Molecular Details of Acetate Binding to a New Diamine Receptor by NMR and FT-IR Analyses

Article abstract of DOI:10.1021/acs.jpca.6b01078

Acetate anion plays an important role in several biochemical functions such as enzyme reaction, antibody response, and action of receptor molecules. This investigation reports the synthesis and molecular details of a unique receptor, 2-amino-N-(2-amino-be

Full text of DOI:10.1021/acs.jpca.6b01078

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Article
Molecular Details of Acetate Binding to a New
Diamine Receptor by NMR and FT-IR Analyses
Biswadip Banerji, Moumita Chatterjee, Uttam Pal, and Nakul Chandra Maiti
J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 31 Mar 2016
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Molecular Details of Acetate Binding to a New Diamine
Receptor by NMR and FT-IR Analyses
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Biswadip Banerjia*, Moumita Chatterjeea, Uttam Palb, and Nakul Chandra
Maitib*
aDepartment of Organic & Medicinal Chemistry, CSIRꢀIndian Institute of Chemical Biology;
bStructural Biology and BioInformatics, CSIRꢀIndian Institute of Chemical Biology; 4, Raja
S.C. Mullick Road, Kolkata, Country. Indiaꢀ700032; Fax: (+) 91 33 24735197, 91 33
24723967; Tel: (+) 91 33 24995709
Eꢀmail: biswadip.banerji@gmail.com
Eꢀmail: ncmaiti@iicb.res.in
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Abstract
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Acetate anion plays an important role in several biochemical functions such as enzyme
reaction, antibody response and action of receptor molecules. This investigation reports the
synthesis and molecular details of a unique receptor, 2ꢀAminoꢀNꢀ(2ꢀaminoꢀbenzyl)ꢀ
benzamide (R) that senses selectively acetate via simultaneous involvement of one aromatic
amine group and an amide proton of the receptor molecule. Solution state NMR, steady state
fluorescence and FTꢀIR examinations established that the acetate anion binds to the receptor
with 1:1 ratio with high specificity. The binding was stabilized by two Hꢀbonds formation
between the oxygen atoms of acetate anion and two Hꢀatoms, one from amide group and the
other from the amine group of the receptor. The binding interaction caused significant
changes in the chemical shift of the receptor protons and the evaluated affinity constant, from
the NMR measurements was found to be 1.87×104 Mꢀ1. DFT analysis further showed a
significant rotation of one of the two aromatic rings leading to formation of a ten member
ring involving the acetate anion, amide proton and the one amine group attached to aromatic
ring. The Hꢀbond patterns observed in the crystal structure were significantly changed due to
complex formation. However, the changes in the geometrical arrangement in the complex
caused a small but a significant increase of the fluorescence emission. Acetate geometry and
unique positioning of the amide and amine groups of the receptor render the recognition
feasible and DFT analysis estimated ~30 kJ Mꢀ1 stabilization due to 1:1 complexation. Such
positioning and geometrical arrangement may make the receptor very specific to bind acetate
anion and as such became a very relevant molecule in detection and function of the acetate
anion present in complex biochemical systems.
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Introduction
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Anions play a vital role in various chemical and biological phenomenon’s, such as osmotic
pressure maintenance1, activating signal transduction pathways2, maintaining cell volume3,
production of electrical signals etc. Apart from the recognition of anion, artificial receptors
involve in other processes as membrane transmit carrier4, enzyme catalyst synthesis5 etc.
Different signalling pathways, such as photoꢀinduced electron transfer (PET)6,7, excimer or
exciplex8,9, intramolecular charge transfer (ICT)10, metalꢀtoꢀligand charge transfer
(MLCT)11,12, fluorescence resonance energy transfer (FRET)13, excited state intramolecular
proton transfer (ESIPT)14 etc. have been followed during anion sensing. Moreover there is a
high demand for developing new methods of sensing anionic pollutants exist in the
environment. Thus the applicability of anion sensors has spread out from laboratory to in
vitro and in vivo studies. Consequently, design and synthesis of anion sensing receptors
become an area of immense research interest15,16 and several receptors those are able to sense
anions of different kinds are reported.17,18
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In this report we showed a preparation of a unique receptor 2ꢀAminoꢀNꢀ(2ꢀaminoꢀ
benzyl)ꢀbenzamide (R) that was capable to specifically bind acetate anion among many
others. The molecule contains an amide linkage formed between anthranilic acid and 2ꢀamino
benzyl amine, however, only one ring (anthranalic acid moiety) produce fluorescence. The
molecule itself was new (no entries in PubChem or ChemSpider) and showed potential to
selectively bind acetate anion.
