Emergent Molecular Recognition through Self-Assembly: Unexpected Selectivity for Hyaluronic Acid among Glycosaminoglycans

Article abstract of DOI:10.1002/anie.201511564

Oligophenylenevinylene (OPV)-based fluorescent (FL) chemosensors exhibiting linear FL responses toward polyanions were designed. Their application to FL sensing of glycosaminoglycans (heparin: HEP, chondroitin 4-sulfate: ChS, and hyaluronic acid: HA) revealed that the charge density encoded as the unit structure directs the mode of OPV self-assembly: H-type aggregate for HEP with 16-times FL increase and J-type aggregate for HA with 93-times FL increase, thus unexpectedly achieving the preferential selectivity for HA in contrast to the conventional HEP selective systems. We have found that the integral magnitude of three factors consisting of binding mechanism, self-assembly, and FL response can amplify the structural information on the target input into the characteristic FL output. This emergent property has been used for a novel molecular recognition system that realizes unconventional FL sensing of HA, potentially applicable to the clinical diagnosis of cancer-related diseases. An unexpectedly selective fluorescent (FL) chemosensor for hyaluronic acid (HA) was developed, where structural information on the target critically directs the mode of chemosensor self-assembly leading to the characteristic FL response (see picture). This system is potentially applicable to the clinical diagnosis of functional disorders such as cancer-related diseases involving the overexpression of HA.

Full text of DOI:10.1002/anie.201511564

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201511564
German Edition: DOI: 10.1002/ange.201511564
Fluorescent Probes
Emergent Molecular Recognition through Self-Assembly: Unexpected
Selectivity for Hyaluronic Acid among Glycosaminoglycans
Takao Noguchi,* Bappaditya Roy, Daisuke Yoshihara, Junji Sakamoto, Tatsuhiro Yamamoto,
and Seiji Shinkai*
[4]
Abstract: Oligophenylenevinylene (OPV)-based fluorescent
FL) chemosensors exhibiting linear FL responses toward
property of molecular self-assembly, its utilization toward
practical applications still remains unexplored.
(
polyanions were designed. Their application to FL sensing of
glycosaminoglycans (heparin: HEP, chondroitin 4-sulfate:
ChS, and hyaluronic acid: HA) revealed that the charge
density encoded as the unit structure directs the mode of OPV
self-assembly: H-type aggregate for HEP with 16-times FL
increase and J-type aggregate for HA with 93-times FL
increase, thus unexpectedly achieving the preferential selectiv-
ity for HA in contrast to the conventional HEP selective
systems. We have found that the integral magnitude of three
factors consisting of binding mechanism, self-assembly, and
FL response can amplify the structural information on the
target input into the characteristic FL output. This emergent
property has been used for a novel molecular recognition
system that realizes unconventional FL sensing of HA,
potentially applicable to the clinical diagnosis of cancer-related
diseases.
Recently, we reported a molecular recognition system
coupled with self-assembly and fluorescence (FL) signalling
by using an oligophenylenevinylene (OPV)-based FL chemo-
[5]
sensor. Therein, a subtle difference in guest structures
dramatically alters the resulting FL responses. For example,
l- and meso-tartarate afforded the distinct FL difference in
the OPV sensor as well as the totally different self-assembly
morphologies: fibrous structure with l-tartarate and finite
particle with meso-tartarate. It is known that the preferable
conformations of l- and meso-tartarate in water are anti and
gauche conformers, respectively. Therefore, we demonstrated
that only a conformational preference originating from their
stereochemical difference determines the J- or H-type self-
assembly path of the OPV sensor, which leads to the
characteristic self-assembly morphologies and the consequent
FL responses. In other words, an initial input of guest
structural information into the OPV sensor critically directs
the OPV self-assembly process, leading to a FL response
characteristic to the guest input (incommensurable output).
This schematic concept highlights a key feature of the
emergent property in the self-assembly-based FL sensory
system, and more importantly, achieves precise molecular
recognition that has never been accomplished by the conven-
tional molecular recognition based on the traditional key-
S
elf-assembly is ubiquitously seen in nature, ranging from
biological systems to artificial nanomaterials, which is
[
1,2]
strongly associated with the origin of the functionality.
