L-cysteine recognition triggered by Zn2 + complexation with ligand

Article abstract of DOI:10.1016/j.inoche.2013.08.006

A new type of benzimidazole-based imine-linked fluorescence chemosensor for Zn^{2 +} was synthesized. Coordination of Zn^{2 +} with the receptor led to enhanced fluorescence while other metals quenched the receptor fluorescence. The resulting complex had a supramolecular assembly with a 1:1 host-guest ratio with variable size. The (1.Zn^{2 +})_n assembly showed high selectivity towards the complexation of L-cysteine even in the presence of other potentially competitive biologically important anionic species.

Full text of DOI:10.1016/j.inoche.2013.08.006

Inorganic Chemistry Communications 36 (2013) 96–99
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
L-cysteine recognition triggered by Zn2+ complexation with ligand
a,
Min Joung Kim a, Hemant Sharma b, Narinder Singh b, , Doo Ok Jang
⁎
⁎
a
Department of Chemistry, Yonsei University, Wonju 220-710, Republic of Korea
b
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India
a r t i c l e i n f o
a b s t r a c t
A new type of benzimidazole-based imine-linked fluorescence chemosensor for Zn2+ was synthesized. Coordi-
nation of Zn2+ with the receptor led to enhanced fluorescence while other metals quenched the receptor fluores-
cence. The resulting complex had a supramolecular assembly with a 1:1 host-guest ratio with variable size. The
Article history:
Received 11 May 2013
Accepted 10 August 2013
Available online 17 August 2013
(1.Zn2+
) assembly showed high selectivity towards the complexation of L-cysteine even in the presence of
n
other potentially competitive biologically important anionic species.
Keywords:
© 2013 Elsevier B.V. All rights reserved.
Chemosensor
Molecular recognition
Supramolecular assembly
L-cyteine
Zn(II)
Research on the interactions of biomolecules with metal ions has
grown [1]. Such interactions include the role of metal ions as cofactors
that promote the catalytic activities of enzymes. Inspired by these
types of interactions, supramolecular chemists have been using these
concepts for the construction of various types of supramolecular devices
including chemosensors, molecular logic gates, and others [2,3]. Metal
ion coordination with organic receptors may provide: a) additional
binding sites for the recognition of anions through electrostatic interac-
tions with the metal center allowing recognition of anions in polar sol-
vents and b) better steric orientation for selective binding of anions [4].
In short, metal ion binding with an organic receptor may trigger a pos-
itive cooperative effect for anion recognition [5].
sensitivity, accuracy, and quick analysis time. Although great efforts
have been devoted on fluorescent chemosensors for cysteine [13], the
reports on turn-on fluorescent chemosensors remain rare, which are
beneficial for bioimaging the distribution of cysteine in cellular processes
[14]. Here, we present a new type of benzimidazole-based imine-linked
fluorescent chemosensor for Zn2+, which was further utilized in the
development of a turn-on fluorescent chemosensor for L-cysteine.
Receptor 1 was prepared according to Scheme 1. Compound 2 [15]
was treated with 2-aminobenzimidazole under basic conditions to af-
ford compound 3 that was converted into receptor 1 by reaction with
salicylic aldehyde in the presence of a catalytic amount of Zn(ClO4)2 in
MeOH.
The second most important transition metal in human body is zinc.
Its deficiency and excess may lead to several disorders like apoptosis,
epilepsy, Alzheimer's disease, growth retardation, diarrhea and impo-
tence [6]. Therefore, developing selective and sensitive probes for the
detection of Zn2+ is a matter of interest.
Sulfur-containing amino acids and peptides have drawn a great deal
of attention due to their essential, and significant functions in biological
systems [7]. Cysteine comes under the category of an essential amino
acid, and a deficiency of cysteine may lead to hair depigmentation,
liver damage, skin lesions, and stunted growth [8]. The thiol group of cys-
teine is involved in many biological oxidation and reduction reactions [9].
High-performance liquid chromatography (HPLC) and electrophoresis
are generally used for the detection of cysteine [10–12], methods that
are time-consuming and quite expensive. Fluorescence spectroscopy
has several advantages to these analytical methods because of its high
The photophysical properties of receptor 1 were investigated by
UV–vis absorption profile as well as its fluorescence emissions. The
UV–vis absorption spectra recorded at a 10 μM concentration of recep-
tor 1 in DMSO/CH3CN (1:9; v/v) exhibited a band at 372 nm (Fig. S1).
This band appeared to be a consequence of charge transfer between
\OH and \CH_N-linkages [16]. Upon excitation at 372 nm, the solu-
tion revealed a dual channel emission at 440 and 545 nm. This dual
channel emission is the consequence of keto–enol tautomerism involv-
ing the \OH and \CH_N-functional groups of the fluorophore [17].
To investigate different analytes, receptor 1 was monitored with a
library of 25 anions/dicarboxylic acids/thiols. In this context, a solution
of receptor 1 (10 μM) in DMSO/CH3CN (1:9, v/v) was prepared. Aliquots
of anions/dicarboxylic acids/thiols (40 μM) were added, and the respec-
tive emission spectra were measured. A sufficient time (approx. 45 min)
was allowed for each solution to attain equilibrium before recording the
fluorescence profile. Most of the tested species had little effect on the
fluorescence intensity of receptor 1. The most probable reason for
weak coordination of these species is due to competition between the
analyte and solvent for receptor binding sites.
⁎
Corresponding authors. Tel.: +911881242176.
E-mail addresses: nsingh@iitrpr.ac.in (N. Singh), dojang@yonsei.ac.kr (D.O. Jang).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.08.006

