Smart D-π-A Type Near-Infrared Aβ Probes: Effects of a Marked π Bridge on Optical and Biological Properties

Article abstract of DOI:10.1021/acs.analchem.7b02246

To expand the scope of D-π-A based near-infrared (NIR) probes for detecting β-amyloid (Aβ) plaques and to systematically explore the relationship among their structural characteristics, optical properties, and biological properties, three series of smart NIR probes with different aromatic rings and up to seven trans double bonds were synthesized and evaluated. Marked correlations between the conjugated π system and properties of these probes, such as optical data, binding ability, and brain uptake, were observed. One probe, PHC-4, displayed improved properties as a NIR probe for the in vivo detection of Aβ plaques.

Full text of DOI:10.1021/acs.analchem.7b02246

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Article
Smart D-#-A type near-infrared A# probes: effects of
marked # bridge on optical and biological properties
Kaixiang Zhou, Hongcun Bai, Liang Feng, Jiapei Dai, and Mengchao Cui
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02246 • Publication Date (Web): 31 Jul 2017
Downloaded from http://pubs.acs.org on August 1, 2017
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Analytical Chemistry
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Smart DꢀπꢀA type nearꢀinfrared Aβ probes: effects of marked π bridge
on optical and biological properties
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Kaixiang Zhoua, Hongcun Baib, Liang Fenga, Jiapei Daic and Mengchao Cuia,*
a Key Laboratory of Radiopharmaceuticals, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing
100875, China
b Key Laboratory of Highꢀefficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan,
Ningxia 750021, China
c Wuhan Institute for Neuroscience and Neuroengineering, SouthꢀCentral University for Nationalities, Wuhan 430074, China
ABSTRACT: To expand the scope of DꢀπꢀA based NIR probes for Aβ plaques, and explore the relationship between structure
characteristics, optical properties, and biological properties systematically, three series of smart NIR probes with different aromatic
rings and transꢀdouble bonds up to seven were synthesized and evaluated. Marked correlations between conjugated π system and
properties, such as optical data, binding ability and brain uptake were observed for these probes. One probe, PHCꢀ4, displayed imꢀ
proved properties as NIR probe for in vivo detection of Aβ plaques.
INTRODUCTION
systematic enough, which may lead to unreliable and limited
conclusions. Therefore, there is a need to explore the relationꢀ
ship between structure and properties with large and compreꢀ
hensive sample volumes. Herein, three series of compounds
with different aromatic rings (benzene, pyridine, and pyrimiꢀ
dine) and extended transꢀdouble bonds, up to seven, were
designed, synthesized, and evaluated as smart NIR probes for
Aβ plaques (Figure 1). Systematic studies on the relationship
between structure characteristics, optical properties, and bioꢀ
logical properties will be discussed in detail. We expect that
this study will provide guidance for the development of new
NIR probes with improved Aβ detection capability.
Alzheimer’s disease (AD) is a neurodegenerative disorder,
associated with aging, which gradually worsens over time, and
is characterized by dementia, disorientation, cognitive imꢀ
pairment, and irreversible memory decline. According to the
amyloid cascade hypothesis, the deposition and accumulation
of βꢀamyloid (Aβ) peptides, resulting from the cleavage of
amyloid precursor protein (APP) by βꢀ and γꢀsecretases in the
brain, is the initiating event in AD pathogenesis1ꢀ3. Therefore,
nonꢀinvasive detection of Aβ plaques in the brain may faciliꢀ
tate the clinical diagnosis of AD, and evaluate the therapeutic
effects of antiꢀAβ drugs4,5.
In the past decade, several NIR probes with diverse chemical
structures have been designed to detect Aβ plaques in the
brain, such as oxazineꢀderivative AOIꢀ9876, bithiophene deꢀ
rivative NIADꢀ47, boronꢀdipyrromethene derivative BAPꢀ18,
difluoroboronate curcumin derivatives CRANADꢀ2/39,10 and
other derivatives with different scaffolds11ꢀ15. All these probes
have a common structural feature: a πꢀconjugated system with
terminal electronꢀdonor (D) and electronꢀacceptor (A) moieꢀ
ties to generate DꢀπꢀA or DꢀπꢀAꢀπꢀD architectures. Recently,
we reported a series of polymethine derivatives with different
donors, acceptors, as well as different lengths of πꢀbridge
Figure 1. Smart Aβ probes for NIR imaging based on pullꢀpush
architecture with different donors and multiple conjugated double
bonds.
