Polymeric monosaccharide receptors responsive at neutral pH

Article abstract of DOI:10.1021/ja905652w

(Figure Presented) We present the first detailed report of the synthesis of Wulff-type styrenic monomers and their polymerization by radical addition-fragmentation chain transfer (RAFT) methods. The resulting polymers and block copolymers exhibit sugar-re

Full text of DOI:10.1021/ja905652w

Published on Web 09/10/2009
Polymeric Monosaccharide Receptors Responsive at Neutral pH
Kyoung Taek Kim, Jeroen J. L. M. Cornelissen, Roeland J. M. Nolte, and Jan C. M. van Hest*
Institute for Molecules and Materials, Radboud UniVersity Nijmegen, Nijmegen, The Netherlands
Received July 9, 2009; E-mail: j.vanhest@science.ru.nl
Scheme 1. Synthesis and Polymerization of Wulff-Type Boronic
Acid Monomer, Its Pinacol Boronate Ester
Binding of saccharides to organoboronic acids via the formation
of reversible covalent bonds has been studied extensively due to
its practical importance in the development of sensors and drug
releasing platforms for glucose-related human disorders such as
diabetes.1 In particular, polymers and block copolymers containing
boronic acid moieties are attracting recent interest as stimuli-
responsive materials that switch their solubility in water in
correspondence to the glucose concentration in the medium.2-4
However, the sugar-responsive behavior of these materials requires
aqueous media of relatively high pH (>9) because the hydrolysis
of boronate esters is favored at neutral pH conditions.5 This high
pH medium requirement for stimuli-responsiveness is a disadvan-
tage for the use of boronic acid containing polymers under
physiological conditions.
Wulff ingeniously showed that aromatic boronic acids having
o-dialkylaminomethyl groups (Wulff-type boronic acids) exhibit a
lower pKa value due to the intramolecular interaction between B
and N atoms, thus stabilizing boronate esters at neutral pH.6 Shinkai
and co-workers utilized this concept to develop fluorescent sensors
detecting glucose in a methanol/water mixture at neutral pH.7 The
incorporation of these Wulff-type boronic acids into polymers would
be a highly attractive method to create sugar-responsive materials,
which could operate under physiological conditions. However, until
now, these materials have hardly been studied, presumably due to
the unavailability of procedures for the synthesis of monomers that
can be polymerized in a controlled manner.8 Here we present the
first detailed report of the synthesis of Wulff-type styrenic
monomers and their polymerization by radical addition-frag-
mentation chain transfer (RAFT) methods. The resulting polymers
and block copolymers exhibit sugar-responsive solubilization in
aqueous buffer solutions (pH ) 7.4-7.8) in the presence of
monosaccharides such as D-fructose and D-glucose.
for 24 h. Precipitation of the aqueous solution into dioxane gave 7
as a white powder. Polymer 7 was insoluble in most common
organic solvents except in methanol. Protection of 7 with pinacol
only gave partially protected polymers because of the steric reason.
Molecular weight determination of these materials by GPC (CHCl3)
was unsuccessful due to the high affinity of the polymer to the
GPC column material. However, size exclusion chromatography
in 70% formic acid (pH ) 1) showed a single peak indicating that
the polymerization was successful (Figure 1B).
Wulff-type styrenic boronic acid 5 was synthesized from
3-bromo-4-bromomethylbenzonitrile (1) in five steps (Scheme 1,
Supporting Information (SI)). To facilitate purification and char-
acterization, boronic acid 5 was reacted with pinacol to yield the
pinacol boronate ester 6. Compound 6 was obtained as a white
crystalline solid in 65% yield from 5. X-ray crystallographic studies
on 6 (Figure 1A) revealed the presence of a N-B dative bond
(1.735 Å) and a strained geometry of the B center (tetrahedral
character 58.3%).9
Compound 6 could not be polymerized by any free radical or
controlled radical polymerization method. We presumed that the
steric hindrance imposed by the bulky pinacol boronate on the
m-position with respect to the vinyl group prevented any further
propagation after initiation. Therefore, we tried a free radical
polymerization of 6 in an acidic water/methanol mixture (1:1 by
volume, pH ≈ 3). Under these conditions, pinacol boronate was
hydrolyzed in situ, yielding 5, which could be consequently
polymerized. The polymerization was performed at 80 °C for 8 h.
After raising the pH of the reaction mixture to ca. 9 by adding
aqueous 1 M NaOH, the solution was dialyzed against pure water
Figure 1. (A) X-ray crystal structure of 6 (ellipsoids are shown at 50%
probability). Selected bond lengths (Å) and angles (deg): B1-N1 1.735 (2),
C1-B1 1.607(2), B1-O2 1.432(2); C1-B1-N1 95.57(11), O2-B1-O1
