Hemicelluloses - American Chemical Society

Chapter 11

Isolation and Properties of Xylan: Rediscovery and Renewable Resource

Downloaded by UNIV OF ARIZONA on January 11, 2013 | http://pubs.acs.org Publication Date: October 7, 2003 | doi: 10.1021/bk-2004-0864.ch011

Robert H . Marchessault Department of Chemistry, McGill University, 3420 University, Room 201, Montreal, Quebec H3A 2A7, Canada

In the past 50 years wood chemists have learned much about the composition and physical properties of hardwood xylans. Partially acetylated glucuronoxylans are model native hardwood xylans. They exhibit thermoplasticity, film forming properties, crystallization potential and are oriented in the secondary cell wall. Their crystal structure has been determined and the hydration of this crystalline polysaccharide has been defined. The structural regularity of these abundant polysaccharides can be interpreted using the principle of optical superposition.

© 2004 American Chemical Society

In Hemicelluloses: Science and Technology; Gatenholm, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

159


Introduction In 1943 the Chemistry Department at McGill University set a strong course for the study of carbohydrates and wood polysaccharides when Clifford B. Purves became E . B . Eddy Professor of Industrial and Cellulose Chemistry at McGill, and Director of Wood chemistry research in the Pulp & Paper Research Institute of Canada. About ten years later he was joined by Dr. T . E . Timell from Sweden and hemicellulose analysis prevailed for the next decade. Timell focused his attention on wood hemicellulose and generously collaborated with the writer since he was a consultant for American Viscose Corp. where I was employed. M y immediate superior was Dr. Wayne Sisson and his experience in x-ray diffraction on cellulose organic polymers encouraged me to learn that technique and apply it to wood hemicelluloses (7,2). American Viscose was the dominant rayon company at the time and wellpurified dissolving grade wood cellulose was constantly improved thanks to fundamental research on methods of carbohydrate analysis. Purves had originally studied at St. Andrews University where Haworth, Hirst, Purdy and Irvine pioneered the methylation techniques which were essential for chemical structure determination. The rayon business eventually went through a serious downturn and investment in fundamental research related to cellulose and its purification were neglected. Today, in the Green Chemistry era, sustainability concepts and focus on renewable resources means that natural polysaccharides such as xylan are being rediscovered not only in wood but also in all kinds of biomass. In essence, biomass is destined to be a source of carbon for the world's plastics industry. The search for sustainability will promote this replacement.

Molar Rotation of Xylans Since the days of Pasteur and Freudenberg, the value of optical rotation: [a] , as a means of characterizing polysaccharides has been appreciated. Timell isolated and purified glucuronoxylans and arabinoxylans to provide a series of samples whose optical rotation could be recorded in 10% aqueous sodium hydroxide. Figure 1 is a schematic of the molecular structure of the 4-O-methylglucuronoxylans which he isolated with a variable content of uronic acid. Figure 1 also shows the Equation which we used to account for the observed molar rotation of xylans (3). D


160


As can been seen in Figure 2 the plots of reduced molar rotation for the fractions which varied in their xylose/aldobiuronic ratio are linear. However, the slope for the arabinoxylans and glucuronoxylans are of opposite sign, in keeping with the different chemistry of the uronic acid linkage and the arabinose to the xylan backbone. In both cases the slope represents the molar rotation of the disaccharide unit in the chain (3) and since the a-D-configuration of 4-0methylglucuronosyl xylose is well established, the opposite slope favors the assignment of a a-L-configuration for the arabinosyl xylose unit, which is known to contain an L-arabinofuranose sugar. These linear plots are essentially physical proofs of structure of the respective polysaccharides and can be used for analytical purposes. Someone once referred to the Equation in Figure 1 as the "Marchessault equation" which I accept humbly but in fact the derivation follows directly from the teachings of C.S. Hudson and K . Freudenberg, used for proof of structure of cellulose. However, the optical rotation of the neutral xylan chain, derived from these data, is much greater than that of cellulose.