M.J. Kim et al. / Inorganic Chemistry Communications 36 (2013) 96–99
97
N
of the fluorescence intensity at 545 nm that originates from the keto
form of receptor 1 and enhancement at 412 nm, which was assigned
to the enol form of the receptor. These results indicate that the keto
and enol forms of pure receptor 1 are in equilibrium. Zn2+ coordination
led to binding with the enol form and thus, removal of the keto form
from the equilibrium. A comparison of binding of receptor 1 with
Zn2+ (Fig. S2) and titration of receptor 1 with Zn2+ (Fig. 1B) showed
differences in their spectra. The addition of 40 μM of Zn2+ in a single
portion to 10 μM of receptor 1 led to enhanced fluorescence intensity
of receptor 1 at only 440 nm (Fig. S2), while the stepwise addition of
Zn2+ to a solution of receptor 1 had affected the shape and position of
the emission originally at 440 nm. We previously reported a similar
effect highlighting the “tuning of the fluorescence profile of a sensor”
through the addition of the same analyte but varying its time of addition
[18]. The binding constant for the 1.Zn2+complex was calculated to be
H2N
Zn(ClO4)2’ MeOH,
reflux, 68%
N
H
O
O
N
N
Cl
Cl
N
N
N
N
KOH, acetone, reflux,
98%
HO
H
N
N
NH2 H2N
2
3
O
N
N
OH
N
O
N
N
N
N
N
OH
1
7.9 (
[19].
0.9) × 10 M−1 using the Benesi–Hildebrand method (Fig. S3)
Scheme 1. Synthesis of compound 1.
In order to evaluate potential interference from other metal ions for
Zn2+ estimation, a competitive binding test was performed, which
showed that other metal ions did not affect Zn2+ recognition with
receptor 1, as shown in Fig. 2. Although Fig. 1A shows the influence of
several metal ion on the fluorescence profile of 1; however, Fig. 2 advo-
cates no influence of any of these metals ions on the fluorescence profile
of 1 for Zn2+. This highlights the much stronger binding affinity of 1 for
Zn2+ as compare to other metal ions.
In another set of experiments, the impact of different cations on the
fluorescence profile of receptor 1 was tested. These typical experiments
involved the metal recognition properties of receptor 1 by mixing a
10 μM solution of receptor 1 with a metal nitrate salt (40 μM) in
DMSO/CH3CN (1:9, v/v) solvent. The fluorescence spectrum of each so-
lution was measured with excitation at 372 nm. Although most of the
tested metal ions quenched the emission intensity of receptor 1, Zn2+
coordination led to enhanced fluorescence (Figs. 1A and S2). This
enhancement in intensity is due to the greater molecular rigidity of
the complex, which makes non-radiative decay less probable from the
excited state, and coordination from imine linkages, which otherwise
prevails due to cis-trans isomerism.
To confirm the stoichiometry of the 1.Zn2+ complex, a Job's plot
analysis was performed, which showed a 1:1 stoichiometry for the com-
plex (Fig. S4) [20]. CHN analysis also supported the formation of a 1:1
complex. A comparison of IR spectra of receptor 1 and 1.Zn2+ showed
that there were shifts of the imine linkages, \CH_N−, as well as the
\OH band upon complexation with Zn2+ (Fig. S5). It was concluded
that the nitrogen atom of the \CH_N− group and the oxygen atom
of the \OH group were involved in metal coordination. Surprisingly,
the mass spectrum of 1.Zn2+ did not indicate formation of a 1:1 metal
complex as the mass obtained was substantially higher than expected.
This contemplates us to evaluate the metal complex with transmission
electron microscopy (TEM) and results suggested the formation of
nano-aggregates of the metal complex (32–50 nm) (Fig. 3). These
types of metal complex could be expected since receptor 1 has two
well-defined pseudocavities, which are able to form a supramolecular
assembly similar to the one adopted by bi-functional groups [21].
To gain further insight into the sensor activity of receptor 1 for Zn2+
,
a titration was performed with 10 μM of receptor 1 and sequential addi-
tion of Zn2+ (0 –120μM) in DMSO/CH3CN (1:9, v/v) solvent (Fig. 1B).
Successive additions of Zn2+ to receptor 1 solution led to quenching
440nm
A 0.8
0.6
0.4
0.2
0
This leads us to conclude that the formation of the (1.Zn2+
)n supra-
molecular assembly occurred as shown in Scheme 2, where the size of
the assembly was dependent on the mixing time of the precursors
-0.2
-0.4
(Fig. S6). The supramolecular assembly of (1.Zn2+
)n was thought to be
an excellent candidate for recognition of anions/dicarboxylic acids/thiols,
since metal coordination could provide better steric orientation and
additional binding sites through electrostatic interactions between the
metal center and an anionic analyte. For the recognition of anionic
-0.6
Ag (I) Ba (II) Cu (II) Co (II) Ca (II)Fe (III) K (I) Mg (II) Ni (II) Na (I) Zn (II) Cd (II)
45
40
35
B
160
140
30
25
20
15
10
5
120
100
80
60
40
20
0
0
500
550
600
Wavelength (nm)
395
445
495
545
595
645
695
Wavelength (nm)
Fig. 1. (A) Fluorescence ratio ((I − I0)/I0) of receptor 1 (10 μM) upon addition of
metal nitrate salts (40 μM) in a DMSO/CH3CN (1:9, v/v) solvent system excited at
372 nm; (B) Changes in fluorescence spectra of receptor 1 (10 μM) in a DMSO/CH3CN
(1:9, v/v) solvent system excited at 372 nm upon consecutive additions of Zn2+ ions
(0–120 μM) (inset represents the decrease in fluorescence intensity around 545 nm).
Fig. 2. Detection of Zn2+ with receptor 1 in the presence of other metal ions (Na+, K+
Mg2+, Ca2+, Ba2+, Fe3+, Co2+, Ni2+, Cu2+, Ag+, Cd2+) in DMSO/CH3CN (1:9, v/v).
,

