C O M M U N I C A T I O N S
excimer emission. In our case, the large association constant and
selectivity of PPi can be explained by two cooperative factors. As
explained by Hong et al., the total anionic charge density of the
four O-P oxygen atoms involved in the complexation between ATP
and the two Zn sites is smaller than that of the four O-P oxygen
atoms of PPi.6c,d In addition to the four zinc binding sites for PPi,
the favorable π-π interaction of two flat aromatic center moieties
can induce tight 2+2 type binding between compound 1 and PPi,
which makes this the first example of a PPi fluorescent sensor based
on 2+2 type excimer formation.
In conclusion, we report a highly selective fluorescent chemosen-
sor for PPi that can function in a 100% aqueous solution. This
sensor shows an excimer peak at 490 nm only in the presence of
PPi. Four zinc sites as well as a π-π interaction induced the unique
2+2 type excimer in the presence of PPi. Furthermore, the detection
of PPi is selective over ATP or Pi.
Figure 3. Fluorescent titrations of 1 (6 µM) with PPi at pH 7.4 (0.01 M
HEPES) (excitation at 383 nm).
Acknowledgment. This work was supported by the Korea
Research Foundation Grant (KRF-2006-311-C00091), the SRC
program of the Korea Science and Engineering Foundation (KO-
SEF) (R11-2005-008-02001-0), and BK21. S.K.K. thanks the Seoul
Science Fellowship. K.M.K.S. is thankful to KOFST for a Brain
Pool Fellowship.
Scheme 2. Proposed Binding Mechanism of Chemosensors 1
with PPi
Supporting Information Available: Experimental details of the
synthesis, fluorescence spectra, UV absorption spectra, and NMR
spectra (PDF); X-ray crystallographic data (CIF). This material is
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1
There was a severe broadness in the H NMR spectrom of 1 in
D2O when PPi was added (SI S-Figure 8). These results suggest
the binding of compound 1 with PPi as a 2+2 complex, as explained
in Scheme 2.
From the fluorescent titration experiments (Figure 3), the
association constant was calculated as 4.1 × 105 M-1 (error
<15%).9 The titration experiments were also repeated in the
presence of 100 equiv of Pi or 10 equiv of ATP (SI S-Figures 3
and 4).
There are two previous examples of PPi fluorescent sensors based
on the formation of an excimer. Teramae et al. utilized a
guanidium-pyrene system to detect PPi.5g However, this system
was examined only in methanol, and the selectivity for PPi was
compared only with Pi. On the other hand, Hong et al. reported a
pyrene-Zn2+ complex as a PPi-selective fluorescent chemosensor
based on excimer formation.6d However, in that system, the presence
of a small amount of ATP (0.4 equiv) also induced a considerable
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