Transferases in Polymer Chemistry - Enzymatic Polymerisation - page 37

grafting from oligosaccharide-terminated poly(ethylene oxide) and studied the
solution properties of these amphiphilic block copolymers by static and dynamic
light scattering [ 176 , 177 ].
It was also shown that the enzymatic polymerization of amylose could be started
from oligosaccharide-modified polymers that are not soluble in the medium of poly-
merization (aqueous buffers). Amylose- b -polystyrene block copolymers could be
synthesized by attaching maltooligosaccharides to anionically synthesized amino-
terminated polystyrene, and subsequent enzymatic elongation to amylose [ 178 ,
179 ] . Block copolymers with a wide range of molecular weights and copolymer
compositions were synthesized via this synthetic route. The solution properties of
star-type as well as crew-cut micelles of these block copolymers were studied in
water and THF, and the according scaling laws established [ 180 ] . In THF, up to
four different micellar species were detectable, some of them in the size range of
vesicular structures, whereas the crew-cut micelles in water were much more de-
fined. Bosker et al. studied the interfacial behavior of amylose- b -polystyrene block
copolymers at the air-water interface using the Langmuir-Blodgett technique [ 181 ] .
Recently, two groups reported controlled radical polymerizations starting from
maltooligosaccharides (ATRP [ 182 ] and TEMPO-mediated radical polymeriza-
tion [ 183 ] ), which will certainly lead to new synthetic routes towards amylose-
containing block copolymers.
Even though the products are not block copolymer structures, the work of
Kadokawa and colleagues should be mentioned here. In a process that the authors
named “vine-twining polymerization” (after the way that a vine plant grow helically
around a support rod), the enzymatic polymerization of amylose is performed in the
presence of synthetic polymers in solution, and the authors showed that the grown
amylose chains incorporate the polymers into its helical cavity while polymerizing
[ 184 - 191 ] .

3.2 Branching Enzymes

→

The formation of the 6) glucosyl branches of amylopectin and glycogen is

synthesized by branching enzymes (systematic name: (1

α -(1

→ →

4)- α - D -glucan:(1 4)-

α - D -glucan 6- - D -glucano]-transferase; EC 2.4.1.18).

This enzyme catalyzes the formation of

α - D -[(1 → 4)- α

α -(1 → 6) branching points by cleaving an

α 4) glycosidic linkage in the donor substrate and transferring the nonreducing

end-terminal fragment of the chain to the C-6 hydroxyl position of an internal glu-
cose residue, which acts as the acceptor substrate [ 192 ] . Depending on its source,
branching enzymes have a preference for transferring different lengths of glucan
chains [ 193 - 196 ] .
Recently, the in vitro synthesis of amylopectin- or glycogen-like structures via a
tandem reaction of phosphorylase and branching enzyme was reported [ 197 - 201 ] .
Phosphorylase catalyzes the polymerization of glucose-1-phosphate in order to
obtain linear polysaccharide chains with

-(1 →

α -(1 → 4) glycosidic linkages; the glycogen