Luminescent heptadentate Tb3+ complex with pendant aza-15-crown-5 showing recognition of lactate and salicylate in aqueous solution

Article abstract of DOI:10.1016/j.tetlet.2004.06.063

The coordinatedly unsaturated neutral complex TbL1 that possesses two labile metal-bound water molecules provides linear response to lactate in the range of 0-3.0mM at the simulated extracellular background with the variations of Tb luminescence lifetime

Full text of DOI:10.1016/j.tetlet.2004.06.063

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 6055–6058
3
Luminescent heptadentate Tb complex with pendant
+
aza-15-crown-5 showing recognition of lactate and salicylate
in aqueous solution
q
Cong Li and Wing-Tak Wong*
Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and
Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China
Received 5 April 2004; revised 26 May 2004; accepted 2 June 2004
Abstract—The coordinatedly unsaturated neutral complex TbL1 that possesses two labile metal-bound water molecules provides
linear response to lactate in the range of 0–3.0 mM at the simulated extracellular background with the variations of Tb luminescence
lifetime as output; the maximal amplification of the luminescence intensity of TbL1 reaches a factor of 135 upon titration with
aromatic antenna salicylate in the physiological pH window.
ꢀ
2004 Elsevier Ltd. All rights reserved.
The selective recognition of anions by artificial receptors
1
plays a significant role in supramolecular chemistry.
tively charged complexes in aqueous solution due to the
3a
electrostatic attraction. However, the large osmolality
Despite the importance of bio-active anion recognition
in physiological surroundings, a majority of synthetic
receptors perform their behaviors only in organic sol-
vents due to the large free hydration energies DGhyd of
difference between the cationic complex solution and the
body fluid causes pain and tissue sloughing, which limit
the applications of positively charged lanthanide com-
5
plexes in vivo. Hence, the neutral complex with high
2
these anions. Recently, luminescence sensors based on
binding constants to the specific anion is the best choice.
In this work, we report a neutral complex, TbL1,which
signals two important bio-active anions lactate and
salicylate in aqueous solution.
3
þ
3þ
well-defined heptadentate Eu and Tb macrocyclic
complexes with two metal-bound water molecules have
3
attracted much attention. The stepwise displacement of
water molecules by an anion is signaled by an increase in
the luminescence lifetime and the emission intensity due
to the removing of diffusing OH (or NH) oscillators,
which quench the excited states of Eu and Tb effec-
The synthesis of TbL1 is depicted in Scheme 1. Treat-
ments of the electrophiles N-chloroacetyl-aza-15-crown-
5
1
with 2.5 equiv 1,4,7,10-tetraazacyclododecane
(cyclen) gave mono N-alkylated product 2 with satis-
4
tively. Moreover, additional advantages of such lan-
6
thanide luminescence sensors include in their long-lived
excited states (ꢀms), long emission wavelengths, large
stokes shifts, and line-like emission bands in ambient
conditions. Previous work showed that the positively
charged lanthanide complexes have much higher bind-
ing constants to anions than those of neutral or nega-
factory yield. The amide in 2 was reduced to amine in
succession. The resulting product 3 reacted with excess
tert-butyl bromoacetate to afford the corresponding
alkylated product 4. Through the deprotection step, the
7
ligand L1 was obtained. Treatment of L1 with
8
3 3
Tb (CO ) gave the complex TbL1. Moreover, to
2
clarify the role of pendant crown ether played in the
9
recognition process, TbL2 with pendant ethyl amine
was prepared. All intermediates and complexes were
well characterized using conventional methods. Fur-
thermore, the hydration number q of both TbL1 and
Keywords: Recognition; Lactate; Salicylate; Terbium; Luminescence.
q
Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2004.06.063
Corresponding author. Tel.: +852-2859-2157; fax: +852-2547-2933;
e-mail: wtwong@hkucc.hku.hk
3þ
*
TbL2 was evaluated as two by measuring their Tb
10
luminescence lifetime in H O and D O, respectively.
