polymer from the activated polyprenyl sugar phosphate 10, which is in turn synthesized from glucose via 5-phosphoribose pyrophosphate (pRpp) [46-48]. Elongation of the polymer chain is believed to involve a family of arabinosyltransferases (AraT s) that recognize both 10 and arabinofuranoside-based acceptors of differing structures (Fig. 5) [18,19,49,50]. In AG biosynthesis, the entire polysaccharide appears to be assembled as a polyprenol diphosphate intermediate, which is transferred to peptidoglycan prior to the addition of the mycolate esters [18]. In LAM biogenesis, the arabinan portion is believed to be synthesized as a polyprenol phosphate that is transferred to lipomannan [51]. 

As showed by Cardoso et al. [13, 37], the two-phase olive pomace contains considerable amounts of arabinan-rich pectic polysaccharides, as evaluated in an ethanolic insoluble residue (Fig. 2). These polymers contain a typical (1—>5)-l-At backbone, substituted at 0-3 with a linkage composition of 5 4 3 1 for (1—>5)-l-Ara/, T-L-Ara/, (l- 3,5)-L-Ara/, and (1—>3)-l-At, respectively. These pectic arabinans are also characterized by the occurrence of T-p-L-Ar (l—>5)-linked to (1 3,5)-L-Ar residues and of branched and linear blocks in their backbone. The proposed structure for the olive pomace pectic polysaccharides is represented in Fig. 5. It implies arabinan structures composed by an estimated average of 13 Ara residues linked to the rhanmogalacturonan backbone. This is found for distinct olive cultivars [37, 38]. 

The ot-L-Ara/-(l->5)-arabinan, oiDP 45-80, covalently attached to rhamnose residues, can be cleaved and isolated, and forms reasonably well-defined crystallites, which, however, cannot be aligned for fibre X-ray. Recent powder X-ray work has indicated that the polymer crystallises in a monoclinic unit cell with one chain per unit cell. It adopts a single two-fold helix with the... 

Pectic substances are the glues to plant cells and exist predominately in middle lamella between cell walls. The American Chemical Society defined the terms protopectin, pectinic acid and pectic acid (21). The native protopectin is a long chain, alpha-1, 4-D-galacturonic acid polymer, interspersed with alpha-1,2-L-rhamnose residues. Shorter polymers, galactans and arabinans exist and are bonded covalently with the main chain (22). Many types and different amounts of side chains exist in various plant tissues. 

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