Acidic Xylan Research Articles

Isolation and characterisation of cell wall polymers from olive pulp ( Olea europaea L.)

Cell wall material (CWM) was prepared from olive pulp, which is rich in oil. Polymers were solubilised from the CWM by sequential extraction with trans-1,2-cyclohexanediamine- N,N,N′,N′-tetraacetic acid, sodium salt; Na 2CO 3; 1 M KOH; 4 M KOH; and 4 M KOH+borate to leave the cellulose-rich residue (CR). A suspension of this residue, on neutralisation, released a pectic material, to give residue CR1. The residue CR1 contained cross-linked pectic polysaccharides and some cell wall glycoproteins which were solubilised with chlorite-acetic acid. The polymers from the various extracts were fractionated by graded precipitation with ethanol prior to anion-exchange chromatography, and selected fractions were subjected to methylation analysis. Closely related pectic polysaccharides rich in arabinose were the major components of the cell walls and they differed in their ease of extraction from the wall matrix. Significant amounts of acidic xylans were isolated from the precipitates obtained on neutralisation of the 1 M KOH extracts, and a large proportion of these would have been derived from the lignified sclereids; the xyloglucans from the supernatant fractions had xylans associated with them. However, significant amounts of xyloglucans associated with glucomannans, but virtually free of xylans, were isolated from the ‘neutral fractions’ of the supernatant solutions from the 4 M KOH and 4 M KOH+borate extracts. These ‘neutral fractions’ also contained significant amounts of hydroxyproline-rich cell wall glycoproteins. The anomeric configurations of the sugars in the glucuronoxylans were determined by 13C NMR spectroscopy. The structural features of the wall polymers are discussed in relation to the structure of the cell wall of the fruit pulp.

Raw and extruded fibre from pea hulls. Part II: Structural study of the water-soluble polysaccharides

The soluble fraction (9%) obtained by aqueous extraction of an extruded pea hull fibre gave 3 peaks on DEAE-Sepharose-6B. The first population (25%) was not bound to the gel and contained xylose, arabinose and glucose (15:6:1), the second one (12%), eluted for an ionic strength of 0·08 m, contained xylose, arabinose and glucuronic acid (18:1:1), and the third one (63%), eluted for an ionic strength between 0·3 m and 0·8 m, contained galacturonic acid, arabinose, xylose, rhamnose and galactose (5·3:3:1·8:1·7:1). The structure of these fractions was studied through enzymic degradation, methylation, 1H-NMR and 13C-NMR. The first one was composed mainly of a heteroxylan having a (1 → 4)-β- d-xylan backbone (average d.p. 103) with only very few branching points at O-2, and of a slightly branched arabinan. The second one was an acidic xylan. Both heteroxylans were acetylated. The third one was pectic in nature with a main chain consisting of (1 → 4) linked galacturonic acid, partly substituted at the O-2 and O-3 positions, and (1 → 2) linked rhamnose residues which were mainly substituted at the O-4 position. This acidic fraction was appreciably hydrolyzed by an endopolygalacturonase, indicating the presence of homogalacturonan regions in the molecules. Arabinose, galactose and xylose were present as side-chains.

Cell wall changes in immature Asparagus stem tissue after excision

Cell wall material (CWM) was prepared from sections of fresh and aerobically-stored asparagus ( Asparagus officinalis, L. cv. Connovor Collossus) stems. Polymers were solubilized from the CWM by successive extraction with cyclohexane- trans-1,2-diamine- NNN′N′-tetraacetate (CDTA), Na 2CO 3 and KOH to leave the α-cellulose residue which contained a significant amount of cross-linked pectic polysaccharides. The polymers were fractionated by anion-exchange chromatography and selected fractions were subjected to methylation analysis. The storage-related decrease in (1–4)-linked Gal p was detected in all the fractions rich in pectic polysaccharides, particularly in the CDTA, Na 2CO 3, 0.5 M KOH fractions and α-cellulose residue. A smaller decrease in Ara f was also observed. This was mainly due to a decrease in (1–5)-linked Ara f in the Na 2CO 3-1-soluble polymers, and terminal Ara f in the α-cellulose residue. There was evidence for the occurrence of significant amounts of complexes containing pectic polysaccharides and xylans having a relatively low degree of polymerization in the dilute alkali-soluble polymers, and some of these contained phenolic compounds; the storage-induced increase in (1–4)-linked Xyl p was confined to these polymers. Interestingly, no free acidic xylans could be detected in the 1 M and 4 M KOH-soluble polymers; instead, the bulk of the hemicellulosic polysaccharides appeared to be mixtures of xyloglucans and xylans in which the ratio of xyloglucan to xylan increased with increasing strength of alkali used for extraction of the polymers. The non-degradative extraction and fractionation procedures revealed heterogeneity in pectic polysaccharides, pectic polysaccharide-xylan complexes and xyloglucans in close association with xylans. The possible relationship between pectic polysaccharide-xylan-phenolic complexes and the onset of lignification in maturing tissues is discussed.

