Galactosyluronic Acid Research Articles

Extraction of Pectin from Citrus Peels - A Review

Pectin is complex polysaccharides contains 1, 4-linked x and β galactosyluronic acid residues extracted using alcohol precipitation method from citrus peels. The color of pectin from orange peel was pale yellow. Pectin is soluble in hot and cold alkaline water. The pectin is polysaccharide use as a stabilizer in foods. The pectin was extracted from various citrus fruit peels and its optimization by varying pH using hydrochloric acid and citric acid. Pectin can be used to improve the mouth feel and the pulp stability in juice-based drinks and as a stabilizer in acidic protein beverages. Extraction of pectin from citrus fruit peels like Orange peels, sweet orange and lemon gives higher yield of pectin. % Yield and rate of extraction for pectin from orange and sweet lemon (Mosambi) peels depends on parameters such as pH, temperature, solvent, time of extraction. Extraction carried out at pH 1.5 at 60 min with 60,70 & 80 OC temperature. The higher yield of pectin at pH value 1.5 and contact time 60 min. The extraction of pectin such as Citric Acid, Hydrochloric Acid, Sulphuric Acid, Nitric Acid and oxalic acid (C2H2O4). The time of extraction increased to an extreme there is less effect on yield of pectin reported and also decreased from maximum level due to thermal degradation of the extracted pectin.

A galacturonic acid-containing xyloglucan is involved in Arabidopsis root hair tip growth.

Root hairs provide a model system to study plant cell growth, yet little is known about the polysaccharide compositions of their walls or the role of these polysaccharides in wall expansion. We report that Arabidopsis thaliana root hair walls contain a previously unidentified xyloglucan that is composed of both neutral and galacturonic acid-containing subunits, the latter containing the β-D-galactosyluronic acid-(1→2)-α-D-xylosyl-(1→ and/or α-L-fucosyl-(1→2)-β-D-galactosyluronic acid-(1→2)-α-D-xylosyl-(1→) side chains. Arabidopsis mutants lacking root hairs have no acidic xyloglucan. A loss-of-function mutation in At1g63450, a root hair-specific gene encoding a family GT47 glycosyltransferase, results in the synthesis of xyloglucan that lacks galacturonic acid. The root hairs of this mutant are shorter than those of the wild type. This mutant phenotype and the absence of galacturonic acid in the root xyloglucan are complemented by At1g63450. The leaf and stem cell walls of wild-type Arabidopsis contain no acidic xyloglucan. However, overexpression of At1g63450 led to the synthesis of galacturonic acid-containing xyloglucan in these tissues. We propose that At1g63450 encodes XYLOGLUCAN-SPECIFIC GALACTURONOSYLTRANSFERASE1, which catalyzes the formation of the galactosyluronic acid-(1→2)-α-D-xylopyranosyl linkage and that the acidic xyloglucan is present only in root hair cell walls. The role of the acidic xyloglucan in root hair tip growth is discussed.

Effects of a slight acidic arabinoxylan from Plantago ovata seed husk on human skin cells

The Plantago ovata, Forssk, Plantaginaceae, seed husk has a high content of water-soluble polysaccharide identified as an arabinosyl (galactosyluronic acid) rhamnosylxylan further denominated as P1. The P1 activity on the cell proliferation, migration and cell cycle of keratinocytes (NHEK, HaCaT) and fibroblasts (NHDF) was analyzed by flow cytometry. Internalization studies were done to elucidate the effectiveness of P1. Enzymatic hydrolysis was done to investigate the necessary structure responsible for the biologic activity. Subfractions of P1 were obtained using α-L-arabinofuranosidase (P11-P18), endo-1,4-ß-xylanase (P21-P24) and both enzymes in combination (P31-P36).

Characterization of pectin methyltransferase from soybean hypocotyls.

