Dextrans network structures

Kurisawa M and Yui N. Dual-stimuli-responsive drug release from interpenetrating polymer network-structured hydrogels of gelatin and dextran. J. Control. Rel. 1998 54 191-200. 

The discussion of the structural studies in this Section are divided into two parts the first deals with the chemical elucidation of dextran structures, and the second, with the application of physicochemical measurements to the examination of network structures formed by association of dextran molecules. 

Network structures formed by dextrans in the presence of aqueous solvents have not, as yet, been investigated in a systematic maimer. The conclusions contained in the second part of this Section are, therefore, based upon experience with a limited number of randomly selected dextrans and, as such, should be accepted with due reservation. 

To facilitate the examination of the network structures that are formed by dextrans, the salient features of polysaccharide networks will initially be summarized. 

Streptomycin, chemistry of, 3, 337-384 Structural chemistry, of fungal polysaccharides, 23, 367-417 of the hemicelluloses, 14, 429-468 Structure, molecular, of cellulose, 19, 219-246 of dextran, 15, 341-369 of glycogens, 12, 261-298 of polysaccharide gels and networks,... 

Antigens and their corresponding antibodies precipitate by cross-linking to form an insoluble network. Polysaccharides have multiple, repetitive immunodeterminants and virtually none have demonstrable tertiary structure in solution (except, perhaps, under viscous stress). The number of these immunodeterminant groupings on each macromolecule is large. In the case of dextran, for instance, there are several thousand of them (if the dextran has a molecular weight of several million), even if the determinant involves the hep-tasaccharide. There is, thus, ample opportunity to form a precipitating, crosslinked complex with divalent (or polyvalent) antibody molecules. 

This approach—which uses Brinkman s equation, with an appropriate correlation to permit estimation of the hydraulic permeability from the structural characteristics of the medium—provides a straightforward method for estimating the influence of hydrodynamic screening in polymer solutions predicted diffusion coefficients for probes of 3.4 and 10 nm in dextran solutions (Pf = 1 nm) are shown in Figure 4.9. This approach should be valid for cases in which probe diffusion is much more rapid than the movement of fibers in the network, although it appears to work well for BSA diffusion in dextran solutions, even though the dextran molecules diffuse as quickly as the BSA probes [54]. 

Electron-microscope observations and chemical analyses of Streptococcus mutans OMZ 176 dextran indicated that the gel formed by this water-insoluble dextran is composed of two distinct networks, one formed by association of (l->3)-linked chain segments [structures 26 (p. 393) and 38 (p. 3 )], and the other by association of (l->6)-linked chain segments [structures 6 (p. 386) and 36 (p. 396)]. It remains to be determined whether this and similar gels are formed from a single polysaccharide type (and are, therefore, continuous), or from at least two different polysaccharides. 

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