Araban

Arabans have been isolated from the cell walls of many dicotyledonous plants. Until recently, no homo-araban has been isolated specifically from primary cell walls. However, methylation analysis of the walls of suspension-cultured sycamore 123) and pea (55) cells strongly suggested that these primary cell walls possess arabans which are structurally similar to arabans obtained from other tissues or organelles. Recently, an araban, essentially free of other polysaccharides, has been isolated from a methylated sycamore cell wall polysaccharide fraction 5). 

Nuclear magnetic resonance analysis of Rosa glauca araban has provided additional evidence that the arabinosyl residues are a-5-linked and in the furanose configuration. The C-1 resonance expected of a-arabinofuranosyl residues, but not the C-1 resonances expected of the P-arabinofuranosyl or a- and P-arabinopyranosyl residues, was detected by C-NMR analysis 68b). The proton NMR spectrum is consistent with a- or P-furanosyl residues and P-pyranosyl residues, but not with a-pyranosyl residues 68b). The a-anomeric nature of these linkages is confirmed by the negative optical rotations, from —181 to —108, exhibited by such arabans 11, 68b). 

Arabinosyl linkage Soybean Lemon Mustard s Cotyledons Mustard S seed Rape- seed Rose s Sycamore White willow Aspen  

Isolated from mustard (Sinapis alba L.) cotyledon 108a). 

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Pectin also contains araban and galactan. Il is present in fruits, root vegetables and other plant products, and confers on jams their typical gelling property. Pectin is manufactured as a white powder, soluble in water, and used to assist the setting of jams and jellies, and for numerous other purposes. Low melhoxyl pectins, with under 7% methoxyl, give firmer gels than pectins proper. 

The plant is harvested by hand sickle and, after defoUation, is stripped and scraped by hand or machine decorticated. Because of the high gum (xylan and araban) content of up to 35%, retting is not possible. The fibers are separated chemically by boiling in an alkaline solution in open vats or under pressure, then washed, bleached with hypochlorite, neutralized, oiled to facUitate spinning, and dried. 

According to some, pectin appears to be a mixture of methyl esterified galacturonan, galactan, and araban in which some of the galacturonan molecules... 

The bulk of the heteropolysaccharides in the green coffee bean are formed from mannan, galactan, and araban.95 96 Three fractions can be... 

The glycoproteins and water-soluble polysaccharides contain mainly galactan with some araban.95... 

The present authors and their collaborators have also investigated arabans derived from apple and citrus pectins and here again it is to be noted that the arabans are associated in nature with galactan and pectic acid. The methylated derivatives of all these arabans have very similar properties and on hydrolysis give approximately the same proportions of 2,3,5-trimethyl-L-arabofuranose, 2,3-dimethyl-L-arabinose and... 

The only other araban which has hitherto been examined from the point of view of its chemical structure is the one present in gum traga-canth.38 This araban has properties similar to those of the araban just described, but in addition to L-arabinose it contains a small amount of D-galactose. 

We may return now to the polysaccharides present in the peanut for a brief consideration of the relationship of the other components present in the pectic materials to the araban constituent. All the evidence indicates that the pectic acid portion of the peanut is identical with normal pectic acid and, as was indicated in the previous section, this material, which is very stable to acid hydrolysis and possesses a high positive rotation contains a main chain which is built up of D-galac-turonic acid residues of the pyranose type. If, therefore, the araban associated with the pectic acid had been derived directly from the pectic acid by decarboxylation without intermediate hydrolysis of the poly-galacturonide, the sugar residues in the araban should also be in the pyranose form. The experimental evidence shows clearly, however, that the arabinose residues in araban are furanose in type and it follows that any hypothesis concerning the direct conversion of pectic acid into the araban by decarboxylation is untenable. 

These observations have a bearing also on certain claims37 which have been made concerning a supposed increase in the pentosan content when pectin is heated in alkaline solution. It is obvious that any increase in the proportion of pentosan in the product cannot be due to decarboxylation of the pectic acid molecule with the formation of the araban. The... 

So far as we are aware, the galactan associated with pectin has hitherto been isolated in a reasonable degree of purity in one instance only,2 namely from the seeds of Lwpinua albus. This source contains a comparatively high proportion of the galactan component ([< ]d + 35° in water) and the isolation of the galactan is thus facilitated. The araban was partially removed by repeated reprecipitation from aqueous solution... 

L-arabinose molecules combine together in the furanose form to produce an araban. Since enzymes capable of hydrolysing pectic acid to n-galacturonic acid are known to be present in certain plant juices, and since the comparatively low temperature and slightly acid condition of plant materials would tend to favor the transformation of arabinose into the furanose form, the occurrence of the furanose structure in the polysaccharide derived from arabinose is not altogether unexpected. 

Alternatively, if we suppose that the hexose polysaccharide is produced first and is then transformed into pectic acid and araban, at least two types of galactan, other than the one already isolated from pectic materials, would be required to account for the types of structure present respectively in pectic acid and araban. This would seem to be somewhat unlikely since, on this view, one would expect to encounter other types of pectic acid and araban, whereas on the evidence now available the pectic acid isolated from all samples of pectic materials examined appears to be the same, and a similar conclusion applies in the case of the araban. 

It will be clear that in this nomenclature arabans and galactans are not included as components of pectinic or pectic acids. They have been found in pectic preparations, but it is not known whether they represent chemically bound constituents such as side chains or are to be regarded as separate substances closely associated physically with the predominating pectinic or pectic acids. The nomenclature is thus based upon an ideal conception and should be regarded as provisional and subject to more accurate specification as knowledge increases. 

Determination of xylan is frequently made by estimation of furfural production. Data so obtained, even when appropriately corrected for furfural that arises from uronic acid, may be high if araban is present in the polysaccharide preparation. In the absence of interfering carbohydrates, furfural estimation may lead to accurate xylan values. 

Other workers140 have also examined the products derived by hydrolysis of periodate-oxidized xylan. Both wheat straw and com cob xylan, after oxidation and hydrolysis, yield small amounts of L-arabinose and D-xylose. These sugars are obtained even after the xylans have been subjected to extended periods of oxidation. It is concluded that the D-xylose constituted branch points in the xylan. Likewise, the L-arabi-nose molecules must not have been terminal units in a xylan chain but must have been either interior units in the xylan molecule or have constituted an araban-like polysaccharide which is in combination or admixture with the xylan polysaccharide. 

The two most important natural pentoses, 1 -arabinose and 1 -xylose, occur in nature as polymeric anhydrides, the so-called pentosans, viz. araban, the chief constituent of many vegetable gums (cherry gum, gum arabic, bran gum), and xylan, in wood. From these pentapolyoses there are produced by hydrolysis first the simple pentoses which are then converted by sufficiently strong acids into furfural. This aldehyde is thus also produced as a by-product in the saccharification of wood (cellulose) by dilute acids. Furfural, being a tertiary aldehyde, is very similar to benzaldehyde, and like the latter undergoes the acyloin reaction (furoin) and takes part in the Perkin synthesis. It also resembles benzaldehyde in its reaction with ammonia (p. 215). 

Browne, C.A. and Tollens, B., Uber die Bestandtheile des Mais-Marks und des Hollander-Marks und das gleichzeitige Vorkommen von Araban und Xylan in den Pflanzen. Berl. Dtsch. Chem. Ges., 1902, 35, 1457. 

The glucose tetra-acetate was used as an initiator and results in each araban chain (average degree of polymerisation 23.6) terminating in a glucosyl reducing unit. 

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