Pectin methyltransferase (PMT) catalyzing the transfer of the methyl group from S-adenosyl-L-methionine (SAM) to the C-6 carboxyl group of galactosyluronic acid residues in pectin was found in a membrane preparation of etiolated hypocotyls from 6-d-old soybean (Glycine max Merr.). The enzyme was maximally active at pH 6.8 and 35-40 degrees C, and required 0.5% (w/v) Triton X-100. The incorporation of the methyl group was significantly enhanced by addition of a pectin with a low (22%) degree of methyl-esterification (DE) as exogenous acceptor substrate. The apparent Michaelis constants for SAM and the pectin (DE22) were 0.23 mM and 66 microg x ml(-1), respectively. Attachment of the methyl group to the carboxyl group of the pectin via ester linkage was confirmed by analyzing radiolabeled product from incubation of the enzyme with [14C]methyl SAM and the acceptor pectin. Size-exclusion chromatography showed that both enzymatic hydrolysis with a pectin methylesterase and a mild alkali treatment (saponification) led to the release of radioactive methanol from the product. Enzymatic hydrolysis of the product with an endopolygalacturonase degraded it into small pectic fragments with low relative molecular mass, which also supports the idea that the methyl group is incorporated into the pectin. The soybean hypocotyls were fractionated into their cell wall components by successive extraction with water, EDTA, and alkali treatment. Among the resulting polysaccharide fractions, high PMT activity was observed when a de-esterified polysaccharide derived from the EDTA-soluble fraction (the pectic fraction) was added as an alternative acceptor substrate, indicating that the enzyme may be responsible for producing methyl-esterified pectin in vivo.

Changes in pectin structure and localization during the growth of unadapted and NaCl‐adapted tobacco cells

SummaryTobacco cells adapted to grow in high concentrations of NaCl exhibit a drastically altered growth physiology that results in cells whose fully expanded volume is only one‐fifth to one‐eighth those of unadapted cells. Comparison between NaCl‐adapted and unadapted tobacco cells provides an opportunity to evaluate current concepts of the structural and mechanical determinants of cell wall expansion. Both biochemical studies of pectic polymers and the ultrastructural localization of pectic epitopes at three specific phases of cell culture, maximal cell division, maximal elongation, and stationary phase are reported here.One‐half of the galactosyluronic acid units in wall polymers of NaCl‐adapted cells are esterified throughout the culture period, while wall polymers of unadapted cells show a rise in esterified polygalacturonic acid from 50 to 80% during elongation and then a decrease to 70% at stationary phase. Methyl esters account for only a proportion of the total esterified polygalacturonic acid at any stage in both unadapted and NaCl‐adapted cell walls. Using monoclonal antibodies, we show differences in the localization of relatively methyl‐esterified and unesterified pectic epitopes at different stages of growth and corroborate the chemical determinations. Fourier transform infrared (FTIR) microspectroscopy of representative walls of both NaCl‐adapted and unadapted cells confirms, at the single cell wall level, that results obtained from chemical analysis of bulk samples are applicable to the entire cell population.FTIR microspectroscopy also reveals an increase in wall protein in the walls of adapted cells. Images obtained by the fast‐freeze, deep‐etch, rotary‐shadowed replica technique show clearly different cell wall architectures in NaCl‐adapted compared with unadapted cells; walls of elongating unadapted cells contain long, thin fibres that show a net orientation with respect to the long axis of the cell, whereas walls of adapted cells have thicker, flatter bundles of fibres with no clear net orientation. Polarized FTIR microspectroscopy indicates that, in unadapted tobacco cells during elongation, pectin molecules may be oriented within the wall in a similar manner to cellulose. Possible ways in which pectin structure and conformation may affect the behaviour of the cellulose‐xyloglucan network are discussed.

Isolation and structural characterization of β- d-glucosyluronic acid and 4- O-methyl β- d-glucosyluronic acid-containing oligosaccharides from the cell-wall pectic polysaccharide, rhamnogalacturonan I

