Conversion of molecular information by luminescent nanointerface self-assembled from amphiphilic Tb(III) complexes

Article abstract of DOI:10.1021/ja2057924

A novel amphiphilic Tb³⁺ complex (TbL⁺) having anionic bis(pyridine) arms and a hydrophobic alkyl chain is developed. It spontaneously self-assembles in water and gives stable vesicles that show sensitized luminescence of Tb³⁺ ions at neutral pH. This TbL ⁺ complex is designed to show coordinative unsaturation, i.e., water molecules occupy some of the first coordination spheres and are replaceable upon binding of phosphate ions. These features render TbL⁺ self-assembling receptor molecules which show increase in the luminescence intensity upon binding of nucleotides. Upon addition of adenosine triphosphate (ATP), significant amplification of luminescent intensity was observed. On the other hand, ADP showed moderately increased luminescence and almost no enhancement was observed for AMP. Very interestingly, the increase in luminescence intensity observed for ATP and ADP showed sigmoidal dependence on the concentration of added nucleotides. It indicates positive cooperative binding of these nucleotides to TbL⁺ complexes preorganized on the vesicle surface. Self-assembly of amphiphilic Tb³⁺ receptor complexes provides nanointerfaces which selectively convert and amplify molecular information of high energy phosphates linked by phosphoanhydride bonds into luminescence intensity changes.

Full text of DOI:10.1021/ja2057924

ARTICLE
pubs.acs.org/JACS
Conversion of Molecular Information by Luminescent Nanointerface
Self-Assembled from Amphiphilic Tb(III) Complexes
Jing Liu,†,‡ Masa-aki Morikawa,†,§ and Nobuo Kimizuka*,†,§,^
†Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku,
Fukuoka 819-0395, Japan
‡College of Physics & Information Technology, Shaanxi Normal University, Xi’an 710062, P. R. China
§JST CREST
^International Research Center for Molecular Systems, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395
S Supporting Information
b
ABSTRACT: A novel amphiphilic Tb3+ complex (TbL+) having anionic
bis(pyridine) arms and a hydrophobic alkyl chain is developed. It sponta-
neously self-assembles in water and gives stable vesicles that show sensitized
luminescence of Tb3+ ions at neutral pH. This TbL+ complex is designed to
show coordinative unsaturation, i.e., water molecules occupy some of the first
coordination spheres and are replaceable upon binding of phosphate ions.
These features render TbL+ self-assembling receptor molecules which show
increase in the luminescence intensity upon binding of nucleotides. Upon
addition of adenosine triphosphate (ATP), significant amplification of
luminescent intensity was observed. On the other hand, ADP showed
moderately increased luminescence and almost no enhancement was observed for AMP. Very interestingly, the increase in
luminescence intensity observed for ATP and ADP showed sigmoidal dependence on the concentration of added nucleotides. It
indicates positive cooperative binding of these nucleotides to TbL+ complexes preorganized on the vesicle surface. Self-assembly of
amphiphilic Tb3+ receptor complexes provides nanointerfaces which selectively convert and amplify molecular information of high
energy phosphates linked by phosphoanhydride bonds into luminescence intensity changes.
’ INTRODUCTION
quantitative complexation is expected in water. Pyridine ligands
also serve as antenna moieties which exert efficient energy trans-
fer to coordinated lanthanide ions upon ultraviolet illumination.4
2. To impart self-assembling nature to the lanthanide com-
plex, a hydrophobic alkyl chain is introduced to the ligand
L2ꢀ. It would drive amphiphilic self-assembly of lantha-
nide complexes in water.
3. Stoichiometric (1:1) coordination between the dianionic
ligand L2ꢀ and trivalent lanthanide ion give a monocationic
lanthanide complex. Tb3+ ion was selected as lanthanide ion
because of its distinct emission properties with a large Stokes
shift and long luminescence lifetime.5 Coordinative unsatura-
tion is expected for this 1:1 complex, that is, water molecules
would occupy the rest of coordination spheres. The complexes
are aligned regularly at the aqueous interface, and water
molecules in the coordination spheres are replaceable by
coordinating anions such as phosphates. As coordinating
water molecules generally quench luminescence of lanthanide
ions via deactivation of the excited states through OꢀH
vibrational modes of coordinated water molecules, such
replacement may allow detection of phosphate coordination
by increased luminescence intensity.6
Biological anions such as nucleotides constitute important
family of receptor ligands which play indispensable roles in storage,
transformation of genetic information and in intracellular signaling.
