A-D-glucans

PuUulan [9057-02-7] first described in detail in 1959, is a water-soluble extracellular a-D-glucan elaborated by the fungus yiureobasidiumpullulans (formerly Eullulariapullulans) (285). It is a linear polymer of maltotriose units linked from the reducing end of one trisaccharidic unit to the nonreducing end of the next trisaccharidic unit by a(l — 6) linkages (286) ... 

Fig. 22.—Antiparallel packing arrangement of the 2-fold helices of (1— 3)-a-D-glucan (21). (a) Stereo view of two unit cells approximately normal to the aoplane. The two chains in the back (open bonds) are antiparallel and so are the chains in the front (filled bonds). Each helix is stabilized by 2-OH 0-4 hydrogen bonds across the bridge oxygen atoms. Interchain hydrogen bonds are formed in sheets along the a direction, (b) An axial projection of the unit cell shows that the sheets in the front and back are also joined by hydrogen bonds.

Extensive studies have been performed on the (1- 6)-)8-D-glucan (pustulan) and the (l- 4)-a-D-glucan (amylose). These are linear polysaccharides that may exist as helical polymers in aqueous solution, as demonstrated by c.d. spectroscopy. Characteristic of the helical structure of these glucans is a negative band at 182 nm, a crossover at 177 nm, and a more intensely positive band at shorter wavelengths (see Figs. 8 and 9). 

Benzyl derivatives of (1 6)-a-D-glucan, (l->6)-a-D-mannan, and (l-> 6)-a-D-galactan have been studied in 1,4-dioxane. These derivatives have complex and interesting c.d. spectra due to the ww transition of the chromophore with resolved vibrational structure. However, a conformational interpretation of these interesting spectra is not possible at this time. 

Cadexomer-H is an iodophor similar to povidone-iodine. It is a 2-hydroxymethylene crosslinked (1-4) a-D-glucan carboxymethyl ether containing iodine. The compound is used especially for its absorbent and antiseptic properties in the management of leg ulcers and pressure sores where it is applied in the form of microbeads containing 0.9% iodine. 

The synthesis of linear 4 —> 1-a-D-glucans from D-glucopyranosyl phosphate by the action of phosphorylases has been shown by comparison of results of methylation and end-group assay and viscosity determination,209 and by potentiometric, iodine titrations82 on the product. The chain length of the enzymic product (100 to 200 D-glucose units) is less than that of the natural component. Whether this is due to impure enzymes cannot yet... 

Fig. 10.—13C-N.m.r. Spectrum at 68 MHz of Branched a-D-Glucan from Streptococcus mutans OMZ-176. [Solvent, DsO, pD 14, at 40° chemical shifts are based on external tetramethylsilane (8C = 0).]...

In the field of polymer science, the most extensively used transferase is phosphorylase (systematic name (1 4)-a-D-glucan phosphate a-D-glucosyltransferase EC 2.4.1.1). Although this enzyme is responsible for the depolymerization of linear a-( 1 4) glycosidic chains in vivo it can also be used to synthesize linear a-( 1 4) glycosidic chains (amylose) in vitro. 

Another very valuable glycosyltransferase for the construction of well-defined polysaccharides is glucansucrase (dextransucrase systematic name sucrose (1 6)-a-D-glucan 6-a-D-glucosyltransferase EC 2.4.1.5). 

Polysaccharides disaccharidea,a-trehalose,a-D-glucan, (3-D-glucan, a-L-fuco-a-D-... 

L-arabinose, have also been reported to be in direct linkage with a protein. L-Arabinose may also be linked to L-serine and L-threonine in a hyaluronic acid-polypeptide complex from ox vitreous-humor.750 D-Mannose is reported to be glycosidically linked in a glucoamylase [(l->4)-a-D-glucan glucohydrolase, EC 3.2.1.3] from Aspergillus... 

The production has been studied415-4 9 of (1— 4)-a-D-glucan gluco-hydrolase (glucoamylase) by Hansenula beckii (.Endomycopsis bi-spora). 

A / -D-glucan has been isolated from the cell walls of 3-day-old, mung-bean hypocotyls.76 Extracts of cell walls of older hypocotyls were deficient in this polysaccharide. The glucan contained 3-linked and 4-linked glucopyranosyl residues in the molar ratio of 1.0 1.7. However, the hypocotyl tissue from which the glucan was extracted contained both primary and secondary walls, and therefore, the polymer cannot be definitely characterized as a primary cell-wall component. 

Subsequent to publication of an earlier article,1 several enzymic procedures have been applied to studies on aspects of the structures of (1 - 4)-(1 - 6)-a-D-glucans. 

Enzymic hydrolysis of dextrans has provided information both on the linkage types and the disposition of different linkages as established by methylation. Since the publication of the earlier article,1 enzymic studies on Leuconostoc dextrans have continued, and investigations of Streptococcus a-D-glucans have shown that there are significant, structural differences between some of these and those of Leuconostoc spp. Reviews on dextrans have appeared.419,420... 

Hydrolysis by endo-( 1 -> 3)-a-D-glucanase has demonstrated the presence of consecutive a-(l - 3) linkages in a number of streptococcal a-D-glucans (mutans). Insoluble a-D-glucans produced by S. mutans OMZ 176 and OMZ65 were hydrolyzed by endo-(l- 3)-a-D-glucanase, whereas the soluble polysaccharides from the same sources, and dextran T-2000, were... 

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