Bioorganic & Medicinal Chemistry Letters
Enzymatic synthesis of a 6-sialyl lactose analogue using a pH-responsive
water-soluble polymer support
Wenjun Wang a, Lei Li a, Chen Jin a, Yujie Niu a, Sen Li a, Ji Ma a, Linfeng Li a, Yu Liu a, Li Cai c, Wei Zhao a,
,
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Peng George Wang a,b,c,
⇑
a College of pharmacy, Nankai University, Tianjin 300071, PR China
b State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin 300071, PR China
c Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH 43210, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The Letter describes a strategy for the enzymatic synthesis of glycans based on a pH-responsive
water-soluble polymer. In neutral condition, the polymer is water-soluble and convenient for in-solution
enzymatic synthesis, whereas in acidic condition (pH lower than 4.0), the polymer disconnects with the
product and becomes insoluble, which can be easily removed. A 6-Sialyl lactose analogue was synthe-
sized as a model reaction using this approach.
Received 7 March 2011
Revised 17 April 2011
Accepted 19 April 2011
Available online 24 April 2011
Ó 2011 Published by Elsevier Ltd.
Keywords:
Carbohydrate
pH-responsive polymer
Enzymatic synthesis
Solid supports, such as trityl resins and Controlled-Pore Glass
(CPG), are widely used for the synthesis of peptides, DNA, and
other polymers or oligomers that are assembled in a linear fash-
ion.1 There are two distinct advantages of solid-phase synthesis.
Firstly, excessive material can be used to drive reactions to comple-
tion; Secondly, products are easily harvested by simple filtration.
For the synthesis of oligosaccharides, enzymatic approaches are
more attractive than traditional chemical methods, which are pla-
gued with multiple steps of protection/deprotection steps as well
as poor regio/stereoselectivity.2 Pioneer works in the solid-phase
enzymatic synthesis of carbohydrates were based on various kinds
of supports including CPG,3,4 water-compatible polyacrylamide
gel,5,6 sepharose7,8 and polyethylene glycol polyacrylamide
(PEGA).9 However, researchers are plagued with such problems
as non-linear kinetics, low reactivity, stereo-chemical complexity
and analytical difficulty.10
Recently, Wong and co-workers designed a thermo-responsive
water-soluble polymer support to overcome these problems.10 In
their technique, oligosaccharides were synthesized on the polymer
through enzymatic glycosylation in homogenous phase and har-
vested by elevating the temperature. However, the polymer cannot
be recovered by simply raising the temperature of the solution for
sialic acid containing glycans, due to its hydrophilic character. To
compensate for this drawback, we design herein a pH-responsive
polymer using 3-pentene-1,3,4-tricarboxylic acid, cyclic 3,4-anhy-
dride as a switchable linker. A trisaccharide, associated with the
infection of flu virus, a 6-sialyl lactose analogue was synthesized
in a proof-of-concept experiment (Scheme 1).11,12
The polymer modified by 3-pentene-1,3,4-tricarboxylic acid,
cyclic 3,4-anhydride was synthesized as described in Scheme 2.13–
15 3-Pentene-1,3,4-tricarboxylic acid, cyclic 3,4-anhydride was ob-
tained from
a-ketoglutaric acid diethyl ester and triethyl 2-phos-
phonopropionate through Wadsworth–Emmons reaction. N-
Isopropylacrylamide and acrylic acid N-hydroxysuccinimide ester
were allowed to polymerize after activation by AIBN. To avoid the
crosslink in the final product, an excessive amount of 1,2-ethylene-
diamine was used. In the second step, the 3-pentene-1,3,4-tricar-
boxylic acid, cyclic 3,4-anhydride was activated by oxalyl chloride
and coupled with the free amine group of the synthesized polymer.
Due to the facts that acyl chloride is more reactive than the anhy-
dride and excessive amount of acyl chloride used, the linear product
was exclusively formed. The reaction mixture was acidified to pH 2
and purified to afford our desired polymer 7. The IR spectrum con-
firmed the structure of polymer 7 (Fig. 1). Red shift of the wave num-
ber of stretching vibration (1765.0, 1715.3 cmÀ1) of the anhydride
can be observed because of the dampness of the polymer backbone.
When reacted with an amine group in neutral condition, the poly-
mer could generate the same amount of carboxyl groups which
make the resulting polymer highly hydrophilic and soluble in water.
Moreover, when the pH value becomes lower than 4.0, 3-pentene-
1,3,4-tricarboxylic acid, cyclic 3,4-anhydride could be regenerated
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Corresponding author. Tel./fax: +086 22 23501834 (W.Z.); tel./fax: +86 22
23500261 (P.G.W.).
0960-894X/$ - see front matter Ó 2011 Published by Elsevier Ltd.