2
2
0
040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.06.063

6056
C. Li, W.-T. Wong / Tetrahedron Letters 45 (2004) 6055–6058
O
O
O
O
O
O
O
O
O
N
N
tBuOOC
N
O
O
O
O
O
NH
N
NH
N
N
N
N
N
O
c
a
b
d
O
N
Cl
NH HN
O
NH HN
O
tBuOOC
t
O
COO Bu 4
1
2
3
O
O
O
O
O
O
O
Tb
O
O
O
N
N
NH2
H2O
H2O
HOOC
HOOC
O
O
N
N
N
N
N
N
N
N
N
N
N
e
Tb
H2O
H2O
O
O
N
COOH
O
O
O
O
O
L1
TbL1
TbL2
Scheme 1. Reagents and conditions: (a) 2.5 equiv cyclen, K
tert-butyl bromoacetate, Na CO /THF–H O, rt, 4 h, 84%; (d) TFA/CH
2
CO
3
/CH
3
CN, 50–60 ꢁC, 12 h, 86%; (b) BH
Cl , rt, 2 h, 96%; (e) Tb (CO
3
ÆTHF/THF, reflux, 12 h, 81%; (c) 3.3 equiv
, H
O, 80 ꢁC, 10 h, 92%.
2
3
2
2
2
2
3
)
3
2
Monitoring the concentration of lactate in vivo plays an
important role in the observation, diagnosis, and treat-
tivity to these anions. The changing patterns of lumi-
ꢁ
nescence lifetime upon addition of lactate, HCO
ꢁ
3
, and
are quite similar, and 20%, 14% and 11%
11
ment of diseases such as ischemia, stroke, and epilepsy.
To investigate the selective recognition of the Tb
H
2
PO
4
3
þ
enhancements of luminescence lifetime were recorded in
the presence of 14 equiv of anion.
complexes to bio-active anions, a series of titration
experiments were carried out in buffered aqueous solu-
tions (pH ¼ 7.4). As shown in Figure 1, the luminescence
lifetime of TbL1 increased by 48% from 1.45 ms to its
maximum of 2.15 ms upon the addition of 3.0 equiv of
Changes in the Tb luminescence lifetime of TbL1 and
TbL2 were further investigated upon the addition of
lactate in a stimulated extracellular ionic background
ꢁ
ꢁ
lactate. Concomitantly, its q value decreased from 2 to
(0.1 M NaCl, 30 mM HCO
3
,
0.9 mM
H
2
PO
4
,
ꢁ
0
. In the titration with HCO3 , the lifetime of TbL1 only
0.13 mM citrate, pH 7.4, 0.05 M Tris/HCl). As illus-
trated in Figure 2, in comparison to the lifetime of TbL2
increasing by about 9% in the presence of 4 mM lactate,
the lifetime of TbL1 increased linearly by 35% from 1.72
to 2.32 ms as the concentration of lactate increased from
0 to 3 mM. It is noteworthy that the concentration of
lactate in physiological fluids is in the range of 0.35–
increased by about 19% to 1.73 ms even after 14.0 equiv
of anion was added. Similar behaviors were observed
ꢁ
ꢁ
upon the addition of H PO4 and AcO , and the life-
2
time of TbL1 achieved 12% and 6% enhancements,
respectively, in the presence of 14 equiv of anion.
ꢁ
Meanwhile, the addition of Cl did not lead to any
significant changes of luminescence lifetime or the q
value. In the case of TbL2, it showed no obvious selec-
12
2.3 mM, which means TbL1 can be used as a potential
probe for monitoring the concentration of lactate
in vivo. To clarify the selective recognition of TbL1 to
lactate, binding constants K of TbL1 and TbL2 for
a
Figure 1. Plots of the luminescence lifetime of TbL1 (kem ¼ 545 nm,
k
ex ¼ 228 nm) in H
2
O as a function of the ratio [anions]/[TbL1]; black
ꢁ
Figure 2. Plots of the luminescence lifetime of TbL1 (filled circle) and
TbL2 (open circle) as a function of added lactate (1.0 mM complex,
pH 7.4, 0.05 M Tris/HCl, in a simulated extracellular background:
ꢁ
square, lactate; red circles, HCO
ꢁ
3
; green triangles, H PO ; open
circle, AcO ; purple square, Cl (1.0 mM complex, pH 7.4, 0.1 M Tris/
HCl, 295 K).
2
4
ꢁ
ꢁ
ꢁ
100 mM NaCl, 30 mM HCO
3
, 0.9 mM H
2
PO
4
, 0.13 mM citrate).