The sustained and warped helicoidal pattern of a xylan-cellulose composite: the stony endocarp model

The study was devoted to the microstructure of the thick walled cells of the endocarp of prune (Prunus domestica L.), cherry (Prunus cerasus L.), walnut (Juglons regia L.). The tissue is formed of closely associated cells showing a homogeneous development characterized by an intense constructive activity of ordered walls with a typically twisted pattern (cholesteric-like). The arced layers are produced in tens, each corresponding to a 180° full rotation of the molecules (axis of rotation oriented radially) and their succession gives rise to a basic regular and monotonous periodicity. On the other hand, observation of the tissue revealed the large capacity of the helicoidal morphogenesis to adjust itself under the influence of two topological contingent constraints: (1) the spherical shape (and derivated shapes) of the cell and (2) the numerous pit canals which maintain the symplastic transport and produce a recess during the construction of the wall. Spherical shape (closed surfaces) and recess both introduce additional internal strains which are relieved by deviations of the molecular array in the basic pattern (moire and knotty aspects). Special attention was given to the defects integrated in the spherical twist. The defects emerging in the angled stacks of microfibrils (disclinations, distortions) were a diagnostic feature of an actual liquid crystal behaviour under mechanical constraints. The abundance of such defects, of cusps and spiral motions strengthened the hypothesis that a transient fluid phase, later on consolidated and stiffened, operates during the cellulose ordering. The saddle-like figures developed in the complex polylobed situation of walnut were particularly demonstrative. The fractionation of the secondary wall yielded the glucidic matrix in the same ratio as cellulose. The bulk of this embedding matrix was composed of acidic xylans more or less tightly bound to the microfibrils. The coat of negatively charged polysaccharides visualized by the binding of cationic gold to wall strips might be expected to act as a surfactant generating an electrostatic repulsion between microfibrils. This could be a cooperative mechanism for the self-positioning (aligment in sheets and progressive rotation) of the composite.

Isolation and characterization of an acidic xylan from Gobo(Arctium lappa L.).

(1991). Isolation and Characterization of an Acidic Xylan from Gobo (Arctium lappa L.) Agricultural and Biological Chemistry: Vol. 55, No. 4, pp. 1139-1141.

Composition of the cell walls of different asparagus (Asparagus officinalis) tissues

Portions of stems from the base of asparagus spears (Asparagus officinalis L. cv. Connovor Collossus) were dissected to give the following tissues: (1) pith, which was free of vascular bundles, (2) two surrounding layers, parenchyma and fibre I and II (PFI and PFII), containing parenchyma and vascular bundles, (3) sclerenchyma sheath, (4) epidermis and sub‐epidermal layers and (5) asparagus vascular fibre (AVF). The alcohol‐insoluble residues (AIRs) from these tissues were shown to be free of starch. They were analysed for moisture and protein, and the component sugars were released by two hydrolytic procedures, which helped to distinguish the sugars from non‐cellulosic polysaccharides and cellulose. The AIRs from pith and epidermal tissues were relatively low in xylose, but were rich in cellulosic glucose, and sugars associated with pectic polysaccharides such as galacturonic acid, galactose and arabinose. Their major component polysaccharides (in decreasing amounts) were inferred to be pectic polysaccharides, cellulose, and hemicelluloses. AIR from sclerenchyma was rich in glucose and xylose, suggesting the presence of much cellulose and (acidic) xylans. The AIRs of PFI, PFII and AVF contained significant amounts of xylose in addition tn other sugars, and the major polysaccharides inferred to be present were pectic polysaccharides, cellulose and hemicelluloses, a significant proportion of which may be acidic xylans. Methylation analysis of the AIRs confirmed the above inferences. The bulk of the glucosyl residues were (1–4)‐linked, and there were small but significant amounts of (1–4, 6)‐linked glucosyl residues (the linkage characteristic of xyloglucans) in all the preparations. The presence of (1–4)‐linked galactosyl, (1–5)‐linked arabinosyl, terminal galactosyl, terminal arabinosyl, (1–2)‐ and (1–2, 4)‐linked rhamnosyl residues in all the AIRs except that from sclerenchyma, confirmed the presence of significant levels of pectic polysaccharides in all the parenchyma tissues. All the preparations containing vascular tissues contained significant amounts of (1–4)‐linked xylosyl residues, probably derived from acidic xylans. Even in the AIR of pith, a significant amount of (1–4)‐linked xylosyl residues were detected. This may be due to the ability of these cells and the parenchyma cells associated with the vascular bundles, to undergo lignification in mature asparagus plants.