For example, self-assembly of protein subunits into well-
defined superstructures is a prerequisite for vital biological
functions such as transcription, translation, metabolic, and
[3]
immune systems. Therein, the biological functions are
driven by a key signal (initial input), whereas the mutation
in protein structures causes fatal biofunctional disorders. Such
a critical impact of the initial input evokes the emergent
[
6]
and-lock binding technique. In this study, we utilize this
emergent property for the design of a novel FL sensing system
applicable to biologically relevant polymers, glycosaminogly-
cans (GAGs) such as heparin (HEP), chondroitin 4-sulfate
(ChS), and hyaluronic acid (HA), as shown in Figure 1.
GAG is an important class of biopolymer that plays a key
role in the biofunctional regulation including cell growth and
[4]
phenomena. Learning from the biological systems, we can
acquire a chance to apply this emergent property to the
creation of self-assembly-based functional materials.
Although there exists one report describing the emergent
[
7]
differentiation, angiogenesis, and immune response. For
clinical purposes, HEP has been mostly utilized as a respon-
sible anticoagulant during surgery, and its efficient and
reliable sensing methodologies have been developed through
an appropriate design of assembly-based binders and colori-
[
*] Dr. T. Noguchi, Dr. B. Roy, Prof. Dr. S. Shinkai
Institute for Advanced Study, Kyushu University
7
44 Moto-oka, Nishi-ku, Fukuoka 819-0395 (Japan)
[
8,9]
E-mail: tnoguchi@mail.cstm.kyushu-u.ac.jp
shinkai_center@mail.cstm.kyushu-u.ac.jp
metric or fluorescent chemosensors.
cation of cationic aggregation-induced emission dyes was
More recently, appli-
[10]
Dr. T. Noguchi, Dr. D. Yoshihara, Dr. J. Sakamoto, Dr. T. Yamamoto,
Prof. Dr. S. Shinkai
Nanotechnology Laboratory, Institute of Systems, Information
Technologies and Nanotechnologies (ISIT)
offered because of their strong cohesive force with the
[11]
polyanions.
In any case, most reports described the
selective detection of the most anion-charged HEP owing to
the preferential association of cationic chemosensors. Their
applications were therefore limited to HEP sensing and have
rarely been explored to FL sensing of other GAGs such as
HA in spite of the clinical importance related to cancer
4
-1 Kyudai-Shinmachi, Nishi-ku, Fukuoka 819-0388 (Japan)
Prof. Dr. S. Shinkai
Department of Nanoscience, Faculty of Engineering, Sojo University
4
-22-1 Ikeda, Kumamoto 860-0082 (Japan)
[12]
diagnosis involving its overexpression and excretion. In this
context, we have developed a novel polyanion sensing system
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201511564.
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cates that p-stacked aggregates are formed. The ratiometric
plot (A406/A414) also revealed a linear relationship (Figure 2d),
correlating with the FL titration result (Figure 2b). These
results support that the FL response with the linear relation-
ship stems from the formation of p-stacked aggregates of
OPV-G1 on the polyanionic platform of PA.
Here one fundamental question arises: why is the linear
FL response observed in spite of the self-assembly system?
This result is rather surprising for us because we have
reported, to date, the nonlinear FL response accompanied
[5,13]
with the molecular self-assembly process.
In order to
address this query, we performed a titration experiment
where PA concentration was fixed at 30 mm and OPV-G1
concentration was varied. Upon increasing concentration of
OPV-G1, the FL intensity was increased nonlinearly and
saturated at 30 mm which is equivalent to PA concentration
(Figure S3a,c in the SI). When PA is absent, the FL intensity
of OPV-G1 was increased linearly with its increasing concen-
tration (Figure S3b,c in the SI). These results indicate that the
nonlinear FL response stems from the cooperative associa-
[14]
[14]
tion of OPV-G1 to PA. According to Hill equation, the
apparent global association constant (LogK ) was estimated
a
to be 8.65 (Figure S4 in the SI). As the nonlinear FL increase
was saturated at 1.0 equivalent (Figure S3c in the SI), we
Figure 1. Chemical structures of a) assembly-based fluorescent chemo-
sensors: OPV-G1 and OPV-G2, b) polyacrylic acid (PA), and c) repre-
sentative glycosaminoglycans: heparin (HEP), chondroitin 4-sulfate
could apply this LogK value to explain why the linear FL
a
(ChS), and hyaluronic acid (HA).
