the pyrophosphate may be capable of effective ditopic hydrogen
bonding to the two pyrrole moieties of the sensor. Such ditopic
binding could account for the strong complexation of pyr-
ophosphate. This feature is a subject of continuing studies.
Figure 6 shows schematic representation of the complex
(energy minimized using MM+, HyperChem 5.1).
Fig. 4 The photograph under UV light of the 1.0 µM in CH2Cl2 solutions of
the sensor S1 (1) in the presence of the following anions (100 eq.): fluoride
(2), chloride (3), pyrophosphate (4), and phosphate (5).
Further insight and quantitative evaluation of the sensing
capability of sensors S1–S4 was obtained from UV-vis and
fluorescence quenching experiments. Representative titration
curves are shown in Fig. 5. In general, the addition of fluoride
and pyrophosphate anions to the solution of sensors S1–S4 in all
cases results in the decrease of the absorption intensity in the
400–450 nm region together with the appearance of a strong
band centered at 500–550 nm, which is responsible for the red
color of the solution. The addition of fluoride and pyr-
ophosphate resulted in a significant quenching ( > 95%) of the
fluorescence emission intensity.
Fig. 6 Proposed structure of the pyrophosphate–DPQ complex (A: side
view, B: front view).
In conclusion, we demonstrated that the chromophore
extension in simple fluorescence-based anion sensors such as
DPQ may result in dramatic improvement of sensor perform-
ance as judged by the enhanced fluorescence output and
improved affinity for anionic substrates. The fact that simple
sensors such as DPQ can selectively bind pyrophosphate is
scientifically noteworthy since such sensors may serve as a
departing platform for the development of sensors for other
biologically important anions. Efforts toward the synthesis of
materials capable of anion sensing in the presence of water are
under way.
Financial support from Kraft Foods, the Petroleum Research
Fund (ACS-PRF #38110-G), NSF (NER 0304320) and BGSU
to P.A. and McMaster Fellowship to D.A. is gratefully
acknowledged.
Notes and references
‡ Fluorescent quantum yields (Ff) referenced to quinine sulfate in 0.1 N
H2SO4 (Ff = 0.545)11 as a standard.
§ The anions used were in the form of tetrabutylammonium salts (TBA)
hydrates. The degree of hydration was determined from elemental analyses
of the respective salts.
Fig. 5 Left panel: changes in UV-vis spectra of S1 (10 µM in CH2Cl2) upon
addition of pyrophosphate TBA salt (0–1.0 mM). Right panel: decrease in
fluorescence emission intensity of S1 (2 µM in CH2Cl2) upon the addition
of pyrophosphate (0–86 µM); (lexc = 446 nm) Inset: linearized binding
isotherm used for a binding constant determination.
The emission quenching data was used to calculate the
respective binding constants summarized in Table 1. One can
clearly see that DPQ-sensors with extended chromophores,
namely S1 and to a lesser extent also S2–S4, show substantially
increased affinity (sensitivity) for fluoride and pyrophosphate
compared to the parent DPQ sensor. More importantly, all
sensors S1–S4 show a reversed fluoride/pyrophosphate se-
lectivity (KPP ≈ 2KF) compared to DPQ (KPP 4 KF) (Table
1).
1 L. Stryer in Biochemistry, 4th Ed, W. H. Freeman & Co., New York,
1995.
2 (a) Riegel’s Handbook of Industrial Chemistry, Ninth Edition, ed. J. A.
Kent, Van Nostrad Reinhold-International Thomson Publishing, New
York, 1992; (b) Ullmann’s Encyclopedia of Industrial Chemistry, 6th
Edition, Eds. J. E. Bailey; M. Bohnet; C. J. Brinker; B. Cornils; T.
Evans; H. Greim; L. L. Hegedus; J. Heitbaum; W. Keim; A. Kleemann;
G. Kreysa; J. Löliger; J. L. McGuire; A. Mitsutani; L. Plass; G.
Stephanopoulos; D. Werner; P. Woditsch; N. Yoda; 1999 Electronic
Release, Wiley VCH, New York, 1999.
Table 1 Anion binding constants (Ka)a (M21) obtained from titrations of the
sensors with anions
3 Comprehensive Supramolecular Chemistry, Eds. J. L. Atwood; J. E.
Davies; D. D. MacNicol; F. Vögtle; D. N. Reinhoudt; J.-M. Lehn;
Pergamon-Elsevier Science, Oxford, 1996.
Anionb
4 (a) P. D. Beer and P. A. Gale, Angew. Chem. Int. Ed., 2001, 40, 486; (b)
J. L. Atwood and J. W. Steed, Supramol. Chem. Anions, 1997, 147.
5 (a) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. Huxley,
C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97,
1515; (b) S. C. Burdette, G. K. Walkup, B. Spingler, R. Y. Tsien and S.
J. Lippard, J. Am. Chem. Soc., 2001, 123, 7831.
6 (a) C. B. Black, B. Andrioletti, A. C. Try, C. Ruiperez and J. L. Sessler,
J. Am. Chem. Soc., 1999, 121, 10438; (b) P. Anzenbacher Jr., A. C. Try,
H. Miyaji, K. Jursíková, V. M. Lynch, M. Marquez and J. L. Sessler, J.
Am. Chem. Soc., 2000, 122, 10268; (c) J. L. Sessler, H. Maeda, T.
Mizuno, V. M. Lynch and H. Furuta, Chem. Commun., 2002, 862; (d) J.
L. Sessler, H. Maeda, T. Mizuno, V. M. Lynch and H. Furuta, J. Am.
Chem. Soc., 2002, 124, 13474.
7 Y. Tsubata, T. Suzuki and T. Myashi, J. Org. Chem., 1992, 57, 6749.
8 J. K. Stille, Angew. Chem. Int. Ed. Engl., 1986, 25, 508.
9 All aryl-tributyltins except phenyl were synthesized according to
procedure from: S. S. Zhu and T. M. Swager, J. Am. Chem. Soc., 1997,
119, 12568.
10 K. A. Connors, Binding Constants, John Wiley and Sons, New York,
1987.
11 J. R. Lakowicz, Principles of fluorescence spectroscopy, 2nd ed.,
Plenum, New York, 1999.
32
2
F2
Cl2
HP2O7
H2PO4
Molar Ratio Water-Anion 6:1
2:1
2:1
2:1
DPQ6a
S1c
S2
S3
S4
18200
< 50
< 100
< 100
< 100
< 50
14300
93700
58900
57300
39000
< 100
< 200
< 100
< 100
< 50
51300
24700
25600
27500
a All errors are ± 15%. All binding constants are reported as the average of
3 trials. b Anions were used as TBA salts. c Ka were determined by
fluorescence quenching in CH2Cl2. Fitting was carried out according to
Connors.10 The 1:1 stoichiometry of the complexes was determined from
Job-plots3 carried out at different sensor concentrations.
While the strong affinity for fluoride is not unexpected given
its small size and resulting high surface charge density, the high
binding constants for the pyrophosphate anion are particularly
interesting in light of a potential application in the sensing of
nucleotide di- and triphosphates. Such remarkable selectivity
may be explained by a different binding mode. We propose that
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