Specific chiral sensing of amino acids using induced circularly polarized luminescence of bis(diimine)dicarboxylic acid europium(III) complexes

Article abstract of DOI:10.1021/ic500196m

The circularly polarized luminescence (CPL) from [Eu(pda)₂] ^- (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) and [Eu(bda) ₂]^- (bda = 2,2-bipyridine-6,6-dicarboxylic acid) in aqueous solutions containing various amino acids was investigated. The europium(III) complexes exhibited bright-red luminescence assignable to the f-f transition of the Eu^III ion when irradiated with UV light. Although the luminescence was not circularly polarized in the solid state or in aqueous solutions, in accordance with the achiral crystal structure, the complexes exhibited detectable induced CPL (iCPL) in aqueous solutions containing chiral amino acids. In the presence of l-pyrrolidonecarboxylic acid, both [Eu(pda) ₂]^- and [Eu(bda)₂]^- showed similar iCPL intensity (g_lum ～ 0.03 for the ⁵D₀ → ⁷F₁ transition at 1 mol·dm^-3 of the amino acid). On the other hand, in the presence of l-histidine or l-arginine, [Eu(pda)₂]^- exhibited intense CPL (g _lum ～ 0.08 for the ⁵D₀ → ⁷F₁ transition at 0.10 mol·dm^-3 of the amino acid), whereas quite weak CPL was observed for [Eu(bda)₂] ^- under the same conditions (g_lum < 0.01). On the basis of analysis of the iCPL intensities in the presence of 12 amino acids, [Eu(pda)₂]^- was found to be a good chiral CPL probe with high sensitivity (about 10^-2 mol·dm^-3) and high selectivity for l-histidine at pH 3 and for l-arginine at pH 7. The mechanism of iCPL was evaluated by analysis of the fine structures in the luminescence spectra and the amino acid concentration dependence of g_lum. For the [Eu(pda)₂]^--histidine/arginine systems, the europium(III) complexes possess coordination structures similar to that in the crystal with slight distortion to form a chiral structure due to specific interaction with two zwitterionic amino acids. This mechanism was in stark contrast to that of the europium(III) complex-pyrrolidonecarboxylic acid system in which one amino acid coordinates to the Eu^III ion to yield an achiral coordination structure.

Full text of DOI:10.1021/ic500196m

Article
pubs.acs.org/IC
Specific Chiral Sensing of Amino Acids Using Induced Circularly
Polarized Luminescence of Bis(diimine)dicarboxylic Acid
Europium(III) Complexes
Kazuhiro Okutani, Koichi Nozaki, and Munetaka Iwamura*
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
S
* Supporting Information
ABSTRACT: The circularly polarized luminescence (CPL) from
[Eu(pda)2]− (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) and
[Eu(bda)2]− (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid) in aqueous
solutions containing various amino acids was investigated. The
europium(III) complexes exhibited bright-red luminescence assign-
able to the f−f transition of the EuIII ion when irradiated with UV
light. Although the luminescence was not circularly polarized in the
solid state or in aqueous solutions, in accordance with the achiral
crystal structure, the complexes exhibited detectable induced CPL
(iCPL) in aqueous solutions containing chiral amino acids. In the
presence of L-pyrrolidonecarboxylic acid, both [Eu(pda)2]− and
5
[Eu(bda)2]− showed similar iCPL intensity (glum ∼ 0.03 for the D0
7
→ F1 transition at 1 mol·dm−3 of the amino acid). On the other
hand, in the presence of L-histidine or L-arginine, [Eu(pda)2]− exhibited intense CPL (glum ∼ 0.08 for the 5D0 → 7F1 transition at
0.10 mol·dm−3 of the amino acid), whereas quite weak CPL was observed for [Eu(bda)2]− under the same conditions (glum
<
0.01). On the basis of analysis of the iCPL intensities in the presence of 12 amino acids, [Eu(pda)2]− was found to be a good
chiral CPL probe with high sensitivity (about 10−2 mol·dm−3) and high selectivity for L-histidine at pH 3 and for L-arginine at pH
7. The mechanism of iCPL was evaluated by analysis of the fine structures in the luminescence spectra and the amino acid
concentration dependence of glum. For the [Eu(pda)2]−−histidine/arginine systems, the europium(III) complexes possess
coordination structures similar to that in the crystal with slight distortion to form a chiral structure due to specific interaction
with two zwitterionic amino acids. This mechanism was in stark contrast to that of the europium(III) complex−
pyrrolidonecarboxylic acid system in which one amino acid coordinates to the EuIII ion to yield an achiral coordination structure.
applications because of their excellent luminescent features
such as long lifetimes even in aqueous media, high yields, and
characteristic sharp spectra.2,5,18,21−25 Thus, rare-earth com-
plexes are potentially excellent luminescent probes for novel
microscopic chiral-sensing spectroscopy.22 It is thus required to
find luminescent rare-earth complexes that exhibit strong
induced CPL upon interaction with chiral species (induced
CPL, iCPL).
