Sugar-sensitive supramolecular structures based on phenylboronic acid-modified cyclodextrins

Article abstract of DOI:10.1248/cpb.c13-00542

Supramolecular structures were developed from phenylboronic acid-modified cyclodextrins (PBA-CyDs). The intermolecular interaction between the PBA moiety and the CyD cavity was proved using two dimensional (2D)-NMR and powder X-ray diffraction techniques. PBA-α-CyD formed a head-to-tail supramolecular polymer, whereas PBA-β-CyD formed a head-to-head dimer. The supramolecular structures were disintegrated in the presence of sugars owing to the resulting boronate sugar interactions.

Full text of DOI:10.1248/cpb.c13-00542

1188
Note
Chem. Pharm. Bull. 61(11) 1188–1191 (2013)
Vol. 61, No. 11
Sugar-Sensitive Supramolecular Structures Based on Phenylboronic
Acid-Modified Cyclodextrins
,a
Kiminobu Nakamura,a Tomohiro Seki,a Yuya Egawa,* Ryotaro Miki,a Yoshiki Oda,b
Takashi Yamanoi,b and Toshinobu Sekia
a Faculty of Pharmaceutical Sciences, Josai University; 1–1 Keyakidai, Sakado, Saitama 350–0295, Japan: and
b The Noguchi Institute; 1–8–1 Kaga, Itabashi-ku, Tokyo 173–0003, Japan.
Received July 9, 2013; accepted August 12, 2013; advance publication released online August 21, 2013
Supramolecular structures were developed from phenylboronic acid-modified cyclodextrins (PBA-CyDs).
The intermolecular interaction between the PBA moiety and the CyD cavity was proved using two dimen-
sional (2D)-NMR and powder X-ray diffraction techniques. PBA-α-CyD formed a head-to-tail supramolecu-
lar polymer, whereas PBA-β-CyD formed a head-to-head dimer. The supramolecular structures were disinte-
grated in the presence of sugars owing to the resulting boronate sugar interactions.
Key words cyclodextrin; phenylboronic acid; supramolecular chemistry; sugar recognition
Cyclodextrins (CyDs) are cyclic oligosaccharides composed an acid-base equilibrium between the molecular form and the
of six, seven, or eight glucopyranoside units and are named ionic form of PBA at a certain pH. The addition of α-CyD in-
as α-, β-, or γ-CyD, respectively. CyDs consist of a hydro- duces a complexation between α-CyD and the molecular form
phobic cavity in which hydrophobic guest molecules could be of PBA because it is more hydrophobic than the ionic form.
enclosed to form an inclusion complex. When such a guest The molecular form of PBA is consumed by the complexation.
molecule is covalently attached to the CyD, the guest-modified In order to maintain the acid-base equilibrium, the ionic form
CyD forms an intermolecular complex resulting in supramo- of PBA transforms to the molecular form. Accordingly, the
lecular assemblies, thus attracting great interest because of total amount of the non-charged form of PBA and included
their unique structures.1,2)
PBA increase, which results in the rise of apparent pKa. The
Harada et al. have reported chemical-responsive supra- increase in the pKa also proves that ester bonds are not formed
molecular structures based on guest-modified CyD deriva- between PBA and α-CyD. In the same procedure, the pKa of
tives.3–6) The supramolecular structure became disintegrated PBA was decreased to 7.8 in the presence of 100mM D-glucose
when an excess amount of another type of guest was added to (Glu), which is known to make ester bonds with PBA.
it. This chemical stimuli-sensitivity could be explained based
on a competition between the covalently attached guest moiety guest for the CyD cavity, the PBA moiety was covalently at-
and the other type of guest for the CyD cavity. tached to the CyD through a carboxylic ester linkage by the
After confirming that the molecular form of PBA acts as a
In this study, a new type of chemical stimuli-sensitive reaction of one hydroxyl group of CyD with p-chlorocarbonyl
guest-modified CyD was developed based on a structural phenylboronic acid (Chart 1). PBA-α-CyD and PBA-β-CyD
change of guest moiety, where phenylboronic acid (PBA) was were successfully synthesized, and their solubilities were de-
used as the guest molecule. PBA reacts with cis-diol func- termined (Table 1). The lower solubility of PBA-α-CyD and
tional groups of sugars to form a five- or six-membered ring PBA-β-CyD than that of the parent CyDs strongly indicated
through an ester bond.7,8) Consequently, PBA derivatives have the intermolecular interactions of PBA-CyDs and the forma-
been widely investigated as sugar-recognition motifs in chemi- tion of supramolecular structures.3–5)
cal probes for sugar analysis9) and sugar-sensitive insulin
In order to confirm the presence of intermolecular inter-
release systems.10–12) If the PBA-modified CyDs (PBA-CyDs) actions in the dissolved state, two dimensional (2D)-NMR
form a supramolecular structure, then the structure will be techniques were used. The rotating Overhauser enhance-
transformed by a chemical stimulus caused by varying the ment and exchange spectroscopy (ROESY) spectrum of
sugar concentration. This stimuli-responsiveness is derived PBA-CyDs showed the nuclear Overhauser effect (NOE)
from the structural change of the guest moiety rather than interaction between the protons of the phenyl group and H-3
owing to the competition between the two types of guests for of the CyD (Fig. S1), indicating that the PBA moiety was
the CyD cavity.3–6)
shallowly included from the wide secondary hydroxyl group
First, we confirmed that PBA acts as a guest for CyDs by side of CyDs. The diffusion ordered spectroscopy (DOSY)
monitoring the pKa of PBA, which is a Lewis acid with a pKa spectrum of PBA-α-CyD shows the presence of three species
to coordinate with hydroxide ion. The pKa of PBA was esti- in a D2O solution (Fig. S2), corresponding to the monomer,
mated by monitoring the change in absorbance at 267nm, cor- dimer, and trimer of PBA-α-CyD.14) The powder X-ray dif-
responding to an n→π* transition. By using curve fitting anal- fraction (PXRD) analyses were carried out to investigate the
ysis, the pKa of PBA was calculated to be 8.8.9) In the pres- supramolecular structure of PBA-CyDs in the solid state.
ence of 100mM α-CyD, the pKa of PBA increased to 9.1. This The PXRD of PBA-α-CyD shows a similar pattern as that of
result is interpreted by the formation of a host–guest complex mono-6-azide-6-deoxy-α-CyD, which is known to form a heli-
between the molecular form of PBA and α-CyD.13) There is cal supramolecular polymer through the head-to-tail intermo-
lecular interactions15) (Fig. S5). From these results, we propose
The authors declare no conflict of interest.
that PBA-α-CyD forms an oligomer through the head-to-tail
© 2013 The Pharmaceutical Society of Japan
*To whom correspondence should be addressed. e-mail: yegawa@josai.ac.jp

