Lower rim 1,3-di{4-antipyrine}amide conjugate of calix[4]arene: Synthesis, characterization, and selective recognition of Hg2+and its sensitivity toward pyrimidine bases

Article abstract of DOI:10.1021/jo2022372

The structurally characterized lower rim 1,3-di{4-antipyrine}amide conjugate of calix[4]arene (L) exhibits high selectivity toward Hg²⁺ among other biologically important metal ions, viz., Na⁺, K ⁺, Ca²⁺, Mg

Full text of DOI:10.1021/jo2022372

Article
pubs.acs.org/joc
Lower Rim 1,3-Di{4-antipyrine}amide Conjugate of Calix[4]arene:
Synthesis, Characterization, and Selective Recognition of Hg2+
and Its Sensitivity toward Pyrimidine Bases
Jayaraman Dessingou,† Khatija Tabbasum,† Atanu Mitra,† Vijaya Kumar Hinge,‡ and Chebrolu P. Rao*,†,‡
‡
†Bioinorganic Laboratory, Department of Chemistry and Department of Biosciences and Bioengineering, Indian Institute of
Technology Bombay, Powai, Mumbai 400076, India
S
* Supporting Information
ABSTRACT: The structurally characterized lower rim 1,3-di
{4-antipyrine}amide conjugate of calix[4]arene (L) exhibits high
selectivity toward Hg2+ among other biologically important metal
ions, viz., Na+, K+, Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+,
Cd2+, Hg2+, Pb2+, and Ag+ as studied by fluorescence, absorption,
and ESI MS. L acts as a sensor for Hg2+ by switch-off fluorescence
and exhibits a lowest detectable concentration of 1.87 0.1 ppm.
The complex formed between L and Hg2+ is found to be 1:1 on
the basis of absorption and fluorescence titrations and was con-
firmed by ESI MS. The coordination features of the mercury com-
plex of L were derived on the basis of DFT computations and found that the Hg2+ is bound through an N2O2 extending from
both the arms to result in a distorted octahedral geometry with two vacant sites. The nanostructural features such as shape and
size obtained using AFM and TEM distinguishes L from its Hg2+ complex and were different from those of the simple mercuric
perchlorate. L is also suited to sense pyrimidine bases by fluorescence quenching with a minimum detection limit of 1.15 0.1
ppm in the case of cytosine. The nature of interaction of pyrimidine bases with L has been further studied by DFT computational
calculations and found to have interactions through a hydrogen bonding and NH-π interaction between the host and the guest.
calix[4]arene (L), wherein the antipyrine unit is connected
through an amide link for the purpose of selectively recognizing
Hg2+ in methanol. The L also shows selectivity toward pyrim-
idine bases by fluorescence quenching.
INTRODUCTION
■
Development of selective chemosensors for the determination
of transition and heavy metal ions is an important area of re-
search, because metal ions such as Pb2+, Cd2+, and Hg2+ have
lethal effects on living systems in addition to their effect in the
environmental pollution.1 Supramolecular systems like calixar-
enes2 functionalized with suitable binding cores can act as good
hosts for the recognition of ionic and molecular species,3
though such systems are limited in the literature in case of Hg2+
recognition.4 Calix[4]arenes functionalized with aryl-azo,5
benzothiazole-azo,6 benzimidazole,7 and fluorescent probes
such as pyrene, rhodamine,8 and dansyl fluorophoric groups9
have been used as ion receptors in the literature. Therefore, the
development of molecular receptors that can selectively recognize
Hg2+ is an important area of research. Recognition of nucleic bases
is important in DNA detection, as biosensors and chips are being
widely used in biological sciences for the same.10 A gold electrode
based sensor array for selective recognition of purines and pyrim-
idines has been reported.11 Ruthenium and osmium complexes of
bipyridyl ligands have been used for electrochemical and/or
luminescent labeling of biomolecules and hence are sensitive in
the recognition of the nucleic acids.12 Silver nanoparticles capped
with a conjugate of calix[4]arene have been reported to dis-
criminate purines from those of pyrimidines.13 A calix[6]arene-
acid derivative is shown to be effectively extracting adenine by
forming a complex.14 Therefore, herein we report the devel-
opment of a lower rim 1,3-di{4-antipyrine}amide derivative of
RESULTS AND DISCUSSIONS
■
Synthesis of the Receptor Molecule (L). The receptor
molecule, L, was synthesized via four known steps15 starting
from p-tert-butyl calix[4]arene as shown in scheme 1. The calix-
[4]arene acid chloride derivative L4 was made by reacting the
diacid derivative, L3, with SOCl2, followed by coupling with
4-aminoantipyrine to result in L. All these molecules including
L were characterized satisfactorily by 1H and 13C NMR, ESI MS,
HRMS, FTIR, and elemental analysis (Supporting Information
01, 02). The cone conformation of L has been revealed by
1H NMR spectroscopy and was further confirmed by establish-
ing the structure using single crystal XRD.