Crystalline Structure and Conformation of a (4-O-methylglucuronoxylan) Since native xylans are partially acetylated they dissolve in water and can be cast into flexible films. Strips of these films were hot-stretched into oriented fibers and deacetylated with alkaline methanolic solutions to obtain x-ray fiber diagrams from which to derive a unit cell and observe the pitch of the xylan chain along the fiber axis. Figure 3 shows the derived left-handed helical conformation of the chain corresponding to 3 residues per turn within a distance of 14.85A (4). The handedness of the pitch was further confirmed as improved x-ray fiber diagrams became available. The structure responsible for this fiber diagram is a xylan dihydrate and in fact glucuronoxylans are water sensitive and take up increasing amounts of water with relative humidity. The baseplane of the unit cell of xylan hydrate is shown in Figure 4 where the hexagonal unit cell is built around a column of water. The opening shown by the circle in Figure 4 most likely can accommodate the randomly occurring uronic acid substituents so that the latter co-crystallize, as it were, with the xylan chain in the unit cell. Nevertheless, this was not part of our proposed hydrate unit cell which is shown in Figure 5 with the helical hydrogen bonded water chain (5). The pronounced susceptibility of xylan to hydration is nature's method of plasticizing and the random acetyl groups assist further by preventing crystallization. Polarized infrared spectroscopy provided evidence that the xylan chains are oriented along the tree axis and one can conclude that this is equivalent to axial orientation in the fiber. (6) The schematic in Figure 6, illustrates the proposed crystalline and non crystalline state of xylan in the S2 layer of fibers. The crystalline xylan state develops when deacetylation happens without xylan removal or when epitaxial


161

-x{x-x-x-x-xj-x-

GA


Let: n m u

be number of unsubstituted xylose units be molar rotation of xylose unit be molar rotation of aldobiuronic acid

a cp

be (xylose / aldobiuronic) ratio be the molar rotation of a xylan chain with a ratio a

m*n + w y ^-T

D

n(l + / ) /

1 9 0 3 5

rf* = W

=

(132 + c r +

/ )

(7

/

-) 1

G

Figure 1. Schematic of glucuronoxylan chain segment and optical superposition equation to fit the observed rotation [a] to glucuronic acid content. D

tv x10"

GLUCURONOXYLANS

0.4

1.2

2.0

.08

.16

Figure 2. Reduced molar rotation plot as a function of l/o for arabino and glucuronoxylans.(Adapted from ref. 3)



162

Figure 4. Baseplane unit cell projection of white birch xylan hydrate. The unit cell is indexed as hexagonal: a=b=9.16 A; c (fiber axis) = 14.85 A. The circle represents the position of the water column.(By permission from ref. 5)


163


C

Figure 5. Projection of lefthanded 3, helices of xylan hydrate onto 110 plane. Helical dotted line represents hydrogen-bonded water chain. Twofold rotation axes, relating antiparallel chains, are shown. (By permission from ref. 5)

CRYSTALLIZED XYLAN

NATIVE FIBER

Figure 6. Schematic of oriented xylan in secondary wall of hardwood xylan for native state and crystalline state. (Adapted from Ph.D. thesis of W.J. Settineri, College ofForesty, Syracuse N.Y., 1966)


164 crystallization of xylan, due to redeposition, happens during pulping. Different extents of these two organizations can exist in wood depending on history of the material and location.