Among various significant anionic analytes, acetate anion is the most important owing
to its functions in biochemical, environmental and pharmaceutical science. Production rate of
acetate anion and oxidation have been commonly used as an indicator for decomposition of
organics during marine sediments and transꢀmetalation of tetrapyrroles. Moreover,
carboxylates have been regularly used in paper, plastic, dye, paint as well as in nylon
industry. Acetate anion plays a crucial role in various metabolic processes and in enzyme
action in biological processes. It has a unique potential to form strong Hꢀbond because of its
suitable geometry and basic property and thus it specifically binds some enzyme and
modulates its function.19,20 Its content in physiological environment is therefore very
important. However, the binding and sensing of the ion requires the presence of acidic
hydrogen, due to the basic nature of acetate anion, in the recognition moiety of the receptor.
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Thus, some receptors that are able to detect acetate ion have been reported, however, most of
them contained OH group that bind to the anion.21ꢀ23
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Several spectroscopic methods and techniques are popular to investigate the
receptor/ion interactions including UVꢀVisible spectral changes24,25, colorimetric26ꢀ28 and
fluorescence change.29 Many examples of both colorimetric and fluorescent
chemosensors30,31 for acetate anion are reported. FTꢀIR spectroscopy is another well
recognized method in studying receptor/ligand complex formation via several bond vibrations
and bonding properties.32,33 Similarly, NMR is also frequently used to determine the
molecular details of the binding interaction and receptor anion recognization.34,35 In this
investigation the binding interaction of the receptor with the acetate (tetrabutylammonium
acetate) and other anions were investigated by measuring the spectroscopic (UVꢀvis,
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fluorescence), FTꢀIR and HNMR methods. Further we performed DFT analysis to asses the
geometrical alignment of the receptor/acetate complex and the effect of complexation to the
electronic signature of the receptor molecule.
Materials and Methods
Reagents
All chemicals were purchased from Acros Organics, and used without further purification
unless otherwise stated. Solvents were freshly distilled by the standard procedures prior to
use. Flash chromatography was performed on silica gel (Merck, 60ꢀ120 mesh) with the
indicated eluant. Mass spectra were obtained on a Jeol MS station 700. All 1H and 13CꢀNMR
spectra were recorded on Bruker 300 MHz NMR spectrometer. All FTꢀIR spectra were
recorded on Bruker TENSOR27 spectrometer. Density Functional Theory (DFT) calculation
were performed by using Gaussian 09 revision C.01 and GaussView 5.
Synthesis of receptor R (2-Amino-N-(2-amino-benzyl)-benzamide)
2ꢀnitrobenzoic acid, 1 (0.8 gm, 4.79 mmole) was added to a well stirred mixture of 1ꢀEthylꢀ
3,3ꢀdimethylaminopropyl carbodiimide hydrochloride (1.836 gm, 9.57 mmole) and
hydroxybenzotriazole anhydrous (0.77 gm, 5.69 mmole) in tetrahydrofuran (THF) under
nitrogen atmosphere. After stirring for 5 min, triethylamine (4 ml, 28.75 mmole) was added
under cooling condition followed by 2ꢀaminobenzyl amine 2, (0.643 gm, 5.27 mmole) and
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the reaction was continued for 3hrs at room temperature (monitoring via TLC). The reaction
mixture was concentrated to dryness and then extracted with ethyl acetate (3×20 ml) from
aqueous layer. Evaporation of solvent left the crude product which was purified by column
chromatography over silica gel (hexane:ethyl acetate::70:30) to provide the corresponding
compound 3 as yellow solid (yield = 75%). The nitro group of that generated amide was
reduced by adding Fe powder (0.24 gm, 4.3 mmole) and ammonium chloride (0.59 gm, 11
mmole) to a stirred solution of compound 3 (0.30gm, 1.1 mmole) in aqueous ethanol (1:1)
and the reaction mixture was heated under reflux for 2hrs (monitoring via TLC). The reaction
mixture was concentrated to dryness, further diluted with ethyl acetate and filtered through
celite bed with ethyl acetate. Then ethyl acetate part was washed with brine and dried with
anhydrous sodium sulfate (Na2SO4). Finally, evaporation of the solvent left the crude product
which was purified by column chromatography over silica gel (hexane:ethyl acetate::80:20)
to provide the corresponding pure receptor, R (yield = 55%) as white solid.