(conjugated transꢀdouble bonds), including DANIRs16ꢀ19
,
EXPERIMENTAL SECTION
MCCADs20, and BBTOMs21.
General Information. All reagents used for chemical syntheꢀ
sis were commercial products and used without further purifiꢀ
cation unless otherwise stated. Reactions were monitored by
thin layer chromatography: Silica gel 60 F254 plates (Merck,
German), Flash column chromatography was carried out on
silica gel (45 ꢀ 75 ꢁm, Yantai Industry Research Institute, Chiꢀ
na). Synthetic Aβ1ꢀ42 was purchased from PEPTIDE
INSTITUTE, Inc. (Osaka, Japan), and the aggregation method
was using the previously reported procedures for in vitro asꢀ
says. 1H NMR and 13C NMR spectra were recorded on Bruker
In our previous work, we found that the length of polyenic
chains has significant impact on the optical properties of these
DꢀπꢀA based NIR probes, as well as biological properties such
as Aβ binding affinity and brain kinetics. In addition, some
computational studies based on our NIR probes provided the
theoretical relationship between photophysical properties and
chemical structures, especially the length of polyenic chains,
and the computational results are well matched with our exꢀ
perimental data22,23. However, the sample volumes in both
experimental and computational models are small and not
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Avance III (400 MHz or 100 MHz), JEOL JNMꢀECZ400R/S1 enals was isolated as the major product in yields of 6.6–
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(400 MHz or 100 MHz), JNMꢀECZ600R/S3 (600 MHz or 150
MHz) NMR spectrometer in CDCl3, DMSOꢀd6 or CF3COOD
solutions at room temperature. Chemical shift (δ) is recored in
ppm downfield from tetramethylsilane and coupling constants
(J) are reported in Hertz (Hz), and the multiplicity is defined
by s (singlet), d (doublet), t (triplet), or m (multiplet). MS and
HRMS spectra were obtained by the Surveyor MSQ Plus
(ESI) (Waltham, MA, USA) instrument. Fluorescence spectra
were determined by RFꢀ5301PC spectrofluorophotometer
(Shimadzu, Japan). The absolute fluorescence quantum yields
of all samples were measured by the Absolute PL Quantum
Yield Spectrometer C11347 (Hamamatsu, Japan). UVꢀvisible
spectra were conducted on the UVꢀ2450/3600 UVꢀVis specꢀ
trophotometer (Shimadzu, Japan). HPLC was conducted on an
Agilent 1260 Infinity Quaternary LC (BonnaꢀAgilent Techꢀ
nologies, China) and Hitachi Primaide system. The sample
was analyzed on a Venusil MP C18 reverse column (Agela
Technologies, 5 ꢁm, 4.6 mm×250 mm) eluted with an isocratꢀ
ic system at flow rate of 1.0 mL/min. The mobile phase B was
acetonitrile while A was water. Fluorescence observation was
performed by the Axio Observer Z1 inverted fluorescence
microscope equipped with DAPI, AF488, AF546 and CY5
filter sets (Zeiss, Germany), EVOS FL imaging system
equipped with DAPI, GFP, RFP, CY5, IRD680, CY7 filter
sets was also used (Life, USA). Male ICR mice (5 weeks, 18ꢀ
22 g) used for brain uptake experiments were purchased from
Vital River Laboratories (Beijing, China). Double transgenic
mice (C57BL6, APPsw/PSEN1, 14ꢀmonth old) used as an
Alzheimer’s model and age matched control mice (C57BL6,
14ꢀmonth old) were purchased from the Institute of Laboratory
Animal Sciences, Chinese Academy of Medical Sciences.
Postmortem brain tissues from autopsy confirmed cases of AD
patients (91ꢀyear old, male; 64ꢀyear old, female; 71ꢀyear old,
female; 85ꢀyear old, male; temporal lobe) were obtained from
the Chinese Brain Bank Center (CBBC). All protocols requirꢀ
ing the use of animals were approved by the animal care
committee of Beijing Normal University.
