Polysaccharide stretched conformation

Interestingly, polysaccharide bilayers are considerably thicker than polypeptide ones. To understand these differences, it has to be taken into account that the relative layer thickness depends on several factors, mainly charge density, hydrophilicity, M, residue size, etc. On the one hand, polysaccharides display higher chaige densities, and tend to adopt a stretched conformation to minimize the repulsion between charges, which would lead to thinner layers. On the other hand, they present increased hydrophilicity and consequently adsorb more water, which would result in an increase of the thickness. Furthermore, a rise in the PE M and chain length typically causes an increase in the amount of material adsorbed. Overall, it is found that the adsorption of the polysaeeharides results in the formation of thicker bilayers. 

The conformation of di-, oligo-, and polysaccharides center on the glycosidic link, for which several distinct ROA signatures have been identified. The most important and critical one is a couplet centered at 917 cm , positive at lower and negative at higher wavenumbers, which originates in C-O-C stretch modes of the common q -(1 4) glycosidic link present in maltose. 

Just as the primary structure of heparin and heparan sulfate has a wealth of fine detail, depending on its exact provenance and function, so the exact conformation of a stretch of polysaccharide depends on its exact location in the chain. The key to this conformational flexibility lies in iduronic acid residues, which can adopt either the C4 or the conformation (the glucosamine-derived residues are firmly in the C conformation, as are glucuronic acid residues). Interpretation of vicinal proton proton coupling constants of IdoA residues in terms of an equilibrium between just and conformations suggests the equilibrium changes from 60 40 for internal IdoA residues to 40 60 for terminal residues (Figure 4.84(a)). " ... 

The x-ray structure of oriented fibers from the carrageenan fractions, k and X, was examined by Bayley over a decade ago. The fractions differ in the sulfate substitution. The level of order in his stretched fibers appears to have been of the paracrystalline type, but an ordered conformation along the backbone was clearly present. He drew the important conclusion that the x-ray pattern from the whole carrageenan does not represent the sum of those from the separate k and X components, and that, therefore, the two components must exist in a unique structural relationship with respect to one another. Only continued, intensive efforts on the native and pure components will permit a full appreciation of this complex structure. The same comment applies to all of the acidic and ester polysaccharide studies mentioned in this Section. A review of chemical work on carrageenans mentions that Bayley interpreted his diagrams in terms of incorrect structures for both components. 

Solid films have been examined frequently for infrared analysis in biochemical work, for example, in structural studies of proteins, polypeptides, and polysaccharides. Such films have been of particular value for studying polarization spectra of macromolecules in intact films and in oriented ones (stretched, rolled, or stroked), thereby permitting knowledge to be gained concerning spatial arrangements within the molecule and conformational effects among molecules. (See The Use of Polarized Infrared Radiation and the Measurement of Dichroism, p. 73, for a detailed discussion.) A few workers have discussed the film technique (Lecomte, 1948 Randall et al., 1949 Hacskaylo, 1954). 

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