INTRODUCTION
■
The detection and recognition of molecular chirality is relevant
to a wide range of scientific fields including chemistry, biology,
and medical science. The development of methods for the
detection of chiral species is one of the central issues in these
fields. In particular, in vivo imaging of chiral species in living
cells may provide a means for an in-depth understanding of
biochemical systems and the development of relevant nano-
technologies.1−9
Although both circular dichroism (CD) and circularly
polarized luminescence (CPL) spectroscopy may be used to
detect chiral species, CPL is superior as a highly spatially
resolved spectroscopic technique relative to CD9−11 because
the use of luminescence probes facilitates the application of
commonly utilized microscopic techniques.12−15
Rare-earth complexes are advantageous as luminescence
probe molecules for such chiral sensing because their f−f
transitions have an optical anisotropy factor (g value) that is
potentially much higher than those of organic com-
pounds.14,16−21 Furthermore, complexes of rare-earth ions
(such as Eu3+ and Tb3+) are widely used as luminescent probe
reagents or emitting sensors in medical and biochemical
Certain racemic rare-earth complexes such as [Tb(dpa)3]3−
(dpa = dipicolinic acid) are known to exhibit detectable iCPL
in solutions containing chiral species.11,14,19,26−31 iCPL
originates from the Pfeiffer effect,31,32 i.e., perturbation of the
racemic equilibrium between Δ- and Λ-[Tb(dpa)3]3− by
interaction with the chiral molecules. Thus, the structure of
[Tb(dpa)3]3− itself is not changed by the chiral agents, as
confirmed by the fact that the spectral profile of the total
luminescence (TL) was independent of the presence of the
chiral agent. Recently, we reported detectable iCPL from an
achiral emissive europium(III) complex, [Eu(bda)2]− (bda =
Received: January 25, 2014
Published: May 12, 2014
© 2014 American Chemical Society
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Materials. 2,2′-Bipyridine-6,6′-dicarboxylic acid (bda) was ob-
tained by oxidation of 6,6′-bi-2-picoline by CrO3 in the manner
described previously.33,39 1,10-Phenanthroline-2,9-dicarboxylic acid
(pda) was synthesized according to the literature method, except for
adjustment of the reflux temperature (100 °C) and the concentration
of nitric acid (63%).40 The purity of pda was checked using UV-
absorption spectroscopy, NMR, and elemental analysis. Found: C,
57.5; H, 3.61; N, 9.70. Calcd as C14H6N2O4·1.5H2O: C, 57.3; H, 3.09;
N, 9.55. Aqueous solutions of [EuL2]− (L = pda, bda) were prepared
by mixing stoichiometric amounts of aqueous solutions of EuCl3 and
L. EuCl3 was purchased from Sigma-Aldrich Co. Single crystals of
(C6H16N)[Eu(pda)2] were obtained by recrystallization from a
mixture of an aqueous solution of EuCl3 and a methanolic solution
of pda containing a small amount of triethylamine.
2,2′-bipyridine-6,6′-dicarboxylic acid; Figure 1), in an aqueous
solution of chiral pyrrolidonecarboxylic acid (PCA), although
Figure 1. Ligands of [EuL2]−.
Reagent-grade arginine, histidine, ornithine, alanine, isoleucine,
threonine, phenylalanine, lysine, glutamine, leucine, and PCA were
purchased from Wako Pure Chemical Industries, Ltd., and proline was
from Tokyo Chemical Industry Co. These materials were used without
further purification. The pH of the sample solutions was adjusted using
NaOH or HCl solutions using a MW101-type smart pH meter
(Milwaukee Japan Co.).
no CPL was observed in solutions without chiral amino acids.33
On the basis of the fact that the changes in the TL spectra were
accompanied by iCPL under conditions where no ligand
substitutions of [Eu(bda)2]− were recognized, it was concluded
that CPL was induced by structural distortion of [Eu(bda)2]−
by interaction with the amino acid. This was caused by
distortion of the structure from achiral to chiral in the
molecular level, which is in contrast to the case of the Pfeiffer
effect. This demonstrates that [Eu(bda)2]− may be advanta-
geous as a chiral-sensing probe for a luminescence microscopy
detection system compared with the racemic iCPL probe. One
advantage is that this system can recognize chirality even at the
single molecule level, which has not been accomplished by the
racemic probes. As far as we know, this is the first example of
the iCPL probe using structural changes from achiral to chiral
by interactions with chiral agents. Another advantage is that
further enhancement of the sensitivity or selectivity as a CPL
probe using relevant lanthanide complexes should be promising
because of the availability of various bis(diimine)dicarboxylic
acid derivatives.34−36
RESULTS AND DISCUSSION
■
1. Absorption and Emission Spectra. Both [Eu(pda)2]−
and [Eu(bda)2]− showed intense absorption bands in the
region of 300 nm, assignable to an intraligand π−π* transition
(Figure 2). Upon irradiation at the wavelength of the π−π*
For development of the chiral-sensing method using achiral
CPL probes, it is important to know what kind of chiral agents
are detectable and, furthermore, to find out what kind of
chemical modifications of the ligand of the probe molecules are
effective for improving the sensitivity and/or selectivity of the
chiral agents. In this paper, iCPL of [Eu(pda)2]− (pda = 1,10-
phenanthroline-2,9-dicarboxylic acid) and [Eu(bda)2]− is
examined in aqueous solutions containing various chiral
amino acids. Recently, the EuIIIpda complex has been employed
as an emissive pH sensor, and europium(III) complexes with
pda derivatives have been examined as DNA binding
markers.35,37 [Eu(pda)2]− exhibits bright-red f−f emission and
its structure has plane symmetry, i.e., an achiral structure, in the
crystal as in the case of [Eu(bda)2]−.37 Although [Eu(bda)2]−
showed relatively weak iCPL for most of the amino acids, it is
demonstrated herein that [Eu(pda)2]− exhibits easily detectable
iCPL signals only for arginine and histidine, and therefore the
complex can be used as a selective and sensitive molecular
probe for these chiral amino acids. The mechanism of iCPL in
the europium(III) complexes was examined based on analysis
of the fine structures in the luminescence spectra and the pH
and concentration dependence of the iCPL intensity.
Figure 2. Absorption (blue) and emission spectra (red) of aqueous
solutions of (upper) [Eu(pda)2]− and (lower) [Eu(bda)2]− (λex = 310
nm, pH ∼8, and [Eu] = 1.0 × 10−4 mol·dm−3 for emission spectra).
transition, both complexes exhibited bright emission assignable
5
7
to the D0 → Fj (j = 0−6) transitions of Eu3+ in the visible
region, indicating that the ligands work as photosensitizers
because of facile energy transfer to the metal center.
Figure 3 shows the emission spectra of the aqueous
[Eu(pda)2]− solution and crystalline (C6H16N)[Eu(pda)2].
