Novel water-soluble red-emitting poly(p -phenylenevinylene) derivative: Synthesis, characterization, and fluorescent acetylcholinesterase assays

Article abstract of DOI:10.1021/jp206930v

A new cyano-substituted poly(p-phenylenevinylene) (PPV) derivative, MEOPS-CNPPV, is synthesized through Knoevenagel condensation of anionic diacetonitrile and neutral dialdehyde and characterized by ¹H NMR, IR, elemental analysis, and gel-permeation chromatography (GPC). To our knowledge, the polymer is the first water-soluble red-emitting PPV derivative. The absorption and emission wavelength of this water-soluble conjugated polymer (CP) depend on the solvent. In buffer solution, the fluorescence of MEOPS-CNPPV is quenched by cationic dinitrobenzene derivatives. Further research indicates that dinitrobenzene derivative with a more flexible structure exhibits a larger K_sv. Making use of the charge reversal of dinitrobenzene-modified substrate, a "turn-on" method is developed for AChE activity assay with the new polymer as a fluorophore. This convenient and direct fluorometric assay thus provides a platform for novel red-emitting sensory systems.

Full text of DOI:10.1021/jp206930v

ARTICLE
pubs.acs.org/JPCB
Novel Water-Soluble Red-Emitting Poly(p-phenylenevinylene)
Derivative: Synthesis, Characterization, and Fluorescent
Acetylcholinesterase Assays
Wenjun Zhang, Linna Zhu, Jingui Qin, and Chuluo Yang*
Department of Chemistry, Hubei Key Laboratory on Organic and Polymeric Optoelectronic Materials, Wuhan University,
Wuhan 430072, People's Republic of China
S Supporting Information
b
ABSTRACT: A new cyano-substituted poly(p-phenylenevinylene) (PPV) derivative, MEOPS-
CNPPV, is synthesized through Knoevenagel condensation of anionic diacetonitrile and neutral
dialdehyde and characterized by 1H NMR, IR, elemental analysis, and gel-permeation
chromatography (GPC). To our knowledge, the polymer is the first water-soluble red-emitting
PPV derivative. The absorption and emission wavelength of this water-soluble conjugated
polymer (CP) depend on the solvent. In buffer solution, the fluorescence of MEOPS-CNPPV is
quenched by cationic dinitrobenzene derivatives. Further research indicates that dinitrobenzene
derivative with a more flexible structure exhibits a larger Ksv. Making use of the charge reversal of
dinitrobenzene-modified substrate, a “turn-on” method is developed for AChE activity assay
with the new polymer as a fluorophore. This convenient and direct fluorometric assay thus
provides a platform for novel red-emitting sensory systems.
’ INTRODUCTION
in Alzheimer's disease (AD).27,28 AChE activity is traditionally
monitored by UVÀvis spectroscopy,29 but such assay methods
show low sensitivity. Recently, fluorometric approaches with good
sensitivity have been developed to sense the activity of AChE.30À35
However, most of them adopt indirect and discontinuous
methods, which are not suitable or precise enough for enzymo-
logical studies. For example, chemiluminescent probes for the
AChE activity assay have been achieved by using an enzyme-
involved cascade reaction in which AChE assay was converted
to the detection of H2O2.30,31 Zhang's group developed two
“turn-on” fluorescent probes based on the nucleophilicity of
thiocholine.32,33 However, thio-containing compounds such as
cysteine and reduced glutathione will interfere on the assay. Very
recently, Wang's group reported two fluorescent probes which
used a direct method, but the emission wavelength is in blue-
emission region.35 Therefore, it will be of significance to develop
direct methods for AChE activity assay with long wavelength
fluorescence.
In this paper, we designed and synthesized a red-emitting
water-soluble PPV derivative for the first time. The fluorescence
of the red-emitting polymer can be quenched by dinitrobenzene
compounds. In view of the fact, we synthesized two substrates
containing dinitrobenzene moiety for the AChE detection. The
assay for AChE activity was successfully developed through
fluorescence “onÀoffÀon” of the red-emitting CP.
