Polysaccharide helical conformation

The gummy polysaccharide from the conn sacs of Watsonia pijra-midata crystallizes in a trigonal unit-cell, the base-plane dimensions of which depend on the relative humidity (r. h.). At 76% r.h., the dimensions are a = b = 1.40 mn, and, for the dry form, a = h = 1.34 nm. A 3(-0.495) helical conformation was proposed. The richness of the X-ray pattern led to the conclusion that the substitution is highly regular in the crystalline regions. 

The sodium salt form of the Klebsiella K5 polysaccharide crystallizes in a 2(1.35) helical conformation. The intrachain hydrogen-bonds are (i) OH-3-(GlcpA)—0-5-(Manp), (ii) OH-3-(Glcp)—0-5-(GlcpA), and (Hi) OH-2-(Manp)—0-2-(Glcp). 

The nonbonded energy (van der Waals) is computed for isolated helical amylose chains as a function of the dihedral angles (, relative orientations of the glucose residues in the polysaccharide chain. In conformity with x-ray data, different helical conformations ere proposed for different crystalline modifications of amylose. 

Several studies have been made of LB films of esters of naturally occurring polysaccharides. Kawaguchi et al. [242] formed long chain esters of cellulose which, however, could only be formed into multilayers by the horizontal lifting technique. Schoondorp et al. [243] studied LB multilayers of esters of amylose and showed that materials with short alkyl side chains have a helical conformation at the air/water interface and that this structure can be transferred into multilayers. As in the case of the isotactic polymethylmethacrylate, the helical structure appears to lead to an oriented structure in the LB film. These two families of materials are illustrated in Figure 5.9. 

It has also been reported from circular dichroism (CD) studies [36] that polysaccharide-based CSPs can induce chirality in enantiomeric guests such as (4Z,15Z)-bilirubin-Ixoc (BR) (Fig. 5). Although not optically active, BR has two enantiomeric helical conformations maintained by six intramolecular hydrogen bonds between two carboxylic acid moieties and two pyrromethenone — NH— protons. These (R)- and (5)-helical conformers are in dynamic equilibrium in an achiral solution [37], but some optically active compounds can enantioselectively bind to BR to induce CD spectra in solution [38-40]. A significant induced CD... 

The final Klebsiella polysaccharide that will be mentioned specifically is KZS, whose chemical structure was determined by Niemann et al (24) (Figure 30). This polysaccharide is of particular interesT because its backbone consists of a similar alternating 1, 3 diequatorial, 1, 4 diequatorial glycosidic linkage geometry to that found in the connective tissue polysaccharides, hyaluronic acid, chondroitin sulphate and dermatan sulphate (25). All these three polysaccharides have exhibited 3-fold helical conformations with axially projected chemical repeats in the range 0.95 - 0.97 nm which is comparable to 0.97 nm found in K25 which also forms a 3-fold helix (14). Further, left-handed helices were found to be more favourable in K25 as has previously been observed in the connective tissue polysaccharides. The similarities between these various different structures is apparent in Figure 31 which shows projections down the axis of K25 and several of the connective tissue polysaccharides. 

Finally, there is a pressing need for more-detailed information on the molecular fine-structure, or conformation, of polysaccharides in solution. The potential of small-angle x-ray diffraction for distinguishing between a random coil and a broken or partial helix in solution has been established. A better definition of the helical conformation of dissolved polysaccharides, which, because of basic chemical asymmetry, have a favored chirality ( handedness ), is awaited. This area of x-ray study is only beginning to be developed. ... 

The fiber diffraction technique has been used to determine the structures of a wide variety of synthetic and biological molecules including structural proteins such as collagen and keratin, a range of helical conformations of the nucleic acids, DNA and RNA, and polysaccharides. In the case of the B form of DNA, early fiber diffraction patterns, which were not fully crystalline, still provided sufficient information to show... 

Physicochemical studies of other aliphatic, carboxylic esters of polysaccharides include (a) assessment of the gel-permeation properties of the series cellulose propionate-cellulose heptanoate, (b) dissolution of cellulose acetate phthalate, (c) monolayer properties of amylose acetate, and (d) melting and transition temperatures of the series amylose acetate-amylose hexanoate. Amylose acetate assumes a helical conformation at air-water interfaces and in oriented films, and the X-ray-analysis data for mannan acetate imply a threefold screw-axis... 

It seems that the kind of linkage is extremely important in determining the conformation of the chain which in turn will be responsible for the secondary and tertiary structure adopted by the polysaccharide. Obviously, the ribbon-like units of chitin can pack readily into dense, rectangular cross-sectioned microfibrils ideal as the skeleton material of cell walls. Therefore, helical conformation, secondary and tertiary structure are dictated by well-defined variations in the main chain chemistry. 

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