Polysaccharide double helice

More than half a century ago, Bawden and Pirie [77] found that aqueous solutions of tobacco mosaic virus (TMV), a charged rodlike virus, formed a liquid crystal phase at as very low a concentration as 2%. To explain such remarkable liquid crystallinity was one of the central themes in the famous 1949 paper of Onsager [2], However, systematic experimental studies on the phase behavior in stiff polyelectrolyte solutions have begun only recently. At present, phase equilibrium data on aqueous solutions qualified for quantitative discussion are available for four stiff polyelectrolytes, TMV, DNA, xanthan (a double helical polysaccharide), and fd-virus. 

Fig. lla-c. Phase boundary mass concentrations for aqueous solutions of three stiff polyions a xanthan (a double-helical polysaccharide) [78] b fd-virus [24] c tobacco mosaic virus (TMV) [23], Circles, experimental results curves, predictions by the first-order perturbation theory (see text)... 

The zero-shear viscosity r 0 has been measured for isotropic solutions of various liquid-crystalline polymers over wide ranges of polymer concentration and molecular weight [70,128,132-139]. This quantity is convenient for studying the stiff-chain dynamics in concentrated solution, because its measurement is relatively easy and it is less sensitive to the molecular weight distribution (see below). Here we deal with four stiff-chain polymers well characterized molecu-larly schizophyllan (a triple-helical polysaccharide), xanthan (double-helical ionic polysaccharide), PBLG, and poly (p-phenylene terephthalamide) (PPTA Kevlar). The wormlike chain parameters of these polymers are listed in Tables... 

Since its introduction several years ago, the virtual bond, constrained optimization method has proved very useful in studies of polysaccharide crystal structure. Notable among the successes that can be ascribed to it are the structural determinations of the double-helical amylose (.11), the cellulose polymorphs of different chain polarities (.12, 13), and of a number of other polysaccharides and their derivatives. As described in a review of amylose structures elsewhere in this volume, the use of this refinement method has produced structural detail that has previously been unavailable (ll). These results have provided much-needed... 

Figure 2 Illustration of the defibrillation of a polysaccharide double helix as a function of temperature (T) in a unit volume of solvent (water), flowing under shear rate (y) and pressure (t). Elongation of the single helices exposes a smaller cross-sectional area, resulting in birefringence and a lower circumferential resistance to flow (lower T]). As a result of defibrillation (e.g., doubling of the microfibrils), number-average but not weight-average properties increase.

Current methods take root in the early 1960s, when the conformational analysis of macromolecules became of general interest [29-30]. Anderson et al. [31] used model building and X-ray diffraction studies to determine the double helical structures of polysaccharides using crystalline structure data as an initial set of coordinates followed by computational sampling of new structures by rotation around selected covalent bonds. The details of these so-called hard-sphere calculations are described in Rees and Skerrett [32] and Rees and Smith [33]. This approach was also applied to carbohydrate conformations in the analysis of bacteria and polysaccharidic structures and linkages [34-35]. 

It has been known for almost 200 years that starch gives a deep blue color when a solution of potassium iodide and iodine is added [47]. More than a century later it was suggested that the complex consisted of a helical polysaccharide, with triiodide in the center of the helix [48]. Using flow dichroism, it was demonstrated that the triiodide was stacked in a linear structure, as required for the helical model [49]. Another study of the optical properties of crystals of the amylose-triiodide complex showed it to be consistent with a helical structure [50] and X-ray diffraction showed the triiodide complex gave the dimensions of a unit-cell of a helix with six glucose residues per turn [51]. This confirmed a helical structure for the amyiose complex with triiodide that predated the helical models proposed by Pauling for polypeptides [52] and the double helical model for DNA by Watson and Crick [53] by 10 years. 

A common type of junction is shown in Figure 17.12a. Many polysaccharides form double helices below a given temperature and at given physicochemical conditions. Apparently, the helices often involve two molecules. This may be difficult to achieve because of geometrical constraints the parts of a molecule not incorporated in the helix then would also become twisted to the same extent (but in the opposite sense), which is largely prevented by the entanglements in the system. However, in many polysaccharides, complete rotation (i.e., by 360°) about the bonds between monomers appears to be possible, at least at some positions along... 

The salt-free crystalline polysaccharides reviewed by Bluhm et al. [15] are stabilized in characteristic crystalline unit cells by specific amounts of water. Two kinds of locations have been proposed for the water molecules one is unique, i.e., the water lies clustered in an existing interstitial cavity between double helices of B-starch. The other has water bound at specific sites within each unit cell. Additional water in this second type expands one or more unit cell dimension. This almost continuous expansion of the unit cell with increasing content of water may represent a more ordered aspect of the same interaction that occurs between water and accessible, disordered surfaces of celluloses crystallites (and other imperfectly crystalline polysaccharides). 

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