Conjugated polymers (CPs) have been extensively studied as
materials in optical and electronic applications including light-emit-
ting devices,1,2 photovoltaic cells,3,4 and thin-film transistors.5À7 CPs
have also attracted interest in sensing areas in recent years. A key
advantage of CP-based sensors over devices using small molecule
elements is the potential of the CPs to exhibit collective proper-
ties that are sensitive to very minor perturbations.8À10 Recently,
water-soluble fluorescent CPs have attracted much attention as
novel biosensors for detection of proteins, nucleic acids, and
proteases.11À15 Up to now, most of the fluorescent CPs, such as
poly(p-phenylenevinylene) (PPV),1,2 poly(p-phenyleneethynylene)
(PPE),16À19 and polyfluorene (PF),13,20À22 emit blue or green
fluorescence upon ultraviolet excitation. However, for many cell
and neurobiological applications, red-emitting biosensors would
offer distinctive advantages over blue-emitting biosensors. Red
emission excited by visible light would permit a better discrimi-
nation against cellular autofluorescence, an enhancement of the
optical transparency of tissue, and the ability to interrogate thicker
specimens.23 Cyano-substituted poly(p-phenylenevinylene)
(CN-PPV) has been known as a red-emitting CP and widely
used both as electron-transporting material and emissive layer
in optoelectronic devices.2,24À26 Nonetheless, water-soluble
CN-PPV has not been reported to date. Furthermore, there
is little work involved in the application of CN-PPV as a
biosensor.
The hydrolysis of acetylcholine (ACh) by acetylcholinesterase
(AChE) is a key process for the regulation of the neural response
system, and its breakdown in the brain can accelerate the
assembly of amyloid β peptides into amyloid fibrils, which results
Received: July 20, 2011
Revised:
September 2, 2011
Published: September 14, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of MEOPS-CNPPV
’ EXPERIMENTAL SECTION
3F-HF mass spectrophotometer. Liquid chromatography mass
spectra were measured on a Waters ZQ-Mass electrospray
ionizer (ESI). Elemental analyses of carbon, hydrogen, and
nitrogen were performed on a Carlorerba-1106 microanalyzer.
Gel permeation chromatography (GPC) analyses were con-
ducted on an Waters 2690D separations module equipped with
Waters 2410 refractive index detector, using polystyrene as
standard and dimethylformamide (DMF) as eluent. Fourier
transform infrared (FT-IR) experiments were carried out on a
Perkin-Elmer spectrometer with KBr pellets. UVÀvis absorption
spectra were recorded on Shimadzu UV-2550 recording spectro-
photometer. PL spectra were recorded on a Hitachi F-4500
fluorescence spectrophotometer at 37 °C.
Materials. 2-(2-(2-Methoxyethoxy)ethoxy)ethyl-4-methyl-
benzenesulfonate, 1-methoxy-4-(2-(2-(2-methoxyethoxy)ethoxy)-
ethoxy)benzene (2), 1,4-bis(chloromethyl)-2-methoxy-5-(2-(2-
(2-methoxyethoxy)ethoxy)ethoxy)benzene (3), 3-(4-methoxy-
phenoxy)propan-1-ol (5), and 3-(2,5-bis(chloromethyl)-4-
methoxyphenoxy)propan-1-ol (6) were prepared according to
literature procedure.36,38 AChE was purchased from Sigma, and
the solution of the enzyme was cooled in ice before use. Other
chemicals were purchased from commercial sources and were
used without further purification unless otherwise noted.