Rhamnogalacturonan I (RG-I), a pectic polysaccharide isolated from the walls of suspension-cultured sycamore cells, was shown by glycosyl-residue composition analysis to contain d-glucosyluronic acid (Glc pA) residues (1 mol%) and 4- O-methyl- d-glucosyluronic acid (4- O-Me-Glc pA) residues (0.5 mol%). These monosaccharides were shown, by glycosyl-linkage analysis, to be present in RG-I as terminal nonreducing residues. The glycosyl sequences containing Glc pA and 4- O-Me-Glc pA were determined by structurally characterizing the acidic oligosaccharides released by partial acid hydrolysis of RG-I. Six acidic oligosaccharides were purified by semipreparative high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and characterized by glycosyl-residue and glycosyl-linkage composition analyses, GLC-CIMS, GLC-EIMS, electrospray MS (ESMS), and 1H NMR spectroscopy. We propose that three of the acidic oligosaccharides characterized, 4- O-Me-β- d-Glc pA-(1 → 6)- d-Gal, β- d-Glc pA-(1 → 6- d-Gal, and β- d-Glc pA-(1 → 4)- d-Gal, originate from the galactosyl-containing side chains of RG-I. The three other acidic oligosaccharides characterized, α- d-Gal pA-(1 → 2)- l-Rha, α- d-Gal pA-(1 → 2)-α l-Rha p-(1 → 4)-α- d-Gal pA-(1 → 2)-α- l-Rha, and α- d-Gal pA-(1 → 2)-α- l-Rha p-(1 → 4)-α- d-Gal pA-(1 → 2)-α- l-Rha p-(1 → 4)-α- d-Gal pA-(1 → 2 )-α- l-Rha, were generated by partial hydrolysis of the RG-I backbone. No evidence was obtained for the presence of galactosyluronic acid in the side chains of RG-I. To our knowledge this is the first report that rhamnogalacturonan I contains glycosyluronic acid or 4- O-methyl glucosyluronic acid residues.

Structural characterization of endo-glycanase-generated oligoglycosyl side chains of rhamnogalacturonan I

Rhamnogalacturonan I (RG-I) has been isolated from the walls of suspension-cultured sycamore cells ( Acer pseudoplatanus), and additional structural features of the polysaccharide were elucidated. Treatment of RG-I with a purified endo-(1 → 5)-α- l-arabinanase released a series of arabinose-containing oligosaccharides with degrees of polymerization (dp's) between 2 and 20. These oligosaccharides were shown, by glycosyl-linkage composition analysis, to contain terminal, 5-, and (3 → 5)-linked Ara f residues. These results provide evidence that a branched arabinan is attached to the backbone of RG-I. RG-I was freed of 95% of its arabinosyl residues by treating the polysaccharide with a combination of endo-(1 → 5)-α- l-arabinanase and α- l-arabinosidase. No galacturonic acid was released by these enzymes, which is evidence that the arabinosyl-containing portions of the side chains do not contain galactosyluronic acid residues. The galactose-containing portions of the side chains of RG-I were not fragmented by an endo-(1 → 4)-β- d-galactanase. However, approximately 85% of the galactose and small amounts of galacturonic acid were released by digestion of arabinose-depleted RG-I with a combination of endo- and exo-β- d-galactanases. The galacturonic acid may have been released by small amounts of an exo-α-galactosyluronidase contaminating the galactanases. Treatment of RG-I with this mixture of endo- and exo-glycanases resulted in a relatively size-homogeneous, almost side chain-free backbone composed of the O-acetylated diglycosyl repeating unit → 4)-α- d-Gal pA-(1 → 2)-α- l-Rha p. A combination of 1H NMR spectroscopy and periodate oxidation established that the backbone repeating unit contained a single O-acetyl substituent on C-2 or C-3 of each galactosyluronic acid residue.

Changes in Esterification of the Uronic Acid Groups of Cell Wall Polysaccharides during Elongation of Maize Coleoptiles

Cell walls of grasses have two major polysaccharides that contain uronic acids, the hemicellulosic glucuronoarabinoxylans and the galactosyluronic acid-rich pectins. A technique whereby esterified uronic acid carboxyl groups are reduced selectively to yield their respective 6,6-dideuterio neutral sugars was used to determine the extent of esterification and changes in esterification of these two uronic acids during elongation of maize (Zea mays L.) coleoptiles. The glucosyluronic acids of glucuronoarabinoxylans did not appear to be esterified at any time during coleoptile elongation. The galactosyluronic acids of embryonal coleoptiles were about 65% esterified, but this proportion increased to nearly 80% during the rapid elongation phase before returning to about 60% at the end of elongation. Methyl esters accounted for about two-thirds of the total esterified galacturonic acid in cell walls of unexpanded coleoptiles. The proportion of methyl esters decreased throughout elongation and did not account for the increase in the proportion of esterified galactosyluronic acid units during growth. The results indicate that the galactosyluronic acid units of grass pectic polysaccharides may be converted to other kinds of esters or form ester-like chemical interactions during expansion of the cell wall. Accumulation of novel esters or ester-like interactions is coincident with covalent attachment of polymers containing galactosyluronic acid units to the cell wall.