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Scheme 1: Schematic of synthesis route of the receptor R. Reagents and Conditions: (i)
EDC.HCl, HOBT, TEA, THF, 0 oC to r.t, 3hrs (ii) Fe powder, NH4Cl, EtOH, reflux, 2hrs.
UV-vis Absorption spectroscopic study: The UVꢀvis absorption spectra of the prepared
receptor R and UVꢀvis titration of the receptor R with anions were acquired by using a
JASCO Vꢀ630 Spectrophotometer (JASCO International Co. Ltd, Japan). The spectra were
scanned in the wavelength range of 200 to 500 nm. A high quality quartz cuvette was used to
record the spectra.
Fluorescence emission and excitation: The intrinsic fluorescence of the receptor R and the
fluorescence titration of the receptor R with anions were measured on an Agilent Technology,
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Cary Eclipse fluorescence spectrophotometer. The excitation wavelengths and emission
measurements were selected according to the sample solution, with both the excitation and
emission slit widths keeping at 5 nm each. Excitation spectra were recorded on Agilent
Technology, Cary Eclipse fluorescence spectrophotometer. The emission wavelengths and
excitation measurements were selected according to the sample nature. A quartz cuvette of (4
cm × 1 cm × 1 cm) volume with a path length of 1 cm was used. All the experiments were
carried out at room temperature (25°C).
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FT-IR: The FTꢀIR spectra of the samples were recorded on a Bruker TENSOR27
spectrometer by the KBr disc technique. Solid sample (receptor R, tetrabutylammonium
acetate) was mixed with KBr in a clean glass pestle and compressed to obtain a pellet. For
recording the FTꢀIR of the mixture of receptor R with tetrabutylammonium acetate, we took
solid receptor R with acetate salt and mixed them at 1:1 ratio, and then KBr added to this
mixture in a clean glass pestle and compressed to obtain a pellet of this mixture. The spectra
were recorded from 400–4000 cmꢀ1 at 4 cmꢀ1 resolution. Background spectra were obtained
with a KBr pellet for each sample. Solution state FTꢀIR spectra were recorded in acetonitrile
solvent. Bruker software was used for data processing. FTꢀIR experiments were carried out at
room temperature (25°C).
1
NMR method: H and 13C NMR spectroscopic data were recorded with a Bruker DPX
300MHz spectrometer. Chemical shifts (δ) were reported in parts per million (ppm) and
tetramethylsilane (δ = 0.00) used as the internal standard. All the 1H and 13C NMR spectra of
the compound recorded in DMSOꢀD6 solvent having 1H NMR, δ = 2.50 ppm (s), 13C NMR, δ
= 40 ppm (m) and all data were reported as follows: chemical shift, integration, multiplicity
(s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet) and coupling constant (s) in
hertz (Hz). All the experiments were carried out at room temperature (25°C).
Crystallization study: Concentrated solution of receptor R (6mg/0.5ml) was prepared in
dichloromethane solvent in a clean sample vial and then 0.2 ml of hexane was added down
the side of the vial to form a discreet layer of two solvent. This sample vial was covered with
a perforated cap and left in unperturbed condition for slow mixing of the two solvent systems
i.e. dichloromethane and hexane. After 4 days, crystals of the receptor R were obtained with
good diffracting quality. Crystal structure was solved by BRUKER KAPPA APEX II Xꢀray
crystallography machine. The data was submitted to Cambridge Crystallographic Database
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and the accession number is CCDC 1400035. The detailed crystallographic data of the
receptor R can be obtained from the accession number CCDC1400035.
