Chemodosimetric Hg2-selective signaling by mercuration of dichlorofluorescein derivatives

Article abstract of DOI:10.1021/ol8013446

(Chemical Equation Presented) The chemodosimetric behavior of dichlorofluorescein derivatives toward Hg²⁺ ions was investigated. Simple chemodosimetric systems showed selective and efficient signaling behaviors toward micromolar concentrations

Full text of DOI:10.1021/ol8013446

ORGANIC
LETTERS
Chemodosimetric Hg2+-Selective
Signaling by Mercuration of
Dichlorofluorescein Derivatives
2008
Vol. 10, No. 17
3717-3720
Myung Gil Choi,† De Hun Ryu,† Hye Lim Jeon,† Sunyoung Cha,† Janggeun Cho,†
Hyun Hye Joo,† Kwan Soo Hong,‡ Chulhyun Lee,‡ Sangdoo Ahn,† and
Suk-Kyu Chang*,†
Department of Chemistry, Chung-Ang UniVersity, Seoul 156-756, Korea, and MRI
Team, Korea Basic Science Institute, Ochang, Cheongwon, Chungbuk 363-883, Korea
skchang@cau.ac.kr
Received June 14, 2008
ABSTRACT
The chemodosimetric behavior of dichlorofluorescein derivatives toward Hg2+ ions was investigated. Simple chemodosimetric systems showed
selective and efficient signaling behaviors toward micromolar concentrations of Hg2+ ions over other common interfering metal ions in an
aqueous environment. The signaling mechanism is selective mercuration of the 4′,5′-position of the xanthene moiety, which results in efficient
chromogenic and fluorogenic signaling of Hg2+ ions in aqueous environment.
Recently, a number of ingenious chemosensing systems to
detect important chemical and biological species have been
developed.1 Among many widely used signaling approaches,
chemodosimeters are particularly attractive due to their
advantages of high selectivity and a characteristic accumula-
tive effect for analyte determination. Based on the classic
fluorescent chemodosimeter of rhodamine B hydrazide for
selective Cu2+ signaling,2 many unique signaling systems
to detect Hg2+, Cu2+, and F- ions have been successfully
Fluorescein and its related derivatives have been widely
employed as signaling handles for molecular imaging4 and
chemosensing applications,5 including the detection of metal
ions like K+, Mg2+, Zn2+, Cd2+, Pb2+, and Hg2+.6 Mercurated
fluoresceins have been employed as key intermediates for
the preparation of arsenic derivatives in the site-specific
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Yook, K.-J.; Tae, J. J. Am. Chem. Soc. 2005, 127, 16760. (c) Song, K. C.;
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3109.
3
devised.
† Chung-Ang University.
‡ Korea Basic Science Institute.
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10.1021/ol8013446 CCC: $40.75
Published on Web 07/24/2008
2008 American Chemical Society