response is observable for the PA sensing shown in Figure 2b:
OPV-G1 quantitatively associates PA with LogK = 8.65 until
a
the plot reaches the plateau (Figure 3a). Owing to this linear
relationship, the detection range of PA concentration
becomes adjustable by the initial OPV-G1 concentration
using new self-assembly-based FL chemosensors (OPV-G1
[
5]
and OPV-G2, Figure 1a) for a representative polyanion,
polyacrylic acid (PA, Figure 1b), that exhibits
a linear FL response toward the PA concen-
tration. In the course of its application to GAG
(
Figure 1c) sensing, we unexpectedly found
that OPV-G2 shows the selective FL detection
and sensing of HA, in contrast to the conven-
[8,9,11]
tional HEP-selective sensing systems.
Here, we wish to emphasize that such unex-
pected selectivity does appear when the emer-
gent property of self-assembly is integrated as
a novel molecular recognition technique.
OPV-G1 was water-soluble and showed
a weak red FL emission maximum at 604 nm
with excitation wavelength at 400 nm. The FL
intensity was enhanced by 5.0-fold by the
addition of PA, the FL intensity being observ-
able by our naked eyes (Figure S1 in the
Supporting Information (SI)). In order to test
the performance level of OPV-G1 for sensing
purposes, a FL titration experiment was per-
formed. Upon increasing concentration of PA,
the FL intensity increased linearly and reached
a plateau at ca. 1.0 equivalent per repeat unit
of PA (Figures 2a,b and S2 in the SI). This
linearity was also confirmed in the UV/Vis
titration result. Upon increasing concentration
of PA, the absorbance of OPV-G1 at 414 nm
decreased with increasing absorbance at
Figure 2. a) Fluorescence titration (l =400 nm) of OPV-G1 upon increasing concentra-
tion of PA (0–60 mm) in HEPES buffered water. b) Plot of the changes in the fluorescence
intensity at 604 nm. c) UV/Vis titration of OPV-G1 upon increasing concentration of PA
ex
(
0–60 mm) in HEPES buffered water. d) Ratiometric plot of A406/A414. Conditions: [OPV-
4
06 nm (Figure 2c). This spectral change indi- G1]=30 mm, [HEPES]=10 mm (pH 7.4), 258C.
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anions in HAwhile that of OPV-G1/HEP was still observable,
indicating that HEP selectivity is significantly improved
(Figure S10b in the SI). These results provide us an insight
that the adjustment of the medium pH is very effective for
improving the selectivity for GAGs.
A dramatic alteration in selectivity was observed when
[15]
OPV-G2 sensor was used.
In its application to GAG
sensing, we observed the unexpected selectivity for HA, the
maximum FL increase being 93-fold (Figures 4b and S12 in
the SI). This selectivity trend of HEP < ChS < HA is in direct
contrast to the conventional selectivity trend of HEP > ChS >
HA shown in Figure 4a. This marked selectivity inversion
suggests that the affinity is not simply governed only by the
electrostatic interactions as is the conventional case. The UV-
vis titration provided a clear view that in the OPV self-
assembly two different aggregation modes exist: that is, H-
type aggregate with HEP and J-type aggregate with HA
(
Figures 4c and S13 in the SI) as supported by previous
[5,13d,16]
literatures.
As mentioned above, one can consider that
the selectivity of OPV-G2 for the GAGs is controllable by
adjusting the medium pH condition. When the medium pH
was adjusted to pH 1.5, the FL intensity of OPV-G2/HA
indeed reached the level of the background. As a result, ChS
selectivity is most enhanced (Figure S14 in the SI). These
results give us a new rationale to explain the selectivity: the
lower charge density in the unit structure of the GAGs directs
the J-type mode of OPV-G2 self-assembly, affording an
Figure 3. a) Schematic illustration of PA sensing that exhibits apparent
1
1
:1 linear relationship. b) Dependence of OPV-G1 concentration (blue:
0 mm, green: 30 mm, red: 60 mm) on the detection range toward PA
(0–1.0 equivalent).