98
M.J. Kim et al. / Inorganic Chemistry Communications 36 (2013) 96–99
Complex
A
2-Aminothiophenol
4-Aminothiophenol
2-Mercaptothiazoline
L-Cysteine
600
2-Mercaptopyridine
2-Mercaptopyrimidine
2,5-Dimercapto-1,3,4-thiadiazole
Thioacetic acid
Succinic acid
500
400
300
200
100
0
Malonic acid
Terephthalic acid
Isophthalic acid
Glutaric acid
Adipic acid
Suberic acid
Pimelic acid
380
430
480
530
580
630
Wavelength (nm)
240
B
Complex
Fluoride
Chloride
Bromide
Iodide
210
180
150
120
90
Fig. 3. TEM analysis of the 1.Zn2+ complex.
Cyanide
Hydrogen Sulphate
Perchlorate
analytes, the assembly (1.Zn2+)n was prepared in situ by mixing recep-
tor 1 (10 μM) with Zn2+ in a molar ratio of 1:10. Of the various anions
(F−, Cl−, Br−, I−, NO−3 , CN−, HSO4−, ClO−4 , CH3CO−2 , H2PO−4 ), dicarbox-
ylic acids (succinic acid, malonic acid, terephthalic acid, isophthalic
acid, oxalic acid, glutaric acid, adipic acid, suberic acid, pimelic acid)
and thiols (thioacetic acid, 2-aminothiophenol, 4-aminothiophenol,
2-mercaptothiazoline, L-cysteine, 2-mercaptopyridine, 4,6-diamino-
2-mercaptopyrimidine, 2-mercaptopyrimidine, 2,5-dimercapto-1,3,4-
thiadiazole, 3-mercaptopropionic acid) tested for their interaction with
assembly (1.Zn2+)n, only L-cysteine had a pronounced signal with
approximately four-fold enhancement in the intensity of assembly
(1.Zn2+)n as shown in Fig. 4A and B.
Acetate
Dihydrogen Phosphate
Nitrate
60
30
0
383
433
483
533
583
633
Wavelength (nm)
Fig. 4. (A) Changes in fluorescence intensity of assembly (1.Zn2+
)n (10 μM) upon addition
of a dicarboxylic acid or thiol (200 μM) in DMSO/CH3CN (1:9, v/v); (B) Changes in fluores-
cence intensity of assembly (1.Zn2+
in DMSO/CH3CN (1:9, v/v).
)n (10 μM) upon addition of the test anion (200 μM)
In order to examine kinetic effects on the sensing activity of assembly
(1.Zn2+)n, solutions were kept for 45 min to attain equilibrium before
recording spectra. These solutions were again scanned after standing
overnight, and it was found that 45 min was an appropriate time to elim-
inate any kinetic effect. The effect of salt on the fluorescence profile of
1:10 was titrated with L-cysteine through continuous small additions
(0 to 200 μM). It was observed that addition of L-cysteine to the solu-
tion of assembly (1.Zn2+
) leads to an enhancement in intensity at
assembly (1.Zn2+
) was also investigated. Fluorescence spectra were
n
n
440 nm (Fig. S10). The association constant was calculated using the
recorded with different concentrations of tetrabutylammonium perchlo-
rate salt, and there was no change in the spectral shape or intensity.
Hence, these results ruled out the occurrence of a salt effect. Furthermore,