107.67(13), O2-B1-N1 112.49(12), O2-B1-C1 117.38(13), C15-N1-C14
109.09(14). (B) SEC traces of 7 and 8 (70% formic acid, pH ) 1) compared
to PEG (Mn ) 2000 g/mol).
To investigate the possibility of applying this monomer in the
synthesis of stimulus responsive block copolymers, we performed
a RAFT polymerization of 6 by using a poly(ethylene glycol)-chain
transfer agent (PEG-CTA, Mn ) 2000 g/mol)10 (Scheme 1). For
polymerization to occur, compound 6 was again hydrolyzed in situ
in an acidic water/methanol mixture (pH ≈ 3). The polymerization
was conducted for 24 h at 80 °C ([M]:[CTA]:[I] ) 100:1:0.2,
conversion >90% by 1H NMR), and the resulting block copolymer
was thoroughly purified by dialysis and washing with CHCl3 to
9
13908 J. AM. CHEM. SOC. 2009, 131, 13908–13909
10.1021/ja905652w CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. (A) Optical density measurement of the colloidal solution of 8
in PBS (b) and in TRIS (2) buffers. Upon addition of fructose (10 mM,
O) or glucose (20 mM, 4), optical density decreases. (B) DLS results of
Figure 2. A schematic representation of the polymer 7 with monosaccha-
rides in different pH environments. The photograph shows tubes containing
the polymer 7 in different buffers after 18 h. Tube 1: 7 in PBS (pH ) 7.4),
2: in PBS/D-fructose (50 mM), 3: in PBS/D-glucose (100 mM), 4: in TRIS/
D-glucose (100 mM, pH ) 7.8).
the colloidal solutions (b: 8 in PBS, Davg ) 335 nm; 2: 8 in TRIS, Davg
)
194 nm) and dissociated 8 with monosaccharides in the same buffers (O,
4: Davg ) 9.2 nm).
remove any residual impurities. The degree of polymerization of
acids of 7 and 8 must contribute to the solubility change of the
polymer. These results demonstrate that Wulff-type boronic acid
containing polymers may be utilized as sugar-responsive materials
under physiological conditions, which removes one of the main
limitations of the currently applied materials. Following up on the
synthetic procedures described here, we are currently working on
the design of polymers with well-defined architectures to be used
in glucose sensing and drug delivery.
1
the purified block copolymer 8 was ∼200 as determined by H
NMR integration (estimated Mn ) 43 000 g/mol, Figure S1 in SI).
The resulting block copolymer 8 was soluble in pure water and
methanol but remained insoluble in neutral pH buffers. SEC of this
block copolymer (Figure 1B) clearly indicated that 8 was success-
fully prepared by showing a sharp signal free from any residual
PEG-CTA.
Boronic acid containing polymers increase their solubility in
water upon binding of saccharides to the boronic acid moieties.3,4
We first screened the solubility of polymer 7 in aqueous media at
varying pH’s.11 The polymer was well soluble when the pH of the
medium was in a weakly acidic (<6) or in a moderately basic (>9)
region. Polymer 7 remained insoluble in a PBS buffer (pH ) 7.4)
until D-fructose (50 mM, 3.4 equiv to boronic acid) was added.
Addition of D-glucose (100 mM, 6.8 equiv to boronic acid) did
not solubilize polymer 7 in PBS but rendered 7 to be completely
soluble in a TRIS/HCl buffer (pH ) 7.8) (Figure 2). Dynamic light
scattering results on the buffer solutions of 7 with monosaccharides
showed average hydrodynamic diameters (Davg) of 11.2 nm for the
fructose/PBS solution and 12.6 nm for the glucose/TRIS solution,
which indicates the presence of molecularly dissolved species
(Figure S2 in SI). The solubilized 7 in the buffer/monosaccharide
turned insoluble upon dialysis against buffers (MW cutoff: 13 000
g/mol, 24 h), indicating that the sugar responsiveness of 7 is
reversible with respect to the concentration of monosaccharides.
For comparison, we checked the solubility of poly(styrene-4-boronic
acid) (PSBA, Scheme 1) in the same monosaccharide/buffer media.
The polymer remained insoluble after a prolonged time (3 weeks).
When 8 was dispersed in PBS and TRIS/HCl buffers with the
aid of THF (15% by volume) colloidal suspensions were formed.12
As Figure 3 shows, these suspensions remained stable when no
monosaccharides were added. Upon addition of monosaccharides
(10-20 mM), the colloidal particles of 8 dissociated into smaller
objects (Davg ) 9.2 nm), and the turbidity of the solution disappeared
with time. The results therefore indicate block copolymer 8 exhibits
similar sugar responsiveness as the homopolymer 7 in neutral pH
buffers.
Acknowledgment. We thank NWO ACTS and Encapson for
financial support for this work. K.T.K. thanks Sanne Schoffelen
for SEC experiments and Jan Smits for X-ray crystallography.
Supporting Information Available: X-ray crystallographic data of
6 (CIF) and synthetic details and characterization. This material is
available free of charge via the Internet at http://pubs.acs.org.
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