93.3%. Three series of NIRF probes (PHCs, PYCs, and
PMCs) with different lengths of transꢀdouble bonds (n = 0–6)
were obtained from the Knoevenagel condensation between
trans arylpolyenals and malononitrile in moderate yields
(19.6–99.5%). Consistent with previous reports, the condensaꢀ
tion reaction is operationally simple; the final products could
be separated from the reaction mixture as pure compounds
1
with trans stereochemistry as characterized by H NMR, 13C
NMR, and HRMS spectrums. Further purification was perꢀ
formed by recrystallization from CH2Cl2–MeOH as needed.
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Scheme 1. Synthesis of the DꢀπꢀA based NIR probes. Regents
and conditions: a. (1) anhydrous THF, 18ꢀcrownꢀ6, NaH, r.t,
12 h; (2) 1 M HCl, r.t., 30 min; b. K2CO3, MeOH, reflux.
Optical Properties
In general, the energy gap between the highest occupied moꢀ
lecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) is closely associated with the absorption and
emission wavelengths of fluorescence probes5. The most efꢀ
fective approach for the construction of a NIR probe is to put
electronꢀdonors (e.g. N,NꢀDimethylamino group) and electronꢀ
acceptors (e.g. dicyano group) to the end of a conjugated π
system (e.g. trans polyenic chains) to form a “pushꢀpull” arꢀ
chitecture16,24. In addition, we can easily tune the energy gap
by adjusting the length of conjugated double bonds, therefore,
shifting the absorption and emission maximum to NIR range.
As shown in Table 1 and Figure 2, in accordance with our
previous results, significant bathochromic shift was observed
for the three series of probes. As the π conjugated system inꢀ
creases, the redꢀshift of absorption and emission maximum
gradually rises, and finally approaches saturation (Figure 2A,
B). This could be explained by the fact that the degree of intraꢀ
molecular charge transfer (ICT) was weakened by multiple
torsions and cisꢀtrans isomerizations of the growing polyenic
chains25, thereby influencing the energy gap. In addition, unꢀ
der the same length of conjugated double bonds, probes with
more electronegative nitrogen atoms in the aromatic ring disꢀ
played short absorption and emission maximum (pyrimidine <
pyridine < benzene). Introduction of nitrogen atoms to the
conjugated π system may reduce the electron donating ability
of N,Nꢀdimethylamino group, and finally weaken the ICT efꢀ
fect. Moreover, our experimental data is in agreement with the
calculated energy gaps (Table 1). Absolute quantum yields of
these probes in CH2Cl2 increased first and then decreased with
lengthening of the polyenic chain, and the maximum value
was obtained at n = 4 (Figure 2C). In general, a large sensitiviꢀ
ty to solvent polarity is typically observed in ICT fluoroꢀ
phores26. The emission behaviors of these probes were investiꢀ
gated throughout 5 solvents with different polarity (Figure S2ꢀ
4). All of them displayed appreciably positive solvatochromic
effect; for probe PHCꢀ4 (in 12 solvents), the emission waveꢀ
length shifted by 161 nm from hexane to PBS (Figure S8). All
Chemistry. The syntheses of arylaldehydes and NIR probes
with different length of transꢀdouble bonds were outlined in
scheme 1 and S1 in Supporting Information. Details of syntheꢀ
1
sis, H NMR, 13C NMR, MS, and HRMS spectra were deꢀ
scribed in Supporting Information.
Spectroscopic Evaluations. Ultraviolet absorption spectrums,
fluorescence spectrums and absolute quantum yields of these
probes in different solvents were measured using conventional
methods. The fluorescence enhancement of probes with Aβ1−42
aggregates and bovine serum albumin (BSA) were measured
according to our previously reported procedures.16,18
Biological Evaluations. In vitro neuropathological staining on
brain sections, in vitro inhibition binding assay using Aβ1−42
aggregates, brain uptake and kinetics in normal mice, in vivo
NIR imaging and ex vivo fluorescence staining in transgenic
and control mice were conducted according to our previously
reported procedures, with detailed description in Supporting
Information.16,18
RESULTS AND DISCUSSION
Synthesis
N,NꢀDimethylamino substituted arylpolyenals were prepared
by iterative Wittig reaction starting from the arylaldehyde or
transꢀcinnamaldehyde followed by acidic hydrolysis of the
intermediate acetal. Allꢀtrans configuration of the arylpolyꢀ
these
results
indicated
that
the
conjugated
π
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Analytical Chemistry
Table 1. Selected spectroscopic and binding data for the synthesized NIRFs.