C. Li, W.-T. Wong / Tetrahedron Letters 45 (2004) 6055–6058
6057
a
Table 1. Binding constants (log K
a
ꢂ 0:2) for 1:1 complex formation
excited at 324 nm. A maximal luminescence enhance-
ment factor of ca. 135 was recorded for the band at
Tb
b
and the hydration numbers, q (±15%) (295 K, 0.05 M Tris/HCl
buffer, pH 7.4)
5
7
5
45 nm as the D fi F transition. Moreover, the
4
5
Tb
ꢁ1
Anion
Log K
a
(q
)
binding constant log K was estimated as 3.9 (±0.2) M .
a
TbL1 emission upon benzoate addition was also mea-
sured. The maximal amplification factor (kem ¼ 545 nm)
of ca. 12 was detected, and the binding constant was
TbL1
TbL2
Lactate
ꢁ
4.1 (0)
3.1 (0.4)
2.6 (1.1)
1.9 (1.3)
3.0 (1.0)
HCO
AcO
PO
3
2.6 (1.0)
2.0 (1.2)
3.3 (0.8)
ꢁ
1
ꢁ
about 2.9 (±0.2) M . During the binding process with
salicylate, the two liable coordination sites of the Tb
3þ
ꢁ
H
2
ꢁ
4
c
c
Cl
Benzoate (1)
Salicylate (2)
n.d. (1.8)
2.9 (0.7)
3.9 (0)
n.d. (1.8)
2.7 (0.7)
2.9 (0.4)
center can be taken up by the two oxygen atoms of the
carboxylate, to form a four-membered ring chelate, or
via one oxygen atom from carboxylate and the other
from phenolic oxygen to form a six-membered ring
chelate (Fig. 3, inset A). However, in the case of ben-
zoate, only a four-membered ring chelate can be
achieved through the binding of two oxygen atoms in
carboxylate with a Tb center. For TbL2, the excited
triplet state can also be sensitized efficiently by benzoate
and salicylate, and the maximal intensity enhancement
factors of 14 and 21 were measured, respectively.
Moreover, the binding constants between TbL2 and
these two aromatic anions were calculated as 2.7 and
a
Values of K
a
were the mean of three determinations in the presence of
3þ
ꢁ3
1
· 10 M Tb complexes.
b
c
Tb
Values of q were measured in the presence of 10 equiv of anion.
Not detected.
anionic species were determined by plotting the emission
values against the concentration of anions, and the re-
sults are summarized in Table 1. The binding constant
of TbL1 for lactate is not only larger than those for
ꢁ
ꢁ
ꢁ
ꢁ
HCO3 , H PO4 , AcO , and Cl , but also larger than
2
ꢁ
1
that of TbL2 for lactate, indicating that the pendant
crown ether promotes the binding specificity to lactate in
aqueous solution.
2.9 M .
The results of the titration experiments with bio-active
anions suggest that TbL1 with pendant aza-15-crown-5
prefers to bind with anion, which can form a larger
membered ring chelate with lanthanide center. For
ꢂ
As the active form of Aspirin , salicylate performs
various pharmacological actions in clinical use, such as
anti-inflammation, anti-thrombopoiesis, and potential
3
þ
5
example, more efficient population of the Tb
D
4
13
anti-tumor activities. The recognition of TbL1 and
TbL2 for the aromatic antenna benzoate and salicylate
was conducted in pH 7.4 Tris/HCl buffer. These two
aromatic anions not only possess triplet states that are
excited state and larger binding constants can be
achieved through the six- or five-membered ring chelates
3þ
between the Tb center and salicylate or lactate. This
specificity determined by binding mode indicates the
existence of sterical interaction between the crown ether
and the specific anion in the axial binding sites of Tb
3þ
5
ꢁ1 14
high enough to sensitize Tb
but they can also form ternary complexes with the
heptadentate Tb ion. As expected, TbL1 was emissive
silence in the absence of the chromophores when excited
at 270–350 nm (Fig. 3). During the titration process,
TbL1 became emissive upon addition of salicylate while
D
4
(E ¼ 20500 cm ),
3
þ
15
center, and further investigations into these features
are currently in progress.
In this work, TbL1 with pendant aza-15-crown-5 shows
recognition of two important bio-active anions: lactate
and salicylate in physiological surroundings with the
variation of luminescence lifetime and intensity,
respectively. Moreover, as far as we know, this is the
first time that the recognition of an anion in aqueous
solution with the variation of long-lived lanthanide
luminescence lifetime has been observed.
Acknowledgements
We thank the Hong Kong Research Grants Council for
support (Grant No. HKU 7119/00P and HKU 7116/
0
29). This work was also supported by the Area of
Excellence Scheme of the University Grants Committee
Hong Kong).
(
Figure 3. Emission trace of TbL1 upon addition of salicylate in water
3þ
(
k
em ¼ 545 nm, kex ¼ 324 nm). Inset A: Tb emission at 545 nm as
functions of [salicylate]/[TbL1] ‘filled circles’ and [benzoate]/[TbL1]
open circles’; the lines present the fit to a 1:1 binding mode (0.1 mM
References and notes
‘
complex, pH 7.4, 0.05 M Tris/HCl). Inset B: plots of the excitation
trace of TbL1 upon the addition of salicylate.