Crystal Structure of L. Colorless, diffraction quality single
crystals of L were grown from its EtOH/CHCl3 (1:4) solution.
It crystallizes as monoclinic with space group P21/c. The corres-
ponding details of the structure determination and refinement
data are given in Table 1. The crystal structure of L given
in Figure 1a confirms cone conformation of the conjugate that
Received: October 31, 2011
Published: January 4, 2012
© 2012 American Chemical Society
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a
Scheme 1. Synthesis of L
a
(a) Bromoethylacetate/K2CO3/acetone; (b) NaOH/C2H5OH, reflux; (c) SOCl2/benzene, reflux; (d) 4-aminoantipyrine/Et3N/THF. R = tert-
butyl.
Table 1. Crystal Data and Structure Determination Data
for L
parameter
value for L
empirical formula
formula weight
wavelength (Å)
T/K
C70H82N6O8, C2H6O, H2O
1199.51
0.71073
120
crystal system
space group
a (Å)
monoclinic
P21/c (No. 14)
22.45(7)
11.99(3)
25.56(8)
90
b (Å)
c (Å)
α (deg)
β (deg)
107.797(4)
90
γ (deg)
V (Å3)
Figure 1. (a) X-ray crystal structure of the receptor molecule,
L·EtOH.H2O. (b) Intramolecular C−H···π interactions observed
between the arms. (c) C−H···π interactions observed between ethanol
and the calix[4]arene cavity.
6554.7(4)
4
Z
absorption coefficient (mm−1)
0.081
F(000)
2576
Dcalcd (g/cm3)
total reflections
unique reflections
Rint
1.215
55285
11527
0.051
no. of parameters refined
final R (I > 2σ)
wR2 (I > 2σ)
857
0.0460
0.0855
was also predicted on the basis of NMR studies in solution. The
metric data for L is given in Supporting Information 03.
The circular hydrogen bonding observed between the
donor···acceptor pairs at the calix[4]arene lower rim are
2.7619(19) and 2.7790(19) Å with the corresponding angles of
170(2)° and 171(2)°, respectively. L has been crystallized with
a molecule each of ethanol and water. Ethanol was found to be
trapped inside the aromatic cavity of calix[4]arene while the
water is present in the lattice. There exists hydrogen bonding
between the −CO of antipyrine and O−H of water with an
O···O distance of 2.848(2) Å and hydrogen bond angle of
172(3)° (Figure 2). One of the arms is bent by forming an
intra-rim N−H···O hydrogen bond with an N···O distance of
3.10 Å. The amide CO of the bent arm points outside the
Figure 2. Dimer formed from two receptor (L) molecules via lattice
water and C−H···π interactions in the crystal lattice.
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Figure 3. Fluorescence titration of L with metal ions. (a) Spectral traces of L as a function of added Hg2+ to result in [Hg2+]/[L] mole ratio from
0 to 10.0 in methanol. (b) Relative fluorescence intensity (I/I0) versus [Hg2+]/[L] mole ratio at 400 nm. Inset: Job’s plot obtained from fluorescence
2+
data, where IL is fluorescence intensity of receptor L and IL+Hg is fluorescence intensity of the mercury ion complex. nL is mole fraction of L. (c)
Histogram of highest I0/I ratio indicating the quenching fold for the titration of L by different metal ions. Error bars were given on the basis of three
different measurements.
calix[4]arene cavity while the other amide CO points toward the
calix[4]arene core. The centroid to centroid distance of the aryl
moieties of the antipyrine lies at a distance of 6.87 Å with a tilt
angle of 9.5° and hence cannot exhibit any intramolecular π···π
interaction in L (Figure 1b). However the crystal packing structure
shows (Figure 1c) intermolecular C1−H···π interaction with the
centroid of the ring at a distance of 3.19 Å. In the lattice, the two
neighboring L are connected through two water molecules (O18
and O17) by exhibiting hydrogen bonds with the amide CO at a
distance of 2.84 and 2.93 Å and hydrogen bond angles of 147° and
Figure 4. Absorption titration of L with Hg2+. (a) Spectral traces ob-
172°, respectively. The ethanol molecule that is trapped in the
served in the titration of L with Hg2+ in methanol at varying mole ratio.
calix[4]arene cavity exhibits CH···π interaction with all of the four
aromatic moieties, with CH···π distances of 2.73 − 2.91 Å from
the center of the aromatic ring (Figure 1c).
