Ammonium-bearing dinuclear copper(II) complex: A highly selective and sensitive colorimetric probe for pyrophosphate

Article abstract of DOI:10.1021/ol5007339

Two dinuclear copper complexes with and without ammonium moieties were synthesized. The complexes exhibited selective binding affinity to pyrophosphate in aqueous solution. The dinuclear copper complex, with ammonium arms, showed a ca. 527-fold enhancement in pyrophosphate binding affinity compared with its analogue without ammonium units.

Full text of DOI:10.1021/ol5007339

Letter
pubs.acs.org/OrgLett
Ammonium-Bearing Dinuclear Copper(II) Complex: A Highly
Selective and Sensitive Colorimetric Probe for Pyrophosphate
Wenxiang Yu,† Jian Qiang,† Jun Yin,‡ Srinivasulu Kambam,† Fang Wang,† Yong Wang,†
and Xiaoqiang Chen*,†
†State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech
University, Nanjing 210009, P. R. China
‡Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University,
Wuhan 430079, P. R. China
S
* Supporting Information
ABSTRACT: Two dinuclear copper complexes with and
without ammonium moieties were synthesized. The complexes
exhibited selective binding affinity to pyrophosphate in
aqueous solution. The dinuclear copper complex, with
ammonium arms, showed a ca. 527-fold enhancement in
pyrophosphate binding affinity compared with its analogue
without ammonium units.
he development of recognition systems for anions has
hydrogen bonding, and proton transfer.7 Therefore, introduc-
Tattracted increasing attention in recent years because of ing some hydrogen-bonding donors to a dinuclear metal
complex as the PPi receptor is a desirable strategy to enhance
the binding affinity with PPi. In this study, we report two
dinuclear copper complexes, namely, Cu−La and Cu−Lb. We
used bis-2-((pyridin-2-ylmethylamino)methyl)phenol as the
coordinated unit (Figure 1) for the detection of PPi using
their potential application in clarifying the function of anions in
biological processes.1 Pyrophosphate (P2O74−, PPi) plays an
important role in bioenergetic and metabolic processes.2 Metal
complex based chemosensors for PPi in conjunction with
indicator displacement assays (IDAs) are highly interesting to
researchers because the sensing assays can be carried out in
aqueous media.3 Among various indicators, pyrocatechol violet
(PV) is the most popular for anion detection. PV exhibits a
remarkable color change between blue and yellow, which
corresponds to associated and disassociated forms with metal
complexes, respectively, thus allowing naked eye recognition
and sensing. This IDA strategy is based on the displacement of
the indicator attached onto the metal complex by a noncovalent
model of the analyte ion, thus avoiding the synthesis through
covalent linking with the receptor and signal reporter. In the
past, most of the receptors based on zinc complexes bearing a
dipicolylamino structure as phosphate-coordinated pockets
have been reported,4 where the bis-dipicolylamino-based zinc
complexes exhibited high affinity and selective binding with PPi
and other phosphates. Conversely, few transition-metal
complexes derived from bis-2-((pyridin-2-ylmethylamino)-
methyl)phenol were used as receptors for PPi even though
the related dinuclear Cu, Zn, and Fe complexes have been
reported.5
Figure 1. Structures of complexes Cu−La and Cu−Lb.
the IDA method. The ligand based on bis-2-((pyridin-2-
ylmethylamino)methyl)phenol is easier to modify by approach
compared with the bis-dipicolylamino-based donors. The
design of the new receptor complex Cu−Lb with two
ammonium arms in the scaffold was inspired by nucleases.
We expected Cu−Lb to show stronger binding affinity with PPi
through electrostatic interactions, hydrogen bonding, and
proton transfer.
La was synthesized according to a previous report.5b The
synthesis of Lb is depicted in Scheme 1. The synthesis starts
with 2-aminomethylphenol, stirred with phthalic anhydride to
In nature, the high affinity and reactivity of nucleases is
attributed to cooperation between metal ions and several
functional groups of the amino acid side chains present in the
active site.6 The positively charged residues of amino acids are
thought to assist and stabilize the binding status between metal
ion centers and phosphates through electrostatic interactions,
Received: March 10, 2014
Published: April 2, 2014
© 2014 American Chemical Society
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Letter
demonstrating the stronger binding affinity of Cu−Lb (ca. 53-
fold) to PV compared with the Cu−La complex.