The intracellular signaling is initiated by molecular recognition events
on biomembranes, where receptor proteins and lipids are dynamically
self-assembled into specific supramolecular complexes successively to
the binding of receptor ligands. They activate signaling pathways,
leading to the amplification into cellular response.1 The sophisticated
interfacial functions of biomembranes manifest the importance of
preorganized supramolecular systems which dynamically convert and
amplify molecular information to the other physicochemical signals.
Although synthetic molecular receptors for biological phosphate
molecules has been extensively reported,2 their integration into
molecular assemblies and to construct signal-responsive nanointer-
faces have met limited success.3 It remains challenges to develop such
supramolecular interfaces which are capable of receiving and con-
verting molecular information of biological anions synergistically into
physicochemical outputs. We herein report a novel supramolecular
luminescent receptor for nucleotides that is self-assembled from
amphiphilic lanthanide complex TbL+ (Figure 1). The ligand L2ꢀ
is composed of bis(pyridine) anionic arms and a long alkyl chain,
based on the following considerations.
1. The bis(pyridine) anionic arms are introduced in the ligand L2ꢀ
since they secure high stability constant to lanthanide ions and
Received: June 22, 2011
Published: September 20, 2011
r
2011 American Chemical Society
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Figure 1. Schematic representation for self-assembly of amphiphilic TbL+ complexes in water and binding of ATP molecules.
As biological phosphate molecules, nucleotides were chosen.
Nucleotides carry one or more phosphate groups which are linked in
series by phosphoanhydride bonds. Rupture of these phosphoanhy-
dride bonds releases large amounts of useful energy. Above all
nucleotides, adenosine triphosphate (ATP) transfers chemical
energy in hundreds of different cell reactions, rendering it the
molecular unit currency of intracellular energy transfer.7 The
transformation of ATP into ADP or AMP (adenosine diphosphate
or monophosphate) provides energy to power cell reactions such as
protein synthesis or cell movement. It also serves as (co)substrates
for signal-regulating enzymes like kinases and adenylate cyclase.
Thus, they play essential roles in the storage and retrieval of
biological information in the cell. Although fluorescent8 or
colorimetric9 molecular receptors for ATP have been extensively
developed, molecular self-assemblies responsive to ATP has been
limited to surface monolayers with guanidinium-receptors,10 and
recent binuclear zinc(II) cyclen complex embedded in liposomes.3f
Surface monolayer systems show changes in the surface pressureꢀ
area isotherms which reflect interactions at the airꢀwater
interface,10 but are not suitable to rapid and qualitative sensing of
guest molecules. In conventional vesicular system, on the other
hand, receptor molecules do not exert self-assembling properties by
themselves and they require lipid matrices to be stably dispersed in
water.3f Such membrane-embedded molecular receptors generally
do not show cooperative or synergistic responses, which provide the
basis for conversion and processing of molecular information. Self-
assembly of amphiphilic receptor complexes may provide a rational
strategy toward this issue. The interaction between preorganized
receptors and anionic guest molecules especially with multiple
receptor-binding sites may lead to selectivity and cooperative
interactions, as a consequence of adaptation by aligned receptors.
These features would give nanointerfaces the capability to convert
and amplify molecular signals.