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DFT analysis:
Geometry optimization of the receptor R, AcOꢀ and the complex was
performed using density functional theory (DFT) in Gaussian 09 revision C.01. Crystal
structure coordinates for receptor R was used as input structure for the geometry optimization
of R. Acetate and the complex structure was drawn on GaussView 5. The crystal structure of
receptor R was used to draw the complex. Initial approximation of the complex structure is
given in the supporting information. Geometry optimization was done using triple zeta basis
set B3LYP/6ꢀ311+G(2d,p). Coordinates and the absolute energies of the optimized
geometries for receptor R, AcOꢀ and the complex are given in the supporting information.
The vibrational frequency and NMR calculations were performed on the geometry optimized
structures using the same level of theory. For the NMR calculations, gauge independent
atomic orbital (GIAO) method was implemented and TMS was used as reference. Theoretical
IR and Raman spectra for the receptor R, AcOꢀ and the complex are given in the supporting
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information
.
Results and Discussion
We synthesized a new and unique receptor 2ꢀAminoꢀNꢀ(2ꢀaminoꢀbenzyl)ꢀbenzamide (R) with
the aim that it can selectively bind some specific anion, particularly acetate analyte that plays
many specific roles in enzyme function and other cellular activities. Figure 1 shows the
crystal structure and molecular model of the freshly synthesized receptor. The receptor (R)
contains two electron rich aromatic amine groups in two different aromatic ring and the two
aromatic groups are connected each other by a CONH (amide) and CH2 groups. The detail of
the synthesis is provided in the materials and method sections and in the supporting material
sections.
The crystal structure shows two intramolecular Hꢀbonds to the carbonyl group via
hydrogen of NH2 of each group. The measured Hꢀbond distances were 2.108Å (HbꢀꢀꢀꢀO) and
2.359Å (HaꢀꢀꢀꢀO), respectively. An intermolecular Hꢀbond was also observed in the
crystalline form, shown as HcꢀꢀꢀꢀO in the figure 1 with a bond length 2.251Å.
Figure 2 displays the absorption and emission spectra of the receptor R in acetonitrile
solvent. The molecule contains two separate conjugated systems with independent photo
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activity
.
The anthranilic acid (as amide) moiety absorbs at ~325 nm and the peak at ~300 nm
in the absorption spectra is originated from the other conjugated system. Figure 2 also
illustrates the fluorescence emission and excitation spectra of the molecule. The fluorescence
peak appeared at ~396 nm. The excitation spectra showed a presence of excitation peak at
325 nm. The excitation spectrum of receptor R was largely overlapped with the absorption
spectrum (~325 nm). The excitation spectra shows the mirror image relationship of
absorption and indicated that fluorescence was mainly from the first excited state of the
molecule and there was no major change in the structural reorganization in the excited state
of the molecule due to absorption at ~325 nm. We, however, observed no such band near 300
nm in excitation spectra. It clearly indicated that the fluorescence emission was mainly
originated due to absorption of the anthranilamide moiety. UVꢀvis titration of the receptor R
showed small but positive increment of absorbance in the presence of acetate anion in water
medium (Figure 3a).
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Fluorescence being a very sensitive spectroscopic method, binding of acetate to the
receptor was monitored following the intrinsic fluorescence of the receptor molecule. The
fluorescence titration spectra showed minor but positive increment of fluorescence intensity
of the receptor R in the presence of acetate anion (Figure 3b) in water medium. However,
fluorescence of the receptor R was not effected to a significant level in the presence of many
3ꢀ
other anions such as PO4 , CNꢀ, Fꢀ, Clꢀ, Brꢀ, Iꢀ. The anions were added as their
3ꢀ
2ꢀ
tetrabutylammonium salts (Fꢀ, Clꢀ, Brꢀ, Iꢀ, OAcꢀ) and, as potassium salts (PO4 ,SO4 ). It was
observed that the fluorescence intensity was not enhanced significantly in the presence of the
above mentioned anions (Figure 4a) except acetate anion. It indicated that the receptor R
preferred to bind acetate anion compared to others. For further confirmation of the acetate
anion binding with the receptor, we performed fluorescence measurement of the receptor in
the presence of different acetate salts with varying counter cations such as copper acetate,
sodium acetate etc. We observed positive increment of fluorescence intensity of the receptor
(Figure 4b). However, in comparison to tetrabutylammonium acetate (TBOAc), copper
acetate and sodium acetate salts caused less fluorescence increment. It could be due to the
lesser solubility of these acetate salts compared to TBOAc in acetonitrile solvent. Therefore,
it suggested that the fluorescence increment of the receptor R was due to the acetate anion
3ꢀ
only and not for the cations (TBA+, Cu2+, Na+ and K+) and the other anions (PO4 , CNꢀ, Fꢀ,
Clꢀ, Brꢀ and Iꢀ). These fluorescence studies, thus, strongly indicated that the acetate anion
selectively interacted with the receptor R. Further NMR studies as discussed below also
confirmed that the receptor showed selective preference to bind acetate anion.
