Biology Reference
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enzymatic reactions in aqueous media. A third point is that a C-F
linkage in the glycosyl fluoride can be cleaved readily because of a
good leaving group of the fluoride. However, sugar oxazolines with
unprotected form were also found to be an efficient glycosyl donor
for enzymatic glycosylation [7].
By enzymatic polymerization, using glycosyl fluorides,
cellulose, amylose, xylan, and related polysaccharides have been
synthesized [11-16], and enzymatic polymerization using sugar
oxazolines produced chitin, hyaluronan, chondroitin, and related
aminopolysaccharides [11-16,18-20]. The former proceeds via
the polycondensation through leaving hydrogen fluoride, whereas
the ring-opening polyaddition is conceived in the polymerization
manner of the latter case.
The synthesis of cellulose was achieved in 1991 by enzymatic
polymerization of
-cellobiosyl fluoride catalyzed by cellulase (Fig.
2.4a) [21]. The polymerization proceeded via polycondensation with
liberating hydrogen fluoride, where the monomer behaved as both
the glycosyl donor and acceptor for cellulase. Mixing organic solvents
with buffer was necessary as a medium to prevent the hydrolysis of
produced cellulose by cellulase catalysis. Because the suppression of
the hydrolysis was incomplete as long as the wild-type cellulase was
used, the mutant cellulase was designed such that the polymerization
would take place more efficiently [22]. A novel approach for the
enzymatic polycondensation using cellobiose as a starting substrate
without activation of the anomeric carbon was reported [23]. This
was achieved using a cellulase/surfactant complex as the catalyst in
a nonaqueous LiCl/
β
-dimethylacetamide system. Cellobiose was
polymerized by the catalyst of the complex to give cellulose with
high molecular weight. The enzymatic polymerization of
N,N
α
-maltosyl
fluoride catalyzed by an endotype enzyme,
α
-amylase, which
catalyzes random hydrolysis of
4)-glucan chain, proceeded
in a mixed solvent of methanol/buffer to give amylose oligomers
(Fig. 2.4b) [24].
α
-(1
-Xylobiosyl fluoride was designed as a monomer
for the synthesis of xylan, a
β
4)-linked xylose polysaccharide,
by the enzymatic polymerization catalyzed by cellulase, because its
commercial-grade enzyme has been known to show xylanase activity
(Fig. 2.4c) [25]. Consequently, the cellulase-catalyzed enzymatic
polymerization of this monomer proceeded in a mixed solvent of
acetonitorile/buffer to give a synthetic xylan.
β
-(1