’ RESULTS AND DISCUSSION
The ligand L2ꢀ was synthesized and purified according to the
procedure described in Supporting Information. Self-assembling
characteristics of L2ꢀ and its interaction with TbCl3 were
investigated in HEPES buffer (10 mM, pH = 7.5). Addition of
Tb3+ ion to aqueous dispersion of L2ꢀ gave bright green lumines-
cence under UV irradiation (Figure S1 inset, Supporting In-
formation). In the emission spectrum, luminescence peaks are
observed at 489 (5D4 f 7F6), 545 (5D4 f 7F5), 586 (5D4 f 7F4),
and 621 nm (5D4 f 7F3) with the 545 nm band having the highest
intensity, which are characteristics of Tb3+ ion (Figure S1, Support-
ing Information).11 As neither of the ligand L2ꢀ nor Tb3+ ions alone
reveal any luminescence under these conditions, the observed
luminescence is sensitized by the coordinating ligand L2ꢀ. This is
confirmed by the excitation spectrum monitored at 545 nm which
displayed a peak of pyridine unit at 272 nm. The composition of the
complex Tb3+:L2ꢀ was determined to be 1:1, as revealed by plotting
luminescence intensity as a function of [Tb3+]/([Tb3+] + [L2ꢀ])
(Job’s plot, Supporting Information Figure S2) and elemental
analysis (Supporting Information). To investigate the self-assem-
bling characteristics of TbL+, dynamic light scattering (DLS)
experiments were conducted. Figure S3a (Supporting Information)
shows dependence of light scattering intensity plotted as a function
of TbL+ concentration. A flection point is observed around at
0.35 μM, which corresponds to the critical aggregation concentra-
tion (CAC) for the 1:1 complex (TbL+). This value is almost by 3
orders of magnitude lower than that determined for the aqueous
ligand L2ꢀ alone (0.18 mM, Figure S3b in Supporting Information).
The order of CAC value obtained for aqueous L2ꢀ is typical of
micelle-forming amphiphiles, which is consistent with the molecular
structure. These observations clearly indicate that the formation of
TbL+ complex promotes self-assembly (Figure 1). Morphology of
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Figure 2. (a) Particle size distribution, (b) SEM image, and (c) TEM image of [TbL]Cl (50 μM): 10 mM HEPES buffer (pH = 7.5). In TEM
measurement, the sample was poststained with uranyl acetate.
The ability of TbL+ vesicles to interact with nucleotides and
their influence on luminescence characteristics were then inves-
tigated. Very interestingly, upon addition of ATP, aqueous TbL+
showed a significant increase in luminescence intensity as shown
in Figure 3a. The increase in luminescence intensity of TbL+ (I/
I0 = IATP+TbL/ITbL) reached a maximum value of ∼1.54 when
ATP was added beyond the concentration of ∼10 μM
(Figure 3b). The concentration of 10 μM corresponds to 0.2
molecular equivalents to the total concentration of TbL+. By
considering that ATP would bind only to TbL+ receptors self-
assembled on the outer surface of aqueous vesicles, the molar
composition on the vesicle surface is estimated as TbL+
/
vesicle surface
ATP ≈ 3:1. Formation of this [TbL+]3(ATP) complex was
supported by ESI-MS measurement (Figure S5, Supporting In-
formation). The binding of ATP to TbL+ vesicles was also confirmed
by changes in zeta potential (Figure S6 in Supporting Information).
The zeta potential obtained for TbL+ vesicles (ζ, +6.89 mV) showed
pronounced decrease upon the addition of ATP, and reached a lowest
ζ value of ꢀ41.3 mV above the ATP concentration of ∼10 μM. This
threshold coincides with that observed for the ATP-induced increase
in luminescence intensity (Figure 3), which supports binding of ATP
molecules on outer vesicle surfaces. The evidence for outer vesicle
surface-binding model was further provided by luminescence titration
experiments conducted for molecularly dispersed TbL+ in HEPES
buffer/THF (40 vol%) (Figure S12, Supporting Information).