Inset: Job’s plot of nL versus A·nm, where nm is mole fraction of Hg2+ ion
and A is absorbance. (b) Plot of absorbance vs mole ratio of [Hg2+]/[L]
□
○
▽
= 315 nm).
for different bands ( = 280, = 256 nm and inset
As the arms of L possesses groups that are well suited for the
interaction with the incoming guest species, besides exhibiting fluo-
rescence emission, the L has been explored for its ion and molecular
recognition studies using absorption and fluorescence spectroscopy.
Metal Ion Binding Studies in Methanol. The metal ions,
viz., Na+, K+, Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+,
Cd2+, Pb2+, and Ag+ were studied for their binding properties
with L at 20 μM concentration by carrying out titrations as
given in the Experimental Section. The studies were carried out
by exciting the solutions at 280 nm and recording the fluo-
rescence spectra in the range of 295−530 nm in methanol.
During the titration, the fluorescence intensity of L at 400 nm
decreases as a function of the concentration of Hg2+ added
(Figure 3) and attains saturation at ∼2 equiv. Thus the titration
of L with Hg2+ results in a stoichiometric reaction. The emis-
sion λmax of L is shifted to red by 25 nm upon complexation with
Hg2+. However, similar titrations carried out with other metal
ions, such as Na+, K+, Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+,
Zn2+, Cd2+, Pb2+, and Ag+, exhibited minimal or no change in the
fluorescence intensity as can be seen from Figure 3 (Supporting
Information 04). The binding constant of L with Hg2+ was
derived based on the Benesi−Hildebrand equation to yield a
Ka of 10550 700 M−1. The quantum yields of L and its Hg2+
complex were found to be 0.010 and 0.004 by using naphthalene
as standard. The minimum detection of Hg2+ by L is found to be
(1.87 0.1) ppm (Supporting Information 05) as derived on the
basis of fluorescence studies.
An isosbestic point was observed at 294 nm supporting the com-
plex formation. Job’s plot obtained using the fluorescence and
absorption spectra indicated the formation of a 1:1 complex
between Hg2+ and L. However the absorption titration carried
out with all the other metal ions showed no significant change,
indicating their noninteractive nature with L. Even the ions
from the same group, viz., Zn2+ and Cd2+, do not bind to L
(Supporting Information 06)
Electrospray Mass Spectra of L with Hg2+. The complex
formed between L and Hg2+ has been further confirmed to be
1:1 based on ESI MS. The mass spectra yielded a molecular ion
peak for 1:1 complex at m/z of 1335.7, where its isotopic peak
pattern supports the presence of Hg2+ (Figure 5; Supporting
Information 07).
UV−vis Absorption Studies. UV−vis absorption titrations
were carried out between L and Hg2+ at a concentration of
63 μM in methanol (see Experimental Section for details), and the
study shows a decrease in the absorption intensity at 280 nm
and an increase in the absorbance at 315 and 256 nm (Figure 4).
Figure 5. ESI MS spectrum of [L-Hg2+] complex. (a) Theoretical
isotopic pattern for [L+Hg2+] ion. (b) Experimentally observed
molecular ion peak.
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Competitive Metal Ion Titration by Fluorescence
Spectroscopy. In order to evaluate the efficiency of the re-
ceptor L in recognizing Hg2+ in the presence of other metal ions,
competitive titrations were carried out by fluorescence spectroscopy.
The spectra were recorded for [L + xMn+] against Hg2+ in the
presence of 30 equiv of biologically relevant metal ions, such as Na+,
K+, Ca2+, Mg2+, or 5 equiv of Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+,
Cd2+, Pb2+, or Ag+ ions. The presence of excess equivalents of metal
ions did not affect the quenching characteristics and the emission
intensity of L when titrated with Hg2+. Further, the titrations carried
out between [L + 2Hg2+] and Mn+ did not exhibit any significant
change in the fluorescence intensity as can be noted from Figure 6.