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Job’s plots (continuousꢀvariation plots) were prepared to confirm the stoichiometry of
the receptor/acetate complex formation. Figure 5 shows the changes in fluorescence against
the molar fraction of the acetate. As shown, the maximum value of the change was observed
at ~0.5 of the acetate. It indicated that receptor molecule and acetate anion formed a 1:1
complex.
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To provide an insight into the interaction between receptor and the acetate we
measured the solution state 1H NMR of the receptor in DMSOꢀd6 in the presence of different
concentration of acetate anion (Figure 6 and Table 1). In the absence of acetate anion the
receptor shows two sharp singlet peaks at 5.13 ppm and 6.41 ppm for Ha and Hb respectively
as the two amine groups are in two different chemical environment. The amide proton (Hc) in
the receptor appeared at 8.63 ppm as triplet and the methylene protons appeared at 4.26 ppm.
The aromatic protons of the receptor arose at 7.50 ppm, 7.14 ppm, 7.02 ppm, 6.95 ppm,
6.695 ppm, 6.615 ppm, 6.50 ppm, 6.50 ppm respectively (Table 1). As shown in Figure 6, the
amine protons signals at 5.13 ppm was broadened and intensity decreased sharply with
increasing acetate (TBOAc) concentration from 0 to 0.5 equiv. It was further diminished up
to addition of 7 equivalent of acetate anion. However, Hb protons were not effected that
much. It suggested that one of the amine groups was interacting with the anion.
Simultaneously noticeable downfield chemical shift was observed in case of amide proton
(NHc) peak from δ 8.63 ppm to δ 9.12 ppm, on addition of acetate anion. Some aromatic
protons shifted by 0.03ꢀ0.09 ppm towards down field and some of the peaks shifted upfield
by 0.02ꢀ0.04 ppm in the presence of the acetate anion to the receptor (Table 1). It was
observed that only two protons i.e, Ha and Hc significantly perturbed and the aromatic
protons was effected slightly in the presence of acetate anion. These changes of proton signal
suggested the binding of acetate ion with the aromatic amine proton (Ha) and the amide
proton (Hc) simultaneously and indicated the possible formation of a complex (ten member
rings) via hydrogen bond formation. Some aromatic proton shift was due to change in
chemical environment of the aromatic ring attached to both amine (Ha) and the amide
(CONHc). Chemical shift of Hb proton was too small to involve in binding on addition of
acetate ion. Although, three types of acidic protons were available in receptor R for acetate
anion binding, but it was observed that only two types of proton i.e, aromatic amine proton
Ha and amide proton Hc participated in bonding with acetate ion. But the other aromatic
amine proton Hb remained inert towards the acetate anion. The particular Hb proton of
N(Hb)2 may fail to attain the required geometrical positioning to form a stable complex with
the anion.
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NMR experiments were also performed in the presence of bromide (Brꢀ) and iodide
(Iꢀ) and we observed no major changes in the proton peak positions of the receptors. (Figure
7 and 8) It indicated that bromide and similar other anions disfavored strong and specific
binding to the receptor molecule. This indicated that the binding was driven not only by the
electrostatic force, but also geometry and size of the acetate anion play a major role.
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NMR analysis in the presence of different concentration of acetate showed that amide
proton (Hc) and protons from one of the two amine groups were effected substantially.