Oligogalacturonides are able to induce flowers to form on tobacco explants

SummaryOligogalacturonides (α‐1,4‐D‐galactosyluronic acid oligomers) are fragments of the homogalacturonan component of the primary cell walls of higher plants. Treatment of cell walls with endopolygalacturonase (EPG) releases the polysaccharides rhamnogalacturonan‐1 (RG‐I) and rhamnogalacturonan‐II (RG‐II), and variously sized oligogalacturonide fragments of homogalacturonan. The EPG‐released sycamore cell wall components are able to regulate several morphogenetic processes of tobacco thin‐cell layer (TCL) explants. We followed one of these morphogenesis‐regulating activities, namely, the induction of flower formation on TCLs, to purify the biologically active component from EPG‐released material. Saponification of the methyl and acetyl esters of the EPG‐released material did not reduce the flower‐inducing activity. However, EPG treatment of the deesterified EPG‐released material destroyed the flower‐inducing activity, establishing that the active substance contained several, consecutive α‐1,4‐linked galactosyluronic acid residues that are required for the flower‐inducing activity. The flower‐inducing activity was purified and shown to be α‐1,4‐linked Oligogalacturonides with a degree of polymerization (DP) of 12‐14, which exhibited half‐maximum activity at approximately 0.4 μM. Smaller oligogalacturonides, RG‐I and RG‐II, did not, even at higher concentrations, induce flowers to form. The ability of oligogalacturonides to stimulate the formation of flowers on tobacco explants provides further evidence of the pleiotropic nature of this oligosaccharin.

Investigation of the distribution of methyl ester groups in pectin by high-field 13C NMR

Fully methyl-esterified, water-soluble citrus pectin was partially hydrolysed with alkali to different degrees of esterification (DE). High-field 13C-NMR spectroscopy of the products showed that ester carbonyl carbons appeared as four signals due to different influences from neigh-bouring sugar residues (free or methyl-esterified galactosyluronic acid). Based on the relative peak areas of these signals, it was deduced that alkaline de-esterification occurred randomly. The distribution of methyl ester groups was also found to be random in water-soluble native pectins of tobacco (DE 20) and papaya (DE 70).

Rhamnogalacturonan I, a pectic polysaccharide that is a component of monocot cell-walls

Pectic polysaccharides were solubilized from the primary walls of suspension-cultured cells of maize (Zea mays) and rice (Oryza sativa), and shown by glycosyl-linkage analysis to be similar to the pectic polysaccharide rhamnogalacturonan I (RG-I) that had previously been solubilized from walls of suspension-cultured sycamore (Acer pseudoplatanus) cells. Maize and rice are monocots; sycamore is a dicot. The backbone of sycamore RG-I had previously been shown to consist of the disaccharide repeating unit →4)-α- d-Gal pA-(1→2)-α- l-Rha p-(1→. Side chains containing arabinosyl, galactosyl, and fucosyl residues are attached through O-4 of about half of the backbone rhamnosyl residues. To compare the structures of monocot and dicot RG-I, galactosyluronic acid residues in the maize and rice polysaccharides were selectively cleaved by treatment with lithium in ethylenediamine, and the structural characteristics of the oligoglycosylalditol products compared to those of oligoglycosylalditols previously isolated from sycamore RG-I. The strong resemblance of the glycosyl-linkage compositions of lithium-treated RG-I from maize, rice, and sycamore indicated the overall structural likeness of the three polysaccharides. Spectra obtained by fast-atom-bombardment mass spectrometry (f.a.b.-m.s.) of lithium-treated maize and sycamore RG-I showed that most of the oligoglycosylalditols derived from the side chains of the two polysaccharides were the same. Nine oligoglycosylalditols partially purified from maize RG-I were similar to the corresponding components of sycamore RG-I. F.a.b.-m.s. of the maize reaction-products indicated the presence in maize RG-I of an additional eight oligoglycosylalditols previously isolated from sycamore RG-I. On the other hand, seven oligoglycosylalditols were partially purified from maize RG-I that had not been observed in previous studies of sycamore RG-I, and maize RG-I was shown not to contain fucosyl residues (as does sycamore RG-I). The results described here suggest that, despite the observed differences, many of the structural features of RG-I have been conserved in the primary cell-walls of monocots and dicots.