Thus L can recognize Hg2+ even in the presence of other metal ions.
far away from the predicted binding region. The optimization
of the corresponding complex between L with Hg2+ was started
in a cascade fashion by going through AM1 → HF/CEP-121G →
B3LYP/CEP-121G. In the optimized complex (Figure 7b), the
Hg2+ was bonded to the two amide-N atom. The bond length
and bond angle around the Hg2+ center are N1/N2···Hg = 2.17 Å
and N1···Hg···N2 = 165°, respectively. The two amide-O atoms, viz.,
O1 and O2, also provide additional interactions at a Hg···O distance
of 2.562 and 2.768 Å, which is within the sum of vander Waal’s radii.
Thus, the optimized structure reveals a distorted octahedral
geometry around Hg2+ with two vacant sites of which one lies in
the equatorial plane (Figure 7c) and other lies trans to O2. Such
structure around the Hg2+ indeed exhibited ∼7.6 kcal/mol
additional stabilization when compared to a binding model of
N−Hg−O2Camide (Supporting Information 08).
Nanostructural Features of L and Its Mercury
Complex by AFM. AFM images of L and its Hg2+ complex
shows that the particles were well spread over the mica sheet.
The morphological features of free L and this in the presence of
Hg2+ are quite distinct in their sizes. While L alone forms almost
uniform particles of size 250
30 nm with good separation,
those observed in the presence of Hg2+ were shortened sub-
stantially to have particles with sizes of 45 5 and 80 10 nm
with higher packing density in the latter case. However, both of
these particles are close to spherical in shape. Thus, the presence of
Hg2+ induces the formation of smaller particles as shown in Figures
8 and 9. Therefore, the larger size of the particles observed in case
of L may be attributable to the aggregation due to the presence of
hydrophobic groups, and these become much small in the presence
of Hg2+ owing to the coordination ability of L through amide nitro-
gen and carbonyl oxygen centers. In effect, the structural features
observed in AFM for L differs clearly from its complex of Hg2+.
The structural features observed for L and its mercury complex
differs substantially from that observed with the simple salt, viz.,
mercury perchlorate. The height of the particles of L exhibits an
average value of 90 10 nm. However, in the presence of Hg2+ the
Figure 6. Histograms showing quenching fold. Titration of (a) [L + x
equiv Mn+] against Hg2+ and (b) [L + 2 equiv Hg2+] against Mn+,
x = 30 for Na+, K+, Ca2+, Mg2+ and x = 5 for other metal ions.
Computational Study of the Structural Features of
the 1:1 Complex of L with Hg2+. As no single crystals could
be obtained for the complex of L with Hg2+, its structure could
not be established. Therefore, in order to understand the inter-
actions present between L and Hg2+, computational studies
were carried out in a systematic fashion by starting from semi-
empirical and then going through HF followed by DFT, in a
cascade fashion. As the formation of 1:1 species was already
established by Job’s plot and ESI MS, the [L-Hg2+] complex
was modeled using the Gaussian 03 package.16 Prior to assum-
ing the initial guess model for the computational calculations,
structure of L was optimized (Figure 7a) taking the coordinates
from the crystal structure and chopping off the tert-butyl groups
to reduce the computational time. The structure was then opti-
mized by using different theories. The Hg2+ ion has been placed
height of these particles decrease substantially to result in 12
and 4 2 nm for the larger and smaller size ones, respectively.
2
Nanostructural Features of L and Its Mercury
Complex by TEM. The shape and size of the particles of L,
its Hg2+ complex, and simple mercury perchlorate observed in
TEM micrographs can be seen from Figure 10. L shows spherical
particles with two different size distributions, one ∼550 nm and
the other ∼900 nm, and the particles possess a dark core region
that is surrounded by a lighter shell (Figure 10a,b,g). However in
Figure 7. DFT optimized structures: (a) L; (b) [L-Hg2+] complex; and (c) the binding core around Hg2+. The bond lengths given in panel (c) are in Å.
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Figure 8. AFM micrographs: (a, d) L; (b, c) [L-Hg2+] complex; (c, f) Hg(ClO4)2.
Figure 9. Particle size distribution: (a) L, (b) bigger particles of L in presence of Hg2+, and (c) smaller particles of L in presence of Hg2+.