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Chart 1. Synthesis of PBA-CyDs
Table 1. Solubility (mM) of CyDs in the Presence and Absence of Sugar
(10mM) at 37°C
CyD
No sugar
With Glu
With Fru
α-CyD
β-CyD
270
37
—
—
—
—
PBA-α-CyD
PBA-β-CyD
3.0
0.23
3.8
0.26
6.3
1.2
(a) Head-to-tail supramolecular polymer based on PBA-α-CyD, (b) head-to-head
dimer based on PBA-β-CyD.
Chart 2. Proposed Supramolecular Structures of PBA-CyDs in Solid
State
interactions in the dissolved state, which is demonstrated the
results of the ROESY and DOSY spectra. Possibly, more con-
tinuous interactions lead to the formation of insoluble supra-
molecular polymer of PBAα-CyD (Chart 2a).
A nuclear Overhauser enhancement and exchange spec-
troscopy (NOESY) spectrum of PBA-β-CyD shows the NOE
interactions between each of the two kinds of protons of
PBA moiety and each of the interior H-3 and H-5 protons of
Fig. 1. Sugar-Dependent Turbidity Change of PBA-CyDs in PBS (pH
7.4) at 37°C
(a) PBA-α-CyD, (b) PBA-β-CyD; solid line (Glu), dashed line (Fru).
β-CyD (Fig. S3), indicating that the PBA moiety was deeply solid state of PBA-CyD (PBA-α-CyD: 11µmol, PBA-β-CyD:
included into the β-CyD cavity. The DOSY spectrum shows 3.9 µmol) was suspended in the Dulbecco’s phosphate buff-
the existence of two species (Fig. S4), which corresponds to ered saline (PBS) solution (pH 7.4, 2.0mL) at 37°C and the
the monomer and dimer of PBA-β-CyD. The PXRD patterns resulting turbidity was studied by monitoring the absorbance
of PBA-β-CyD closely resembled that of the inclusion struc- at 700nm wavelength. After the turbidity became constant,
tures, i.e., a dimer structure formed through a head-to-head a stock sugar solution (1.00M) was added to the suspended
interaction (Fig. S6).16) From these results, we propose that solution. The turbidity decreased with increasing concentra-
PBA-β-CyD forms a dimer through the head-to-head interac- tion of sugar (Fig. 1), indicating that the solid form of the
tions in the dissolved state, which is demonstrated the results supramolecular structure was disintegrated and dissolved in
of the NOESY and DOSY spectra, and the dimer forms the water owing to the effect of sugar. To achieve 100mM sugar
channel crystalline structures thorough hydrogen bonding of solution, 222µL of the sugar stock solution was used. When
the secondary hydroxyl groups (C2b).
222µL of buffer solution without sugar was added to the
Next, we investigated the sugar response of PBA-CyDs. A PBA-α-CyD suspended solution, the turbidity was decreased

