Structure of dextran

Of great interest is the possible determination of structures of pneumococcus polysaccharides by comparison with a known chemical structure such as dextran. The extent of the cross-precipitation reactions43 of dextran with various pneumococcus antisera indicates the closeness of the relationships between the structures of dextran and of the various pneumococcus polysaccharides. 

Fig. 16 a Chemical structures of dextran immobilized on a QCM plate through an avidin-biotin linkage, and b hydrolysis schemes of the dextran on the QCM plate catalyzed by isomalto-dextranase (wUd type, D198N, D266N, and D313N) and kinetic parameters obtained in this work... 

Fig. 22.16. General structure of dextran. Glu-cosyl residues are linked by a-1,3, a-1,6, and some a-1,4 bonds.

To date the ehemical struetures of dextrans synthesized by Leuconostoc species have been the most thoroughly investigated, although an increasing interest is being evinced in the chemical structures of dextrans produced by streptococci, because of their implication in the development of oral disease (see p. 433). Serological tests - have indicated that Lactobacilli convert sucrose into dextrans few confirmatory structural studies have, however, been reported. - ... 

Chemically defined, highly branched dextrans have been studied by a spin-lattice relaxation method. Correlations were made to the positions of carbon atoms associated with branching residues, permitting the values of O-substitution to be established for D-glucopyranosyl residues. Fourier-transform-i.r. difference spectra of dextrans have been correlated to the type and degree of branching, which had been established previously by permethylation for these polysaccharides. A combination of n.m.r.- and Fourier-transform-i.r.-spectroscopy has been used to give complementary information on the structure of dextrans. 

FIGURE 53.3 Chemical structure of dextran, where m and n stand for whole numbers signifying the number of monomers present in the polymeric form. 

Dextransucrases.—Dextransucrase has been purified from Leuconostoc mesenteroides and examined. The structures of dextrans enzymatically synthesized therefrom were studied by periodate oxidation. It was found that the content of (1 -> 6)-glucosidic linkages in synthesized dextran was raised considerably as the purification of enzyme proceeded, and that the content of branch links was increased by adding to the purified enzyme a fraction obtained from the culture broth of the organism by chromatography. 

Dextran is a nontoxic water-soluble polysaccharide consisting mainly of a-1,6 linked D-glucopyranose residues with a low percentage of a-1,2, a-1,3, and a-l,4-linked side chains (Figure 2.37) [269]. It is mainly used as a blood plasma substitute [270]. The chemical structure of dextran includes a-(l -> 6) linkages that can vary from 97% to 50% of total glucosidic bonds [271, 272]. Dextran has wide applications in novel drug delivery systems as a polymeric carrier. 

Figure 6.16. Structures of dextrans and related glucans A, L. mesenteroides B-512F dex-tran B, L. mesenteroides B-742 regular comb dextran.

---