Figure 10. TEM micrographs: (a, b) L; (c, d) L in the presence of Hg2+; (e, f) simple Hg(ClO4)2 that is used as control. Size distribution of the
particles: (g) L; (h) L in presence of Hg2+; and (i) simple Hg(ClO4)2.
the presence of Hg2+, the average size of the particles is reduced
and is found in the range of 200−400 nm. In the presence of
Hg2+, the particles are deformed from spherical shape, resulting in
dumbbell shapes, and are well interconnected. When the particles
of L come closer in the presence of Hg2+ with increased con-
nectivity, the dark core region of L disappears. This may result
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Figure 11. Fluorescence titration of L by pyrimidine bases. (a) Spectral traces of L as a function of added [cytosine] at varying mole ratios.
■
▼
●
(b) Relative fluorescence intensity (I/I0) versus [nucleic base]/[L] mole ratio at 400 nm ( = cytosine, = thymine, and = uracil). (c) Histogram
of I0/I ratio indicating highest quenching fold. Error bars were given based on three different measurements.
Figure 12. Absorption titration of L with pyrimidine bases. (a) Spectral traces of L as a function of added [cytosine] at varying mole ratios in
■
●
▼
methanol. (b) Absorbance versus [nucleic base]/[L] mole ratio at 280 nm ( = cytosine, = thymine, and = uracil).
from the coordination of Hg2+with L. The greater communication
among the particles with increased connectivity in the presence of
Hg2+ may be of further research interest. However, the mercury
perchlorate control, under the same conditions, exhibits pre-
dominantly mono-dispersed particles of ∼8 nm size. Thus the
nanostructural features of L differ distinctly from those of its Hg2+
complex, and the size and shape of both of these differ from those
of simple mercury perchlorate
Fluorescence Titrations with Nucleic Bases. The pyrim-
idine bases cytosine, uracil, and thymine were studied for their
recognition by L. The studies were carried out by exciting the
solutions at 280 nm and recording the fluorescence spectra in
the range of 295−530 nm in methanol by taking 20 μM solu-
tions of L (Figure 11; Supporting Information 09). Control
experiments were carried out simultaneously with the bases
alone, and the corresponding data were subtracted from the
original spectra. The results showed quenching behavior for
all the three nucleic bases, and the extent of quenching follows
the order cytosine ≥ thymine ≫ uracil. However, the adenine
and guanine could not be studied owing to their strong
emission when excited at 280 nm. The minimum detectable
concentration based on fluorescence quenching is 1.15 0.1
Computational Study of the Structural Features of
the Interaction of Nucleic Bases with L. As substantial
fluorescence quenching was noticed in the titration of cytosine
and thymine with L, the nature of the interaction between the
nucleic base and L were modeled by DFT computational studies
as carried out using Gaussian 03 package.16 Prior to assuming the
initial guess model, the nucleic bases were independently opti-
mized. In order to generate the L-pyrimidine base complex, the
bases were placed far away from the receptor L. The optimi-
zation of the corresponding complex between L with cytosine
and thymine were carried out at B3LYP/CEP-121G. In both the
cases, the bases were bound to the receptor molecule through a
H-bonding and an N−H···π interaction as can be seen from
Figure 13 (Supporting Information 12 and 13).
and 1.41
0.1 ppm for cytosine and thymine, respectively
(Supporting Information 10).
Absorption Titrations with Nucleic Bases. Absorption
titrations were carried out with L at 20 μM with varying equiv-
alents of the nucleic bases cytosine, thymine, and uracil in
methanol (Figure 12). Control experiments were also carried
out simultaneously with the bases alone, and the corresponding
data were subtracted from the original ones. The results ex-
hibited an increase in the absorbance at 280 nm band upon
addition of these nucleic bases (Supporting Information 11).
Figure 13. DFT optimized structures: (a) L-cytosine complex, and (b)
L-thymine complex. The distances given in the figure are in Å. The H-
bonding angle between O···H−CAr is 177.1° and 164.3°, respectively.
At B3LYP/CEP-121G level, the stabilization energies com-
puted using the formula ΔEs = EA − [EL + ENB] {where EA is
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124.8, 127.3, 129.6, 130.1, 134.2, 151.8, 156.2, 168.1. ESI MS: m/z
(intensity %, fragment) 1135.83 (100, [M + H]+). HRMS (EI): calcd
for C70H83N6O8 [M + H]+ m/z 1135.6272, found m/z 1135.6227.