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Vol. 61, No. 11
Chart 3. Proposed Mechanism of Sugar-Induced Disintegration of Supramolecular Polymer Structure of PBA-α-CyD
Table 2. Binding Constants (M−1) of PBA-CyDs (60µM) in PBS (pH 7.4) not interact with sugar. The sugar-bound PBA-CyD becomes
at 37°C
too bulky to be included in the cavity of other CyDs. Thus,
the consumption of the dissolved free form of PBA-CyDs
CyD
Glu
Fru
induces the further disintegration of the supramolecular
structures to compensate for the lack of free PBA-CyDs. The
mechanism is applicable to both PBA-α-CyD and PBA-β-CyD
structures.
PBA-α-CyD
PBA-β-CyD
22
29
740
720
In conclusion, we have demonstrated the formation of su-
to 86%. These results show that the turbidity decrease in the pramolecular structures of PBA-CyDs. PBA-α-CyD formed
presence of sugar was not only due to the simple dilution the supramolecular polymer structures through the head-to-
effect but also the effect of sugar. The effect of D-fructose tail interactions, whereas PBA-β-CyD formed a dimer through
(Fru) was found to be larger than that of Glu. This difference the head-to-head interactions. The resulting supramolecular
in response demonstrated that the PBA moiety acts as a sugar- structures showed a low solubility in water; however, the ad-
recognition motif because PBA is known to strongly react dition of sugar induced an increase in their solubility. This is
with Fru than with Glu.7–9) The solubilities of PBA-CyDs in a new concept for chemical-responsive materials based on the
the presence of sugar were measured, and the increase in the use of guest-modified CyDs, and it has potential applications
solubility of PBA-CyDs with increasing sugar concentration in drug delivery systems.
was confirmed (Table 1).
To better understand the mechanism of the sugar response
Acknowledgment This work was supported by JSPS
of PBA-CyDs, the binding constants between PBA-CyDs KAKENHI Grant Number 25860027.
and sugars were measured. The change in the UV-visible
absorption spectra was applied to a curve fitting analysis.9)
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