(
Figures 3b and S5 in the SI). From these results, we can
[
17]
rationalize a novel polyanion sensing system utilizing the
assembly-based OPV-G1 chemosensor, where the multiple
electrostatic interactions resulting from the cooperative bind-
ing mechanism dominate over the formation of p-stacked
OPV/PA aggregates quantitatively (Figure 3a). This mecha-
nism is the reason why the system achieves an apparent 1:1
linear relationship in the FL response, as shown in Figures 2b
and 3b.
In order to demonstrate that the polyanion sensing system
has a feature of apparent 1:1 linear relationship, we next
applied this OPV-G1 sensor to the FL sensing of GAGs (HEP,
ChS, and HA shown in Figure 1c). As expected, OPV-G1
showed a linear FL response in the FL titration experiment
intense FL response.
More generally, one can regard that structural informa-
tion on target (in the present case, unit structure of GAGs) is
amplified, via self-assembly of the OPV sensor, into a FL
response characteristic to the structural information input
(incommensurable output). We believe, therefore, that this is
a striking feature of molecular self-assembly known as a sort
of emergent property. This emergent property has now been
utilized for unconventional FL sensing of GAGs, especially
HA, by the integration of the self-assembly concept as a novel
molecular recognition system.
In conclusion, we have developed a novel FL sensing
system orchestrated by binding mechanism, self-assembly,
and FL response, that exhibits apparent 1:1 linear FL
response toward the polyanion concentration. In its applica-
tion to GAG sensing, OPV-G1 shows the conventional HEP
selectivity whereas the unconventional selectivity for HA has
been displayed by OPV-G2 self-assembly. Owing to this
unconventional selectivity, ChS is also detectable by adjusting
the medium pH condition. These results provide us a new
perspective that the selectivity corresponding to the GAG
structural features can be tailored by utilizing self-assembly
(
Figures S6 and S7 in the SI) and revealed that its selectivity is
correlated with the anionic charge density: that is, HEP >
ChS > HA (Figures 4a and S8 in the SI) as the order
[
8,9,11]
conventionally described in the previous reports.
reported previously,
As we
[13d]
the selectivity in this class of measure-
ments can be improved by the influence of the salt interfer-
ence and medium pH. We thus applied this procedure to the
present system. No remarkable influence of salt interference
on the selectivity was confirmed, the FL intensity decreasing
only by 8% under the salt condition ([salt] = 150 mm, salt:
[5]
phenomena. Together with our previous report, we wish to
propose herein a novel molecular recognition system that
realizes unconventional FL sensing via the emergent property
of self-assembly.
NaCl, CsCl, CaCl , shown in Figure S9 in the SI). Although
2
the selectivity for HEP was not improved so much, one can
claim that OPV-G1/GAG complexes with multiple electro-
static interactions have superior tolerance to the highly
competitive salt interference. In contrast, the influence of
medium pH appeared conspicuously. When the medium pH Acknowledgements
decreased to pH 1.5, the FL intensity of OPV-G1/HA
decreased and reached the level of the background (Fig-
ure S10a in the SI) because of the protonation of carboxylate
We are grateful to Dojindo Molecular Technologies, Inc. for
fruitful discussions.
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Kaiser, C. R. Saha-Mçller,
Angew. Chem. Int. Ed. 2011,
5
2
0, 3376 – 3410; Angew. Chem.
011, 123, 3436 – 3473; f) A.
Barnard, D. K. Smith, Angew.
Chem. Int. Ed. 2012, 51, 6572 –
6
6
581; Angew. Chem. 2012, 124,
676 – 6685; g) C. F. J. Faul,
Acc. Chem. Res. 2014, 47,
428 – 3438; h) S. S. Babu,
3
V. K. Praveen, A. Ajayaghosh,
Chem. Rev. 2014, 114, 1973 –
2
129; i) S. Yagai, S. Okamura,
Y. Nakano, M. Yamauchi, K.
Kishikawa, T. Karatsu, A.
