Modeling, polysaccharide chains

Following the identification of KDO as a constituent of LPS, studies by Osborn and her group ( 5) have revealed that, in LPS, KDO (or the KDO region ) is located at the reducing end of the polysaccharide chain, linking the core segment to lipid A. Later, the application of differential color reactions based on thiobarbituric acid (TBA tests) (6, 7) has led to the view that KDO is present, in LPS from Salmonella or E. coli, in the form of a branched trisaccharide (Fig. 1). In this model, a branchpoint KDO residue (KDO I), ketosidically-linked to the second glucosaminyl residue of lipid A, is substituted in position 4 or 5 by a... 

The earliest attempts at model analysis of polysaccharides -typified by the x-ray crystal structure analysis of amylose triacetate - were usually conducted in three steps ( L). In the first step, a model of the chain was established which was in agreement with the fiber repeat and the lattice geometry, as obtained from diffraction data. In the second step, the invariant chain model was packed into the unit cell, subject to constraints imposed by nonbonded contacts. This was followed, in the third step, by efforts to reconcile calculated and observed structure factor amplitudes. It was quickly realized that helical models of polysaccharide chains could be easily generated and varied using the virtual bond method. Figure 1 illustrates the generation of a two-fold helical model of a (l- U)-linked polysaccharide chain. 

The dimensions of the xylan unit cell are slightly different a = b = 1.340 nm, (fibre axis) = 0.598 nm.) Atkins and Parker T6) were able to interpret such a diffraction pattern in terms of a triple-stranded structure. Three chains, of the same polarity, intertwine about a common axis to form a triple-strand molecular rope. The individual polysaccharide chains trace out a helix with six saccharide units per turn and are related to their neighbours by azimuthal rotations of 2ir/3 and 4ir/3 respectively, with zero relative translation. A similar model for curdlan is illustrated in Figure 6. Examinations of this model shows that each chain repeats at a distance 3 x 0.582 = 1.746 nm. Thus if for any reason the precise symmetrical arrangement between chains (or with their associated water of crystallization) is disrupted, we would expect reflections to occur on layer lines which are orders of 1.746 nm. Indeed such additional reflections have been observed via patterns obtained from specimens at different relative humidity (4) offering confirmation for the triple-stranded model. 

The hydrogen bonding in cellobiose and methyl cellobioside as models for that in the celluloses. In the absence of crystal structure analyses of higher oligomers of 1 - 4-linked /5-glucopyranose, cellobiose is frequently used as a model for interpreting the X-ray fiber diffraction patterns of the celluloses, especially since the cellobiose unit is considered to be the repeating unit of the polysaccharide chain. 

It is of interest to note that vhen different polysaccharide chains are compared, the correlation of characteristic ratio and persistence length, normally observed for various models of a particular type of chain, is no longer preserved. Tbus the... 

Figure 3. Model of protein fibers (dots) and polysaccharide chain molecules (lines) for rabbit cornea...

Equation (9) is the basis for investigating how the swelling pressure will vary with hydration, pH, degree of ionization due to radiation, temperature, etc. It is necessary only to adopt a simple model of the tethered polysaccharide chains and evaluate the configurational entropy. 

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