Structural features of acidic xylans isolated from red gram ( Cajanus Cajan) husk

Red gram (Cu@zus ca@n) is a legume commonly consumed in tropical countries, usually after dehusking as &al. Industries have developed in these countries for dehusking red gram seeds and other legumes. Different varieties of red gram that are grown, have differences in dehusking quality that cause problems for these industries’. It is well established that such differences in milling characteristics are due to differences in adherence of husk to cotyledon, and non-starchy polysaccharides are implicated in this process l - 3. This prompted a systematic study of the chemical nature and physical properties of non-starchy polysaccharides of red gram isolated from different parts of the seeds, in varieties that have differences in dehusking characteristics. The present paper deals with structures of glucuronoxylans isolated from a difficult-milling variety of red gram.

Structural features of the cell-wall polysaccharides of the parchment layers of the pods of mature runner beans

Cell-wall material from parchment layers of mature runner-bean pods was delignified and the holocellulose was extracted in sequence with hot water, methyl sulphoxide at 20°, and 0.5 m, m, and 4 m at 20°, to yield the α-cellulose. The hemicelluloses solubilised by hot water and 0.5 m KOH were fractionally precipitated with alcohol, and some of the fractions were resolved further by anion-exchange chromatography. The acidic oligosaccharides released on partial acid hydrolysis of two of the xylans were reduced with NaBD 4, methylated with deuterated iodomethane, and identified by g.l.c.-m.s. The polymers soluble in methyl sulfoxide, m KOH, and 4 m KOH, selected fractions from the graded precipitation with alcohol, and some fractions from anion-exchange chromatography were subjected to methylation analysis. The major hemicellulosic polymers were glucurono- and 4- O-Me-glucurono-xylans with d.p. 90–200 and a ratio of Glc pA to 4-Me-Glc pA of ∼1:5.4. The cell-wall xylans are acetylated, and 7 out of 10 xylose residues of the methyl sulfoxide-soluble xylan are acetylated. Evidence was obtained for the occurrence of small amounts of pectic polysaccharide, possibly in covalent association with some acidic xylans (d.p. ∼30) and lignin.

Structural features of alkali-soluble acidic xylans isolated from the bark of Ceiba pentandra var. indica

The insoluble residue of C. pentandra var. indica, obtained by delignification of the bark, gave an alkali-soluble polysaccharide fraction containing l-fucose, d-xylose, l-arabinose, d-glucose, d-galactose, d-glucuronic acid, and 4- O-methyl- d-glucuronic acid. Fractional precipitation with Cetavlon and barium hydroxide afforded three hemicellulose-type acidic xylans that differed in the contents of uronic acid and mol. wts., and a fucose-containing acidic polysaccharide. Methylation analysis of the carboxyl-reduced acidic xylans suggested a (1→2)- or (1→4)-linked backbone and different patterns of substitution. Partial hydrolysis of one acidic xylan gave 4- O-β- d-xylopyranosyl- d-xylose and 2- O-(α- d-glucopyranosyluronic acid)- d-xylose. The oxidation of this polysaccharide with chromium trioxide and periodate was studied.

Acidic xylan from olive pulp

A purified hemicellulose A, isolated from olive pulp (‘hojiblanca’ variety) contained xylose and 4- O-methylglucuronic acid in the molar ratio 24:1. The molecular weight was estimated to be 9500 by GPC. From the results of partial hydrolysis, enzymatic hydrolysis, methylation analysis and identification of an aldobiuronic acid, it was concluded that hemicellulose A is a polymer of β(1 → 4)-linked xylopyranosyl residues, having branches of 4- O-methylglucuronic acid groups on the O-2 atoms of the main chain.