The europium(III) complexes are known to have achiral D2d
structures in the crystal.41−43 A single peak was observed at
5
7
17220 cm−1 for the D0 → F0 transition of the crystalline
sample, which indicates that there is a single Eu3+ coordination
site for the complex in the crystal. According to the assignment
of Harbuzaru et al., this emission originates from [Eu(pda)2]−
with the structure being very close to D2d.37 On the other hand,
more than three peaks (17260, 17230, and 17215 cm−1) were
EXPERIMENTAL SECTION
■
The TL and CPL spectra were recorded using a laboratory-made CPL
measurement system reported previously.33
Absorption and Emission Spectra. Emission spectra were
recorded using a laboratory-made spectroscopic system reported
previously.38 Absorption spectra were recorded in a 1 cm quartz cell
on a Shimadzu MPS-2000.
5
7
observed in the D0 → F0 emission spectrum of the solution,
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Figure 3. Emission spectra of [Eu(pda)2]− in an aqueous solution (upper, [Eu] = 1.0 × 10−4 mol·dm−3, pH ∼8) and of a (C6H16N)[Eu(pda)2]
crystal (lower) (λex = 310 nm).
as shown in the upper spectrum in Figure 3. Furthermore, the
fine structures of the emission bands of other transitions such
as the 5D0 → 7F2 band were different from those of the crystal.
Such spectral differences between crystal and solution samples
have also been reported for [Eu(bda)2]−.43 The ligand
concentration dependence of the emission spectra from Eu3+
shows that the spectral shapes are independent of the
concentration of pda when the concentration is higher than
twice that of [Eu3+] [see the Supporting Information (SI),
Figure S1], indicating that the dominant emitting species in the
[Eu]:[pda] = 1:2 solution is [Eu(pda)2]−. However, the
5
7
appearance of multiple peaks in the D0 → F0 transition in
the aqueous solutions indicates the presence of several
europium(III) species having structures different from that in
the crystal. Given that the emission of the aqueous solutions at
ambient temperature decayed single-exponentially (τ = 0.80 ms
for [Eu(bda)2]− and 0.79 ms for [Eu(pda)2]−), these species
seem to be in rapid equilibrium, at least on the time scale of the
lifetime of the excited states (∼1 ms). Prospective species
Figure 4. TL and CPL spectra of [Eu(pda)2]− in aqueous solution
containing arginine (λex = 310 nm, [Eu] = 1.0 × 10−4 mol·dm−3, [Arg]
= 0.10 mol·dm−3, and pH ∼6).
5
7
7
16900 cm−1 (5D0 → F1) and −0.005 at 16300 cm−1 (5D0 →
giving rise to additional D0 → F0 peaks in the aqueous
7F2) for the 0.10 mol·dm−3 L-arginine solution. The largest glum
solution would be adducts in which a water molecule interacts
with Eu3+. Horrocks’ analysis [q = 1.11(1/τH O − 1/ τD O
−
5
7
value was observed for the D0 → F1 (ΔJ = 1) transition, in
accordance with the magnetic-dipole selection rule, i.e., allowed
for ΔJ = 0, 1 (except 0 ↔ 0).16 The appearance of CPL
indicates that the coordination structure of the europium(III)
complex is changed to a chiral structure by interaction with the
coexisting arginine. The structural change is also supported by
the fact that the fine structures in the TL spectra are different
from those of the solution without arginine (see also the SI,
Figure S2). It is noteworthy that the overall TL spectra
observed for the solution containing arginine (Figure 4) were
markedly similar to that of the crystal (Figure 3, lower) and,
2
2
0.31), where τH O = 0.79 ms and τD O = 1.5 ms for [Eu(pda)2]−
2
2
and τH O = 0.80 ms and τD O = 1.9 ms for [Eu(bda)2]−],44
2
2
revealing that the number of water molecules coordinated to
Eu3+ (q) was ∼0.4 and suggesting that europium(III)
complexes having one or no coordinated water molecule are
in equilibrium. The three 5D0 → 7F0 peaks mean that there may
be more than two types of coordination of water to Eu3+. The
coordination of OH− to Eu3+ is hardly considerable in neutral
or acidic solutions (see section 4).
2. TL and iCPL Spectra of [Eu(pda)2]− in Aqueous
Solutions Containing Arginine. While no CPL signal was
recorded for [Eu(pda)2]− in aqueous solutions without any
chiral agents, [Eu(pda)2]− in aqueous solutions containing
chiral arginine exhibits detectable CPL (see Figure 4 and the SI,
Figure S2). Figure 4 shows the TL and CPL spectra of
[Eu(pda)2]− in aqueous solutions containing D- and L-arginine
in the region of 14000−17500 cm−1, which involves 5D0 → 7Fj
transitions (j = 0−4) of the Eu3+ ion. The glum value, which is
defined as glum = 2(IL − IR)/(IL + IR), where IL and IR are the
intensities of left and right CPL, respectively, was 0.074 at
5
7
moreover, the single D0 → F0 peak appeared at the same
position (17220 cm−1) as that in the crystal.45 The lifetime of
the f−f emission of [Eu(pda)2]− in the H2O solution
containing arginine (1.33 ms) was the same as that for the
D2O solution, indicating that [Eu(pda)2]− has no coordinated
H2O molecules in the arginine solutions. These findings
indicate that the europium(III) species responsible for iCPL in
the presence of arginine possess coordination structures similar
to that in the crystal, i.e., an eight-coordinate structure, with the
diimine ligands being almost perpendicular to each other. The
structure of the europium(III) species was, however, considered
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to be slightly distorted from the achiral D2d structure because
5
7
the ratio of the integrated emission intensity of the D0 → F1
(magnetic-dipole) transition to that of the 5D0 → 7F2 (electric-
dipole) transition (1:8.3) was greater than that for the crystal
5
7
(1:5.8). Because the D0 → F2 transition is symmetrically
forbidden, the ratio is often used as a measure of distortion in
the coordination structure of the EuIII center.46 Consequently,
whereas [Eu(pda)2]− has a D2d-like structure in the aqueous
solution containing arginine, its structure is slightly distorted to
become chiral because of interaction with arginine molecules,
thus leading to intense CPL. It is noteworthy that the single
5
7
peak of the D0 → F0 transition (Figure 4) indicates that the
emitting europium(III) complex exists as a single species in the
1.0 × 10−2 mol·dm−3 L-arginine solution, indicating that most
of the emitting [Eu(pda)2]− in the solution exists as an
association complex with arginine molecules, which is
consistent with the amino acid concentration dependence of
iCPL described in section 5. Note that the association of
arginine makes the emission brighter because of elongation of
the lifetime (from 0.7 to 1.3 ms) and an increase of the relative
5
7
integrated emission intensity of the D0 → F2 transition (5D0
Figure 5. glum values of iCPL for [Eu(pda)2]− and [Eu(bda)2]− with
various L-amino acids at pH ∼3 (a) and ∼7 (b). [Amino acids] = 0.10
mol·dm−3.