Kitamura, A. Ueno, D. Kuzu-
hara, H. Yamada, T. Seki, H.
Ito, Nat. Commun. 2014, 5,
4
013.
[
[
3] G. A. Petsko, D. Ringe, Protein
Structure and Function, New
Sciences Press, 2004.
4] In this study, emergent phe-
nomenon is defined as the
macroscopic expression of
microscopic initial differences:
Q.-F. Sun, J. Iwasa, D. Ogawa,
Y. Ishido, S. Sato, T. Ozeki, Y.
Sei, K. Yamaguchi, M. Fujita,
Science 2010, 328, 1144 – 1147,
and references therein.
[
[
[
[
5] T. Noguchi, B. Roy, D. Yoshi-
hara, Y. Tsuchiya, T. Yama-
moto, S. Shinkai, Chem. Eur. J.
2
014, 20, 13938 – 13944.
6] J.-M. Lehn, Supramolecular
Chemistry: Concepts and Per-
spectives, VCH, New York,
1
995.
7] I. Capila, R. J. Linhardt,
Figure 4. a) Photograph of OPV-G1 in the absence and presence of HEP (6.25 ppm), ChS (8.50 ppm),
and HA (11.5 ppm), and changes in the fluorescence intensity of OPV-G1 upon increasing concentrations
of HEP (red), ChS (green), and HA (blue). Conditions: [OPV-G1]=30 mm, [HEPES]=10 mm (pH 7.4),
Angew. Chem. Int. Ed. 2002,
4
2
1, 390 – 412; Angew. Chem.
002, 114, 426 – 450.
2
58C, l =400 nm. b) Photograph of OPV-G2 in the absence and presence of HEP (2.5 ppm), ChS
ex
8] a) G. L. Montalvo, Y. Zhang,
T. M. Young, M. J. Costanzo,
K. B. Freeman, J. Wang, D. J.
Clements, E. Magavern, R. W.
Kavash, R. W. Scott, D. Liu,
W. F. DeGrado, ACS Chem.
Biol. 2014, 9, 967 – 975;
b) S. M. Bromfield, P. Posocco,
(5.5 ppm), and HA (8.5 ppm), and changes in the fluorescence intensity of OPV-G2 upon increasing
concentrations of HEP (red), ChS (green), and HA (blue). c) UV/Vis titration of OPV-G2 upon increasing
concentrations of HEP (red), ChS (green), and HA (blue) in HEPES buffered water. Conditions: [OPV-
G2]=10 mm, [HEPES]=10 mm (pH 7.4), 258C, l =388 nm.
ex
Keywords: emergent properties · fluorescent probes ·
molecular recognition · self-assembly ·
supramolecular chemistry
C. W. Chan, M. Calderon, S. E. Guimond, J. E. Turnbull, S. Pricl,
D. K. Smith, Chem. Sci. 2014, 5, 1484 – 1492; c) S. M. Bromfield,
D. K. Smith, J. Am. Chem. Soc. 2015, 137, 10056 – 10059.
[
9] a) A. T. Wright, Z. Zhong, E. V. Anslyn, Angew. Chem. Int. Ed.
2
005, 44, 5679 – 5682; Angew. Chem. 2005, 117, 5825 – 5828;
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5708–5712
Angew. Chem. 2016, 128, 5802–5806
b) W. Sun, H. Bandmann, T. Schrader, Chem. Eur. J. 2007, 13,
7
2
701 – 7707; c) M. C.-L. Yeung, V. W.-W. Yam, Chem. Eur. J.
011, 17, 11987 – 11990; d) Q. Dai, W. Liu, X. Zhuang, J. Wu, H.
Zhang, P. Wang, Anal. Chem. 2011, 83, 6559 – 6564; e) S. M.
Bromfield, A. Barnard, P. Posocco, M. Fermeglia, S. Pricl, D. K.
Smith, J. Am. Chem. Soc. 2013, 135, 2911 – 2914; f) S. M.
