Chain conformations of polysaccharides in different solvents

The variability of primary structure and conformation makes the carbohydrate molecule extremely versatile, for example, for specific recognition signals on the cell surface. In any biological system whatsoever the shape and size adopted by carbohydrates in different solvent environments have been widely demonstrated to be responsible for the biological function of these molecules. 

The rationale for the correct setting of current knowledge about the shape of polysaccharides in solution is based on three factors the correlation between primary structure (i.e., the chemical identity of the carbohydrates polymerized in the chain), intrinsic conformational features dictated by the rotational equilibria (often the major contributions are due to the rotation about the glycosidic linkages) and the interaction with the other mo- 

In this chapter the description of the solvent effect is given within the framework of some specific experimental results and computational methods for studying and predicting oligo- and polysaccharide conformations in solution. It is not the authors intention to make an in depth investigation into the general methodologies which have been widely reported over recent years (and in this book) but rather to provide a step-wise presentation of some conformational features which have upheld theoretical predictions with experimental observations. The number of examples and approaches is necessarily limited and the choice undoubtedly reflects the authors preferences. Nonetheless, the aim is to be as informative as possible about the conceptual difficulties and conceivable results. 

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The chapter, however, does not give extensive references to the experimental determination of the polysaccharide shape and size in different solvents, but rather it attempts to focus on the molecular reasons of these perturbations. A digression is also made to include the electrostatic charges in polyelectrolytic polysaccharides, because of their diffusion and use and because of interesting variations occurring in these systems. Thus, provided that all the interactions are taken into account, the calculation of the energetic state of each conformation provides the quantitative definition of the chain dimensions. 

From the experimental point of view, several polysaccharides with different chain linkage and anomeric configuration have been studied to determine to what extent flic polymeric linkage sfructure and the nature of the monomeric unit are responsible for the preferred solvation and for the chain topology and dimensions. Conversely, since it is generally understood fliat flic sfructure and topology of many macromolecules are affected by solvation, theoretical models must include these solvent effects in addition to the internal flexibihty, in order to estimate changes in the accessible conformations as a result of the presence of the solvent molecules. 

It has to be mentioned that different non-ionic polysaccharides whose chains belong to the periodic-sequence group (Table 2) can also assume ordered conformations in dilute aqueous solution, whose stability is governed by temperature and which moreover, can collapse into random coiled conformations in organic solvents. This is, for example, the case of Schizophyllum commune polysaccharide (Schizophyllan) whose chains consist of linearly linked 8-1,3-D-glucose residues with one B-1,6-D-glucose side chain for every three main chain residues. 

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