Chromene lock , thiol key , and mercury(II) ion hand : A single molecular machine recognition system

Article abstract of DOI:10.1021/ol101771j

A regenerative, molecular machine-like ON-OFF-ON chemosensor based on a chromene molecule with the pyran ring OFF-ON-OFF cycle is reported for the first time. It behaves as a molecular lock that requires a thiol key to open the lock and a mercury(II) ion hand that unlatches the key for unsheathing the key to close the lock.

Full text of DOI:10.1021/ol101771j

ORGANIC
LETTERS
2010
Vol. 12, No. 21
4756-4759
Chromene “Lock”, Thiol “Key”, and
Mercury(II) Ion “Hand”: A Single
Molecular Machine Recognition System
Fang-Jun Huo,† Yuan-Qiang Sun,† Jing Su,‡ Yu-Tao Yang,‡ Cai-Xia Yin,*,‡ and
Jian-Bin Chao†
Research Institute of Applied Chemistry (RIAC), and Key Laboratory of Chemical
Biology and Molecular, Engineering of Ministry of Education, Institute of Molecular
Science (IMS), Shanxi UniVersity, Taiyuan, 030006, China
yincx@sxu.edu.cn
Received July 29, 2010
ABSTRACT
A regenerative, molecular machine-like “ON-OFF-ON” chemosensor based on a chromene molecule with the pyran ring “OFF-ON-OFF”
cycle is reported for the first time. It behaves as a molecular lock that requires a thiol “key” to open the lock and a mercury(II) ion “hand”
that unlatches the key for unsheathing the key to close the lock.
There is considerable interest and intense activity in con-
structing molecular-level devices with molecular recognition
functionality and signal transduction ability in chemistry,1
biology,2 and sensors.3 Especially important and significant
in this regard are sensors that detect sulfhydryl-containing
amino acids and peptides, cysteine (Cys), homocysteine
(Hcy), and glutathione (GSH), as they play many crucial roles
in many physiological processes.4 In recent years, great effort
has been put into the development of thiol detection by means
such as high-performance liquid chromatography,5 capillary
electrophoresis,6 Ellman’s reagent,7 cleavage reactions by
thiol,8 cyclization reactions with aldehyde,9 and others.10
One of the most attractive approaches involves the construc-
tion of receptors of a thiol-based reaction through the powerful
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M.; Katayama, K.; Saeki, K.; Itoh, N. Angew. Chem., Int. Ed. 2005, 44,
2922–2925. (c) Ji, S.; Yang, J.; Yang, Q.; Liu, S.; Chen, M.; Zhao, J. J.
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R. M. J. Am. Chem. Soc. 2004, 126, 438–439. (c) Wang, W.; Rusin, O.;
Xu, X.; Kim, K. K.; Escobedo, J. O.; Fakayode, S. O.; Fletcher, K.; Lowry,
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† RIAC.
‡ IMS.
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A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997,
97, 1515–1566. (b) Pu, L. Chem. ReV. 2004, 104, 1687–1716.
(2) (a) Goncalves, M. S. T. Chem. ReV 2009, 109, 190–212. (b) Liu,
J. W.; Cao, Z. H.; Lu, Y. Chem. ReV. 2009, 109, 1948–1998.
(3) Borisov, S. M.; Wolfbeis, O. S. Chem. ReV. 2008, 108, 423–461.
(4) Chen, X. Q.; Zhou, Y.; Peng, X. J.; Yoon, J. Chem. Soc. ReV. 2010,
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10.1021/ol101771j  2010 American Chemical Society
Published on Web 10/04/2010