Scheme 1. Synthesis Procedure for Lb
Afterward, Cu−La−PV and Cu−Lb−PV ensembles were
utilized in anion assays. Cu−La−PV and Cu−Lb−PV
ensembles were prepared by mixing PV with the equivalent
Cu−La or Cu−Lb complex, and then different anions (10
equiv), such as PPi, HPO42−, Ac−, SO42−, CO32−, F−, Cl−, Br−,
and I−, were added to the solution containing Cu−La−PV (20
μM) or Cu−Lb−PV (20 μM). The absorption spectra of the
Cu−La−PV and Cu−Lb−PV ensembles showed that PPi
induced a remarkable change whereas other anions did not
induce a significant change (Figure 3). PPi also caused a color
yield phthalimidoyl group labled as compound 1. After
formylation, reduction, and chlorination, compound 4 was
obtained. The reaction of 4-methyl-2,6-bis((pyridin-2-
ylmethylamino)methyl)phenol with compound 4 resulted in
compound 5. The target ligand Lb was acquired with
deprotection treatment using N2H4·H2O. These compounds
were characterized by NMR and high-resolution mass spectra
(see Figures S1−S15 in the Supporting Information). Cu−La
or Cu−Lb complexes was obtained by mixing the La or Lb with
2 equiv of Cu(ClO4)2 in aqueous solution.8 The formation of
dinuclear copper(II) complexes was supported by the
corresponding mass spectra (see Figures S16 and S17 in the
Supporting Information).
Figure 3. Changes in the absorbance of Cu−La−PV and Cu−Lb−PV
2−
ensembles upon the addition of various anions, such as PPi, HPO4
,
Ac−, SO42−, CO32−, F−, Cl−, Br−, and I−. The spectra were obtained in
the HEPES buffer (10 mM, pH 7.0): (a) Cu−La−PV (20 μM) in the
presence of concentrations of 10 equiv of anion; (b) Cu−Lb−PV (20
μM) in the presence of concentrations of 10 equiv of anion.
change from blue to yellow in the Cu−Lb−PV ensemble, and
Cu−La−PV showed a color change from light-green to yellow.
In contrast, other anions did not induce any color change (see
Figure 4 and Figure S21 in the Supporting Information). These
results suggested that both Cu−La−PV and Cu−Lb−PV have
good selectivity for PPi detection.
The UV−vis spectra of PV with the increasing addition of
complexes are illustrated in Figure 2. The absorption band
Figure 2. (a) Changes in absorbance of PV (20 μM) upon addition of
various concentrations of Cu−La (0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20,
24, 28, 32, 36, and 40 μM). (b) Changes in absorbance for PV (20
μM) upon addition of various concentrations of Cu−Lb (0, 2, 4, 6, 8,
10, 12, 14, 16, 18, 20, 24, 28, 32, 36, and 40 μM). The spectra were
obtained in the HEPES buffer (10 mM, pH 7.0).
Figure 4. Color of the Cu−Lb−PV ensemble (20 μM) with and
without the addition of various anions (200 μM). From left to right:
no anion, PPi, HPO42−, Ac−, SO42−, CO32−, F−, Cl−, Br−, and I−.