Instruments. 1H NMR and 13C NMR spectra were recorded
on a MECUYRVX300 spectrometer in CDCl3, D2O, and dimethyl
sulfoxide (DMSO)-d6 using tetramethylsilane as an internal
reference, respectively. Mass spectra were measured on a ZAB
Synthesis of the Polymer (MEOPS-CNPPV). Samples of 4
(163 mg, 0.5 mmol) and 10 (173 mg, 0.5 mmol) were dissolved
in DMF (10 mL). Then a solution of t-BuONa (12 mg, 0.1 mmol)
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Scheme 2. Synthesis of AChE Substrates
in t-BuOH (1 mL) was added to the mixture. The reaction mix-
ture was heated at 50 °C for 48 h. After cooling to room tem-
perature, the mixture was poured into acetone and filtrated. The
residue was dissolved in DMF and reprecipitated in vigorously
stirred acetone. The collected solid was dried in vacuum to afford
MEOPS-CNPPV as a brown powder (100 mg, yield: 31.4%). 1H
NMR (DMSO-d6, 300 MHz), δ (ppm): 8.1À7.8 (m, 4H), 7.25
(m, 2H), 4.2 (m, 4H), 3.9À3.8 (m, 7H), 3.6À3.4 (m, 9H), 3.16
(s, 3H), 2.6À2.4 (m, 2H), 2.1 (m, 2H). Anal. Calcd. for
C30H33N2O10S: C, 56.60; H, 5.22; N, 4.40. Found: C, 54.84;
H, 5.69; N, 4.23.
12 (1.62 g, 6.0 mmol) and carbonyldiimidazole (1.07 g, 6.6 mmol)
in DMSO (30 mL) was stirred under Ar for 30 min. The reaction
mixture was then transferred with a syringe to a flask containing a
solution of choline chloride (0.92 g, 6.6 mmol) and 1,8-di-
azabicyclo[5.4.0]undec-7-ene (DBU) (0.9 mL, 6 mmol) in
DMSO (15 mL). The resulting mixture was stirred at 40 °C
for 24 h, and then the solvent was removed under vacuum. The
residue was purified by silica gel column chromatography
(65:25:4 CH2Cl2-MeOH-H2O) to give ACh-DNE (0.49 g, yield:
21%). 1H NMR (D2O, 300 MHz), δ (ppm): 9.07 (s, 1H), 8.84
(s, 2H), 4.48 (br, 2H), 4.13 (s, 2H), 3.59 (t, J = 4.0 Hz, 2H), 3.01
(s, 9H). 13C NMR (D2O, 75 MHz), δ (ppm): 174.9, 162.4,
148.6, 137.5, 128.0, 121.1, 64.10, 57.7, 53.2, 39.5. ESI-MS: m/z
355 (M+-Cl). Anal. Calcd. for C14H19ClN4O7: C, 43.03; H, 4.90;
N, 14.34. Found: C, 43.22; H, 5.03; N, 14.23.
Synthesis of 2-(6-(3,5-Dinitrobenzamido)hexanoyloxy)-N,
N,N-trimethylethanaminium Chloride (ACh-DNH). ACh-DNH
wasobtained from13 by a similar protocol to ACh-DNE with DBU
5% molar equiv, yield 48%. 1H NMR (D2O, 300 MHz), δ (ppm):
9.06 (s, 1H), 8.79 (s, 2H), 4.39 (br, 2H), 3.57 (t, J = 4.0 Hz, 2H),
3.28 (t, J = 6.6 Hz, 2H), 3.05 (s, 9H), 2.31 (t, J = 7.5 Hz, 2H),
1.50À1.47 (m, 4H), 1.27À1.25 (m, 2H). 13C NMR (D2O, 75
MHz), δ(ppm): 174.2, 163.0, 147.4, 136.6, 126.9, 120.1, 64.1, 57.8,
53.4, 39.6, 33.0, 27.7, 25.3, 23.3. ESI-MS: m/z 411 (M+-Cl). Anal.
Calcd. for C18H27ClN4O7: 48.38; H, 6.09; N, 12.54. Found: C,
48.04; H, 6.08; N, 12.56.