“click” chemisty.11,12 A few excellent receptors have been
exploited such as OND,12a squaraine,12b,c coumarin dyes,12d,e
maleimide,12f,g propiolate,12h and quinone.12i,j
subsequent intramolecular Michael addition to the pyran ring
with the mercury(II) ion as a “hand” unsheathing the key to
close the lock.
Very recently, we demonstrated a colorimetric probe for
thiol based on the ring-opening mechanism of the chromene
molecule, 7-nitro-2,3-dihydro-1H-cyclopenta[b]chromen-1-
one, in aqueous solution.13 Herein, we report a new,
regenerative, fluorescence “ON-OFF-ON” probe for thiol
and the mercury(II) ion, 7-chloro-2,3-dihydro-1H-cyclopent-
a[b]chromene-1-one(1)(Figure1,preparedbyBaylis-Hillman
Scheme 1
Scheme 1 depicts the reversible character of the thiol-
chromene “click” nucleophilic pyran ring-opening reaction
of 1 and Hg2+-induced cyclization of 2 to chromene as a
sensing mechanism. The nucleophilic addition of mercap-
topropionic acid (MPA) to the probe 1 leads rapidly to the
formation of the ring-opened, low fluorescent product 2 and
demonstrates a fluorescence quenching effect. 1D 1H NMR,
Figure 1. Structure and thermal ellipsoids of probe 1 are drawn at
the 50% probability level.
1
13C NMR, 2D H-1H COSY, and ESI mass spectrometry
confirmed a thiol-quantitative reaction (Figure S2, Supporting
Information). A kinetic study of the response of MPA to
probe 1 under pseudofirst-order conditions (30 µM probe 1
and 300 µM MPA) is shown in Figure S3 (Supporting
Information). The nucleophilic ring-opening reaction was
completed in less than 3 min at room temperature in aqueous
media.
and intramolecular Michael addition reactions (Figure S1,
Supporting Information)).14 We also report a single molecular
lock based on the following phenomenon: a thiol-chromene
“click” nucleophilic pyran ring-opening reaction with a thiol
key to open the lock and Hg2+-promoted desulfurization and
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Figure 2. (Left) Absorption spectral changes of 1 (30 µM) in Tris-
HCl 10 mM containing 0.15% EtOH, pH 8.0 aqueous buffer upon
addition of Cys. Cys was added gradually at [Cys] ) 0-90 µM.
Each spectrum was recorded 3 min after Cys addition. (Right)
Fluorescence spectral changes of 1 (3 µM) upon addition of Cys
(0-9 µM) (λex ) 380 nm, λem ) 550 nm; slit, 10 nm/10 nm) in 10
mM Tris-HCl containing 0.15% EtOH, pH 8.0 aqueous buffer upon
addition of Cys. Each spectrum was recorded 3 min after Cys
addition. Inset: color (left) and visual fluorescence (right) change
photographs of 1 (100 µM) upon addition of Cys in Tris-HCl/
ethanol (200:1, v/v, 10 mM, pH 8.0) buffer solution under
illumination with a UV 365 nm lamp.
(14) See Supporting Information. The probe 1 molecule: 1H NMR (600
MHz, 25°C, DMSO-d6): δ 7.54 (s, 1H), 7.34 (d, 1H), 7.25 (s, 1H), 6.96 (d,
1H), 5.32 (t, 1H), 2.58-2.63 (m, 1H), 2.45-2.49 (m, 1H), 2.36-2.42 (m,
1H), 2.00-2.08 (m, 1H). 13C NMR (150 MHz, CDCl3): δ 27.99, 37.05,
75.91, 117.87, 123.21, 126.30, 127.07, 129.55, 131.91, 132.73, 157.46,
201.08. Elemental analysis (calcd %) for C12H9ClO2: C, 65.32; H, 4.11.
Found: C, 65.64; H, 4.21. ESI-MS m/z 220.9 (30%, [M]+): calculated 220.7
[M]+. Crystal data for 1: C12H9ClO2, FW ) 220.64, crystal size: 0.1 × 0.1
× 0.1 mm, orthorhombic, space group Pbca (No.61), a ) 15.103(3) Å, b
) 6.0078(12) Å, c ) 21.747(4) Å, ꢀ ) 90.000°, V ) 1973.2(7) Å3, Z ) 8,
Figure 2 (left) shows the change in the UV/visible
spectrum when the Cys solution was added to Tris-HCl
buffer (10 mM, pH 8.0) containing probe 1 (30 µM). With
increasing Cys concentration, the probe 1 absorption peaks
at 250, 302, and 377 nm gradually decreased, and a new
peak appeared at 227 nm (blue-shifted 23, 75, and 150 nm,
respectively) with an isosbestic point at 236 nm, indicating
the formation of the product 2. Figure 2 (right) displays the
T ) 183 K, θmax ) 25.35°, 7615 reflections measured, 1766 unique (Rint
)
0.041). Final residual for 137 parameters and 1575 reflections with I > 2σ(I):
R1 ) 0.0442, wR2 ) 0.1037, and GOF ) 1.09.
Org. Lett., Vol. 12, No. 21, 2010
4757