An acidic xylan from extracellular polysaccharides of suspension-cultured cells of Nicotiana tabacum

An acidic xylan was isolated from the extracellular polysaccharides of suspension-cultured tobacco cells. Its structure was investigated by methylation analysis and 13C NMR spectroscopy and was shown to consist of a main chain of β-(1→4)-linked D-xylopyranosyl residues to which were attached as side-chains, α- D-glucuronic acid residues at O-2.

An acidic xylan from the capsular polysaccharide-complex of Ocimum gratissimum seeds

An acidic xylan composed of d-xylose (48%), l-arabinose (16%), d-galactose (16%), and d-galacturonic acid (∼20%) has been isolated from the capsular, mucilaginous polysaccharide-complex of the seeds of Ocimum gratissimum. Graded, acid hydrolysis afforded O-β- d-galactopyranosyl-(1→3)- l-arabinose, an aldobiouronic acid, and an aldotriouronic acid composed of d-xylose and d-galacturonic acid. Methylation analysis, together with the isolation of a xylan core from the acid-degraded polysaccharide, indicated that the polysaccharide contains a (1→4)-linked xylan backbone with 60% of the xylosyl residues substituted either on O-2 or O-3 with O-β- d-galactopyranosyl-(1→3)- l-arabinofuranosyl, l-arabinofuranosyl, or d-galactopyranosyluronic acid side-chains. These structural features for the polysaccharide accord with periodate-oxidation studies.

Structural investigation of an acidic xylan from rapeseed

An acidic xylan isolated from rapeseed was ∼80% pure, as indicated by sedimentation and sugar-analysis data. Methylation analysis showed that, on average, for every 7 sugar units, the xylan has 1 non-reducing terminal residue of 4- O-methyl- d-glucuronic acid attached to position 2 of a d-xylose residue, with the remaining 5 non-terminal d-xylose residues linked (1→4).

Studies on the chemical structure of tobacco hemicellulose. II. 4-O-methylglucuronoxylan isolated from the midrib of Nicotiana tabacum.

An acidic xylan from the midrib of Nicotiana tabacum was isolated by alkaline extraction and fractionation on a DEAE-cellulose column. Based on the results of methylation analysis, partial acid hydrolysis and Smith degradation, the acidic xylan was concluded to be composed of a linear backbone of β-(1→4)-linked D-xylopyranosyl residues with approximately every ninth residue carrying a terminal 4-O-methyl-α-D-glucopyranosyluronic acid residue linked as a single side chain by (1→2) linkage.

The hemicelluloses of bracken : Part I. An acidic xylan

The acidic xylan from the stem of the fern Pteridium aquilinum is similar to xylans from woods, but has a lower degree of polymerisation. It has a chain of β-(1→4)-linked d-xylopyranose residues, one in every seven of which carries at its C-2 position a 4- O-methyl- d-glucuronic acid residue. A few of the molecules carry l-arabinofuranose residues, probably linked to the main chain by (1→3)-glycosidic links. The degree of polymerisation, based on periodate-oxidation and methylation-analysis studies, is about 67; a lower value was obtained by ebulliometric studies on the methylated xylan.

CORRELATION OF CHEMICAL COMPOSITION AND OPTICAL ROTATION OF NATIVE XYLANS

The principle of optical superposition has been used to derive an equation relating the specific rotations of native xylans with their content of 4-O-methylglucuronic acid and/or arabinofuranose residues. The equation is in good agreement with literature data for the 4-O-methylglucuronoxylans and the arabinoxylans. Based on this correlation, an α-L-linkage is suggested for the arabinofuranose residues in various arabinose-containing xylans. Literature rotations for the 4-O-methylglucuronoarabinoxylans are consistently greater than values predicted by the equation. Finally, the rotatory dispersion behavior of acidic and neutral xylans is examined.

CONSTITUTION OF AN ACIDIC XYLAN FROM TAMARACK (LARIX LARICINA)

An acidic xylan isolated from tamarack wood contained arabinose, xylose, and uronic acid (1:17:3) and was homogeneous on examination by electrophoresis and sedimentation. Molecular weight estimations by four different methods showed a degree of polymerization of 20. Methylation and periodate oxidation showed that the xylan consisted of ca. 16 β-D-xylopyranose units joined by 1 → 4 glycosidic bonds with the possible occurrence of one 1 → 3 linkage. To this unbranched structure were attached as side branches, three single units of 4-O-methyl-D-glucuronic acid through C2 and one unit of L-arabofuranose through C3.

Structure of an Acidic Xylan Isolated from Birch Wood Holocellulose.

Structure of an Acidic Xylan Isolated from Birch Wood Holocellulose.