Change in the chemical shift of amine proton (Ha) was less compared to the amide proton
(Hc). However, amine proton (Ha) peak was broadened compared to the amide proton
indicating fast exchange of the amine protons compared to the amide proton (Hc). To
determine the affinity (binding) constant of the receptor R with acetate we plot the difference
of chemical shift of the amide proton (Hc) of the receptor R at different concentration of the
TBOAc by using the double reciprocal equation i.e.
δH0
Kd
1
=
+
ꢁδH
f
[G
]
f
Where δH0 is the chemical shift of the amide proton (Hc) in the absence of TBOAc,
ꢁδH is the difference in chemical shift in the absence and presence of TBOAc at
ꢁδ Hmax δ H0
concentration [G], Kd is the dissociation constant. f=
/
ratio of the change in
maximum shift with the initial shift. From this double reciprocal plot (Figure 9) we get the
dissociation constant (Kd) of the receptor
R with acetate anion which is 53.31 µM and
affinity constant 1.87×104 Mꢀ1.
1
Thus, the solution state H NMR and fluorescence analysis showed that receptor
R
senses selectively acetate ion as compared to other anions. The selectivity of AcOꢀ over other
competing anions of the receptor
may be attributed to the configuration of the AcOꢀ to be
identical with the cavity of receptor as well as the acidity of two protons of the amine
R
R
[N(Ha)2] and the amide (NHc) moiety, respectively. Apart from that, acetate anion is a ‘Y’ꢀ
shaped oxyanion and planar triangular and the amine and amide are also in the same plane
with the acetate anion, the distance of the two oxygen atoms may fit to two hydrogen atoms
on recognition sites of receptor
R. Finally formation of the stable 10ꢀmembered like ring via
hydrogen bonding interaction36,37 between acetate anion and receptor
R
was responsible for
the selectivity of the receptor
R
toward acetate anion.
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We further engaged FTꢀIR spectroscopic analysis38, which is one of the well
recognized methods to study molecular interaction, to know the binding details of the
receptor to the anion. Figure 10 shows the FTꢀIR spectra of the receptor and the acetate and
were compared to the spectra obtained when they are mixed together in the solid state. A
large number of vibrational bands, as expected were observed for both the acetate and the
receptor. The significant FTꢀIR band positions are marked in the figure 10. The band
positions were assigned (Table 2) based on available information in the literatures.39ꢀ53
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The receptor (R) displays FTꢀIR bands due to different vibrations of aromatic ring,
aliphatic groups (ꢀCH2) and functional groups such as ꢀNH2, ꢀCONH (Figure 10a). The major
vibration bands of the free receptor were observed at 3456, 3354, 1620, 1582, 1527, 1455,
1319, 1261, 1157, 753 cmꢀ1. The bands in the region between 3122ꢀ3500 cmꢀ1 are originated
mainly due to symmetric (lower frequency) and asymmetric stretching vibrations of the two
NH2. 3456 cmꢀ1 band could be due to asymmetric stretching motion of the aromatic NH2
groups. The molecule contains two NH2 groups and the vibration modes of the NꢀH protons
may be overlapped. The molecule also contains one amide NꢀH bond (Figure 10a) and it
showed a single vibration band in this region and overlapped with the bands originated for
symmetric stretching of the NH2 groups. The IR band at 3354 cmꢀ1 accompanied a well
detected hump at 3340 cmꢀ1. The 3354 cmꢀ1 band could be due to symmetric stretching mode
of the amine hydrogens. The band was quite broad compared to the asymmetric stretching
band (3456 cmꢀ1) and contained shoulder/hump at 3340 cmꢀ1 which could be amide mode of
vibrations that assigned to the amide NH vibrations of the –CONH group. The band at 2941
cmꢀ1 was assigned to CꢀH stretching vibrations of the methylene group present in the
molecule. Possible aromatic skeletal vibrations appeared at 1582 cmꢀ1. The vibration bands at
1620 cmꢀ1 was due to amide C=O stretching mode. The band at 1527 cmꢀ1 was assigned to Cꢀ
H bending vibration of the ꢀCH2ꢀ group. Amide II vibrations due to CONH also contribute to
this region.
