Fluorogenic supramolecular complexes formed between pyrenyl-β-cyclodextrin and glyco-rhodamine for the selective detection of lectins

Article abstract of DOI:10.1039/c4cc06356b

Fluorogenic supramolecular complexes formed between tubular-shaped pyrenyl-β-cyclodextrins and glyco-rhodamine are determined to respond to a selective lectin with 'turn-on' fluorescence with excellent selectivity over a range of competing species. This journal is

Full text of DOI:10.1039/c4cc06356b

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Fluorogenic supramolecular complexes formed
between pyrenyl-b-cyclodextrin and glyco-rhodamine
for the selective detection of lectins†
Cite this: Chem. Commun., 2014,
50, 14141
Received 13th August 2014,
Accepted 24th September 2014
Xiao-Peng He,a Ri-Hui Li,‡a Stephane Maisonneuve,‡b Yibin Ruan,b
´
Guo-Rong Chen*a and Juan Xie*b
DOI: 10.1039/c4cc06356b
www.rsc.org/chemcomm
Fluorogenic supramolecular complexes formed between tubular-shaped insights into the advancement of basic science as well as disease
pyrenyl-b-cyclodextrins and glyco-rhodamine are determined to theranostics. Considering that ingenious detection methods
respond to a selective lectin with ‘turn-on’ fluorescence with with low cost, succinct conducting procedures and, meanwhile,
excellent selectivity over a range of competing species.
good sensitivity and selectivity have been rare, we develop here
an uncommon supramolecular complex formed between a
Cyclodextrins (CDs) are cyclic oligosaccharides with a hydrophobic tubular-shaped pyrenyl-b-CD and glycosyl rhodamine (RB) for
cavity that can bind various organic guest molecules. As water- the simple and sensitive recognition of lectins. We determined
soluble polyfunctional natural compounds, CDs have been that the self-assembled supramolecular system showed good
extensively used as versatile platforms to build molecular sensitivity (sub-micromolar limit of detection) and selectivity
assemblies with new functions.1 For example, CDs have been (with no response to a range of competing species) towards a
modified with polar groups, lipophilic groups or conjugated specific lectin (Fig. 1). We believe that this study would provide
with sugars and other molecules for delivery of genes,2–4 anti-HIV brand-new insights into the development of unique supramolecular
drugs5 and pharmacological chaperones.6 They also serve as probes based on CDs for investigation of protein–ligand recognition,
synthetic activators mimicking the lipopolysaccharide effects,7 as more generally.
inhibitors of metal-induced protein aggregation8 and glycosidase,9
and as blocking agents for pore-forming toxins.10 Per-glycosylated
CDs have also been developed for sugar–lectin (proteins that
recognize sugars) interactions.11–13
Despite the extensive biochemical applications of CDs,
CD-constructs that can assemble into unique supramolecular
complexes for bio-recognition have been elusive. With continuing
interest in the chemistry of CDs14–16 and in the probing of sugar–
lectin recognition,17–21 we report here the formation of a pyrenyl-
b-CD construct for the development of a new class of fluorogenic
supramolecular probes for detection of lectins.
Selective sugar–lectin interactions are crucially involved in the
modulation of myriads of physiological and pathological events,22–24
and a recent investigation suggests the role of surface glycans in the
promotion of cancer development.25 As a result, the sensitive and
selective detection of these interactions may provide substantial
a Key Laboratory for Advanced Materials & Institute of Fine Chemicals,
East China University of Science and Technology, 130 Meilong Rd.,
Shanghai 200237, P. R. China. E-mail: mrs_guorongchen@ecust.edu.cn
b PPSM, ENS de Cachan, CNRS UMR 8531, 61 Avenue du Pt Wilson,
F-94235 Cachan, France. E-mail: joanne.xie@ens-cachan.fr
Fig. 1 (a) Synthesis of the target pyrenyl-b-CD and the structure of the
GalNAc-RB (6) and GlcNAc-RB (7). (b) Cartoon depicting the fluorogenic
detection of a selective lectin using the CD–glycosyl RB complex.