Although such a quantitative binding of ATP could reduce the surface
charge of TbL+ vesicles, morphology of these vesicles were stably
maintained after the addition of ATP, as confirmed by DLS and TEM
measurements (Figure S7, Supporting Information). In the case of
ADP, increase in the luminescence intensity was also observed, while
the enhancement is only half of that observed for ATP. The lumines-
cence intensity attained a maximum at higher nucleotide concentra-
tion of ∼13 μM, which is consistent with the less number of negative
charges on ADP. Meanwhile it is noticeable that luminescence
intensity profiles in the presence of ATP and ADP (Figure 3) show
sigmoidal curvature against the concentration of nucleotides, which
indicate cooperative binding. The observed changes in luminescence
intensity were analyzed by using the Hill equation (Figures S8 and S9,
Supporting Information). The Hill coefficient (n) reflects the extent
of cooperativity among multiple binding sites and those determined
for ATP/TbL+ and ADP/TbL+ systems are 2.4 and 2.3, respectively
(Table S1, Supporting Information). The n values greater than one
indicate positive cooperative effect, that is, the binding of phosphate
groups in ATP or ADP occur cooperatively on the surface of TbL+
vesicles. This is further supported by the Scatchard plots constructed
for the binding of ATP and ADP to aqueous TbL+ vesicles (Figure
Figure 3. (a) Luminescence spectra of [TbL]Cl in the absence and
presence of ATP; (b) I/I0 evolution as a function of the concentration of
biomolecules. The concentration of [TbL]Cl for all experiments is
50 μM, in 10 mM HEPES buffer (pH = 7.5), λex = 285 nm.
these nanoparticles was subsequently observed by scanning
electron microscopy (SEM), transmission and cryo-transmis-
sion electron microscopy (TEM, cryoTEM). In SEM and
TEM observation, TbL+ showed spherical nanoparticles with
diameters of 50ꢀ110 nm (Figures 2b, c). These structures are
characteristics of bilayer vesicles, which were also confirmed
by cryo-TEM observation (Figure S4, Supporting In-
formation). The observed diameter of vesicles is consistent
with the DLS measurement (Figure 2a, average diameter, 83.0
( 16.8 nm). It is apparent that TbL+ spontaneously forms
stable self-assemblies in water without the help of any other
matrices such as surfactant micelles or lipid bilayers.
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S10, Supporting Information). These plots possess maxima which are
consistent with the positively cooperative binding.12 The apparent
binding constant (Ka) obtained for the combination of ATP/TbL+
(4.37 ꢁ 104 Mꢀ1) reveal strong interactions between ATP and TbL+
complexes self-assembled on the surface of vesicles, and is larger than
that observed for ADP/TbL+ (1.78 ꢁ 104 Mꢀ1). On the other hand,
the addition of aqueous AMP caused only slight increase in the lumi-
nescence intensity of TbL+ vesicles (Figure 3b), reflecting weak inter-
action without cooperativity (the Hill coefficient estimated as n ≈ 1).
The cooperative binding of ATP and ADP and selectivity
observed for ATP require preorganization of TbL+ receptor
molecules. When vesicle structures are disrupted by addition of
40 vol% THF, luminescence intensity changes I/I0 upon addition
of ATP or ADP showed only monotonous increases (Figure S12
in Supporting Information). The sigmoidal curves and selectivity
observed in aqueous vesicles were lost in the HEPES/THF
mixture. Thus, cooperativity and selectivity are not observed for
molecularly dispersed receptors but they immerge only when
receptors self-assemble to form supramolecular interfaces.
It was also confirmed that nonionic adenosine showed almost
no influence on the emission intensity of TbL+ (Figure 3b).
These data indicates that the presence of tri- or diphosphate
groups linked by phosphoanhydride bonds is prerequisite to the
sigmoidal enhance luminescence intensity of TbL+ vesicles. This
is further supported by the effect of inorganic triphosphate and
diphosphate anions on the luminescence intensity of TbL+
(Figure S13, Supporting Information). The inorganic phos-
phtates enhanced luminescence intensity of TbL+ vesicles, in
the order triphosphate > diphosphate > monophosphate. It is
however noticeable that the addition of ATP and ADP to TbL+
vesicles showed slightly larger normalized luminescence inten-
sities I/I0 compared to those observed for inorganic phosphates
[ATP (maximum I/I0 value, 1.54), ADP (1.32), AMP (1.07);
triphosphate (1.42), diphosphate (1.25), monophosphate (1.07)].