The FTꢀIR spectrum of solid tetrabutylammonium acetate is also shown in Figure
10b. Assignment of some of these bands is provided in Table 2. The band at 2961 cmꢀ1 was
assigned to asymmetric stretching vibration of methyl and methylene groups in the alkyl
chain. The symmetric stretching vibration of methyl and methylene groups was observed at
2875 cmꢀ1. The CꢀH asymmetric bending vibration due to methyl and methylene group was
observed at 1472 cmꢀ1. The frequency at ~1166 cmꢀ1 could be due to methyl and methylene
twisting modes of vibration. 1109 cmꢀ1 was due to rocking mode of the methyl group. 889
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cmꢀ1 was assigned to asymmetric CꢀCꢀC vibration. Inꢀplane bending vibration of methylene
group and CꢀNꢀC and CꢀCꢀN deformation contribute to the band at 738 cmꢀ1. A broad band
that appeared at 3420 cmꢀ1 was possibly due to moisture present in the sample. The C=O
asymmetrical stretching was appeared at 1565 cmꢀ1 and the CꢀO symmetrical stretching52
was assigned to peak at 1410 cmꢀ1.
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When both the receptor and the anion mixed together, the spectrum pattern changed
and the analysis showed the acetate ion strongly interacts with the receptor molecules. The
amide I band of the receptor was noticed to be shifted from 1620 cm−1 (in free receptor) to
1626 cm−1 in the receptor
R/acetate complex. Amide I band arises mainly due to the C=O
stretching vibration. However, some minor contributions come from the outꢀofꢀphase CꢀN
stretching motion, CꢀCꢀN deformation and the NꢀH inꢀplane bending motion also contribute
to the amide I band to some extent. The increasing of C=O stretching in amide I mode of
vibration was due to the strengthening of the carbonyl group in the amide bond. The
molecular vibration frequency depends on its bond length and strength (force); hydrogen
bonding in general lowers the frequency of stretching vibrations of carbonyl group (C=O) as
it lowers the restoring force that involved with the bond. The same phenomenon may
increase the frequency of bending vibrations since it adds some amount of additional
restoring force. Free receptors in solid state remains hydrogen bonded and the amide
carbonyl group is involved in the interꢀ and intraꢀ molecular hydrogen bonding. When the
receptor forms complex with acetate the amide carbonyl group becomes free and the
stretching frequency increases (1620 cmꢀ1 to 1626 cmꢀ1). Thus, the blue shift in the C=O
stretching frequency could be correlated to the loss of two hydrogen bonds observed in the
crystalline solid receptor
R molecule involving this carbonyl and one aromatic amine group
and one amide group (Figure 1b). On the other hand, the FTꢀIR spectra of the receptor in
acetonitrile showed amide carbonyl vibration at 1639 cmꢀ1. In the complex formed with the
acetate in acetonitrile solvent the amide carbonyl stretching frequency of the receptor was
slightly red shifted to 1636 cmꢀ1 (Figure S1). In solution state the carbonyl group of the
receptor may not form two strong hydrogen bonds as observed in the crystal structure (solid
state). In solution state, due to complex formation with the acetate anion through amide NH,
the amide carbonyl group became weak and showed shift to low frequency range. Our
theoretical calculation also suggested weakening of the C=O bond of the receptor due to
complex formation. However, in acetonitrile, the stretching frequency of C=O of acetate was
blue shifted from 1583 cmꢀ1 to 1588 cmꢀ1 after complex formation.
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In order to get better insight into the structural aspects of receptor
R and acetate
interactions, we also performed theoretical studies at density functional level of theory.54
Hydrogenꢀbonded complex of the receptor, acting as two proton donating group
with the acetate anion that acted as proton accepting moiety was modeled using
DFT with triple zeta basis set B3LYP/6ꢀ311+G(2d,p). The optimized structural information
and the coordinates are given in the supporting information.
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The acetate and receptor
R complex were found to get stabilized in the complex by
two hydrogen bond formation (Figure 11a). The hydrogen bond length was measured as the
distance between the proton from the N(Ha)2 and NHc group and the acetate oxygen atoms as
shown in figure 11a. The Hꢀbond length between the amide proton and the acetate oxygen
was 1.82 Å, NꢀHꢀꢀꢀꢀꢀO angle 169°, H—O=C angle 122°. A similar bond length was observed
with amine hydrogen as Hꢀbond length 1.83 Å, NꢀHꢀꢀꢀꢀꢀO angle 164°, H—O=C angle 124°.