emission peak of 1 at 550 nm (λex )380 nm) that decreased
rapidly upon addition of Cys and eventually stopped with a
saturation point around 3 equiv of Cys. When 3 equiv of
Cys was added to the solution of 1, a more than 10-fold
decrease in fluorescent intensity at 550 nm was observed.
Simultaneously, an elaborate assay was carried out by
fluorescence titration (Figure S4, Supporting Information)
and showed a linear response of probe 1 to increasing
amounts of even low Cys concentration from 3 × 10-7 to
3.9 × 10-6 M. GSH and HCy can also play the same roles.
These indicate that probe 1 can respond to thiol at low
micromolar levels. Other amino acids did not affect the
detection of Cys (Figure S5, Supporting Information).
It is very exciting and noteworthy that probe 1 could be
regenerated only by adding Hg2+ ions to the solution
containing compound 2 (a regenerative process of 1, Figure
S6, Supporting Information). Common metal ions such as
Na+, K+, Mg2+, Ca2+, Ba2+, Cr3+, Mn2+, Fe3+, Co3+, Ni2+,
Cu2+, Zn2+, Cd2+, Al3+, Ga3+, Sn2+, Zr4+, Ce3+, Nd3+, Sm3+,
Eu3+, Er3+, Tb3+, Ho3+, Yb3+, Ag+, and Pb2+ did not generate
the same result, which indicates these ions cannot cause the
regeneration of 1 (Figure 3 (above)). These novel facts show
ions trigger C-S bond cleavage of compound 2, and
subsequent intramolecular nucleophilic attack of the phenolic
oxygen on the cyclopentenone group results in the recovery
of the cyclized probe 1 (Scheme 1). Furthermore, the
regeneration of probe 1 after exposure to thiols and Hg2+
was examined by spectra. The change of spectra is regenera-
tive over several cycles of “ON-OFF-ON” (Figure S7,
Supporting Information). The results strongly proved that
probe 1 can be not only used to detect thiols and Hg2+ but
also readily regenerated.
On the basis of the sensor “ON-OFF-ON” cycle, we
propose a cyclic mechanism (Scheme 2 (above), Figure S7,
Scheme 2
Supporting Information). This reversible system of the thiol-
chromene “click” ring-opening process and Hg2+-promoted
pyran ring cyclization behaves as a molecular lock that can
be unlocked with a thiol key and can be locked with an Hg2+
ion “hand”, unsheathing the key (Scheme 2 (below)): probe
1 and product 2 represent the locked states and unlocked
states, respectively.
Figure 3. (Above) Absorption spectral changes (30 µM) and
fluorescence spectral (3 µM) changes of 2 (λex ) 380 nm, λem
)
550 nm; slit, 10 nm/10 nm) in 10 mM Tris-HCl, 0.5 mM EDTA,
containing 0.15% EtOH, pH 8.0 aqueous buffer upon addition of
10 equiv of various metal ions including: Na+, K+, Mg2+, Ca2+
Ba2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Al3+, Ga2+
,
,
Sn2+, Zr4+, Ce3+, Nd3+, Sm3+, Eu3+, Er3+, Tb3+, Ho3+, Yb3+, Ag+,
Pb2+, and Hg2+. (Below) Hg2+ ion was added gradually with [Hg2+
]
On the basis of structure information for compound 2, we
propose a fluorescence mechanism of 1, quenched upon the
addition of thiols. The probe 1 molecule is planar with a
maximum deviation of 0.4914 (2) Å for only the C3 atom
and has a strong fluorescence emission at 552 nm. When
MPA is added to 1, the rigid plane of the chromene 1 is
destroyed because of the pyran ring opening. To clarify this
change, an energy-minimized structure of 2 was produced,
with a molecular mechanical calculation which suggests that
) 0-90 µM for UV/visible spectra with [2] ) 30 µM and [Hg2+
]
) 0-9 µM for fluorescence spectra with [2] ) 3 µM. Each
spectrum was recorded 10 min after Hg2+ addition.
that compound 2 can act as a fluorogenic chemosensor for
the Hg2+ ions with high selectivity and sensitivity (Figure 3
(below)). The detection limit for Hg2+ in aqueous buffer is
low micromolar level, e.g., 0.15-9.0 µM. The regeneration
of probe 1 should involve a Hg2+-promoted desulfurization
such as the one reported by Ros-Lis:12c,15 intramolecular
Michael addition of 2 to the pyran ring. In other words, Hg2+
(15) (a) Nolan, E. M.; Lippard, S. J. Chem. ReV. 2008, 108, 3443–3480.
(b) Ros-Lis, J. V.; Marcos, M. D.; Mart´ınez-Ma´n˜ez, R.; Rurack, K.; Soto,
J. Angew. Chem., Int. Ed. 2005, 44, 4405–4408.
4758
Org. Lett., Vol. 12, No. 21, 2010

the dihedral angle between the cyclopentene plane and the
benzene ring plane is 66.58° (Figure S8, Supporting Infor-
mation).
unlatch the key to close. Further studies on novel molecular
machine-like, regenerative probes and their applications are
now underway in our laboratory.
In summary, the current study has demonstrated for the
first time a regenerative, single molecular machine-like
“ON-OFF-ON” chemosensor, based on a chromene mol-
ecule with a pyran ring “OFF-ON-OFF” cycle process.
The sensor can function as a new, simple fluorescence-off
probe for thiol with very high sensitivity and selectivity
through the thiol-chromene “click” ring-opening reaction in
aqueous media, and it can act as a novel fluorescence-on
probe of the Hg2+ ion with high selectivity based on a Hg2+-
promoted desulfurization, intramolecular Michael addition
to the pyran ring. Thus, these results are significant and
interesting for a new generation of molecular recognition
systems. The above phenomenon behaves as a molecular
lock, which requires an inserted key to open and a hand to
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (No. 20801032,
21072119), Shanxi Provincial Natural Science Foundation
(No. 2009021006-2), the Shanxi Province Foundation for
Returnee (200815), the Program for the Top Young Aca-
demic Leaders of Higher Learning Institutions of Shanxi
(TYAL), and the Shanxi Province Foundation for Selected
Returnee (No. 2010).
Supporting Information Available: Experimental pro-
1
cedures, spectroscopic data, kinetic study, H NMR, 13C
NMR, ESI-MS data, and crystal data (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
OL101771J
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