→ F1:5D0 → F2 = 1:6.1 and 1:8.3 without and with arginine,
7
7
respectively), which is suitable for an emitting sensor.
The D2d-like structure of the europium(III) species clearly
rules out the occurrence of ligand substitution of pda (or bda)
by amino acid molecules in the solutions. This was also
confirmed by examination of the dependence of the ligand
concentration on the CPL spectra in the presence of an excess
of the amino acid.33 When pda was absent, no europium(III)
emission was observed for the solution containing an excess
amount of arginine. In the case where mono-pda-coordinated
europium(III) complexes were dominant in the solution, very
weak TL and iCPL were observed, quite different from TL and
iCPL of the bis-coordinated europium(III) complexes. When
the ligand concentration was more than 2 times higher than
that of the EuIII ion, intense emission appeared and both TL
and iCPL were almost independent of the ligand concentration
(see the SI, Figure S3). Consequently, detectable iCPL should
not originate from the arginine-coordinated europium(III)
species but rather the bis-pda-coordinated species with slight
structural distortion because of interaction with the chiral
agents.33 This mechanism is also operative for iCPL of [EuL2]−
in solutions of L-histidine and PCA described in the next
section (see the SI, Figures S4−S7, and ref 33).
For ornithine, alanine, isoleucine, threonine, phenylalanine,
proline, lysine, glutamine, leucine, and PCA, the observed iCPL
intensities were fairly weak under the same conditions (see the
SI, Table S1). Notably, PCA induced detectable CPL with both
[Eu(pda)2]− and [Eu(bda)2]−, e.g., glum ∼ 0.03, at higher
concentrations of [PCA] = 1 mol·dm−3.
Interestingly, [Eu(pda)2]− exhibited well-detectable iCPL
(glum ∼ 0.07) only in response to L-arginine in neutral solutions
and in the presence of L-histidine in acidic solutions, whereas
such highly selective iCPL was not recognized for [Eu(bda)2]−.
Thus, iCPL of [Eu(pda)2]− is ascribed to a specific interaction
that is very sensitive to the chemical properties of the chiral
species, particularly involving the diimine ligands. These
findings may provide insight for the development of chiral-
sensing systems and also facilitate elucidation of the detailed
mechanism of interaction from which iCPL originates.
The TL and iCPL spectra of [Eu(pda)2]− in the presence of
both L-histidine and PCA were similar to that in the presence of
L-arginine (see Figure 4 and the SI, Figures S2, S4, and S6),
indicating that the coordination structure of the europium(III)
complex from which CPL originates is almost independent of
the associating chiral agents. The results are consistent with the
postulate that the chirality around the Eu3+ center is induced by
the change in the coordination structure of [Eu(pda)2]− to a
chiral configuration such as one in which there is a flattening
distortion from D2d to D2.
The addition of alanine, isoleucine, threonine, phenylalanine,
proline, leucine, lysine, ornithine, glutamine, or PCA to the
solution of [Eu(pda)2]− produces a change in TL although no
(or moderately) detectable CPL was recognized in these
systems in the 0.1 mol·dm−3 solutions. These amino acids
undergo some type of interaction with the europium(III)
complexes to produce a certain structural change, although the
structures are not chiral, or the structural change is too subtle to
induce detectable iCPL. The changes of TL for lysine,
ornithine, glutamine, threonine, and proline are very close to
those in histidine and arginine systems (see the SI, Figure S9).
On the other hand, TL changes in alanine, isoleucine, leucine,
Note that the mechanism of iCPL in [Eu(pda)2]− seems to
be different from that of the [Eu(bda)2]−−PCA system
reported in the previous paper.33 In the latter case, it has
been suggested that direct coordination of the COO− group in
PCA to the Eu3+ ion induced CPL, and thus the structure of
[Eu(bda)2]− emitting iCPL is far from D2d-like, which is
supported by the fact that the iCPL and TL spectra of the
[Eu(pda)2]−−arginine system are different from those of
[Eu(bda)2]−−PCA (see the SI, Figure S8).
3. Comparison of the glum Values for Various Amino
Acids. The intensity of iCPL of [Eu(pda)2]− and [Eu(bda)2]−
varied strongly based on the types of amino acids. Figure 5
summarizes the maximum glum values of the 5D0
→
7F1
transition in iCPL of PCA, L-arginine, L-histidine, and L-
glutamine at a concentration of 0.10 mol·dm−3 in neutral and
acidic conditions. Although [Eu(pda)2]− was found to exhibit
iCPL for various amino acids (e.g., L-arginine and L-histidine),
the spectra acquired in the presence of these amino acids were
not significantly different (see the SI, Figures S2, S4, and S6).
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Figure 6. Classification of the amino acids examined. Group a: amino acids yielding iCPL and considerable changes in the TL spectrum. Group b:
those giving no detectable iCPL but some changes in the TL spectrum. Group c: those giving no iCPL and small changes in TL. Although PCA was
categorized in group c, it exhibits detectable iCPL and TL changes at concentrations higher than 0.5 mol·dm−3. The values in parentheses indicate
the pH values of the solutions for which TL and CPL were measured. Abbreviations: Arg, arginine; His, histidine; Lys, lysine; Orn, ornithine; Gln,
glutamine; Thr, threonine; Pro, proline; Ala, alanine; Ile, isoleucine; Leu, leucine; Phe, phenylalanine; PCA, pyrrolidonecarboxylic acid.
