An amide-containing metal-organic tetrahedron responding to a spin-trapping reaction in a fluorescent enhancement manner for biological imaging of NO in living cells

Article abstract of DOI:10.1021/ja2048489

Metal-organic polyhedra represent a unique class of functional molecular containers that display interesting molecular recognition properties and fascinating reactivity reminiscent of the natural enzymes. By incorporating a triphenylamine moiety as a brig

Full text of DOI:10.1021/ja2048489

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pubs.acs.org/JACS
An Amide-Containing MetalꢀOrganic Tetrahedron Responding to a
Spin-Trapping Reaction in a Fluorescent Enhancement Manner for
Biological Imaging of NO in Living Cells
Jian Wang, Cheng He,* Pengyan Wu, Jing Wang, and Chunying Duan*
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116012, China
S Supporting Information
b
served as a dominant source of inspiration in the area of
supramolecular chemistry, the functionalization with amide
ABSTRACT: Metalꢀorganic polyhedra represent a unique
class of functional molecular containers that display inter-
esting molecular recognition properties and fascinating
reactivity reminiscent of the natural enzymes. By incorpor-
ating a triphenylamine moiety as a bright blue emitter, a
robust cerium-based tetrahedron was developed as a lumi-
nescent detector of nitronyl nitroxide (PTIO), a specific
spin-labeling nitric oxide (NO) trapper. The tetrahedron
encapsulates molecules of NO and PTIO within the cavity
to prompt the spin-trapping reaction and transforms the
normal EPR responses into a more sensitively luminescent
signaling system with the limit of detection improved to 5 nM.
Twelve-fold amide groups are also functionalized within the
tetrahedron to modify the hydrophilic/lipophilic environ-
ment, ensuring the successful application of biological
imaging in living cells.
groups, the characteristic structural motif of protein, on the
metalꢀorganic cages should be a useful attempt to achieve
functional metalꢀorganic polyhedra with suitable hydrophilic/
lipophilic characterization.14 By incorporating robust amide-
containing tridentate chelating sites into the three arms of a
triphenylamine fragment, the often-used energy donor and bright
blue emitter,15,16 we obtained a cerium-based neutral molecular
tetrahedron, 1. This tetrahedron is able to encapsulate 2-phenyl-
4,4,5,5-tetra-methylimidazolineyloxyl-3-oxide (PTIO), one of
the most stable nitronyl nitroxides known and a specific spin-
labeling NO trapper used for detecting NO in biological sys-
tems.17ꢀ21 The confinement of the cavity of tetrahedron 1 can
force the NO and PTIO species close to each other, prompting
the specific spin-trapping reaction and restoring the lumines-
cence of compound 1. With compound 1, the biological detec-
tion of NO at a limit of 5 nM and the biological imaging of NO in
living cells are realized, whereas other reactive nitrogen and
oxygen species do not give any detectable luminescence responses.
Ligand H6TTS was synthesized from 4,40,400-nitrilotribenzo-
carbohydrazide with salicylaldehyde in methanol solution and
characterized by elemental analysis and spectroscopic methods.
Diffuse CH3OH to a DMF solution of H6TTS containing
etalꢀorganic polyhedra (MOPs), discrete molecular archi-
M
tectures constructed through the coordination of metal
ions and organic linkers, have attracted considerable attention for
their potential for a variety of applications due to their high
symmetry, stability, and rich chemical/physical properties.1ꢀ4
These molecules are synthesized by using modular and high-
yield metal-directed self-assembled methods, so that the geo-
metric and electronic characteristics embedded within the in-
dividual components have collectively allowed the construction
of the supramolecular entities in a controllable way.5ꢀ8 The
architectures generating well-defined cavities with gated pores
provide specific inner environments for selective uptaking and
binding of guest molecules and catalyzing their reactions.9,10
In parallel to the impressive development of this field in the past
two decades, the research of fluorescent biological imaging has
also grown into a mature field.11,12 In comparison, the science at
the interface between the well-confined MOP systems and the
detection and imaging techniques by luminescence response in
biological cells has received relatively little attention.
