Enantiopure, octadentate ligands as sensitizers for europium and terbium circularly polarized luminescence in aqueous solution

Article abstract of DOI:10.1021/ja076005e

Tb and Eu complexes of enantiopure ligands with a new modular design show strong overall luminescence and CPL activity in aqueous solution. Copyright

Full text of DOI:10.1021/ja076005e

Published on Web 11/22/2007
Enantiopure, Octadentate Ligands as Sensitizers for Europium and Terbium
Circularly Polarized Luminescence in Aqueous Solution
Michael Seitz,† Evan G. Moore,† Andrew J. Ingram,‡ Gilles Muller,‡ and Kenneth N. Raymond*,†
Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460, and Department of Chemistry,
San Jose´ State UniVersity, San Jose´, California 95192-0101
Received August 9, 2007; E-mail: raymond@socrates.berkeley.edu
Circularly polarized luminescence (CPL) is the emission analogue
of circular dichroism (CD). While CD spectroscopy has been widely
used to investigate the configurational as well as conformational
changes in biological systems, CPL also has great, albeit currently
under-developed, potential due to the general sensitivity of lumi-
nescence measurements combined with the high specificity of the
signal for the chiral environment.1 Lanthanide luminescence
(especially Eu(III) and Tb(III)) with its advantageous characteristics
(large Stokes shift, long lifetimes, narrow emission bands) is an
Figure 1. Enantiopure, octadentate ligands.
ideal candidate for the development of chiral CPL probes.2
Scheme 1. Synthesis of Ligands H41 and H42a
We have earlier reported the 2-hydroxyisophthalamide (IAM)
motif as a highly efficient sensitizer for the luminescence of four
different Ln(III) cations (Sm, Eu, Tb, Dy).3 In an extension of this
work, enantiopure versions were recently successfully developed
for use as CPL probes.4 While these species retain the excellent
brightness of their nonchiral analogues, the insolubility in physi-
ologically relevant media remains a limitation for analytical
applications. In order to address this problem, enantiopure, octa-
dentate ligands with decreased hydrophobicity have now been
developed. As an additional feature of this new approach, the
stereogenic centers are introduced in the ligand backbone instead
of incorporating them into the sensitizer units, thus separating the
chiral information from the chromophore and allowing for a much
more generally applicable, modular synthesis of chiral ligands for
CPL applications. 1-Hydroxy-2-pyridinone (1,2-HOPO), which has
recently proven to be a good sensitizer for Eu(III) luminescence,5
and IAM (for Tb(III)) were chosen as model chromophores (Figure
1).
a Reaction conditions: (a) (R)-2-ethyl-N-tosylaziridine,7 benzene, 68%;
(b) HBr (48%)/HOAc, phenol; (c) ion exchange (DOWEX 1 × 8, OH-
form), 46-56% (over two steps); (d) benzyl-protected 1,2-HOPO acid
chloride,11 CH2Cl2, NEt3; 39%; (e) HCl concd/HOAc, 89%; (f) N-
tosylaziridine,10 benzene, 69%; (g) protected IAM activated carboxylic acid
derivative,6,12 CH2Cl2, NEt3, 31%; (h) BBr3, CH2Cl2, 80%.
The chiral information in H41 (the first chiral ligand with the
1,2-HOPO motif) and H42 is easily accessible from either enan-
tiopure amino acids or by resolution of chiral diamines. In the case
of H41, the four stereogenic centers are located in the tetrapodal
arms, whereas H42 displays vicinal stereocenters in the central
portion of the oligoamine backbone. The synthesis of the ligands
is outlined in Scheme 1.6 The hexamine backbone of H41 was
prepared by selective ring opening of enantiopure (R)-2-ethyl-N-
tosylaziridine7 with ethylene diamine, followed by deprotection of
the tosyl groups, and ion exchange chromatography in analogy to
previous reports.8 Similarly, the backbone of H42 was prepared from
enantiopure (R,R)-1,2-diaminocyclohexane9 and N-tosylaziridine.10
Coupling of the two backbones to protected, activated carboxylic
acid derivatives11,12 of the respective chelating moieties, followed
by deprotection, afforded the free ligands H41 and H42 in reasonable
yields.6
The lanthanide complexes [Eu(H1)(H2O)] and [Tb(H2)]13 were
synthesized by standard procedures6 and were isolated in analyti-
cally pure form.14 Upon complexation to the lanthanide, a new
stereogenic element is usually introduced in addition to the fixed
chirality at the asymmetric carbon centers of the ligand backbone.15
This often times occurs in the form of a helical twist of the ligand,
with the potential result of diastereomeric species. In order to assess
the diastereopurity of the complex formation, 1H NMR spectroscopy
was performed on [Eu(H1)(H2O)] in CD3OD (Figure 2).16 The
terbium complex [Tb(H2)] could not be investigated in this way
due to its much stronger paramagnetic nature. This issue will be
addressed in more detail in an upcoming full paper.