Structural features of the plant cell-wall polysaccharide rhamnogalacturonan-II

Rhamnogalacturonan-II (RG-II), isolated from the cell walls of suspension-cultured sycamore cells, has been further characterized. End-group analysis of RG-II showed that the polysaccharide contains about 30 glycosyl residues. Some 28 residues have been found as constituents of well characterized oligosaccharide fragments of RG-II. RG-II was treated with lithium metal dissolved in ethylenediamine to degrade the glycosyluronic acid residues. The major product was isolated, characterized, and shown to be the triglycosylalditol α-Xyl-(1→3)-α-Fuc-(1→4)-β-Rha-(1→3 1)-apiitol. This tetrasaccharide fragment of RG-II has three residues in common with a previously characterized heptasaccharide that had been derived from RG-II by partial hydrolysis with acid. RG-II was found to contain a large number of branched galactosyluronic acid residues that have not yet been identified as components of oligosaccharide fragments, although they are undoubtedly part of an octa(galactosyluronic acid) fragment generated by partial acid hydrolysis. The results of sequential partial acid hydrolysis provided evidence that, in RG-II, the extremely acid-labile 3-deoxy- d- manno-2-octulosonic-acid and 3-deoxy- d- lyxo-2-heptulosaric acid residues are attached to O-3 of 3,4-linked galactosyluronic acid residues, and that the mildly acid-labile apiofuranosyl residues are attached to O-2 of 2,4-linked galactosyluronic acid residues. These and previously published data suggested that RG-II has a highly branched structure, arranged around an α-(1→4)-linked galactosyluronic acid backbone.

Treatment of rhamnogalacturonan I with lithium in ethylenediamine

Rhamnogalacturonan I is a pectic polysaccharide that is solubilized from the walls of suspension-cultured sycamore cells ( Acer pseudoplatanus) by the action of a highly purified endo-1,4-α-polygalacturonanase. Rhamnogalacturonan I has a linear backbone consisting of the diglycosyl repeating unit, →4)-α- d-Gal pA-(1→2)-α- l-Rha p-(1→. Approximately half of the α- l-rhamnosyl residues of the backbone are branched at O-4. Selective cleavage at the galactosyluronic acid residues of the backbone by treatment of rhamnogalacturonan I wit lithium in ethylenediamine resulted in the release of the neutral glycosyl-residue sidechains that had been attached to the backbone. Various analytical techniques, including combined liquid chromatography-mass spectrometry, combined gas-liquid chromatography-mass spectrometry, and 1H-nuclear magnetic resonance spectroscopy, were used to determine the structure of the side chains. The majority of the sidechains were isolated as oligoglycosylalditols, with rhamnitol at the “reducing” end. Terminal 2-, 4-, or 6-linked galactosyl residues were found attached to O-4 of the rhamnitol residues The 2-, 4-, and 6-linked galactosyl residues had terminal or 2-linked arabinosyl, or additional galactosyl, residues attached to them. Based on the results of fast-atom-bombardment mass spectrometry, the side chains were found to range in size from one to fourteen glycosyl residues. The side-chain structures suggest that there are four or more distinct families of side chains attached to the backbone of rhamnogalacturonan I.

Host-Pathogen Interactions

Recent studies have demonstrated that an apparently homogeneous preparation of an alpha-1,4-d-endopolygalacturonic acid lyase (EC 4.2.2.2) isolated from the phytopathogenic bacterium Erwinia carotovora induced phytoalexin accumulation in cotyledons of soybean (Glycine max [L.] Merr. cv Wayne) and that this pectin-degrading enzyme released heat-stable elicitors of phytoalexins from soybean cell walls, citrus pectin, and sodium polypectate (KR Davis et al. 1984 Plant Physiol 74: 52-60). The present paper reports the purification, by anion-exchange chromatography on QAE-Sephadex columns followed by gel-permeation chromatography on a Bio-Gel P-6 column, of the two fractions with highest specific elicitor activity present in a crude elicitor-preparation obtained by lyase treatment of sodium polypectate. Structural analysis of the fraction with highest specific elicitor activity indicated that the major, if not only, component was a decasaccharide of alpha-1,4-d-galactosyluronic acid that contained the expected product of lyase cleavage, 4-deoxy-beta-l-5-threohexopyranos-4-enyluronic acid (4,5-unsaturated galactosyluronic acid), at the nonreducing terminus. This modified decagalacturonide fraction exhibited half-maximum and maximum elicitor activity at 1 microgram/cotyledon (6 micromolar) and 5 micrograms/cotyledon (32 micromolar) galactosyluronic acid equivalents, respectively. Reducing 90 to 95% of the carboxyl groups of the galactosyluronic acid residues abolished the elicitor activity of the decagalacturonide fraction. The second most elicitor-active fraction contained mostly undeca-alpha-1,4-d-galactosyluronic acid that contained 4,5-unsaturated galactosyluronic acid at the nonreducing termini. This fraction exhibited half-maximum and maximum elicitor activity at approximately 3 micrograms/cotyledon (17 micromolar) and 6 micrograms/cotyledon (34 micromolar) galactosyluronic acid equivalents, respectively. These results confirm and extend previous observations that oligogalacturonides derived from the pectic polysaccharides of plant cell walls can serve as regulatory molecules that induce phytoalexin accumulation in soybean. These results are consistent with the hypothesis that oligogalacturonides play a role in disease resistance in plants.