Ce(NO3)3 6H2O generated 1, in a high yield (65%). Magnetic
3
susceptibility measurements suggested a diamagnetic behavior of
the bulk sample from room temperature to 1.8 K, demonstrating
the presence of four CeIV ions of 1. Ce 3d core level XPS
spectrum exhibited a distinct band at about 916 eV, which was
assignable beside the peaks at around 880ꢀ890, 895ꢀ910
distinct regions. The unique peak was assigned to 4f0 orbital
transitions,22,23 which confirmed the presence of oxidation state
CeIV in 1. The bonding of the ligands to the metal ions was also
confirmed by the relatively broadened and shifted resonance
1
signals in H NMR spectra. Precisely, the disappearance of the
phenolic proton signal at ∼11.33 ppm and the significant upfield
shift of aromatic protons in the phenol ring suggested the
coordination occurred between the deprotonated phenolic groups
and the metal ions.24 The downfield shift from 12.10 to 13.17
ppm and the reduction of the proton portion of the amide signal
were indicative of the coordination of amide groups to the metal
ions and the partial deprotonation of amide groups during the
coordination process.
Without doubt, the major challenges for the well-confined
MOP systems applied to biological imaging detection go beyond
achieving molecular recognition, and include the modifying
hydrophilic/lipophilic characterization of MOPs, potentially
helpful to exhibit biocompatibility and cell permeability.13 In this
case, the neutral molecular systems rather than the ionic metalla-
cycles would benefit the biological applications. As nature has
Received: May 26, 2011
Published: July 12, 2011
r
2011 American Chemical Society
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Figure 1. Structure of H6TTS, constitutive/constructional fragments of
the functional tetrahedron 1 showing the sequence of fluorescent
variation of the tetrahedron 1 upon the addition of PTIO and NO.
Figure 2. Families of the luminescence spectra of (a) 1 (15 μM) upon
addition of the free radical PTIO up to 0.45 mM. (b) 1 (15 μM) and
PTIO (0.30 mM) upon addition of NO up to 0.45 mM. (c) Time-
dependent luminescence variations of 1 (15 μM) and PTIO (0.30 mM)
showing the recovery of luminescence upon the addition of the NO with
the concentration 0.1, 0.2, 0.3, and 0.4 mM (black, green, blue, and red
symbols, respectively). (d) Luminescence selectivity of 1 (15 μM) and
PTIO (0.3 mM) treated with various ROS and RNS. NO: 0.5 mM;
H2O2: 1.0 mM; OClꢀ: 1.0 mM NaOCl; 1O2: 1.0 mM H2O2 + 1.0 mM
NaOCl; NO2ꢀ: 1.0 mM NaNO2; NO3ꢀ: 1.0 mM NaNO3; ONOOꢀ:
1.0 mM NaONOO. Intensities were recorded at 470 nm, with the
excitation at 350 nm.
Single-crystal X-ray structural analysis confirmed the forma-
tion of a Ce4(H2TTS)4 tetrahedron in a crystallographic C3
symmetry in the solid state (Figure 1).25 The tetrahedron com-
prised four vertical metal centers and four deprotonated H2TTS
ligands. Each cerium ion was chelated by three tridentate-
chelating groups from three different ligands to form a ternate
coronary trigonal prism coordination geometry with a pseudo-C3
symmetry. The four pseudo-C3 symmetric planar ligands posi-
tioned individually on the four triangle faces of the tetrahedron
defined by four metal ions. The Ce Ce separation was ∼14.9
3 3 3
Å, the inner volume of the tetrahedron is about 360 Å3, and the
rhombic window of the tetrahedron had a size of 6.5 ꢁ 6.5 Å2,
potentially allowing ingress and egress of small molecules. The
CeꢀO (phenol) and CeꢀO (amide) distances of 2.24, and
2.43 Å, respectively, and the CeꢀN distance of 2.67 Å were in
good agreement with the relative compounds,26,27 with the
valences of the two cerium centers calculated as 3.9 and 4.0 for
Ce(1) and Ce(2), respectively.28,29 The CꢀO and CꢀN dis-
tances of 1.26 Å and 1.34 Å, respectively, were intermediate
between formal single and double bonds, indicating extensive
delocalization over the entire molecular skeleton.30 The absence
of any ionic species in the crystal structure suggested that
the tetrahedron 1 was neutral. Referring to the 1H NMR investi-
gation, all the phenol groups and one-third of the amide groups
were deprotonated. The protonated and deprotonated amide
groups are expected to build a suitable hydrophilic/lipophilic
environment of the cavity.
which demonstrates a 1:1 stoichiometric inclusion behavior in
solution.