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Probe
Φ/%[a]
0.1
2.8
4.1
7.8
1.9
0.4
0.3
0.5
0.6
1.0
1.9
1.1
0.5
0.2
0.3
0.5
0.6
0.9
0.7
0.4
0.2
λem (nm)[b]
467
567
659
741
791
780
771
450
558
642
718
771
783
771
412
489
580
662
774
768
766
Gap (eV)[c]
3.02
2.55
2.22
1.98
1.81
1.66
1.53
3.10
2.63
2.30
2.06
1.87
1.72
1.58
3.25
2.75
2.40
2.15
1.95
1.79
1.65
Fold[d]
3.3
Ki (nM)
clogP[e]
2.29
2.95
3.40
4.10
4.72
5.18
5.67
1.57
2.14
2.56
3.11
3.73
4.35
4.89
1.28
1.85
1.91
2.63
3.19
3.75
4.24
PHCꢀ1
PHCꢀ2
PHCꢀ3
PHCꢀ4
PHCꢀ5
PHCꢀ6
PHCꢀ7
PYCꢀ1
PYCꢀ2
PYCꢀ3
PYCꢀ4
PYCꢀ5
PYCꢀ6
PYCꢀ7
PMCꢀ1
PMCꢀ2
PMCꢀ3
PMCꢀ4
PMCꢀ5
PMCꢀ6
PMCꢀ7
>10000
57.2
69.4
62.2
6.4
518.8±29.3
36.9±6.8
14.1±6.4
104.7±23.9
835±175.9
3061±687.7
>10000
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2.2
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2.0
1.0
1.4
>10000
13.0
42.9
2.9
4984±99.7
92.6±5.8
26.6±3.9
88.2±21.7
603±138.7
>10000
2.2
2.0
1.5
6.5
>10000
5.3
4340±1301
978.2±252
162.1±23.6
20.6±5.1
2655±425.8
13.2
10.6
2.6
2.7
[a]Quantum yield measured in CH2Cl2. [b]Emission maxima upon binding to Aβ1ꢀ42 aggregates. [c]Calculated energy gaps.
[d]Fluorescence enhancement upon binding to Aβ1ꢀ42 aggregates. [e]Calculated using the online ALOGPS 2.1 program.
ment upon binding to Aβ1ꢀ42 aggregates (D); Binding affinity (E);
initial brain uptake in normal mice after i.v. injection (F).
system, including different aromatic rings and polyenic chains,
has regular affections on the HOMOꢀLUMO gaps, which
leads to the change of optical properties.
Fluorescence response to Aβ1-42 aggregates
We tested the fluorescence response of our probes to Aβ1ꢀ42
aggregates and BSA in PBS solutions. As shown in Table 1
and Figure 2D, the fluorescence enhancement (1ꢀ to 69ꢀ fold)
in the presence of Aβ1ꢀ42 aggregates was closely related to the
conjugated
π
system,
which
is fairly close to
the changing trend of quantum yields. In each series, certain
lengths of polyenic chains (n = 3 or 4) contributed most to the
enhancement. In addition, upon interaction with BSA, a negliꢀ
gible fluorescence response was observed, indicating low nonꢀ
specific binding. Apart from fluorescence enhancement, a
remarkable hypochromatic shift was also observed for these
probes in the presence of Aβ1ꢀ42 aggregates (e.g. PHCꢀ4, Figꢀ
ure S11). Upon binding to Aβ aggregates, the movements of
the probes are restricted rigidly to form a more planar conforꢀ
mation, which results
in reduced nonꢀradiative transitions,
such as vibrational and rotational processes, and consequently
gives rise to quantum yields5. On the other hand, the bindingꢀ
induced rigid conformation also triggered the modulation
of TICT to ICT state conversion, and thus leads to the redꢀshift
of probes. However, the solvatochromic effect is likely to be
the dominant factor for the final hypochromatic shift, which
also suggested that our probes inserted into the “hydrophobic
channel” of Aβ aggregates. In general, our probes displayed
significant fluorescence changes upon binding to Aβ1ꢀ42 aggreꢀ
gates, especially for PHCꢀ4 and PYCꢀ4, with considerable
fluorescence enhancement and emission maximum greater
Figure 2. Optical and biological properties of probes with the
changing of the conjugated π system. Absorption maxima (A);
emission maxima (B); quantum yield (C); fluorescence enhanceꢀ
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than 700 nm. Compared to PHCꢀ3 (DANIRꢀ2c)16, PHCꢀ4 disꢀ
played 80 nm of redꢀshift upon interaction with Aβ aggregates,
which is very favourable for NIR imaging.