1
The optical purity of both hexamines was confirmed by H NMR
spectroscopy after in situ transformation to the corresponding
tetrakis(urea) derivatives with commercially available, enantiopure
(R)-1-phenylethylisocyanate (Sigma-Aldrich, >98% ee). In each
case, only one set of signals was observed with all four arms being
equivalent on the NMR time scale, consistent with complete regio-
and diastereoselectivity of the aziridine ring-opening reaction.
The aromatic region of the spectrum at ambient temperature (293
K, Figure 2, bottom) shows only the expected three signals of a
† University of California, Berkeley.
‡ San Jose´ State University.
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J. AM. CHEM. SOC. 2007, 129, 15468-15470
10.1021/ja076005e CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Photophysical Properties of the Lanthanide Complexes
(ca. 10-5 M in 0.1 M Tris Buffer, pH 7.4)
λ
max, nm
quantum
yield Φa
lifetime
, msb
1
complex
(ꢀ
, M-1 cm-
)
λ
exc, nm
τ
q
[Eu(H1)(H2O)] 341 (19000c)
[Tb(H2)] 339 (28200)
340
340
0.077 0.48 (0.88)
0.57
0.84
2.28 (2.59) -0.04
a Determined relative to quinine sulfate (Φ ) 0.546) in 0.5 M sulfuric
acid as standard. b In H2O (in D2O). c Saturated solution, estimated ꢀ.
Figure 2. Aromatic region of the 1H NMR spectra (500 MHz) of a saturated
solution of [Eu(H1)(H2O)] in CD3OD at 293 K (bottom) and 213 K (top).
Figure 4. Circularly polarized luminescence (upper curves) and total
luminescence (lower curves) spectra of the 5D0
f
7F1 transition of [Eu-
(H1)(H2O)] (left) and 5D4 f 7F5 transition of [Tb(H2)] (right) in saturated
aqueous solutions at pH 7.4 (0.1 M Tris buffer) and 295 K, upon excitation
at 360 and 350 nm, respectively.
species and therefore highly diastereoselective complex formation
in aqueous solution in each case. Determination of the number of
solvent molecules (water, MeOH) present in the inner coordination
sphere of the lanthanide (q) using Parker’s extension18 of the
Horrock’s relationship19 revealed a value of almost 1 (q ) 0.84)
for the Eu(III) complex of H41. The absence of H2O (q ) -0.04)
in the immediate proximity of the Tb(III) center in [Tb(H2)] results
in an increase of the luminescence lifetime (τ ) 2.28 ms in H2O)
relative to the previously reported enantiopure, octadentate IAM
ligand (in MeOH: q ) 1, τ ) 1.27 ms).4
The quantum yield for [Tb(H2)] (Φ ) 0.57) is remarkably high
in aqueous media and similar to the best octadentate IAM ligands
reported so far (Φ ) 0.59).3 For [Eu(H1)(H2O)], the value of 0.077
is within the range of comparable, commercially available Eu(III)
luminescent probes (Φ ) 0.1-0.02) for use in water.20 It is more
than twice as bright as the complex with the achiral, octadentate
analogue of H41 (q ) 1, Φ ) 0.036).17
Figure 3. Normalized steady-state emission spectra (λexc ) 340 nm, ca.
10-5 M in 0.1 M Tris buffer, pH 7.4).