Crystal Structure Determination by Single Crystal XRD.
Single crystals suitable for X-ray diffraction were obtained for L, and
the crystal data are given in Table 1. Single crystal X-ray diffraction
data were collected on an OXFORD DIFFRACTION XCALIBUR-S
CCD system with graphite−monochromated Mo Kα radiation by ω-
2θ scan mode and the absorption corrections were applied by using
multiscan method. The structure determinations were done by direct
methods, and the refinement of the atomic parameters based on full-
matrix least-squares on F2 were performed using the SHELX-97 pro-
grams. All hydrogen atoms were geometrically fixed and allowed to
refine using a riding model, wherever the difference Fourier did not
result in the position of hydrogen. Most of the O-bound hydrogen
positions were recovered from the difference Fourier maps and were
refined isotropically. All non-hydrogen atoms were refined
anisotropically.
Fluorescence Titrations. All fluorescence titrations were carried
out on a fluorescence spectrometer at 280 nm excitation wavelength in
a 1 cm quartz cell. Bulk solutions (12 × 10−4 M) of L were freshly
made before each set of experiments by dissolving L in CHCl3 (50 μL)
and then making up the desired concentration with methanol. The
metal perchlorate salts were made at 12 × 10−4 M in methanol. The
fluorescence titrations were carried out by exciting the solution at
280 nm after adding the appropriate volume of metal salt solution to
result in the requisite mole ratios of [Mn+]/[L], yet maintaining the
final [L] as 20 μM in a total solution volume of 3 mL achieved by
diluting with solvent.
the total energy of the adduct, EL is the total energy of the
calixarene derivative, and ENB is that of the nucleic base}
yielded −14.5 and −8.7 kcal/mol, respectively, for the
interaction of cytosine and thymine with L.
Conclusions and Correlations. A structurally character-
ized antipyrine conjugate of 1,3-diamido-calix[4]arene (L) ex-
hibits selective recognition of Hg2+. The selectivity has been
demonstrated by fluorescence, absorption, and ESI MS
spectroscopy. The interaction of Hg2+ quenches the fluo-
rescence of L observed at 400 nm. L is selective and sensitive
toward Hg2+ over 14 other biologically important metal ions
studies, viz., Na+, K+, Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+,
Zn2+, Cd2+, Pb2+, and Ag+. Even the other ions from the same
group, Zn2+ and Cd2+, do not bind to L. None of the Mn+ ions
studied impedes the interaction of Hg2+ with L as shown by
competitive fluorescence titrations. The receptor L has been
shown to be sensitive with a minimum detection limit of
1.87 0.1 ppm Hg2+. Fluorescence and absorption spectros-
copy provides information for the formation of a 1:1 complex
between Hg2+ and L, and ESI MS confirms this fact by ex-
hibiting a peak for [L-Hg2+] ion with isotopic pattern for
mercury. The Hg2+ binding characteristics of L has been
studied by computational calculations at DFT level, and the
optimized structure exhibited a N2O2 core with distorted octa-
hedral geometry around Hg2+ with two vacant sites of which
one lies in the equatorial plane and other lies trans to O2. The
nanostructural features observed using AFM and TEM clearly
differentiate L from its complex with Hg2+. L exhibits differ-
ential sensitivity toward the pyrimidine bases by exhibiting
quenching of fluorescence emission to different extents and
thereby provides a means to detect these bases. The fluo-
rescence quenching follows the order cytosine ≥ thymine ≫
uracil. The interaction of pyrmidines with L was further
studied by computational calculations. The computational
results show the nature of interaction of pyrimidnes, viz.,
cytosine and thymine, as primarily through hydrogen bonding
and N−H···π interaction. Thus L acts a potential sensor for
Hg2+ and for pyrimidine nucleic bases.
Absorption Titrations. Bulk solutions were prepared by similar
procedure as in fluorescence studies, but L and metal perchlorate salts
were made at 5 × 10−3 M concentration. Titrations were performed by
varying equivalents of Mn+ from 0.2 to 10 and fixing the concentration
of L at 63 μM.
AFM Studies. Solutions of L and Hg2+ were prepared at 6 × 10−5
M concentration in methanol. The receptor L was initially dissolved in
100 μL of CHCl3 and then made up with methanol to the desired
concentration. The stock solutions of L, L-Hg2+, and Hg2+ were
sonicated for 10 min, after which a 50−70 μL aliquot was taken and
spread over a mica sheet using the drop cast method. The samples
were then dried and analyzed by AFM technique.
