C O M M U N I C A T I O N S
Figure 2. (a) Fluorescence titration of 1a (5.0 µM) with increasing amounts of Gly- (λex ) 377 nm); (inset) dependence of fluorescence intensity with
respect to [Gly-]/[1a]; (b) collective intensity data for R-, â-, γ-amino acid anions (1.0 equiv), AcO- and n-PrNH2 (2.0 equiv)
Acknowledgment. This work was supported by grants from
the CIMS (Grant R11-2000-070-070010), Korea Health Industry
Development Institute (Grant A05-0426-B20616-05N1-00010A),
and Korea Research Foundation Grant (Grant KRF-2005-070-
C00078).
Supporting Information Available: Details for the synthesis of
1a, 1H/19F NMR and ITC titration data, and MO calculation and
experimental results. This material is available free of charge via the
References
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quenching. In contrast, in the case of 1b‚Gly-, HOMO-LUMO
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distributes mostly on the anthracene moiety; hence, the excited
electrons would decay by emitting strong fluorescence. The
calculated data corroborate the experimental results (Figure 3).
Quantum yields measured for a series of mixture between 1a
and glycinate increased as the complex being formed, supporting
that the fluorescence enhancement is due to the adduct formation.
The (1:1)-complex showed a high quantum yield (ΦF ) 0.84),
suggesting a nearly complete restoration of the fluorescence from
the 9,10-substituted anthracene moiety12 upon adduct formation.
Isothermal titration calorimetry (ITC) carried out between 1a
and glycinate in acetonitrile provided thermodynamic parameters
for the binding process. The integration data of the heat evolved
upon titration showed an inflection point near the equivalent point,
again supporting the (1:1)-adduct formation. The “dominant”
binding interaction involved a favorable enthalpy change (∆H° )
-2.0 × 104 cal/mol) accompanied with an unfavorable entropy
change (-T∆S° ) 1.0 × 104 cal/mol, T ) 303 K), from which a
large association constant (Kass ≈ 1.0 × 107 M-1) was obtained
(Supporting Information). The ITC data support the formation of
the cyclic adduct through a cooperative binding by the amine and
carboxylate functions. In the case of â-aminopropionate, a complex
binding equilibrium was suggested, with a smaller association
constant.
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(10) Because of poor solubility of 1b in most organic solvents, we used 1a for
sensing experiments, except for molecular modeling. Synthesis of water-
soluble analogues of 1 and molecular sensing studies are under investiga-
tion.
In summary, we have disclosed a rational approach to fluores-
cence turn-on sensing of amino carboxylates. The approach
primarily relies on the perturbation of the quenching n-π* transition
energy level of the carbonyl ionophore relative to the π-π*
transition energy level of the fluorophore in the sensor, which has
remained as an unexplored approach in the anion sensing so far.
Our anthracene-based bis(trifluoroacetylcarboxanilide) sensor is
structurally simple but selectively senses R-amino acids as their
carboxylate forms over â- and γ-homologues by forming a cyclic
adduct.
(11) The n-π* transition energy level involving the trifluoroacetyl group seems
to intervene in the π-π* transition energy level of the anthracene moiety
(Supporting Information, Figure S7), resulting in the donor-excited photo-
induced electron transfer quenching: Ueno, T.; Urano, Y.; Setsukinai,
K.-i.; Takakusa, H.; Kojima, H.; Kikuchi, K.; Ohkubo, K.; Fukuzumi, S.;
Nagano, T. J. Am. Chem. Soc. 2004, 126, 14079.
(12) The quantum yields of 9,10-dimethylanthracene are 0.63 and 0.98 in
oxygen-containing and oxygen-free ethanol, respectively. See chapter 2
in ref 6c.
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J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008 2395