5
7
Figure 7. pH dependence of the glum values of [Eu(pda)2]− (a−c) and [Eu(bda)2]− (d−f) for the CPL peak of the D0 → F1 transition for L-
arginine (a and d), L-histidine (b and e), and PCA (c and f). The concentrations of the amino acids were 0.10 mol·dm−3 for arginine and histidine
and 0.50 mol·dm−3 for PCA in [Eu(pda)2]− and 0.70, 0.10, and 1.0 mol·dm−3 for arginine, histidine, and PCA in [Eu(bda)2]−, respectively (λex = 310
nm, λmon = 592 nm, and [Eu] = 1.0 × 10−4 mol·dm−3). Data in part f were obtained in the previous work.33
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a
Scheme 1. Acid−base Equilibria of Arginine (a), Histidine (b), and PCA in Aqueous Solutions (c)
a
pKa values are from the literature.26,47
.
phenylalanine, and PCA systems were smaller than the others,
indicating that these amino acids do not interact so well with
the europium(III) complexes (see the SI, Figure S10).
Figure 6 outlines the classification of all of the amino acids
examined in this study into three groups based on the iCPL and
TL spectra of [Eu(pda)2]− in 0.1 mol·dm−3 solutions of the
amino acids. Group a comprises amino acids yielding detectable
iCPL and considerable changes in the TL spectrum. Group b is
made up of those producing no detectable iCPL but some
changes in the TL spectrum, and group c constitutes those
producing no iCPL and changes in the TL are small.
Interestingly, the amino acids belonging to groups a and b
share structural similarity in that both have amino or hydroxyl
groups in their substituents. On the other hand, in the group c
amino acids, such amino or hydroxyl groups are absent. These
facts clearly demonstrate that iCPL of [Eu(pda)2]− originates
from a structure-specific interaction between [Eu(pda)2]− and
the amino acids. In the [Eu(bda)2]− systems, no detectable (or
a minimal) TL change was recognized for the solutions
containing 0.1 mol·dm−3 amino acids, whereas a large amount
of PCA (∼1 mol·dm−3) induced a detectable change in the TL
spectra.33
Notably, small amounts of precipitates were formed after
preparation of the solutions containing 10−4 mol·dm−3 of
[Eu(pda)2]− and more than ∼10−2 mol·dm−3 of the amino
acids, although no precipitate was observed in the case of
[Eu(bda)2]−. To avoid any artifacts in the CPL measurements
due to the precipitates, all of the solutions were filtered
immediately prior to the measurements. The amount of
precipitate was not very significant; thus, the concentration of
the amino acids in the filtrate was almost unchanged from the
original. Judging from the absorbance around ∼200 nm, about
97% of the amino acids remained in the solution (see the SI,
Figures S11−S13).
both the amino acid and the ligand of the europium(III)
complexes. This also shows that iCPL essentially arises from
specific interactions between the diimine ligands and the chiral
species.
To examine the acid−base equilibria involving [Eu(pda)2]−
in aqueous solutions, the pH dependences of the TL and
absorption spectra of [Eu(pda)2]− were examined (see the SI,
Figures S14 and S15). The absorption spectra of the solution
containing [Eu]:[pda] = 1:2 were almost independent of the
pH in the region of pH 3−11 and considerably changed in the
pH region lower than 3. The former spectra could be ascribed
to pda coordinated to the Eu3+ ion because they were quite
different from that of free pda (see the SI, Figure S15). The pH
dependence of the TL spectra for the solution for [Eu]:[pda] =
1:2 was well consistent with that of the absorption spectra: the
spectrum at pH 1.9 was quite different from those in the region
of pH 3−11. Similarity between the TL spectra at pH lower 3
and those for the [Eu]:[pda] = 1:1 solution indicates that
mono-coordinated species ([Eu(pda)]+) would be formed in
the lower pH region. Consequently, bis-pda-coordinated
−
europium(III) complex [Eu(pda)2] is dominantly formed in
the region of pH 3−11 and no acid−base equilibria affecting
the electronic structure of pda exist in the pH region. Small
5
7
changes in the spectrum for the D0 → F0 transition were
recognized at pH higher than ∼9, whereas almost no change
5
7
was observed in the magnetic-dipole transition D0 → F1 in
the region of pH 3−11 (Figure S14 in the SI). The small
5
7
changes in the D0 → F0 transition are probably ascribed to
acid−base equilibria involving H2O molecules coordinated to
Eu3+, although the molecular structures of the europium(III)
species involved in the equilibria are unclear.
In the solutions of [Eu(pda)2]− containing chiral amino acids
in group a, the pH where drastic glum changes were recognized
were pH 6.2 for histidine and pH 3.8 and 9.8 for arginine. It is
5
7
worth noting that the TL spectra for D0 → F1 were also
changed in the same pH (see the SI, Figures S16 and S17). For
histidine, additional changes in the TL spectrum were observed
at around pH 9.2. In both amino acids, all of the pH values at
which considerable changes were observed in the TL spectra of
4. pH Dependence of the glum Value. Because the
chemical forms of amino acids and [EuL2]− in aqueous solution
are dependent on the pH, the pH dependences of TL and iCPL
were examined in order to elucidate the chiral species involved
in the specific iCPL of [EuL2]−. Parts a, b, d, and e of Figure 7
show the pH dependences of the glum value in the region of pH
2−11 for L-arginine and L-histidine. Variation of the glum value
with the pH for PCA was also examined for comparison
(Figure 7c, f) because PCA was used as a chiral agent for
[Eu(bda)2]− in the previous work.33 As shown in Figure 7, the
optimum pH regions for iCPL of [EuL2]− were dependent on
7
5D0 → F1 are very close to their pKa values: 1.8, 6.1, and 9.2
for histidine and 2.2 and 9.0 for arginine. Because the TL
5
spectra of D0 → 7F1 are unchanged in the region of pH 3−11
in the absence of amino acids, these facts strongly suggest that
the ionic states of histidine or arginine dominate its interaction
with [Eu(pda)2]− and thereby affect the coordinating
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structures. In particular, in the pH region where the large glum
values were observed, the ionic interaction seems to be strong
enough to form a single associated species because the solution
5
7
showed a single peak in the D0 → F0 transition (Figure 4),
and the TL and CPL spectra were not changed (see the SI,
Figures S16 and S17).