† Electronic supplementary information (ESI) available: Experimental section
and additional table and figures. See DOI: 10.1039/c4cc06356b
‡ Equal contribution.
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Cu(I)-catalyzed Huisgen azide–alkyne cycloaddition (CuAAC)26,27
was used to obtain the desired pyrenyl b-CD (Fig. 1). Using our
previously developed one-pot procedure,28 pyrene-1-carboxaldehyde
1 was first converted to the alkyne derivative 2 using the Bestmann–
Ohira reagent under basic conditions. A subsequent reaction with
acetylated 6-azido-6-deoxy b-CD 314,15 catalyzed by CuI led to the
desired compound 4 in 92% yield. Removal of the acetyl protecting
group with acetyl chloride in methanol furnished pyrenyl-b-CD 5
with free hydroxyl groups on the secondary face. The two CD
derivatives were characterized by NMR and mass techniques
(see ESI†).
As shown in Table S1 and Fig. S1a (ESI†), pyrenyl-b-CD 4
consistently exhibited a broad absorption band at ca. 347 nm in
all solvents tested. Small blue shifts of the absorption band
were observed in protic solvents (MeOH and EtOH). This might
be attributed to the intermolecular hydrogen bonding between
the nitrogen atoms of triazole and the hydroxyl group of the
protic solvents, leading to the decrease of electron density on
the conjugated system between triazole and pyrene.35 The
extinction coefficient (e) of 4 in MeOH was slightly lower than
those in toluene and dioxane.
Fig. 2 (a) Integrated circular dichroism (upper) and adsorption (lower) spectra
of CD-4 in MeOH (2 mM). (b) Proposed structure of the pyrenyl-b-CD.
With their preliminary photophysical and conformational
information obtained, we further examined the applicability of CD-4
and CD-5 in the bio-recognition of lectins. A previous study
suggested that rhodamine B (RB) can be partially included in
the hydrophobic cavity of an optically active b-CD to produce a
photo-switchable supramolecular system.32 Therefore, we used
previously synthesized N-acetyl-galactosaminyl rhodamine B
(GalNAc-RB, 6) and N-acetyl-glucosaminyl rhodamine
B
(GlcNAc-RB, 7) (Fig. 1a)18 for complexation with the pyrenyl-
CDs. We first observed that the addition of an increasing
amount of CD-5 (Fig. 3a) to 6 and CD-4 to 6 (Fig. 3b) and 7
Different emission patterns of 4 in the different solvents
were observed, with the emission of the pyrene excimer (lmax
=
492 nm) being more predominant than that of the monomer
(Fig. S1b, ESI†). Remarkable monomer emission bands at
388 nm and 410 nm were observed in dioxane, and moderate
emission was observed in THF, toluene and CHCl3. In strong
polar solvents such as DMSO, MeOH and EtOH, the monomer
emission was much weaker than that of the pyrene excimer.
These observations suggest that, in polar solvents, pyrenes
might stack more strongly (to form excimers) than in less
polar solvents. The fluorescence quantum yields (+F) of 4 in
different solvents were determined to range from 0.14 to 0.45,
with the highest value (0.45) in dioxane and the lowest (0.14)
in MeCN.
The above spectral data suggest that the conformation of
CD-4 is dependent on solvent medium. Notably, the excimer
emission was always much stronger than that of the monomer,
suggesting that the seven pyrenyl fluorophores on the narrower
rim of CD are probably arranged in a compactly clustering
manner. To probe the preferential conformation of 4 in more
detail, NMR which is an effective method for structural elucidation
of CDs in solution was used.29 The H NMR of CD-4 showed
1
Fig. 3 Fluorescence quenching (where FI is the Fluorescence Intensity) of
GalNAc-RB 6 (1 mM in 0.05 M Tris-HCl, pH 7.3) in the presence of
increasing (a) CD-5 and (b) CD-4 (0–10 mM), and (c) GlcNAc-RB 7 (2 mM
in 0.05 M Tris-HCl, pH 7.3) in the presence of increasing CD-4 (0–20 mM).