Thermoplasticity of Xylan Perhaps the closest approach to a true "native xylan from wood" has been the native O-acetyl xylan from hardwood which is extracted from a holocellulose with dimethyl sulfoxide. A sample extracted from white birch and having: one 4-O-methyl glucuronic acid group and 3.6 acetyl per 10 xylose units, served to make films whose thermoplasticity was studied. The number average molecular weight was 25,000 g/mol and it is assumed that a linear chain is involved. By drying a 10-20% aqueous solution on a glass plate, a flexible film is obtained which can be used for stretching into oriented filmstrips or for study of thermoplasticity (7). A differential thermal analysis study shows that this film (7), conditioned to room relative humidity, displays endothermic behavior over a broad temperature range from 60 to 2 2 5 ° C with a maximum at 150°C. Since the x-ray diffraction pattern of the starting film is essentially non crystalline, both before and after heating, it can be assumed that the change in chemical composition is not great and that one is witnessing the disappearance of water from the xylan hydrate non-crystalline state. The large endotherm in the D T A curve is most probably related to a distinct softening point for the xylan film which is shown in the thickness-temperature curve in Figure 7. This curve was recorded with a commercial apparatus where the sample is in contact, under pressure, with the thermostat oil and the weight loss, between 15-20% by weight, is mainly due to loss in water. The thermoplasticity phenomenon was also displayed by the arabinoglucurono-xylan films from white pine, but the latter were far more extensible than the native O-acetyl counterparts from white birch. For example, while the white birch O-acetyl xylan could extend no more than 50%, the white birch samples readily allowed up to 200% stretch. There is little doubt that the heterogeneity of the xylans hinders crystallization but the well-established fact that hydrated and dry crystalline states exist should help to understand many of the unusual features of these materials.

Conclusions Polysaccharides have found a wide range of value-added uses. Similarly, starch as an easily isolated pure high molecular weight substance is used industrially both as a fermentation substrate and as a macromolecular substance. Up to now, hemicelluloses have not achieved this same usage. The low



165

Figure 7. Thickness - temperature curves for O-acetyl-4-O-methyl glucuronoxylans. Plasticity develops at "flowpoint". .(Adaptedfrom ref. 7)

molecular weight has been a handicap and the heterogeneity of structure also. Nevertheless, as a carbon source, these features are less important. Hemicelluloses have been studied as a substrate for fermentation and it is estimated that the substrate cost for use in producing the bacterial polyester: poly(3-hydroxybutyrate), P H B , would be similar to that of cane molasses and half that of bulk glucose (8). The development of enzymatic methodology for chemical modification of polysaccharides offers promise of preparing value-added oligomers of xylan. These oligomers custom made to act as macromonomers, could contribute a new "green" family of chiral polymer building blocks.

Acknowledgments The writer is pleased to recognize the important contribution of T . E . Timell (9).

References 1. 2. 3.

Marchessault, R.H.; Timell, T . E . J. Phys. Chem. 1960, 43, 85-100. Bowering, W.D.S.; Marchessault, R . H . ; Timell, T . E . Svensk Papperstidn. 1961, 64, 191-194. Marchessault, R.H.; Holava, H . ; Timell, T . E . Can J. Chem. 1963, 41, 1612-1618.


166 4. 5. 6. 7.

Marchessault, R.H.; Liang, C . Y . J. Polymer Sci. 1962, 59, 357-378. Marchessault, R . H . In Chimie et Biochimie de la Lignine, de la Cellulose et des Hemicelluloses; Les Imprimeries Réunies de Chambéry, France, 1964, pp. 287-301.

8.

Ramsay, J.; Hassan, A . M - C . ; Ramsay, B. Can. J. Microbiol. 1995, 41 Supplement 1, 262-266. Timell, T . E . , Adv. in Carb. Chem. 1964, 19, 247-299; ibid 1965, 20, 410-482.

9. Downloaded by UNIV OF ARIZONA on January 11, 2013 | http://pubs.acs.org Publication Date: October 7, 2003 | doi: 10.1021/bk-2004-0864.ch011

Settineri, W.J.; Marchessault, R . H . J. Polymer Sci. 1965, C-11, 253264. Nieduszynski, I.A.; Marchessault, R . H . Biopolymers 1972, 11, 13351344.


Hemicelluloses - American Chemical Society

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