Xylan crystalline

The aim of this chapter is to summarize some of the research findings on xylan, a natural polymer extracted from corn cobs, which presents a promising application in the development of colon-specific drug carriers. Physicochemical characterization of the polymer regarding particle size and morphology, composition, rheology, thermal behavior, and crystallinity will be provided. Additionally, research data on its extraction and the development of microparticles based on xylan and prepared by different methods will also be presented and discussed. 

Concerning the analysis of crystallinity of xylan, the X-ray diffraction detects a few and small peaks, which indicate that xylan presents a low crystallinity (Figure 6). 

Naturally, very long T,c values are expected for solids as viewed from the expected correlation time at the low temperature side of the T c minimum (0.1-0.2 s) as shown in Figure 1. Indeed, their values turns out to be the order of 10-30 s for carbon sites in the absence of internal fluctuations as in polysaccharides such as (1 — 3)-p-D-glucan and (1 —> 3)-p-D-xylan,46 8 fibrous proteins such as collagen49 and silk fibroin,50 free and metal-complexed ionophores,51 or in some instances up to 1000 s as in crystalline polyethylene.52... 

Yundt64 obtains a crystalline xylan from the xylan-rich fraction of straw or birchwood hemicellulose. The fraction is hydrolyzed with 0.2% oxalic acid solution for five hours at 100°. Upon autoclaving the insoluble residue approximately 0.2% dissolves and precipitates as hexagonal platelets when the filtrate is cooled to 60-70°. Very likely this crystalline material is of low molecular weight. 

Hydrolysis of xylan produces principally D-xylose.71 By hydrolysis of straw xylan in 3% nitric acid, Heuser and Jayme72 obtained crystalline D-xylose in 85% yield. Using a similar procedure, Hampton, Haworth and Hirst70 obtained a yield of 93% crystalline D-xylose from esparto xylan. A practically identical yield was indicated by reducing sugar determination made on the hydrolysis products of corn seedling xylan.68... 

RunUnococcus albus and Ruminococcus flavefadens. These bacteria are important cellulose-degraders found in the rumen of cattle and sheep (2). Most isolated strains ferment cellulose and xylan and all ferment cellobiose. Fermentation of glucose and some other carbohydrates depends on the particular strain. R flavefadens and B. succinogenes can ferment the highly ordered crystalline cellulosic su trates but R albus cannot. No evidence has been found for extracellular cellulase production by R albus, but Ohmiya et al. purified cellobiosidase from this culture 17). Laboratory growth of R albus has been conducted at pH 7.0 and 37 C. 

The properties of the enzymes used in this study have been described in former publications (10,11,15). Important for the following interpretation are their hydrolytic specificities. The xylanase did not hydrolyze either isolated mannans or celluloses—or only to a very small extent (10). The same is true for the mannanase with respect to xylans and celluloses (11,15). The avicelases, which were not purified to the same extent as the xylanase and mannanase, did not hydrolyze mannans, but they degraded xylans besides crystalline cellulose (10). Also, the highly purified cellobiohydrolase C (12) degraded xylan to some extent (Dr. E. K. Gum, Jr., personal communication). 

In Volume 33 of this Series, we presented1 a review of the crystalline structures of polysaccharides published during the period 1967-1974. Detailed accounts of progress in structural studies on specific types of polysaccharides were presented in the Proceedings of the Twenty-sixth Symposium of the Colston Research Society and were subsequently published as a book.2 Precise methods for X-ray diffraction analysis of biopolymer structures were discussed by Hukins.3 The aspects of the structures of cellulose, mannan, and xylan, their organization in the cell wall, and the biosynthesis of cell-wall polysaccharides were described by Mackie.4 Work on the structures of the connective-tissue polysaccharides, O-acetylcellulose, and the various forms of amylose was reviewed by Atkins,5 Chanzy,6 and Sarko,7... 

Hamilton and N. S. Thompson, by fractionation of a partial hydroly-zate of a xylan on filter paper, isolated 4-0-methyl-D-glucuronic acid, the aldobiouronic acid mentioned above, and a crystalline aldotriouronic acid identified as 0-(4-0-methyl-a-D-glucopyranosyluronic acid)-(l —> 2)-0-/3-d-xylopyranosyl-(l 4)-D-xylopyranose. As the isomeric aldotriouronic acid was virtually absent, the authors suggested that, in the xylan polyuronide... 

It is apparent from the foregoing that a more specific method for the determination of xylan would be desirable. To this end, the determination of xylose, after acid hydrolysis of the polysaccharide material, has been attempted. Xylose may be oxidized to xylonic acid which can be precipitated with cadmium bromide as the double salt, but the precipitation is not quantitative. Xylose forms an insoluble, crystalline di-O-benzylidene dimethyl acetal which permits identification in the presence of other sugars, but the necessity for anhydrous reaction conditions precludes the adaptation of this method to ordinary analysis. AVise and Ratliff prepared this derivative of both d- and L-xylose, as well as analogous derivatives from other aromatic aldehydes, and concluded that, with either the di-O-benzylidene or the di-O-(p-isopropylbenzylidene) dimethyl acetal, an excellent, highly specific, qualitative test was available for d- or n-xylose. 

Other polysaccharides whose solutions yield liquid crystalline phases include the bacterial polysaccharide xylan and the extracellular fungal polysaccharide produced by schizophyllum commune... 

---