This can also explain the relationship between binding affinity
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and the length of conjugated double bonds in the three series,
the lipophilicity is descending by adding nitrogen atoms to the
donor ring, but this decline can be counteracted by lengthening
the conjugated double bonds, finally leading to clogP that
fall within the optimal range. It is also worth noting that the
binding affinity of PHCꢀ4 was enhanced three fold comꢀ
pared to DANIRꢀ2c.
In vitro fluorescence staining
To confirm the binding ability of our NIRF probes to Aβ
plaques, in vitro fluorescence staining was carried out on the
brain slices from a transgenic (Tg) mouse (APPswe/PSEN1)
and autopsyꢀconfirmed AD patients. As shown in Figure S14ꢀ
19, in line with our previous results and fluorescence response
to Aβ aggregates16ꢀ19, probes with proper length of polyenic
chains (for PHCs and PYCs: n = 3 – 6; for PMCs: n = 4 – 6)
displayed positive staining on Tg mice, which indicates they
could bind to Aβ plaques. Due to low quantum yields and fluꢀ
orescence enhancement, PMCs showed poor staining of Aβ
plaques on the AD patients. Among these probes, PHCꢀ4 and
PYCꢀ4 were selected for further staining studies using whole
brain transverse sections of Tg mouse and four AD cases. As
shown in Figure 3A and S20A, highꢀcontrast fluorescent spots
in the cortex, cerebellum, and hippocampus regions were obꢀ
served on the Tg mouse, and the staining pattern was coherent
with staining results using ThS (Figure 3B and S20B). Moreoꢀ
ver, they could also recognize three forms of Aβ deposits inꢀ
cluding denseꢀcore (Figure 3C, S20C), diffuse (Figure 3E,
S20E), and vascular amyloid (Figure 3D, S20D), as well as
neurofibrillary tangles (Figure 3F, S20F). The staining results
showed that the conjugated π system exerts influence on the
binding ability of these NIR probes.
Brain uptake and kinetics
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As shown in Figure 2F, the initial brain uptake of these probes
was closely related to the donor rings and number of conjugatꢀ
ed double bonds. The trend of brain uptake changing with the
conjugated π system was similar to the results of fluorescence
enhancement and binding affinity. When the donor ring was
fixed, the initial brain uptake of the three series increased first
and then decreased, by extending the conjugated double
bonds, the peak uptake for PHCs, PYCs, and PMCs were
16.88% ID/g (n = 3), 10.02% ID/g (n = 3), and 7.69% ID/g (n
= 5), respectively. When the number of conjugated double
bonds was fixed (n < 5), the initial brain uptake decreased
significantly with the increase of nitrogen atoms in the donor
ring (e.g. PHCꢀ3: 16.88% ID/g, PYCꢀ3: 10.02% ID/g, PMCꢀ3:
0.98% ID/g). In general, appropriate lipophilicity is one of the
most important factors affecting BBB permeability of small
molecules by passive diffusion (logP value: 2–3.5)28. As
shown in Figure 2F and Table 1, clogP values (2.5 – 4.1) of
the probes with high initial brain uptake (>7.0% ID/g) was
close to the optimal range. The clogP value decreases steadily
with the increase of nitrogen atoms in the donor ring (e.g.
clogP values for PHCꢀ3: 3.40, PYCꢀ3: 2.56, PMCꢀ3: 1.91,
respectively), thus, leading to decreased brain uptake. In addiꢀ
tion, the brain kinetics of these probes expressed as
brain2min/brain60min ratios were negatively correlated to the
number of conjugated double bonds, but positively correlated
to the number of nitrogen atoms in the donor ring, which were
also dominated by the lipophilicity (Figure S22 and Table S6).
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Figure 3. Neuropathological staining of PHCꢀ4 on brain slices
from the Tg mouse (C57BL6, APPswe/PSEN1, 12ꢀmonthꢀold,
male) (A) and AD patients (C, 91ꢀyearꢀold, male, denseꢀcore
plaques), (D, 64ꢀyearꢀold, female, vascular amyloid, arrow head),
(E, 71ꢀyearꢀold, female, diffuse plaques, arrow head), (F, 85ꢀyearꢀ
old, male, neurofibrillary tangles). The adjacent brain sections
were stained with ThS (B).