Synthesis of 2-(3,5-Dinitrobenzamido)acetic Acid (12). A
solution of 3,5-dinitrobenzoic acid (5.30 g, 25 mmol) in thionyl
chloride (30 mL) was added to a flask and refluxed for 4 h. After
thionyl chloride was removed by evaporation, anhydrous dioxane
(20 mL) was added. To a solution of glycine (1.41 g, 18.8 mmol)
in a water/dioxane mixture (1:1, 60 mL) sodium hydroxide
(2.00 g, 50.0 mmol) in H2O (5 mL) and dioxane solution of 3,5-
dinitrobenzoyl chloride were successively added. The mixture
was stirred for 6 h at room temperature and diluted with brine.
The mixture was washed with diethyl ether, acidified with
hydrochloric acid to pH 4, and extracted with ethyl acetate.
The organic solution was washed with brine, dried (sodium
sulfate), and evaporated. The residue was purified by silica gel
column chromatography (1:1 chloroformÀethyl acetate) to give
1
12 as a white solid (4.29 g, yield: 85%). H NMR (DMSO-d6,
300 MHz), δ (ppm): 9.66 (br, 1H), 9.07 (s, 2H), 8.98 (s, 1H),
4.03 (s, 2H). 13C NMR (DMSO-d6, 75 MHz), δ (ppm): 175.2,
162.3, 148.9, 137.7, 128.0, 121.3, 42.3. ESI-MS: m/z 224 (M+-45
(COOH)). Anal. Calcd. for C9H7N3O7: C, 40.16; H, 2.62; N,
15.61. Found: C, 40.36; H, 2.95; N, 15.37.
Synthesis of 6-(3,5-Dinitrobenzamido)hexanoic Acid (13).
Compound 13 was obtained from 6-aminocaproic acid by a sim-
ilar protocol to 12, yield 87%. 1H NMR (DMSO-d6, 300 MHz),
δ (ppm): 12.09 (br, 1H), 9.18 (br, 1H), 9.04 (s, 2H), 8.94 (s,
1H), 3.31 (t, J = 7.5 Hz, 2H), 2.21 (t, J = 7.5 Hz, 3H), 1.58À1.48
(m, 4H), 1.37À1.32 (m, 2H). 13C NMR (DMSO-d6, 75 MHz),
δ (ppm): 175.1, 162.5, 148.8, 137.7, 128.1, 121.3, 40.0, 34.2,
29.2, 26.7, 24.9. ESI-MS: m/z 325 (M+). Anal. Calcd. for
C13H15N3O7: C, 48.00; H, 4.65; N, 12.92. Found: C, 48.19; H,
4.96; N, 13.03.
’ RESULTS AND DISCUSSION
Synthesis and Characterization. The synthesis of the poly-
mer MEOPS-CNPPV and substrates for AChE are outlined in
Schemes 1 and 2, respectively. 4-Methoxyphenol was first alky-
lated by 2-(2-(2-methoxyethoxy)ethoxy)ethyl-4-methylbenze-
nesulfonate or 3-chloropropanol to give 2 and 5, which then
underwent direct chloromethylation to afford 3 and 6, respec-
tively. Oxidation of 3 by dimethylsulfoxide produced the ter-
ephthalaldehyde monomer 4. After reacting with sodium cya-
nide, 6 was transformed into 7. Then 7 was converted to sulfonate
monomer 10 in three steps according to the literature procedure.36
Finally, the polymer was synthesized by the Knoevenagel con-
densation reaction of the neutral dialdehyde 4 with the anionic
diacetonitrile 10. The substrates for AChE were synthesized
by amidation of 3,5-dinitrobenzoic acid with ω-amino acid,
Synthesis of 2-(2-(3,5-Dinitrobenzamido)acetoxy)-N,N,N-
trimethylethanaminium Chloride (ACh-DNE). A solution of
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Scheme 3. Assay of AChE Activity
Figure 1. FT-IR spectrum of MEOPS-CNPPV.