Fluorescence recovery of (d) the CD-5–GalNAc-RB (6) complex and
(e) the CD-4–GalNAc-RB (6) complex in the presence of increasing
soybean agglutinin (SBA, 0–20 mM), and (f) the CD-4–GlcNAc-RB (7) complex
in the presence of increasing wheat germ agglutinin (WGA, 0–50 mM). The
selective fluorescence recovery (where F, Fcom and FRB are the FIs of a CD–RB
complex in the presence of a lectin, that of the CD–RB complex in the absence
of a lectin and that of the RB, respectively) of the (g) CD-5–GalNAc-RB (6),
(h) CD-4–GalNAc-RB (6) and (i) CD-4–GlcNAc-RB (7) complex in the
presence of various selective or unselective lectins (Con A: concanavalin
A, mannose/glucose-selective; LcA: lentil lectin, mannose-selective; PSA:
pea lectin, fucose-selective) (lex = 520 nm). For the original FI spectra of the
histograms, see Fig. S6 (ESI†).
that the CD structure is symmetrical as no cross-coupling
signals were observed between pyrene and CD in the NOE
spectrum (Fig. S2, ESI†). This means that the pyrene moieties
are not included in the CD cavity. The orientation of pyrenes on
the CD rim was further studied by induced circular dichroism
(ICD).30,31 As shown in Fig. 2a, while the 1Bb transition (around
278 nm) of pyrene was polarized along the short axis, a negative
ICD was observed for the 1La transition along the long axis
(around 343 nm). These observations suggest that the pyrene
moieties probably align in parallel with the primary axis of CD.30,31
On the basis of the above spectroscopic data, the preferential
conformation of pyrenyl-b-CDs, featuring a tubular-shaped motif,
was proposed (Fig. 2b).
14142 | Chem. Commun., 2014, 50, 14141--14144
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(Fig. 3c) led to the fluorescence quenching of RB (0.05 M Tris- that the CD–GalNAc-RB complex could serve as a selective lectin
HCl, pH 7.3) in a concentration-dependent manner.
probe with a fluorogenic detection signal.
From these fluorescence spectra the binding constants of
In summary, we have built a tubular-shaped pyrenyl-b-CD
the CD-5–RB-6 complex, the CD-4–RB-6 complex and the CD-4– for the fluorogenic detection of lectins with excellent selectivity
RB-7 complex were determined to be 3.5 Â 105, 4.4 Â 105 and over a range of relevant competing species. This study provides
3.1 Â 105 L molÀ1, respectively, using the Benesi–Hildebrand a new supramolecular platform for the detection of protein–
equation.33 The good linearity yielded by the double reciprocal ligand interactions. Investigation of the exact quenching
plot of 1/[DF] against 1/[C] (where DF is the difference between mechanism and extension of the detection scope are currently
the fluorescence intensity of RB in the presence and absence of underway.
CD and C the concentration of CD) (Fig. S3a–c, ESI†) suggests
We thank the 973 project (2013CB733700), the National Natural
Science Foundation of China (21176076 and 21202045), the Key
the formation of 1 : 1 RB–CD complexes.34
To probe the complexation manner of the system, a compe- Project of Shanghai Science and Technology Commission
tition assay was first performed. We used adamantane that can (13NM1400900) and the Fundamental Research Funds for the
be effectively included in the hydrophobic cavity of b-CD Central Universities.
because of its suitable molecular size, as an ‘inhibitor’ of the
pyrenyl-CD–RB inclusion. To our surprise, addition of an
increasing amount of adamantane up to 40 equiv. (with respect
to GalNAc-RB) did not recover the quenched fluorescence of RB
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