labeling of proteins by utilizing affinity for tetracysteine
motifs.7 However, despite the potential of mercurated
fluoresceins for spectroscopy-based chemosensing, chemo-
sensor studies using these compounds have not been reported,
except for assays of sulfur compounds like hydrogen sulfide,8
disulfide, or thiols by fluorescence quenching.9 In this paper,
we report the selective chemodosimetric Hg2+ signaling
behaviors of simple structured fluorescein derivatives. Dichlo-
rofluorescein and its methyl ester derivative (Scheme 1)
and derivative 3 where the hydroxyl substituents on the
xanthene moiety of 1 are functionalized as pivaloyl esters.
Dichlorofluorescein 1 exhibited characteristic absorption
bands at 475 and 505 nm in acetate-buffered aqueous 10%
DMSO solution (pH 5.0) (Figure 1). Upon treatment with
Scheme 1. Structures of Dichlorofluoresceins Investigated
Figure 1. UV-vis spectra of dichlorofluorescein 1 in the presence
of various metal ions in H2O/DMSO (90:10) at pH 5.0 (10 mM
acetate buffer). [1] ) 1.0 × 10-5 M, [Mn+] ) 1.0 × 10-3 M. Inset:
solution color of 1 in the absence and presence of Hg2+ ions.
showed pronounced Hg2+-selective chromogenic and fluo-
rogenic signaling behaviors in an aqueous environment via
selective mercuration of the 4′,5′-position of the xanthene
moiety. The development of selective and sensitive signaling
systems for the determination of Hg2+ ions10 is very
important due to the toxic impact of mercury on the
environment.11
Hg2+ ions, the absorption bands at 475 and 505 nm were
gradually decreased and red-shifted to 483 and 533 nm
(Figure S1, Supporting Information). The color of the
solution changed from yellowish green to orange (inset of
Figure 1). Other metal ions induced some variation in
absorbance without significantly changing the absorption
maximum. Selectivity toward Hg2+ ions was assessed by the
ratiometric analysis of changes in absorption spectra. An
absorbance ratio (A533/A483) of the two characteristic bands
at 533 and 483 nm was used to illustrate the selective
signaling of dichlorofluorescein toward Hg2+ ions. The ratio
of A533/A483 was 1.02 for Hg2+ ions and varied in a limited
range from 0.058 (Ni2+) to 0.085 (Ag+) for the other metal
ions (Figure S2, Supporting Information).
The fluorogenic behavior of 1 was investigated under
the same conditions, and 1 revealed a strong emission band
around 528 nm (Figure 2). The characteristic fluorescence
spectrum of 1 was effectively quenched upon treatment
with Hg2+ ions. The quenching efficiency can be expressed
by the ratio of Io/I at 528 nm (Io and I represent the
fluorescence intensity of 1 in the absence and in the
presence of metal ions, respectively); Io/I was larger than
1900 for Hg2+ ions (Figure S3, Supporting Information).
Other metal ions did not induce noticeable changes in the
fluorescence emission of 1; Io/I ranged from 1.03 for K+
to 1.27 for Ni2+. The color of the solution significantly
changed from bright green to almost colorless when
illuminated with a hand-held UV lamp (inset of Figure
2). Although this type of ON-OFF fluorescence signaling
is not so appropriate for quantifying analytes, the chro-
mogenic responses of this system facilitate the detection
of Hg2+ ions by the naked eye.
The surveyed compounds are dichlorofluorescein 1, methyl
ester derivative 2 (formed from the lactone moiety of 1),
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3718
Org. Lett., Vol. 10, No. 17, 2008

Figure 2. Fluorescence spectra of 1 in the presence of various metal
1
Figure 3. Partial H NMR spectra of 1 only, 1 in the presence of
ions in H2O/DMSO (90:10) at pH 5.0 (10 mM acetate buffer). [1]
) 5.0 × 10-6 M, [Mn+] ) 5.0 × 10-4 M. λex ) 470 nm (to obtain
a full view of the fluorescence spectra of 1). Inset: solution color
of 1 in the absence and presence of Hg2+ ions under illumination
with a UV lamp.
Hg(OAc)2, and 4 in the presence of NaOAc in deuterated acetate-
buffered 1:1 D2O/DMSO-d6.
upfield-shifted singlet at 6.65 ppm, which is ascribable to
the 1′,8′-protons of the xanthene moiety. In contrast, the
resonances of the phenyl ring bearing the carboxylic group
around 7.1-8.1 ppm were not significantly affected. In the
13C NMR spectra, concomitantly, the resonance at 104.1
ppm, ascribable to the carbon atoms of the 4′,5′-position,
was prominently shifted to 120.8 ppm (Figure S6). These
observations demonstrate that mercuration was effected
selectively on the 4′,5′-positions of the xanthene ring to yield
dimercurated dichlorofluorescein 4.
The Hg2+-selective signaling of 1 is due to selective
mercuration of the xanthene moiety of 1 by Hg2+ ions
(Scheme 2).12 In general, the mercuration of fluoresceins is
Scheme 2.
Hg2+-Selective Chemodosimetric Behavior of 1
The selective Hg2+ signaling of 1 was not influenced by
the presence of common coexisting metal ions. For example,
the fluorescence intensity of the 1-Hg2+ system, which was
obtained by the addition of 10 equiv of Hg2+ ions, varied
by less than 2-fold in the presence of 100 equiv of other,
possibly interfering, metal ions (Figure 4 and Figure S7,
known to be limited to the phenolic residues of the xanthene
moiety, in positions ortho to the hydroxyl group,13 and two
atoms of mercury could be substituted for 1. The selective
transformation of 1 to 4 was confirmed by comparing the
NMR and fluorescence spectra of independently prepared
dimercurated dichlorofluorescein 4 (Figures S4 and S5,
1
Supporting Information). The H and 13C NMR spectra of
the 1-Hg2+ system, obtained by treating 1 with 10 equiv of
Hg(OAc)2 under the same conditions, were almost identical
to those of 4.
The signaling process depicted in Scheme 2 could be
1
followed by H NMR measurements in deuterated acetate-
buffered aqueous 50% DMSO-d6 solution (Figure 3, and
Figure S5, Supporting Information). Upon treatment with 2
equiv of Hg(OAc)2, one of the characteristic aromatic signals
corresponding to the xanthene moiety of dichlorofluorescein
1 at 6.37 ppm completely disappeared, leaving a slightly
Figure 4.
Fluorescence spectra of 1-Hg2+ system in the presence
of various metal ions in H2O/DMSO (90:10) at pH 5.0 (buffered
with 10 mM acetate). [1] ) 2.5 × 10-5 M, [Hg2+] ) 2.5 × 10-4
M, [Mn+] ) 2.5 × 10-3 M. λex ) 470 nm.
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M. T.; Fino, J. R. Anal. Biochem. 1986, 154, 476.
Supporting Information). Furthermore, signaling was com-
pleted within 10 min after the sample preparation (Figure
Org. Lett., Vol. 10, No. 17, 2008
3719

