Helical cellulosics

Triple-Stranded Left-Hand Helical Cellulose Microfibril in Acetobacter xylinum and in Tobacco Primary Cell Wall... 

RUBEN etal. Left-Hand Helical Cellulose Microfibril... 

Mache- Cellulose CECAC-40 XFC Helical Cellulose triacetate... 

Polarized light microscopy of compression wood of spruce (Picea abies), image of helical cellulose fibers in adjacent cell walls (courtesy of Dr I. Burgert, Max Planck Institute of Colloids and Interfaces, Potsdam/Golm, Germany). 

The research listed in Table 2 (81-98) deals mainly with separation problems concerning amino acids, amino acid derivatives, and dipeptides, focusing on the influence of the structure of the chiral support and the eluent temperature on the separation behavior of the racemates. Separation of the aromatic amino acids phenylalanine, P-2-thienylalanine, 4-fluorophenylalanine, and tyrosine could not be achieved on microcrystalline or amorphous cellulose tryptophan isomers, however, could be reproducibly resolved on microcrystalline cellulose layers (83). Lowering the eluent temperature from 30 C to O C enhances enantiomeric resolution. However, developing times of 10 1 h (O C), 7.5 0.5 h (10°C), 5 0.5 h (20°C), and 3.5 h (30°C) have to be tolerated hydrophobic eluent combinations further enhance separation, because they improve formation of the helical cellulose conformation (87). Separation of racemic 3,4-dihydroxyphenylalanine, tryptophan, and 5-hydroxy-tryptophan can be achieved in only 2 h, on a cellulose HPTLC plate (89) these experiments will be described in detail in Section FV.C. 

The small change in stereochemistry between cellulose and amylose creates a large difference in their overall shape and in their properties. Some of this difference can be seen in the strorcture of a short portion of fflnylose in Figure 25.9. The presence of the a-glycosidic linkages imparts a twist to the fflnylose chain. Where the main chain is roughly linear- in cellulose, it is helical in anylose. Attractive forces between chains are weaker in fflnylose, and fflnylose does not form the same kind of strong fibers that cellulose does. 

Derivatized amylose is the basis for the Chiralpak AD CSP. This CSP has been utilized for the resolution of ibuprofen and flurbiprofen, as well as other members of the family of nonsteroidal inflammatory drugs (NSAIDs) [39, 61]. Ibuprofen was not resolved on the Chiralpak AD CSP in LC. Pressure-related effects on stereoselectivity were observed by Bargmann-Leyder et al. on a Chiralpak AD CSP [58]. No corresponding effect of pressure on selectivity was observed with a Chiralcel OD CSP. The authors speculated that the helical conformation of the amylose-based CSP is more flexible than that of the cellulose-based CSP. 

The intermolecular interactions stabilise the helices and greatly influence the properties of exopolysaccharides in solution, ie solubility, viscosity and gel-formation. A strong interaction or good-fit between molecules will lead to insolubility, whereas poor interaction will lead to solubility of exopolysaccharides. The interactions between molecules is influenced by the presence of side-chains. For example, cellulose is insoluble but introduction of a three monosaccharide side-chain into the cellulose chain gives the soluble xanthan. Small changes in the structure of the side-chains can alter the molecular interactions and thus properties of the exopolysaccharide. 

Five articles on polysaccharide helices solved prior to 1979 have appeared in the volumes published between 1967 and 1982.2-6 The first was a review on X-ray fiber diffraction and its application to cellulose, chitin, amylose, and related structures, and the rest were bibliographic accounts. Since then, X-ray structures of several new polysaccharides composed of simple to complex repeating units have been successfully determined, thanks to technological advances in fiber-diffraction techniques, the availability of fast and powerful computers, and the development of sophisticated software. Also, some old models have been either re-... 

Fig. 3.—Parallel packing arrangement of the 2-fold helices of cellulose I (1). (a) Stereo view of two unit cells approximately normal to the ac-plane. The two comer chains (open bonds) in the back, separated by a, form a hydrogen-bonded sheet. The center chain is drawn in filled bonds. All hydrogen bonds are drawn in dashed lines in this and the remaining diagrams, (b) Projection of the unit cell along the c-axis, with a down and b across the page. No hydrogen bonds are present between the comer and center chains.

Fig. 9. — Antiparallel packing arrangement of the 3-fold helices of (1— 4)-(3-D-xylan (7). (a) Stereo view of two unit cells roughly normal to the helix axis and along the short diagonal of the ab-plane. The two helices, distinguished by filled and open bonds, are connected via water (crossed circles) bridges. Cellulose type 3-0H-0-5 hydrogen bonds stabilize each helix, (b) A view of the unit cell projected along the r-axis highlights that the closeness of the water molecules to the helix axis enables them to link adjacent helices.

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