centered at 444 nm decreased, whereas the absorption peaks at
685 nm for Cu−La and 630 nm for Cu−Lb increased when
Cu−La or Cu−Lb complex was added to HEPES buffer (10
mM, pH 7.4) containing PV (20 μM). This result indicated that
free PV bonded with the complexes. Clear color changes from
yellow to light-green for Cu−La and yellow to blue for Cu−Lb
were observed by the naked eye when the complexes were
added to the solution containing PV (see Figure S18 in the
Supporting Information). Using the Job’s plot method, the
stoichiometry of the binding between PV and Cu−La or Cu−
Lb was determined to be 1:1 (see Figure S19 in the Supporting
Information). The absorption at 444 nm of PV depended on
the concentration of Cu−La or Cu−Lb and described as Figure
S20 in the Supporting Information. After fitting of the titration
curves, the association constants (Ka) for PV were found to be
3.54 × 105 M−1 for Cu−La and 1.90 × 107 M−1 for Cu−Lb, thus
The UV−vis absorption spectra were obtained as various
concentrations of PPi were titrated into the HEPES buffer (10
mM, pH 7.0) containing Cu−La−PV (20 μM) or Cu−Lb−PV
(20 μM). Upon the addition of PPi, the absorption peak of the
ensembles at 444 nm increased while the peaks at 685 nm for
Cu−La−PV and 630 nm for Cu−Lb−PV decreased, indicating
that the free PV was released from the ensembles (Figure 5).
We calculated the association constants of complexes Cu−La
and Cu−Lb with PPi (see Figure S22 in the Supporting
Information) based on the curve fittings from the titration data.
The association constants were obtained to be 1.09 × 104 M−1
for the binding between Cu−La and PPi and 5.75 × 106 M−1
for the binding between Cu−Lb and PPi. The introduction of
ammonium moieties to dinuclear copper complex led to a ca.
527-fold enhancement in PPi binding affinity. Higher binding
affinity is observed between Cu−Lb and PPi compared with
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Figure 5. Changes in absorbance of Cu−La−PV and Cu−Lb−PV
ensembles upon the addition of various concentrations of PPi (0, 20,
40, 60, 80, 100, 120, 140, 160, 180, 200, 240, 280, 320, 360, and 400
μM). Spectra were obtained in HEPES buffer (10 mM, pH 7.0): (a)
Cu−La−PV (20 μM) in the presence of various concentrations of PPi;
(b) Cu−Lb−PV (20 μM) in the presence of various concentrations of
PPi.
Figure 6. Time-related ratio of absorption intensity at 444 and 630 nm
in the HEPES buffer of Cu−Lb−PV (20 μM) combined with PPi (200
μM) and different catalysts: PPase (10 units); Mg2+ (20 μM); PPase
(10 units) + Mg2+ (20 μM).
Cu−La. This finding can be attributed to the cooperation
between copper ions and neighboring ammonium moieties
(Scheme 2). The positively charged ammonium ions are
In summary, using ((pyridin-2-ylmethylamino)methyl)-
phenol as a coordinated unit, we reported two dinuclear
copper complexes Cu−La and Cu−Lb for the detection of PPi.
Both complexes showed good selectivity for binding with PPi.
Compared with the complex Cu−La, the complex Cu−Lb
bearing ammonium arms exhibited a 527-fold enhancement
in the binding affinity to PPi, which should be attributed to the
electrostatic interactions, hydrogen bonding or proton transfer
from ammonium ions. Furthermore, Cu−Lb−PV was successful
to monitor the cleavage reaction of PPi catalyzed by PPase.
Scheme 2. Proposed Binding Mechanism of Cu−Lb−PV with
PV and PPi
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details; synthesis and characterization of Lb;
spectroscopic data; Job’s plot of the binding between Cu−La or
Cu−Lb and PV; calculation and fitting of Ka. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: chenxq@njtech.edu.cn.
Notes
The authors declare no competing financial interest.
thought to assist copper ion to binding with PPi through
electrostatic interactions, hydrogen bonding, or proton transfer.
PPi can efficiently displace the indicator of Cu−Lb−PV
ensemble and demonstrate color change from blue to yellow
because of the suitable scaffold and positive effect provided by
ammonium moieties. The addition of PPi also induced the IR
absorption band of Cu−Lb at 3470 cm−1 shift to 3440 cm−1.
We speculated that the binding between ammonium and PPi
caused the change of stretching vibration peak of N−H bond
ACKNOWLEDGMENTS
■
This work was supported by grants from NSF of China
(21376117), the Project of Priority Academic Program
Development of Jiangsu Higher Education Institutions
(PAPD), and the Scientific Research Foundation for the
Returned Overseas Chinese Scholars of State Education
Ministry.
(see Figure S23 in the Supporting Information).
2−
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■
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