In these pH regions, these amino acids are dissolved as
zwitterions possessing one −COO− group and two protonated
amino groups (Scheme 1). In conjunction with the structural
features of the amino acids in group a, this pH dependence
suggests that interaction between [Eu(pda)2]− and the amino
acids plausibly occurs at three different sites: two are on the
asymmetric carbon, i.e., the −COO− group and the −NH3
+
Figure 8. Plots of the glum values at 16900 cm−1 as a function of the
group, whereas the third is another protonated amino group
separated by 6−7 Å from the −NH3+ group on the asymmetric
carbon. The −COO− group is expected to interact with a
positively charged site of the europium(III) complex such as
33
concentration of PCA in aqueous [Eu(bda)2]− (open circle) and
[Eu(pda)2]− (solid triangle) solutions (λex = 310 nm, λmon = 592 nm,
[Eu] = 1.0 × 10−4 mol·dm−3, pH 10 for [Eu(bda)2]−, and pH 3 for
[Eu(pda)2]−).
+
the metal ion. The −NH3 moieties would form hydrogen
bonds with the carboxylate groups of the pda ligands. The
presence of such separated protonated amino groups is
considered as one of the effective factors for highly sensitive
iCPL, given that such an amino group is absent in the amino
acids in group c.
2(IL − IR)
glum
=
=
=
=
IL + IR
2(RL − RR)[A·B]εABkrAB AB
[A·B]εABkrAB AB + [B]εBkrBτB
2(RL − RR)[A·B]
[A·B] + [B]
g1K1[A]0
τ
τ
About the PCA system, we found that iCPL of the
[Eu(bda)2]−−PCA system was observed in the pH region
where the carboxylate exists in the deprotonated form
(−COO−) in the previous paper.33 Thus, it was concluded
that CPL is induced by direct coordination of the amino acid to
Eu3+ via the −COO− moiety. This mechanism, however, is not
applicable for iCPL of [Eu(pda)2]−. For example, for the
[Eu(pda)2]−−PCA system, large glum values were recorded in
the pH region where the carboxylate exists in the protonated
form (−COOH) and, therefore, coordination of −COO− to
the Eu3+ ion should be less important. This is consistent with
the spectral differences in the fine structure of iCPL of
[Eu(pda)2]− relative to [Eu(bda)2]− (see the SI, Figure S8).
Furthermore, the similarity of the CPL spectra of PCA,
arginine, and histidine also indicates that direct coordination of
−COO− to Eu3+ also does not occur in the [Eu(pda)2]−−
arginine, −histidine, and −PCA systems (see the SI, Figures S2,
S4, and S6).
5. Amino Acid Concentration Dependence of the glum
Values. To elucidate the mechanism of iCPL, the amino acid
concentration dependence of the glum value was assessed for
PCA, L-arginine, and L-histidine at the pH where the largest glum
value was observed. It was found that the concentration
dependence of the glum value for the L-arginine− and L-
histidine−[Eu(pda)2]− systems that exhibit highly sensitive
iCPL was markedly different from that of the other systems
such as the [Eu(pda)2]−−PCA system.
K1[A]0 + 1
(2)
where g1 = 2(RL − RR), i.e., the glum values when all of the
europium(III) species are associated with chiral agents (A·B).
RL/R are parameters related to the left and right rotational
strength of A·B (RL + RR = 1), where the value of these
parameters for B is 0.5 because of its achiral structure. εX, kX,
and τX are the molar extinction coefficient, radiative rate
constant, and emission lifetime of species X, respectively.
Equation 2 was obtained based on the following two
assumptions: (1) εX, kX, and τX of the europium(III) complexes
are not affected by association with the chiral agents.33 (2) The
concentrations of the amino acids are unchanged by the
association because their concentrations are higher than those
of the europium(III) complexes. Fitting the data for the glum
values as a function of the concentration of the amino acid gave
the equilibrium constants K1 and g0. The values obtained by the
fitting analysis were not significantly different for the [Eu-
(pda)2]− and [Eu(bda)2]− systems: K1 = 0.81 mol−1·dm3 and g0
= 0.058 for [Eu(pda)2]− and K1 = 0.55 mol−1·dm3 and g0 =
0.077 for [Eu(bda)2]−33 (Table 1).
For [Eu(bda)2]−, the glum value increased with increasing
concentration of PCA (Figure 8).33 Although the pH
dependence of the glum value and the CPL spectra of
[Eu(pda)2]− and [Eu(bda)2]− are different, the glum value for
[Eu(pda)2]− exhibited almost the same concentration depend-
ence of PCA. The plots in Figure 8 were well represented by
the model in which CPL arises from an association complex of
one amino acid molecule (A) and one europium(III) complex
(B), A·B. In this model, the glum values are represented as
follows:33
Table 1. Parameters for Chiral Association Species
chiral
agent
[Eu(bda)2]− PCA
[Eu(pda)2]− PCA
[Eu(bda)2]− L-arginine
[Eu(pda)2]− L-arginine
K1/mol−1
·
K2/mol−2
·
probe
dm3
g1
dm6
g2
a
a
0.55
0.077
0.058
0.053
0.81
0.61
b
b
0
0
4.9 × 104
0.078
a
b
Reference 33. The contribution of A·B-type species was neglected;
the value was roughly estimated because these parameters were hard to
determine from fitting to the current data.
IL/R = RL/R[A·B]εABkrAB AB + 0.5[B]εBkrBτB
τ
(1)
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Figure 9. (a) Variation of the glum values with the L-arginine concentration for [Eu(pda)2]− (red) and for [Eu(bda)2]− (blue). (b) Semilogarithmic
plots of the glum values for the [Eu(pda)2]−−arginine system (red circle). Red and blue indicate the fitting curves using eqs 2 and 4, respectively (λex
= 310 nm, λmon = 592 nm, [Eu] = 1.0 × 10−4 mol·dm−3, and pH ∼7 for [Eu(bda)2]− and [Eu(pda)2]−).