Bromfield, E. Wilde, D. K. Smith, Chem. Soc. Rev. 2013, 42,
9184 – 9195.
[
[
1] G. M. Whitesides, B. Grzybowski, Science 2002, 295, 2418 – 2421.
2] a) G. M. Whitesides, J. P. Mathias, C. T. Seto, Science 1991, 254,
1
312 – 1319; b) J. E. Kingery-Wood, K. W. Williams, G. B. Sigal,
G. M. Whitesides, J. Am. Chem. Soc. 1992, 114, 7303 – 7305; c) S.
Zhang, Nat. Biotechnol. 2003, 21, 1171 – 1178; d) F. J. M.
Hoeben, P. Jonkheijm, E. W. Meijer, A. P. H. J. Schenning,
Chem. Rev. 2005, 105, 1491 – 1546; e) F. Würthner, T. E.
[10] Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40,
5361 – 5388.
Angew. Chem. Int. Ed. 2016, 55, 5708 –5712
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5711

Angewandte
Communications
Chemie
[
[
11] a) M. Wang, D. Zhang, G. Zhang, D. Zhu, Chem. Commun. 2008,
S. Shinkai, Chem. Eur. J. 2014, 20, 381 – 384; d) T. Noguchi, B.
Roy, D. Yoshihara, Y. Tsuchiya, T. Yamamoto, S. Shinkai, Chem.
Sci. 2015, 6, 3863 – 3867.
4
3
469 – 4471; b) X. Gu, G. Zhang, D. Zhang, Analyst 2012, 137,
65 – 369; c) R. T. K. Kwok, J. Geng, J. W. Y. Lam, E. Zhao, G.
Wang, R. Zhan, B. Liu, B. Z. Tang, J. Mater. Chem. B 2014, 2,
[14] C. A. Hunter, H. L. Anderson, Angew. Chem. Int. Ed. 2009, 48,
7488 – 7499; Angew. Chem. 2009, 121, 7624 – 7636.
[15] OPV-G2 also shows a linear FL response to PA, confirming the
similar association mechanism as is the case of OPV-G1; see
Figure S11 in the SI.
4
6
134 – 4141; d) Y. Ding, L. Shi, H. Wei, Chem. Sci. 2015, 6, 6361 –
366.
12] a) T. Pettersson, B. Frçseth, H. Riska, M. Klockars, Chest 1988,
4, 1037 – 1039; b) G. Hillerdal, U. Lindqvist, A. Engstrçm-
9
Laurent, Cancer 1991, 67, 2410 – 2414; c) V. B. Lokeshwar, C.
Öbek, M. S. Soloway, N. L. Block, Cancer Res. 1997, 57, 773 –
[16] a) S.-J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M.-G. Choi,
D. Kim, S. Y. Park, J. Am. Chem. Soc. 2010, 132, 13675 – 13683;
b) S.-J. Yoon, S. Y. Park, J. Mater. Chem. 2011, 21, 8338 – 8346.
[17] This is further supported by the dilution experiment of the HEP
charge with HA; see Figure S15 in the SI.
7
77; d) V. S. Wong, V. Hughes, A. Trull, D. G. D. Wight, J. Petrik,
G. J. M. Alexander, J. Viral Hepat. 1998, 5, 187 – 192; e) N.
Fujimoto, K. Gemba, M. Asano, Y. Fuchimoto, S. Wada, K. Ono,
S. Ozaki, T. Kishimoto, Respir. Investig. 2013, 51, 92 – 97.
[
13] a) T. Noguchi, T. Shiraki, A. Dawn, Y. Tsuchiya, L. T. N. Lien, T.
Yamamoto, S. Shinkai, Chem. Commun. 2012, 48, 8090 – 8092;
b) T. Noguchi, A. Dawn, D. Yoshihara, Y. Tsuchiya, T. Yama-
moto, S. Shinkai, Macromol. Rapid Commun. 2013, 34, 779 – 784;
c) T. Noguchi, B. Roy, D. Yoshihara, Y. Tsuchiya, T. Yamamoto,
Received: December 12, 2015
Revised: February 17, 2016
Published online: April 6, 2016
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