TEM Studies. Solutions of L and Hg2+ were prepared at 3 × 10−2
M concentration in methanol. The receptor L was initially dissolved in
100 μL of CHCl3 and then in methanol. The stock solutions of L,
L-Hg2+, and Hg2+ were sonicated for 10 min, after which a 50−70 μL
aliquot was taken and spread over a carbon-coated grid using the drop
cast method.
EXPERIMENTAL SECTION
■
Synthesis and Characterization Data for the Compounds L1,
L2, L3, and L. The derivatives L1, L2, and L3 have been prepared
by literature reported procedures.15 L was prepared from diacid
chloride derivative, L3 by following the procedure given below. All
the derivatives were characterized satisfactorily using suitable an-
alytical techniques.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 13C NMR spectra of L, ESI-MS and HRMS data for L,
crystal structure details of L, fluorescence titrations spectra with
different metal ions, minimum detection limit of Hg2+ by L,
absorption titration spectra of L with Zn2+ and Cd2+, ESI-MS
spectrum of [L-Hg] complex, computational data, fluorescence
titration spectra of L with thymine and uracil, minimum detec-
tion limit of thymine and cytosine by L, absorption titrations
with nucleic bases, Cartesian coordinates for DFT opti-
mized [L-thymine] complex, and Cartesian coordinates for
DFT optimized [L-cytosine] complex. This material is available
free of charge via the Internet at http://pubs.acs.org.
Receptor L. To a suspension of 4-aminoantipyrine (1.203 g,
5.92 mmol) and Et3N (1.50 mL, 10.75 mmol) was added dry THF
(60 mL), and the mixture was stirred under an argon atmosphere.
Diacid chloride L4 (2.16 g, 2.70 mmol) in dry THF (60 mL) was
added dropwise to this reaction mixture. Immediately, a yellowish
precipitate was formed, and stirring was continued for 48 h at room
temperature. After filtration, the filtrate was concentrated to dryness.
A yellow solid was obtained that was extracted with CHCl3 and
washed with water and then with brine, and the organic layer was dried
with anhydrous MgSO4. The filtrate was concentrated to dryness and
recrystallized from CHCl3/EtOH. Yield 71%. Anal. (% found) C,
74.15; H, 7.35; N, 7.30; (% required) C, 74.05; H, 7.28; N, 7.42. FTIR
(KBr matrix, cm−1): 3430 (νOH); 1694, 1650 (νCO). 1H NMR
(400 MHz, CDCl3, δ ppm): 10.22 (s, 2H); 7.69 (s, 2H); 7.36 (s,
10H); 6.98 (s, 4H); 6.81 (s, 4H), 4.69 (s, 4H); 4.24 (d, 4H, J = 13.2
Hz); 3.30 (d, 4H, J = 13.4 Hz); 3.06 (s, 6H) 2.20 (s, 6H) 1.18 (s,
18H), 0.943 (s, 18H). 13C NMR: (CDCl3, δ ppm): 12.3, 31.0, 31.7,
32.4, 33.9, 34.1, 36.5, 75.4, 109.3, 123.6, 125.4, 126.1, 127.6, 129.0,
132.6, 135.0, 142.5, 147.7, 149.4, 149.9, 150.2, 161.2, 168.2, 124.2,
AUTHOR INFORMATION
■
Corresponding Author
*Tel: +91-22-25767162. Fax: +91-22-25723480. E-mail: cprao@
iitb.ac.in.
1412
dx.doi.org/10.1021/jo2022372 | J. Org. Chem. 2012, 77, 1406−1413

The Journal of Organic Chemistry
Article
Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.;
Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg,
J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
ACKNOWLEDGMENTS
■
C.P.R. acknowledges financial support from DST, CSIR, and
DAE BRNS. A.M. acknowledges CSIR, and K.T. acknowl-
edges UGC for their fellowships. J.D. acknowledges DRDL
for allowing registration for the Ph.D. program at
IIT Bombay. We also acknowledge FIST (Physics)-IRCC
central SPM facility of IIT Bombay for AFM studies and
CRNTS(SAIF) of IIT Bombay for TEM facility. We thank
Dr. Roymon Joseph for some experimental help.
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