On the other hand, [Eu(pda)2]− and [Eu(bda)2]− showed
considerably distinctive concentration dependence of the glum
value when L-arginine was used as a chiral agent (Figure 9). The
glum value of [Eu(bda)2]− increased linearly to 0.013 as the
arginine concentration increased up to 1.2 mol·dm−3, which
was well reproduced by eq 2 using K1 = 0.61 mol−1·dm3 and g1
= 0.053. For the [Eu(pda)2]−−arginine system, however, the
semilogarithmic plot of glum versus the amino acid concen-
tration gave rise to a sigmoidal curve, as shown in Figure 9b: a
steep increase of the glum value appeared around [Arg] = 10−2
mol·dm−3, and the value was limited at glum = 0.08 when the
concentration was higher than 10−2 mol·dm−3. The sigmoidal
curve was not well reproduced by eq 2 (see the red dashed
curve in Figure 9b, which is the best-fit curve using eq 2). The
Figure 10. Semilogarithmic plots of glum values with variation of the L-
fact that the observed concentration dependence was much
steeper than that predicted from eq 2 strongly suggests that
more than one chiral agent should be involved in the
association complex. Provided that the association species of
the two amino acid molecules and [Eu(pda)2]− (A2·B) exhibit
dominant iCPL, the concentration dependence of the glum value
is given as eq 4 under the same assumptions as those used for
eq 2.
histidine concentration for [Eu(pda)2]−. Red, blue, and black lines
indicate the fitting curves using eqs 2, 4, and 5, respectively. The black
curve was obtained for K1 = 280 mol−1·dm3, K2 = 4.5 × 104 mol−2·
dm6, K3 = 5.7 × 107 mol−3·dm9, g1 = −0.14, g2 = 0.20, and g3 = 0.074
(λex = 310 nm, λmon = 592 nm, [Eu] = 1.0 × 10−4 mol·dm−3, and pH
3).
of the glum value. This clearly indicates that there are more than
two types of chiral species in the [Eu(pda)2]−−histidine
system. Of course, the concentration dependence was not
reproduced by eq 2 (see the red dashed curve in Figure 10).
Furthermore, the plots were not well reproduced with eq 4
(blue dashed curve) even without the assumption of K1 = 0
mol−1·dm3 and g1 = 0. One of the possible suggestions is that
the number of chiral species giving rise to iCPL is larger than
two. The black curve shown in Figure 10 was calculated using
the following equation:
IL/R = 0.5[B]εBkrBτB + R1L/R[A·B]εABkrAB AB
τ
+ R2L/R[A2·B]εA2BkrA2B A2B
τ
(3)
2(IL − IR)
IL + IR
glum
=
=
R
2L/R[A2·B] + R1[A·B]
[A2·B] + [A·B] + [B]
3
2
g3K3[A]0 + g2K2[A]0 + g1K1[A]0
2(IL − IR)
IL + IR
2
g2K2[A]0 + g1K1[A]0
glum
=
=
3
2
=
K3[A]0 + K2[A]0 + K1[A]0 + 1
(5)
2
K2[A]0 + K1[A]0 + 1
(4)
For the [Eu(pda)2]−−arginine system, the fitting analysis
gave values of g2 = 0.078 and K2 = 4.9 × 104 mol−2·dm6. The
contribution of A·B-type species was neglected in this analysis,
i.e., K1 = 0 mol−1·dm3 and g1 = 0, because the iCPL intensity at
lower amino acid concentrations (∼10−4 mol·dm−3) was too
low for determination of the K1 values. The obtained
parameters are listed in Table 1.
The histidine concentration dependence of the glum values
was also evaluated for the [Eu(pda)2]−−histidine system
(Figure 10). This system did not exhibit a monotonic increase
This equation, which is applicable for three associated species,
represents iCPL and looks to reproduce the observed
concentration dependence of glum, as shown in Figure 10.
However, it is very difficult to determine the numerous
parameters from the current data, including the number of
associated molecules. Furthermore, the generation of precip-
itations would disturb such a complicated analysis even if the
uncertainty of the amino acid concentration is quite small.
Thus, detailed structural analysis for this system is less fruitful.
Nevertheless, the result definitively indicates that plural chiral
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Figure 11. Schematic picture of the inducement of the chiral structure of [Eu(pda)2]− by the amino acid. (a) Chiral agents (L-arginine). (b) Emitting
probe in the perpendicular structure of ([Eu(pda)2]−). (c) Proposed model structure of the association complex of arginine and [Eu(pda)2]−. The
blue arrows represent electrostatic attraction.
molecules (plausibly three or four) should be placed in the
neighborhood of the probe molecule that exhibits CPL in this
system.
the cationic moiety (see Figure 11b) is about 6.5 Å in the
crystal structure of the europium(III) complex.37 This distance
is close to those between the cationic moieties (∼6−7 Å) not
only in arginine but also in other amino acids in group a (i.e.,
histidine) and most of group b.
It is worth noting that the glum values were converged to
∼0.08 when the concentrations of the amino acids were larger
than ∼10−2 mol·dm−3. This indicates that there are no
europium(III) complexes that are separated from the chiral
agents in these solutions and suggests that the europium(III)
complex exists as a single species in this concentration region.
Next, we pay attention to the results from evaluation of the
amino acid concentration dependence of glum, which suggest
that more than two chiral molecules are associated in the
complex. To represent this mechanism, we propose that the
symmetry of chiral [Eu(pda)2]− in the association complex
belongs to the D2 point group as a distorted structure from true
D2d. If the chiral structure has D2 symmetry, association of one
amino acid molecule would facilitate association of another
amino acid molecule on the opposite side, as shown in Figure
11c because the structural features of the opposite side of
[Eu(pda)2]− should be the same in D2 symmetry. This is
comparable to an allosteric effect in biological systems; i.e., first,
association of the target amino acid to the probe molecule
causes a change of the structure, which promotes coordination
of a second probe molecule.48 This is consistent with the fact
that the amino acids exhibiting highly sensitive iCPL (i.e.,
amino acids belonging to group a: arginine and histidine)
exhibit concentration dependence as it relates to the allosteric
effect. Note that the D2 structure is consistent with the results
of analysis of the TL spectral shape, which suggests that the
structure of [Eu(pda)2]− is close to D2d (but chiral).
