Hybrid polysaccharides polymerization

The examples reviewed in this chapter prove that enzymatic polymerizations using glycosyltransferases and glycosidases are powerful techniques for synthesizing various well-defined polysaccharides ranging from natural saccharides such as cellulose, amylose, amylopectin etc. to non-natural hybrid polysaccharides. 

Scheme 8 Enzymatic polymerizations to a cellulose-chitin hybrid polysaccharide.

Kobayashi et have also investigated other modes of enzymatic polymerization to produce a novel cellulose-chitin hybrid polysaccharide. A sugar fluoride monomer of GlcNAc/3(l 4)Glc (17) was designed as a TSAS monomer for polymerization catalyzed by cellulase from Trichoderma viride. In the polymerization, monomer 17 was recognized by cellulase from T. viride, leading to a cellulose-chitin hybrid polysaccharide 18 with perfect regioselectivity and stereochemistry (Scheme 17). 

Biocatalysis is a key route to both natural and non-natural polysaccharide structures. Research in this area is particularly rich and generally involves at least one of the following three synthetic approaches 1) isolated enzyme, 2) whole-cell, and 3) some combination of chemical and enzymatic catalysts (i.e. chemoenzymatic methods) (87-90). Two elegant examples that used cell-fi-ee enzymatic catalysts were described by Makino and Kobayashi (25) and van der Vlist and Loos (27). Indeed, for many years, Kobayashi has pioneered the use of glycosidic hydrolases as catalysts for polymerizations to prepare polysaccharides (88,91). In their paper, Makino and Kobayashi (25) made new monomers and synthesized unnatural hybrid polysaccharides with regio- and stereochemical-control. Van der Vlist and Loos (27) made use of tandem reactions catalyzed by two different enzymes in order to prepare branched amylose. One enzyme catalyzed the synthesis of linear structures (amylose) where the second enzyme introduced branches. In this way, artificial starch can be prepared with controlled quantities of branched regions. 

Synthesis of Unnatural Hybrid Polysaccharides via Enzymatic Polymerization... 

Figure 1. Synthes of hybrid polysaccharides via enzymatic polymerization.

Characterization data of the resulting cellulose-chitin hybrid polysaccharide 3 are summarized in Table 1. The average molecular wei t value (M ) of the resulting product 3 via cellulase- and chitinase catalyzed polymerization reached to 2840 and 4030, which correspond to 15-16 and 22 saccharide units, respectively. 

In order to polymerize a fluoride-type monomer 10, cellulase and xylanase were chosen for the catalyst. In an acetonitrile-acetate buffer mixed solution, the consumption of 10 was accelerated by the addition of xylanase from Trichoderma viride, giving rise to a white precipitate as the progress of the reaction in a 58% yield. From the analysis of the resulting products, it is a cellulose-xylan hybrid polysaccharide 11 and polymers with the degree of polymerization up to 12 (24 saccharide units) were detected from the MALDI-TOF/MS. 

Chitinase from Bacillus sp. was added to a solution of Xyl (l— 4)GlcNAc oxazoline 13 dissolved into a phosphate buffer. The consumption rate of 13 was accelerated by the enzyme addition and chitin-xylan hybrid polysaccharide 14 was produced. During the polymerization, the reaction proceeded homogenously throughout the reaction. The yield was 76% and its average molecular weight value of 14 was M = 1500 with MJMa = 1.76, determined by GPC. Furthermore, MALDI-TOF mass spectrum of 14 indicated the peaks at every m/z 335, which corresponds to the molecular mass of the repeating disaccharide unit. 

Kobyashi S., Makino A., Matsumoto H., Kunii S., Ohmae M., Kiyosada T., Makiguchi K., Matsumoto A., Horie M., Shoda S.I., Enzymatic polymerization to novel polysaccharides having a glucose-JV-acetylglucosamine repeating unit, a ceUulose-chitin hybrid polysaccharide. Biomacromolecules, 7, 2006,1644-1656. 

A hybrid-type polysaccharide was produced using a monomer consisting of different sugar moieties /9-D-xylosyl-(l 4)-jS-D-glucosyl fluoride was successfully polymerized by the catalysis of xylanase from Tri-choderma viride, giving rise to a cellulose-xylan hybrid polysaccharide (Scheme 31) [142], This hybrid polymer has the alternating structure of (1 -> 4)-/8-D-xyloside and (1 4)-/8-D-glucoside therefore, both natures of cellulose and xylan are expected to be present in a molecule. 

As shown above, hybrid structures bearing amylose blocks can be synthesized by covalent attachment of primer recognition units for phosphorylase and subsequent enzymatic grafting from polymerization. Following the same route we are currently synthesizing hybrid materials bearing (hyper)branched polysaccharide structures as shown in figure 6 with the described tandem reaction of two enzymes. 

Modification of polysaccharides through controlled/living radical polymerization grafting—towards the generation of high performance hybrids, 31,1751-1772. 

The first synthesis of a cellulose-xylan hybrid polymer, a novel polysaccharide having a glucose-xylose repeating imit, has been achieved by the xylanase-catalyzed polymerization of j8-xylopyranosyl-glucopyranosyl fluoride (Fig. 2) (35, 36). Identification of the enzyme fraction promoting the polymerization showed that endoxylanase was highly efficient for production of the hybrid polymer. 

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