Host-Pathogen Interactions XXX. Characterization of Elicitors of Phytoalexin Accumulation in Soybean Released from Soybean Cell Walls by Endopolygalacturonic Acid Lyase

Abstract Endopolygalacturonic acid lyase, purified from the phytopathogenic bacterium, Erwinia carotovora, induces phytoalexin accumulation in soybean (Glycine max L.) cotyledons. This pectin-degrading enzyme releases heat-stable elicitors of phytoalexin accumulation from soybean cell walls, citrus pectin, and citrus sodium polypectate. The most elicitor-active molecules ob­tained by treating soybean cell walls with endopolygalacturonic acid lyase have been purified and characterized. The cell-wall-derived elicitors are α-1,4-linked oligogalacturonides with degrees of polymerization of eight to twelve residues. The molecules with the highest specific elicitor activity were identified as α-1,4 -linked deca- and undecagalacturonides that contained 4,5-unsaturated galactosyluronic acid at the nonreducing termini.

The Mucilage of Opuntia aurantiaca

The mucilage isolated from Opuntia aurantiaca has been shown by methylation and partial hydrolysis studies to possess a highly branched structure. The mucilage is composed of 4-linked galactosyluronic acid, 2-linked rhamnosyl, and 6-linked galactosyl residues. Most of the last-named residues carry branches at C-3 or C-4, or both C-3 and C-4. The branches are composed mainly of (1→3)-linked, (1→5)-linked, and end-group arabinofuranosyl residues, and end-group xylopyranosyl residues. The mucilage possesses both acid-stable and acid-labile galactosyluronic acid residues. The ratio of galacturonic to rhamnose in the degraded polysaccharide is 1:2.

The gel nature and structure of the carbohydrate of ispaghula husk ex Plantago ovata Forsk

Mucilage from the seed husk of Plantago ovata Forsk is an arabinosyl(galactosyluronic acid)rhamnosylxylan. A wide range of molecular sizes was revealed by chromatography of the methylated mucilage on Sephadex LH-60, and the resulting 12 fractions were homologous. Partial hydrolysis with acid and enzymic hydrolysis variously of the mucilage and carboxyl-reduced mucilage led to identification of the β- d-xylosyl residues forming the xylan backbone of the polymer. Similarly, the arabinose, rhamnose, and galacturonic acid residues were shown to be α- l, α- l, and α- d, respectively. A structure for the polysaccharide is proposed and related to gel formation. Electron microscopy of the gel revealed a co-linear association. This association is reversible, and therefore not the result of covalent bonding.

Molecular structures for microbial polysaccharides: x-ray diffraction results from Klebsiella serotype K57 capsular polysaccharide

Fibre X-ray diffraction patterns have been obtained from oriented, semi-crystalline films of the Klebsiella capsular polysaccharide secrotype K57. K57 is a polytetrasaccharide that contains galactosyluronic acid, mannosyl, and galactosyl residues in the backbone, and an additional mannosyl group as a side-appendage. The simplest interpretation of the diffraction pattern is that the molecule crystallizes as a three-fold helix with an axially projected repeat of 1.143 nm which correlates directly with the chemical repeat. The chain is highly extended, even though it incorporates a 1,2-diaxial linkage in the main backbone. Molecular models have been built using least-squares techniques to minimise interatomic compression and simultaneously meet the observed helical parameters. These models have been compared with the experimental data by using cylindrically averaged, Fouriertransform calculations.