Figure 2. Absorption and fluorescence spectra of MEOPS-CNPPV in
different solvents (PBS: 5 mM potassium phosphate buffer with pH of
8.0). Excitation wavelength: MeOH, 402 nm; DMSO, 428 nm; PBS,
520 nm.
followed by esterification with choline chloride. All intermediates
and final products were characterized by 1H NMR, 13C NMR, mass
spectra, and elemental analyses (see the Experimental Section).
The structure of the polymer was characterized by 1H NMR,
IR, elemental analysis, and GPC. Figure 1 shows the FT-IR
spectrum of the polymer. The symmetric and asymmetric SdO
stretching vibrations of sulfonate groups were observed at
1038 cmÀ1 and 1216 cmÀ1, respectively; the characteristic CdC
and CtN stretching vibrations appeared at 1677 cmÀ1 and
2212 cmÀ1, respectively. On the basis of GPC, the number-ave-
rage molecular weight of the polymer is 7.0 Â 104 g/mol by using
a calibration curve of polystyrene standards in DMF, with a low
polydispersity index (PDI; Mw/Mn) value of 1.13. The narrow
distribution can be attributed to the Knoevenagel condensation
reaction.37 MEOPS-CNPPV is soluble in water and polar
solvents with a water solubility of 2 mg/mL.
Figure 3. (a) Fluorescence emission changes of MEOPS-CNPPV upon
addition of ACh-DNE (0.1 mM) or ACh-DNH (0.1 mM). (b) The
dependence of the fluorescence intensity of MEOPS-CNPPV at 617 nm
on the concentrations of ACh-DNE and ACh-DNH. All of the experi-
ments were carried out in PBS buffer (pH 8.0, 5 mM) with 1 μM
MEOPS-CNPPV (in monomer repeat units). The excitation wave-
length is 520 nm.
shift of absorption could be attributed to the aggregation of
the polymer in aqueous solution,38 while the red shift of emission
could be attributed to the synergy of the aggregation and intra-
molecular charge transfer (ICT) of the polymer in very polar
aqueous solution.39 The fluorescence quantum yield of MEOPS-
CNPPV in PBS was found to be 6.5%. The low quantum yield
may be attributed to the red-emission and aggregation of
MEOPS-CNPPV in PBS.
Photophysical Properties. The normalized UVÀvis absorp-
tion and photoluminescence (PL) spectra of MEOPS-CNPPV in
different solvents are shown in Figure 2. In methanol and DMSO,
the polymer shows the absorption peaks at 402 nm and 428 nm,
as well as the emission peaks at 571 nm and 570 nm, respectively.
In contrast, a significant red shift is observed both in the
Fluorescent AChE Assays. The fluorescence assay for AChE
activity is illustrated in Scheme 3. The cationic AChE substrate
absorption and fluorescence maxima (λabs = 520 nm and λem
617 nm) in phosphate buffer saline (PBS). The significant red
=
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(Ksv) that can be determined by the SternÀVolmer equation.
From the titration of fluorescence (Figure 3b), the Ksv for ACh-
DNE and ACh-DNH is determined to be 1.04 Â 105 MÀ1 and
4.05 Â 105 MÀ1, respectively. Presumably, the flexible structure
of ACh-DNH endowed by the longer chain of ACh-DNH,
compared with ACh-DNE, makes it easier for the dinitrobenzene
moiety of ACh-DNH to close to the backbone of the polymer.
Because of the better quenching efficiency of ACh-DNH, it was
used to the subsequent test of AChE activity.
The results of the fluorescence-monitored hydrolysis for ACh-
DNH by AChE are shown in Figure 4. Upon the addition of AChE
to the MEOPS-CNPPV/ACh-DNH complex ([MEOPS-CNPPV] =
1 μM, [ACh-DNH] = 0.1 mM) in PBS (pH 8.0, 5 mM) at 37 °C,
the fluorescence of MEOPS-CNPPV was observed to rise
gradually in 10 min (final [AChE] = 25 units/mL) (Figure 4a).