Compound 1 exhibited an absorption band centered at 395 nm
(log ε = 5.07) assignable to the absorptions endemic to depro-
tonated phenol groups.31 When excited at 350 nm in DMF,
1 (15 μM) exhibited an emission band centered on 470 nm with
a quantum yield of 0.0132 (Figure 2). Upon the addition of free
radical PTIO up to 0.30 mM, the luminescence intensity of the
solution decreased gradually with ∼90% quenching efficiency
and EC50 (the concentration of guest added that half quenched
the luminescence) of about 40 μM. The Hill-plot33 of the
fluorescence titration curves demonstrated a 1:1 stoichiometric
hostꢀguest behavior with an association constant (log Kass) of
4.78 ( 0.2.
Nitric oxide (NO) is an important mediator for both physio-
logical and pathophysiological processes, and a key player in
numerous mammalian functions including vasodilation, immune
responses, and neurotransmission.34 Due to its large diffusivity
and high reactivity with other radicals and metal-containing
proteins in biological systems, the development of methods
capable of detecting NO in biology have been an intriguing
challenge for chemists, biologists, and engineers.35ꢀ37 Nitronyl
nitroxides are stable radicals and have been extensively applied as
NO trappers for the detection of NO in biological systems.17ꢀ21
However this spin-trapping technique is limited by the low
sensitivity of the EPR technique and the low resolution of EPR
image resolution as well as the large sample size of the EPR
detection.34 Interestingly, introducing NO (0.45 mM) into the
mixture containing 15 μM 1 and 0.3 mM PTIO in DMF/H2O
ESI-MS (negative) spectrum of 1 in DMF/CH3OH solution
exhibited two intense peaks at m/z = 1155.78 and 1734.14 with
the isotopic distribution patterns separated by 0.33 and 0.50 Da,
respectively. By comparison of the experimental peaks with the
simulation results obtained based on natural isotopic abun-
dances, the peaks were assigned to the species [Ce4 (HTTS)3-
(H2TTS)]3ꢀ and [Ce4(HTTS)2(H2TTS)2]2ꢀ, respectively, de-
monstrating the formation of M4L4 species in the solution. Upon
the addition of the free radical PTIO, ESI-MS exhibited a new
peak at m/z ∼1233.80 with the isotopic distribution patterns
separated by 0.33 Da. This peak is assignable to the hostꢀguest
complexation species [Ce4(HTTS)3(H2TTS) ⊃ PTIO]3ꢀ
,
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(9:1) media at room temperature immediately restored the
luminescence of 1 with a 12-fold increase of intensity within
5 min. Since fluorescence illumination and observation has been
one of the most rapidly adopted imaging technologies in
medicine and biological sciences, this new probing approach
would be a useful attempt to transform the spin-trapping
responses into a luminescence signaling system.
EPR spectra of the mixture containing 1 (1 mM) and PTIO
(30 mM) at room temperature clearly exhibited change in the
EPR signal pattern of PTIO from the characteristic five-line
pattern with first derivation peak height ratios of 1:2:3:2:1 to a
seven-line one with corresponding ratios of 1:1:2:1:2:1:1 upon
the addition of 40 mM NO. This result is well consistent with
that of PTIO itself without the presence of the tetrahedron and
confirms the occurrence of the reaction shown in eq 1 in the
solution.19
Figure 3. Fluorescence imaging of compound 1 (15 μM, suspended in
DMF) and PTIO (20 μM, suspended in DMF) induced by sodium
nitroprusside (2.0 mM). (Bottom row) Brightfield images of MCF-7
cells shown in top panel. Fluorescence image of MCF-7 cells incubated
with compound 1 (top left); incubated with PTIO in top row (middle);
further incubated with sodium nitroprusside (top right). Excited at
405 nm.
and typically enzymatic-like, governed by the MichaelisꢀMenten
mechanism,44 in which substrate binding is a first equilibrium
prior to the rate-limiting step of the reaction. As a comparison,
addition of PTIO (0.45 mM) to the solution of the free ligand
H6TTS (60 μM) or triphenylamine (60 μM) only caused a less
than 15% intensity decrease of the luminescence at 435 and
405 nm, and further addition of NO to the above-mentioned
solution did not lead to any obvious fluorescence variation within
60 min. The confinement of the cavity of compound 1 as an
analogy of the pocket of an enzyme could accelerate the reaction
through proximity effects and increases the local concentration of
the substrates.1,45
To test its applicability in living system, we investigated 1 in
MCF-7 cells using fluorescence microscopy. The cells were
incubated with a 15 μM solution of compound 1 (suspended
in phosphate-buffered saline [PBS]) for 30 min at room tem-
perature. Then the cells were washed with PBS three times and
mounted on a microscope stage. As shown in Figure 3, the cells
display blue luminescence after incubation by compound 1. The
cells remained viable throughout the imaging experiments
(about 3ꢀ4 h). Incubated with 20 μM PTIO, the luminescence
of the system was not visible, indicating that PTIO in the cell was
fluorescently detected by compound 1. The solution was further
incubated with sodium nitroprusside (2.0 mM) for another
30 min at room temperature; the intense blue fluorescence of
cells was resumed. These experiments proved that our system
could be used for monitoring intracellular NO, and compound 1
is the first example of metalꢀorganic polyhedra successfully used
for biological imaging in living cells. The combination of spin-
trapping agents within the cavity of the luminescent metalꢀ
organic cages thus represents a more powerful approach to the
high-sensitivity luminescent detections than the traditional EPR
response methods.