Therefore, the presence of ribose and nucleobase in ATP and ADP
seems to provide additional interactions which promote their
binding to the vesicular receptors. This effect was however negligible
for AMP, since it showed the same degree of intensity changes as
observed for monophosphate. To check specificity of the current
vesicular receptors to phosphate anions, various mono- and divalent
anions such as Fꢀ, Brꢀ, Iꢀ, NO3‑, CH3COOꢀ, ClO4ꢀ, CO32ꢀ, and
SO42ꢀ were added to aqueous TbL+ vesicles (Figure S14, Support-
ing Information). As a result, significant changes were not observed
in the emission intensity of TbL+ vesicles upon addition of these
anions. It supports that the luminescence of TbL+ vesicles is
selectively amplified for high energy phosphate anions linked in
series by phosphoanhydride bonds.
Table 1. Luminescence Lifetime τ and Rate Constants k
Determined for [TbL]Cl in H2O and D2O, in the Presence or
Absence of ATPa
samples
τ (ms)
k (msꢀ1
)
q
[TbL]Cl(H2O)
1.16
1.72
2.53
3.54
0.86
0.58
0.40
0.28
1.11
[TbL]Cl(D2O)
[TbL]Cl+ATP(H2O)
0.26
[TbL]Cl+ATP(D2O)
a q values were calculated according to the eq 1.
lifetime in H2O (τ = 1.16 ms) is considerably shorter than that
observed in D2O (τ = 1.72 ms), which supports the quenching
mechanism described above. The q value was determined as 1.1
(Table 1), indicating that almost one water molecule is coordi-
nated to the TbL+ complex in water. In contrast, luminescence
decay measurements for [TbL]Cl in the presence of ATP gave
longer luminescence lifetimes of 2.53 ms in H2O and 3.54 ms in
D2O, which are consistent with the observed increase in lumi-
nescence intensity. A q value of 0.26 was obtained in the presence
of ATP (Table 1), and it is obvious that the number of
coordinating water molecules is reduced as a consequence of
the replacement by triphosphate groups (Figure 1).
In conclusion, we have developed a novel amphiphilic recep-
tor TbL+ complex which spontaneously self-assembles in water
and form stable vesicles. The aqueous vesicles of TbL+ com-
plexes show luminescence in water as a consequence of energy
transfer from the coordinating bis(pyridine) units to Tb3+ ion.
Upon addition of varied nucleotides, sigmoidal increase in
luminescence intensity was observed for ATP, followed by
ADP whereas almost no enhancement was observed for AMP.
The enhanced luminescence intensity is ascribed to the displace-
ment of coordinating water molecules by the phosphate groups,
as supported by the luminescence lifetime measurements. Im-
portantly, such sigmoidal increase of luminescence intensity has
not been observed for the structurally relevant water-soluble
lanthanide complexes. A water-soluble Eu3+ complex with bis-
(bipyridylcarboxylate) arms showed luminescence of Eu3+ ions
which was quenched upon coordination with ATP.14 The two-
dimensional self-assembly of receptor complexes play essential
roles in the emergence of positive cooperative binding for
nucleotides carrying high energy phosphate anions linked by
phosphoanhydride bonds. The binding of TbL+ complexes to
each phosphate unit linked by phosphoanhydride bonds occur
cooperatively, probably by adaptively changing their molecular
orientation in the bilayer. They demonstrate synergistic interac-
tions between highly organized receptors (TbL+complex) at the
membrane surface. Self-assembly of receptor molecules to form
nanointerfaces thus open a new strategy to the recognition,
conversion and amplification of molecular information. Together
with adaptive self-assemblies formed in water from nucleotides and
lanthanide ions,5 we envisage these coordination nanointerfaces a
wide range of applications including sensing and diagnostics.