6-substituted 1,2-HOPO derivative. These resonances do not exhibit
a strong paramagnetic shift. At low temperature (213 K, Figure 2,
top), the peaks broaden, presumably due to the increase in viscosity
of CD3OD, and shift very slightly but do not show any additional
peaks, suggesting the absence of a fast equilibrium (at least on
the NMR time scale) of interconverting diastereomeric species at
293 K.
[Eu(H1)(H2O)] and [Tb(H2)] are soluble in aqueous media and
show strong luminescence at physiological pH (Figure 3). The
photophysical properties of the lanthanide complexes show distinct
features (Table 1). The location of the UV absorption maximum
for the n-π* transition in [Eu(H1)(H2O)] is identical to the complex
with the analogous achiral ligand,17 whereas in [Tb(H2)] a slight
blue shift of ca. 11 nm can be seen relative to other octadentate
IAM analogues.3,4 The CD spectra show strong Cotton effects for
the two complexes ([Eu(H1)(H2O)]: -330 nm, +360 nm; [Tb-
(H2)]: -326 nm, +353 nm).6
Finally, the CPL spectra of saturated aqueous solutions at pH
7.4 (0.1 M Tris buffer) of [Eu(H1)(H2O)] and [Tb(H2)] are plotted
5
7
5
7
in Figure 4 in the spectral range of the D0 f F1 and D4 f F5
transitions, which are particularly well-suited for CPL measurements
since they satisfy the magnetic dipole selection rule, ∆J ) 0, (1
(except 0 T 0), respectively. The detection of a CPL signal for
both complexes confirms the presence of stable chiral emitting
Measurements of the luminescence lifetimes gave monoexpo-
nential decays in both cases, like the NMR investigations (vide
supra), also supporting the presence of one dominant emitting
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C O M M U N I C A T I O N S
Table 2. CPL Results for Lanthanide Complexes (Saturated
Aqueous Solutions in 0.1 M Tris Buffer, pH 7.4)
Biomedical Research Support (2 S06 GM008192-24A1) and
Research Corporation Cottrell Science Award (CC6624) for their
financial support.
complex
electronic transition
λ, nm
glum
586.6
594.2
543.6
545.8
551.0
-0.046
-0.12
-0.083
+0.078
-0.051
Supporting Information Available: Experimental procedures for
the synthesis of H41 and [Eu(H1)(H2O)], H42, and [Tb(H2)]. Full
analytical characterization for H41 and [Eu(H1)(H2O)], H42, and [Tb-
(H2)]. Extended spectral characterization for [Eu(H1)(H2O)] and [Tb-
(H2)], as well as experimental details for the spectroscopic measure-
ments. This material is available free of charge via the Internet at http://
pubs.acs.org.
[Eu(H1)(H2O)]
5D0 f 7F1
[Tb(H2)]
5D4 f 7F5
species on the luminescence time scale. We follow the practice of
reporting the degree of CPL in terms of the luminescence
dissymmetry factor, glum(λ), which is defined as follows:
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IL - IR
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and water-soluble Eu(III) and Tb(III) complexes using a modular
ligand design which allows for rapid changes in the chiral
information, located in the hexamine backbone, and sensitizer
properties independently. The solubility in biologically relevant
media, the complete exclusion of water molecules from the inner
coordination sphere in the terbium complex, and the more than
two-fold increase in quantum yield for the europium complex in
comparison to the achiral analogue constitute major advances. The
general luminescence characteristics and the significant CPL activity
in physiologically relevant media make both complexes very
promising candidates for the development of practical CPL probes.
Research in this direction is currently underway.
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Acknowledgment. M.S. thanks the German Research Founda-
tion (DFG) for a research fellowship. The authors thank Michael
D. Pluth for assistance with NMR spectra. This work was supported
in part by NIH Grant R01-HL69832 and by the Director, Office of
Science, Office of Basic Energy Sciences, and the Division of
Chemical Sciences, Geosciences, and Biosciences of the U.S.
Department of Energy at LBNL under Contract No. DE-AC02-
05CH11231. G.M. thanks the National Institutes of Health Minority
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