Finally, the results of evaluation of the ligand dependence
(pda and bda) of iCPL are considered. It has been
demonstrated that the “allosteric effect” is very important for
the highly sensitive chiral recognition system, and thus probes
with highly a symmetrical structure, such as those with D2
symmetry, are very important for this type of chiral sensing.
This effect was observed in the [Eu(pda)2]− systems but not in
the [Eu(bda)2]− system, indicating that the highly symmetrical
structure is not maintained in the association complexes of
[Eu(bda)2]−. This is attributed to the freedom of rotational
motion along the pyridine linker. As observed in the crystal
structures of several rare-earth complexes with the bda
5
7
This is consistent with the TL spectrum of the D0 → F0
transition in the solution of 0.10 mol·dm−3 showing a single
peak (Figure 4).
6. Mechanisms for Highly Sensitive iCPL. Here, we
consider the mechanism underlying iCPL based on the present
experimental results. First, we focus on the results of iCPL
generation with the various amino acids and the pH
dependence. The data suggested that the amino acid molecules
that induce intense CPL have two cationic moieties and one
anionic carboxylate group. Naturally, it is expected that the two
cationic species would bind to the carboxylate moiety on the
ligands of the europium(III) complexes and participate in
hydrogen bonding between the oxygen and nitrogen atoms. In
this association complex, the −COO− moiety on the amino
acid undergoes electrostatic interaction with the metal center.
Thus, this moiety would be placed near the metal center in the
association complex. It is expected that a combination of these
electrostatic interactions would fix the structure of the
europium(III) complex in a chiral orientation. Although the
structures of free [Eu(pda)2]− in aqueous solutions are still
unclear, those of [Eu(pda)2]− associated with arginine should
be close to the crystal structure (D2d-like structure), as
confirmed by the similarity between these TL spectra. A
proposed model structure for the association complex of
arginine and [Eu(pda)2]− based on this concept is depicted in
Figure 11c. In this figure, the amino acids bridging the two
ligands via hydrogen bonding induce the structural change from
the true D2d to D2. Such an association is highly likely to occur
given that the distance between the oxygen atoms that bind to
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ligand,42,43 bda can twist by rotation along this axis, whereas
such motion is not allowed in pda.37,41 Due to the freedom,
structural distortion of [Eu(bda)2]− may occur along the twist
in the ligand in the association complex rather than flattening
(D2d to D2) of the complex. This would make the symmetry of
the complex lower than D2 in the association complex. We
propose this as a plausible explanation of why the allosteric
effect is not operative in the [Eu(bda)2]− system and why the
[Eu(bda)2]− system does not show highly sensitive iCPL.
The current model is highly consistent with the results
obtained for the [Eu(pda)2]−−arginine system, where the
arginine concentration dependence of glum was well reproduced
by the two-arginine-molecule association model. In the
[Eu(pda)2]− system with histidine, the mechanism seems to
be more complicated because the complex contains more than
two histidine molecules, as revealed by the concentration
dependence. The fact that the signs of the glum values are
different in the lower and higher amino acid concentration
regions (Figure 10) indicates that there are, in fact, several
types of associations in the [Eu(pda)2]−−histidine system.
However, the identical fine structure of CPL and TL of the
arginine system and the histidine system of [Eu(pda)2]−
suggests that the structure of [Eu(pda)2]− in the iCPL complex
of histidine is not far from the D2 structure. This indicates that
the mechanism for iCPL in the histidine system is very similar
to that of the arginine system shown in Figure 11.
the precipitation, pH dependence of emission spectra of the
[Eu(pda)2]− aqueous solutions, pH dependence of TL and
CPL spectra of the [Eu(pda)2]− aqueous solutions for selected
amino acids. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: miwamura@sci.u-toyama.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Kakenhi (Grant 23750060), a
Grant-in-Aid for Young Scientists (B), a Grant-in-Aid for
Scientific Research (C) (Grant 22550057), and a First Bank of
Toyama Scholarship Foundation Research Scholarship, 2013.
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7. Summary. Herein, the sensitivity of [Eu(pda)2]− as a
chiral probe for arginine and histidine was found to be higher
than that of [Eu(bda)2]−. Our comparative study of iCPL using
various amino acids revealed that iCPL is significantly affected
by the electrostatic interactions between specific positions in
the chiral molecules and in the probe molecules. The proposed
model (depicted in Figure 11) representing the chiral structure
induced by plural amino acid molecules is definitively
consistent with the observations for the [Eu(pda)2]− systems.
The proposed mechanism postulates a path for the develop-
ment of iCPL probe molecules; i.e., bis(diimine)europium(III)
complexes that have rigid ligands with substituents that interact
+
with −COO− and −NH3 groups would be promising for
highly sensitive CPL probe molecules.
In addition to the quite high sensitivity of iCPL of
[Eu(pda)2]− in the concentration region of ∼10−2 mol·dm−3,
[Eu(pda)2]− offers significant advantages over [Eu(bda)2]− for
application to microscopic spectroscopy. This is because the
commercially available UV-type objective lens does not
transmit light of wavelength shorter than ∼330 nm, whereas
[Eu(pda)2]− absorbs at wavelengths longer than 330 nm but
[Eu(bda)2]− does not. Studies of the CPL spectra of crystal
systems and rare-earth complexes of various phen derivatives,
which will provide more detailed mechanisms, are promising.
ASSOCIATED CONTENT
* Supporting Information
■
S
Ligand concentration dependence of TL spectra for [Eu-
(pda)2]− in the 5D0 → 7Fj transition (j = 0, 1, 2), TL and CPL
spectra of [Eu(pda)2]− in the 5D0 → 7F1 transition for selected
amino acids and their ligand concentration dependence, TL
and CPL spectra of [Eu(pda)2]− and [Eu(bda)2]− in the
solutions containing PCA, glum values of [Eu(pda)2]− and
[Eu(bda)2]− in solutions containing various amino acids (0.10
5
mol·dm−3), TL spectra of [Eu(pda)2]− in the D0 → 7F1
transition for selected amino acids, absorption spectra of
amino acids in the [Eu(pda)2]− aqueous solutions, influence of
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