As described previously, the luminescence of TbL+ complex is
quenched to a certain level by the energy transfer from excited
state of Tb3+ to the higher OꢀH vibration overtones of
coordinating water. In this case, the luminescence lifetime of
TbL+ complex will be increased by replacing coordinating H2O
molecules with D2O. The number of water molecules coordinat-
ing to Tb3+ ion in the first coordination sphere, is represented by
a q value, which can be estimated by using the eq 1.13
’ EXPERIMENTAL SECTION
q ¼ 5:0 ꢁ ðKH O ꢀ KD O ꢀ 0:06Þ
ð1Þ
2
2
Materials. 6-Chloromethylpyridine-2-carboxylic acid ethyl ester
was synthesized according to the reported method.15 All the other
chemicals were purchased and used as received. All solvents used are of
analytical grade and used after purification by standard literature
methods. Water was purified with Direct-Q system (Millipore Co.).
Here, KH O and KD O represent the rate constants of lumines-
2
2
cence decay, which are measured in H2O and D2O, respectively
(the decay curves are shown in Figure S15 in Supporting
Information). In the case of TbL+ complex, its luminescence
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Characterization. Luminescence spectra were measured by using a
PerkinElmer LS 55 fluorescence spectrometer. Luminescence decay
curves were obtained by a streak camera (Hamamatsu, C4334) with a
Nd:YAG laser (λ = 266 nm, LOTIS TII, LS-2132) as an excitation
source. UVꢀvis absorption spectra were conducted on a JASCO V-550
spectrometer; DLS and Zeta potential measurements were conducted
on the Malvern Zeta sizer Nano-ZS. ESI-MS spectrum was obtained for
aqueous mixtures of [TbL]Cl and ATP by using a JMS-T100CS MS.
Pure water was used as solvent to avoid any disturbance of HEPES salt to
the ESI-MS measurement. In addition, 40% of THF was added to the
mixture to reduce high surface energy of water. SEM and TEM were
performed by Hitachi S-5000 (acceleration voltage, 10 KV) and JEOL
JEM-2010 (acceleration voltage, 120 KV) respectively. For SEM and
TEM measurement, the sample solution was drop coated on carbon-
meshed copper grid. After 1 min, the excess solution was removed by
using a filtration paper and the resultant grid was dried in vacuum. In
TEM measurement, the grid was poststained with uranyl acetate. For
SEM measurement, the sample was coated with Pt on HITACHI E-1030
ion sputter. In cryo-transmission electron microscopy (cryo-TEM) obser-
vation, the sample aqueous solution was drop cast onto a holey carbon TEM
grid, held with a pair of forceps. Excess liquid was blotted a way by using a
filter paper and the samples were plunged into cryo preparation chamber
containing liquid nitrogen. The vitrified samples were transferred to the
JEOL JEM-2010. The sample was observed at the acceleration voltage of
120 kv under working temperature below ꢀ170 ꢀC.
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’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis of ligand, synthesis
b
of [TbL]Cl, luminescence of TbL+ complex and its composition,
determination of CAC for [TbL]Cl, Cryo-TEM image of
[TbL]Cl, the profile of zeta potential evolution, ESI-MS spec-
trum of [TbL]Cl/ATP, nucleotides binding studies, TEM and
DLS measurement for ATP/TbL+ vesicle, and luminescence
lifetime measurement. This material is available free of charge via
the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
n-kimi@mail.cstm.kyushu-u.ac.jp
(14) Mameri, S.; Charbonniꢂere, L. J.; Ziessel, R. F. Inorg. Chem.
2004, 43, 1819–1821.
’ ACKNOWLEDGMENT
(15) (a) Fife, T. H.; Przystas, T. J. J. Am. Chem. Soc. 1982,
104, 2251–2257. (b) Fornasier, R.; Milani, D.; Scrimin, P.; Tonellato,
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We thank Prof. Chihaya Adachi and Dr. Kenichi Goushi of
Center for Future Chemistry, Kyushu University for the lumi-
nescence lifetime measurement. We also thank Prof. Kenji
Kaneko, Department of Materials Science and Engineering,
Kyushu University for his assistance in cryo-TEM measurement,
Prof. Yoshio Hisaeda and Dr. Toru Okawara, Department of
Chemistry and Biochemistry, Kyushu University for the ESI-MS
measurement. This research work was supported by the Japan
Society for the Promotion of Science (JSPS) and JST CREST. L.
J. acknowledges the support from the Natural Science Founda-
tion of China (no.20902055).
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