S8, Supporting Information). Other metal ions, such as Ag+,
Cd2+, and Pb2+, did not induce any significant responses even
24 h after the sample preparation. From the changes in Hg2+-
dependent fluorescence intensity (Figure 5), the detection
limit was estimated to be 7.5 × 10-6 M.14
(Figures S10-S12, Supporting Information). However,
compound 2 showed somewhat less sensitive Hg2+ signaling
(detection limit ) 1.5 × 10-5 M) than 1. In contrast, the
responses of the pivaloyl derivative 3, which could not be
mercurated on the xanthene moiety, were insignificant
(Figure S13, Supporting Information), highlighting the
importance of the phenolic moieties of the xanthene in the
signaling process. Fluorescein itself exhibited similar chro-
mogenic and fluorogenic behaviors to 1; however, a disad-
vantage of signaling using fluorescein is that it involved the
multimercuration of the xanthene moiety, which would
consume up to four mercury ions under the present experi-
mental conditions.
In summary, we have developed a simple chemodosimetric
signaling system, based on a readily available dichlorofluo-
rescein backbone, for the selective determination of Hg2+
ions in aqueous solutions. In acetate buffered solution,
dichlorofluorescein and its methyl ester derivative exhibited
selective and sensitive Hg2+-induced chromogenic and fluo-
rogenic behaviors. Signaling is due to the selective mercu-
ration on the 4′,5′-positions of the xanthene ring. Simple
fluorescein-based Hg2+ detection systems could be used to
measure the concentration of Hg2+ ions in the micromolar
range in aqueous environment.
Figure 5
.
Fluorescence titration of 1 with Hg2+ ions. [1] ) 2.5 ×
10-5 M, H2O/DMSO (90:10) at pH 5.0 (10 mM acetate buffer).
λex ) 470 nm.
Acknowledgment. This work was supported by a KBSI
grant (T27041) to K.S.H. and by the Bio-MR Research
Program (E27071) from the Korean Ministry of Science and
Technology.
To gain more insight into the general signaling behavior
of the fluorescein moiety, we also investigated other repre-
sentative fluorescein derivatives. Methyl ester 2 exhibited
similar chromogenic and fluorogenic behaviors to 1 (Figure
S9, Supporting Information): the absorption band was red-
shifted (∆λ ) 35 nm), and fluorescence was exclusively and
efficiently quenched by Hg2+ ions (Io/I at 534 nm ) 501)
Supporting Information Available: Experimental details
and characterization for new compounds, NMR spectra,
UV-vis, and fluorescence data are reported. This material
is available free of charge via the Internet at http://pubs.acs.org.
(14) Shortreed, M.; Kopelman, R.; Kuhn, M.; Hoyland, B. Anal. Chem.
1996, 68, 1414.
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