the effect of medium pH on sensor activity was evaluated through pH
titrations. Fig. S7A and B illustrate that both acidic and basic media influ-
ence the spectra. However, the binding profile of the host remained intact
in the pH range from 6.5 to 11. The anion/dicarboxylic acid/thiol binding
test was also performed with a UVvis spectrophotometer to check for
possible interactions (Figs. S8 and S9). However, no selectivity was ob-
served for particular anions/dicarboxylic acids/thiols.
equation: log (Fmax − F)/(F − Fmin) = log [anion] — log β, where
Fmax and Fmin are the maximum and minimum emissions and F is the
measured emission [22]. The binding constant, log β, was calculated
to be 2.22.
To determine whether assembly (1.Zn2+
) could detect L-cysteine
n
selectively in the presence of other anionic analytes, a competitive bind-
ing assay was performed as shown in Fig. 5. Mixtures of assembly
(1.Zn2+)n, L-cysteine, and anions/dicarboxylic acids/thiols analytes
were prepared in a molar ratio of 1:5:5, and the corresponding fluores-
cence emission spectra were recorded. It was concluded that assemble
To explore the mechanism behind the selectivity of assembly
(1.Zn2+
) selectively recognized L-cysteine even in the presence of a
n
(1.Zn2+
) ) with L-
for L-cysteine, a titration of the assembly (1.Zn2+
n
n
cysteine was performed. A stock solution of the assembly (1.Zn2+)n pre-
number of biologically relevant anions.
pared in situ by mixing receptor 1 (10 μM) with Zn2+ in a molar ratio of
We have synthesized a new type of benzimidazole-based imine-
linked fluorescent receptor such that the receptor platform provides addi-
tional binding sites for metal recognition leading construction of a supra-
molecular assembly. The (1.Zn2+)n assembly consists of a 1:1 host–guest
ratio with a size around 32–50 nm supported by TEM. The (1.Zn2+)n as-
sembly was successfully used for the estimation of L-cysteine (0–200 μM)
even in a medium with potentially competitive biologically important an-
ionic species.
Zn2+
Zn2+
Zn2+
Zn2+
Scheme 2. Diagrammatic representation of assembly (1.Zn2+)n.

M.J. Kim et al. / Inorganic Chemistry Communications 36 (2013) 96–99
99
600
500
400
300
200
100
0
Fig. 5. Estimation of L-cysteine by assembly (1.Zn2+
)n in the presence of dicarboxylic acids/thiols in DMSO/CH3CN (1:9, v/v).
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Acknowledgments
This work was supported by the Korean and Indian Government for
the India-Korea Joint Program of Cooperation in Science & Technology
(NRF-2011-0027710) and the CSIR project (project no: 01(2417)/10/
EMR-II).
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