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1
colorless oil (107.7 mg, yield: 92%). H NMR (400 MHz,
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. Marcus, Y. J. Chem. Soc., Faraday Trans. 1991, 87, 2995–
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13
2
3.56–3.40 (2H, s), 3.32–2.70 (18H, m);
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C NMR
3
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2
),
),
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), 51.2 (2 · CH
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), 50.8 (2 · CH ), 50.0 (2 · CH
2
2
48.8 (2 · CH
2
), 39.1 (CH
2
), 37.3 (CH
2
); ESI-MS m=z 390.2
(M+1) ; HRFAB-MS m=z 390.2357 (M+1) [Calcd for
þ
þ
þ
C
16
H
32
N
5
O
TbL2 was obtained as a white solid, and stored in a
6
(M+1) , 390.2353]. Selected data for TbL2:
desiccator under inert gas (65.5 mg, yield: 90%). ESI-MS
þ
þ
m=z 546.1 (M+H) ; HRFAB-MS m=z 546.1389 (M+H)
þ
44, 4869–4872.
[Calcd for C16
H
29
N
5
O
6
Tb (M+H) , 546.1371]; Anal.
4
. (a) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner,
S.; Parker, D.; Royle, L.; de Sousa, A. S.; Williams, J. A.
G.; Woods, M. J. Chem. Soc., Perkin Trans. 2 1999, 493–
Calcd for C16
H
28
N
5
O
12.05. Found: C, 33.31; H, 5.36; N, 12.32.
6
2
TbÆ2H O: C, 33.05; H, 5.55; N,
10. The hydration number q is calculated by using the
Tb
504; (b) Magennis, S. W.; Craig, J.; Gardner, A.; Fucassi,
Horrocks equation:
q
¼ 5ð1=sH2O ꢁ 1=sD2O ꢁ 0:06Þ,
F.; Cragg, P. J.; Robertson, N.; Parsons, S.; Pikramenou,
Z. Polyhedron 2003, 22, 745–754.
where sH2O and sD2O are the luminescence lifetime of
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þ
Tb complex in H
2 2
O and D O, respectively. The uncer-
5
6
7
. Vehmas, T.; Markkola, A. T. Acta Radiol. 1998, 39, 223–228.
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tris(acetic acid)-1,4,7,10-tetraazacyclodode-cane L1: L1
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1
was obtained as a colorless oil (171.5 mg, yield: 96%). H
O, pD ¼ 7.6): d 4.12–3.96 (4H, br),
NMR (400 MHz, D
3.74–3.62 (4H, br), 3.60–3.42 (14H, m), 3.41–3.12 (14H,
2
1
3
2
m), 3.10–2.79 (10H, br). C NMR (100 MHz, D O): 171.7
(
6
5
C), 169.9 (C), 169.7 (C), 69.4 (2 · CH
2
), 68.9 (2 · CH
), 54.8 (3CH
2
2
),
),
12. Demestre, M.; Boutelle, M. G.; Fillenz, M. J. Phys. 1997,
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8.4 (2 · CH
2.9 (CH ), 51.9 (CH
), 46.4 (CH
2
), 63.5 (2 · CH
2
), 54.9 (2 · CH
2
2
2
), 51.3 (CH
); ESI-MS m=z 592.4 (M+H) ;
2
), 50.7 (3 · CH
2
), 48.6
þ
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(
3 · CH
2
2
þ
ꢂ
HRFAB-MS m=z 592.3560 (M+H)
[Calcd for
A. International Aspirin Award, March 2, 2002, Madrid;
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Ed.; CRC, 1994.
þ
C
26
H
50
N
5
O
10 (M+H) , 592.3558].
8
. Selected data for TbL1: TbL1 was obtained as a white
solid that was very hygroscopic and stored in a desiccator
under inert gas (58.5 mg, yield: 92%). ESI-MS m=z 748.3
(
C
14. (a) Handbook of Photochemistry, 2nd ed.; Murov, S. L.,
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þ
þ
M+H) ; HRFAB-MS m=z 748.2545 (M+H) [Calcd for
þ
26
H
47
N
5
O
10Tb (M+H) , 748.2576]; Anal. Calcd for
C N
C, 38.78; H, 6.86; N, 8.61.
26
H
46
5
O
2
10TbÆ3H O: C, 38.96; H, 6.54; N, 8.74. Found:
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