This result indicates that AChE catalyzed the hydrolysis of
ACh-DNH, consequently leading to a “turn-on”response of MEOPS-
CNPPV fluorescence. The fluorescence recovery of MEOPS-
CNPPV depends on the concentration of AChE (Figure 4b). An
increase in the concentration of AChE leads to a fast cleavage
reaction rate and a high fluorescence recovery. AChE with
concentrations of ca. 12.5 units/mL can be detected. Though
the assay is not as sensitive as the chemiluminesence technique, it
has two advantages: (i) long wavelength excitation and emission
cause less damage and exhibit less autofluorescence background;
(ii) this continuous and direct fluorometric assay is convenient
and suitable for enzymological studies. We believe that the
sensitivity can be improved by designing more substrate struc-
tures with a high binding constant to MEOPS-CNPPV.
’ CONCLUSION
Figure 4. (a) Fluorescence spectra of MEOPS-CNPPV/ACh-DNH as
a function of reaction time for AChE-catalyzed hydrolysis; [MEOPS-
CNPPV] = 1 μM, [ACh-DNH] = 0.1 mM, [AChE] = 25 units/mL. The
experiment was carried out with a fixed concentration of AChE. (b) I/I0
of MEOPS-CNPPV at 617 nm versus reaction time in the hydrolysis of
ACh-DNH with varying concentrations of AChE; [MEOPS-CNPPV] =
1 μM, [ACh-DNH] = 0.05 mM, [AChE] = 12.5 À 50 units/mL.
Fluorescence measurements were made in PBS buffer (pH 8.0, 5 mM)
with excitation at 520 nm.
We have developed a new water-soluble red-emitting polymer
MEOPS-CNPPV. The structural design of the polymer com-
bines the red emission of the CNPPV backbone and the water-
solubility of sulfonic side chain. The polymer is the first example
of water-soluble red-emitting PPV derivative. By taking advan-
tage of charge reversal of a quencher-modified dinitrobenzene
moiety (substrate of AChE), the new red-emitting water-soluble
polymer has been successfully applied to monitor AChE activity.
The red emission of MEOPS-CNPV permits a better discrimina-
tion against cellular autofluorescence and can have a perspective
for polymer-based biosensor. We believe the red-emitting water-
soluble CP will find more application in sensor areas.
can form a complex with the anionic polymer MEOPS-CNPPV
through electrostatic interactions. The fluorescence of MEOPS-
CNPPV is quenched by the nitrobenzene moiety by an electron-
transfer process.40 Upon the addition of AChE, the AChE sub-
strate will be hydrolyzed by the catalysis of AChE, in which
process the substrate is decomposed into choline and a negatively
charged residue that contains the dinitrobenzene moiety. The
quencher moiety is repelled away from MEOPS-CNPPV, and
the fluorescence of the polymer is recovered. Consequently, the
hydrolysis catalyzed by AChE can be monitored by the fluores-
cence recovery of MEOPS-CNPPV.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis of the intermediates
b
4, 7, 8, 9, and 10; H NMR and 13C NMR spectra of the
1
intermediates, the substrates ACh-DNE and ACh-DNH, and the
polymer MEOPS-CNPPV. This material is available free of
charge via the Internet at http://pubs.acs.org.
To choose a better substrate and determine the sensitivity of
the protocol, we initially investigated the fluorescence response
of the new polymer MEOPS-CNPPV to the two different sub-
strates ACh-DNE and ACh-DNH. As shown in Figure 3, after the
addition of ACh-DNE or ACh-DNH, the fluorescence of MEOPS-
CNPPV was quenched more efficiently by ACh-DNH. With a
concentration of 0.1 mM, the fluorescence of MEOPS-CNPPV
was almost completely quenched in the presence of ACh-DNH,
whereas it is slightly quenched by ACh-DNE (Figure 3a). The
quenching efficiency is related to the SternÀVolmer constant
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: clyang@whu.edu.cn.
’ ACKNOWLEDGMENT
We are grateful for financial support from the National Natural
Science Foundation of China (No. 20874077), the program
for Changjiang Scholars and Innovative Research Team in
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