Quantitative binding affinity
Similar to the results of fluorescence enhancement with Aβ1ꢀ42
aggregates and staining on brain slices, binding affinities
gradually increased then decreased with lengthening of the
conjugated double bonds. As shown in Table 1 and Figure 2E
(curves plotted by pKi versus numbers of conjugated double
bonds), peak affinities (Ki value) for PHCs, PYCs, and PMCs
were 14.1 nM (n = 4), 26.6 nM (n = 5), and 20.6 nM (n = 6),
respectively. According to previous studies, reasonable lipoꢀ
philicity is vital for Aβ probes to insert into the “hydrophobic
pocket” of Aβ fibers27. Thus, the lipophilicity of our probes
was estimated as clogP values (Table 1), and we found that
Figure 4. In vivo NIR imaging of Tg and WT mice before and
after i.v. injection of PHCꢀ4 (A) or PYCꢀ4 (C). Brain kinetic
curves of PHCꢀ4 (B) and PYCꢀ4 (D) at selected time points.
In vivo NIR imaging and ex vivo fluorescence staining
Among these probes, the PHCꢀ4, with better properties was
selected for further in vivo studies. The PYCꢀ4, with relatively
better properties, in the PYCs was also selected for compariꢀ
son. The dynamic images shown in Figure 4A and C indicate
that they could penetrate the BBB efficiently, and the fluoresꢀ
cent intensities in the brain regions were decreased gradually
with the passage of time after i.v. injection. However, the
probes with high binding affinity (Ki
<
50 nM)
fall within the optimal range, between 3.4 and 4.1 (clogP).
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clearance curves (Figure 4B and D) plotted by relative fluoꢀ smart NIR probe for the detection of Aβ plaques in the living
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rescent intensity in the region of interest versus time after i.v.
injection showed that the washout rate in Tg mice was signifiꢀ
cantly slower than in wildꢀtype (WT) mice at later time points,
which demonstrated that they could specifically bind to Aβ
plaques in the brain of Tg mice. Due to lower lipophilicity of
the PYCꢀ4 compared to the PHCꢀ4, the PYCꢀ4 displayed fastꢀ
er washout rate from the brain, which is consistent with the
quantitative data of the brain entry. Moreover, compared to
DANIRꢀ2c, the differentiating ability of PHCꢀ4 between Tg
and WT was significantly enhanced.
brain.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Details of experimental methods, additional tables and figures,
spectral data, including 1H NMR, 13C NMR, MS, and HRMS
(PDF)
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In order to further confirm the in vivo specific binding of the
PHCꢀ4 to Aβ plaques, after NIR imaging studies,
fresh frozen horizontal slices were obtained from the brain of
Tg and WT mice, and were observed under fluoresꢀ
cent microscope. A large number of Aβ plaques with negligiꢀ
ble background in the cortex, hippocampus, and cerebellum
regions were observed from a Tg mouse (Figure 5A, C), the
locations of Aβ plaques were further verified by ThS staining
on the same section with high nonꢀspecific signals in the corꢀ
pus callosum, cingulum, and arbor vitae regions (Figure 5B,
D). However, no plaques were found in the WT mouse (Figure
S23).
AUTHOR INFORMATION
Corresponding Author
*Phone/Fax: +86ꢀ10ꢀ58808891. Eꢀmail: cmc@bnu.edu.cn.
Author Contributions
M. Cui, and K. Zhou conceived and designed the experiments. K.
Zhou, M. Cui and H. Bai performed the experiments. K. Zhou, M.
Cui and H. Bai analyzed the data. M. Cui, H. Bai and J. Dai conꢀ
tributed reagents, materials, and analysis tools. K. Zhou and M.
Cui wrote the paper.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was funded by the National Natural Science Foundaꢀ
tion of China (Grant No. 21571022 and 31640034), and the Naꢀ
tional Science and Technology Major Projects for Major New
Drugs
Innovation
and
Development
(Grant
No.
2014ZX09507007ꢀ002).
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Conclusions
In summary, three series of DꢀπꢀA probes, with different doꢀ
nor rings (benzene, pyridine, and pyrimidine) and polyenic
chains, up to seven, were synthesized and evaluated as NIR
probes for Aβ plaques. Marked correlations between conjugatꢀ
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