Usually, nitronyl and imino nitroxides are nonspecifically
reduced by various biological reductants. To avoid the inter-
ference from the redox systems in endothelial cells, nitronyl
nitroxides are usually mixed with the effluent from an endothelial
cell column.38 For our probing system, the probing process could
not be disturbed by the presence of other reactive species in the
EPR measurement, and the luminescence measurement of the
mixture containing 15 μM of compound 1 and 0.3 mM of PITO
in DMF/H2O (9:1) media upon the addition of reactive species
including H2O2, 1O2, ClOꢀ, NO2ꢀ, NO3ꢀ, and ONOOꢀ hardly
exhibited obvious luminescence variation.39 The high selectivity
toward nitric oxide over other species is likely due to the special
environment provided by the cavities of the polyhedron. That is
to say, the encapsulation of spin-trapping agent PTIO within the
cavities of the luminescent metalꢀorganic polyhedron provides a
new probing method with significantly improved selectivity over
the traditional systems.
Importantly, our system is able to respond NO in an apparent
pH range from 5.0 to 9.0 (DMF/H2O = 7:3), with the
fluorescence intensity varying by less than 10%, facilitating the
detection of NO in aqueous media at physiological pH. Under
optimized conditions, the fluorescence intensity of the probing
solution (1.5 μM 1 and 30 μM PTIO) was nearly proportional to
the NO concentration, and the purging of 5 nM NO causes about
15% fluorescent enhancement within 5 min at 298 K. The
detection limit of 5 nM established is comparable to that of the
most sensitive NO sensors reported40,41 and is quite lower than
the practical EPR detection limit (0.1ꢀ0.01 μM).42 Clearly, the
turn-on response manner has sensitivity higher than the tradi-
tional spin-trapping platform. Accordingly, the high sensitivity
and selectivity coupled with the solubility in aqueous media of
the probe made it a superior probe for NO detection by
biological imaging.
Another interesting finding is about the kinetic behavior of the
probing. Typically, the spin-trapping reaction between PTIO and
NO shows a second-order rate kinetic behavior43 with the
In a summary, this work represents the first example of
molecular tetrahedra that enables the application of biological
imaging of NO in living cells. The metalꢀorganic tetrahedron 1
can function as an enzyme-like pocket to encapsulate the spin-
trapping agent molecule and prompt the spin-trapping reaction
with NO occurring in biological systems. In the presence of NO,
the quenched luminescence of triphenylamine fragments was
constant of 5.15 ꢁ 103 Mꢀ1
s
ꢀ1. Our new system exhibited a
pseudo-zeroth-order kinetic behavior under saturation condi-
tions, i.e. having quite large NO and PTIO concentration. It
seems that the spin-trapping reaction is significantly catalyzed
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Journal of the American Chemical Society
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ignited by the spin trapping reaction, which essentially wiped out
the luminescence quenchers. The EPR response was substituted
by the highly selective and sensitive, rapidly adapted, luminescent
imaging technology.
(25) Crystal data: Ce-TTS C178H148Ce4N30O31 [Ce4(C42H29N7O6)4
3
2C3H7NO 4CH3OH H2O], M = 3763.74, Hexagonal, space group R3c,
3
3
black block, a = 30.93(1) Å, c = 86.90(6) Å, V = 71982(65) Å3, Z = 12, Dc =
1.042 g cmꢀ3, μ(Mo KR) = 0.802 mmꢀ1, T = 180(2) K. 14083 unique
reflections [Rint = 0.1135]. Final R1 [withI>2σ(I)] = 0.0692, wR2 (alldata) =
0.2031. CCDC number 798012.
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We gratefully acknowledge financial support from the NSFC
(20801008 and 21025102).
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