The Hꢀbond lengths indicated very strong binding and free energy change was calculated due
to complex formation via two hydrogen bonding was found to be ꢀ29.58 kJ/mol. The
geometrical arrangement of the receptor and the acetate was such that one of the aromatic
ring rotate sufficiently to form the complex. The other amine group not changed much.
The orbital energies of the free receptor and the complex were calculated using
density functional theory with B3LYP/6ꢀ311+G(2d,p) basis set. The HOMOꢀLUMO band
gap energies in the free receptor R and in the complex were compared (Figure 11b). It was
found that the HOMOꢀLUMO energy difference was 4.18 electron volts in the free receptor
R, whereas, in the complex it was increased to 4.23 electron volts. Such change in the
HOMOꢀLUMO band gap suggested ~4 nm blue shift in the absorption maximum. However,
major change in absorption spectra was not observed when it formed complex with the
acetate anion. In the solution state the absorbance bands at 300 and 325 nm of the receptor
R
was slightly enhanced upon gradual addition of the acetate anion to the receptor solution
(Figure 3a), however, no significant changes in absorption peak position was observed.
Vibrational frequency via DFT analysis was also performed on the optimized
geometries of the receptor, acetate anion and the complex. C=O, NꢀH and NH2 stretching
frequencies as obtained by theoretical analysis are listed in Table 3. The detailed theoretical
vibrational spectra are given in the supporting information. The theoretical vibrational mode
calculations enabled us to assign the peaks in FTꢀIR spectra to some extent. The vibration
bands at 1669 cmꢀ1 was due to amide C=O stretching mode of the receptor. The frequency
was shifted to 1658 cmꢀ1 due to complexation of the acetate to the receptor, which makes the
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carbonyl bond weaker. In the experimentally obtained FTꢀIR spectra, amide C=O stretching
was observed at 1620 cmꢀ1. In the complex, the amide bond vibration was affected and the
C=O stretching band shifted to 1626 cmꢀ1. In solid state, the receptor molecule forms intraꢀ
and interꢀ molecular strong hydrogen bonds and the amide carbonyl group is directly
involved in the hydrogen bond formation (Figure 1). When the receptor forms complex with
acetate, the amide carbonyl group becomes free (from Hꢀbond formation) and the stretching
frequency increases. In the complex, amide NH group was directly involved in the acetate
binding and its effect to the carbonyl bond vibration may be less compared to the strong
hydrogen bonds observed in the crystalline structure of the receptor.
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Solution state FTꢀIR experiment showed that the amide carbonyl group frequency
appeared at 1639 cmꢀ1 (Figure S1). It was slightly weakened when the receptor formed
complex with acetate in acetonitrile. This effect could be attributed to the involvement of the
NH of the amide group as suggested earlier. The DFT calculation was performed in vaccuo
with a single receptor molecule and the amide carbonyl stretching frequency was high (Table
3). The complex formation with acetate, which involves the amide NH, weakens the amide
carbonyl stretching. DFT analysis also showed that the symmetric stretching frequency of
acetate blue shifted upon complex formation (Table 3) which was consistent with the solid
state and solution state FTꢀIR experiments (Figure 10 and S1). We observed that stretching
frequency of the bound acetate in solution state appeared at 1588 cmꢀ1 and the free acetate
showed the peak position at 1583 cmꢀ1 (Figure S1).
The bands in the region between 3000ꢀ3700 cmꢀ1 were originated mainly due to
symmetric (lower frequency) and asymmetric stretching vibrations of the two NH2 groups
present in the molecule. The NH2 has two major bands between 3000ꢀ4000 cmꢀ1, symmetric
stretching frequency centered near 3400 cmꢀ1 and asymmetric stretching frequency appeared
near 3480 cmꢀ1 and it depends on the characteristics of attached groups. The bands at 3466
cmꢀ1 and 3521 cmꢀ1 originated due to symmetric stretching motion of the aromatic NH2
groups of the receptor. This band was also observed in the experimentally obtained FTꢀIR
spectra. In the complex we observed a complex behavior of NH2 bond vibrations. The
molecule interacted with the hydrogen atom of NH2 and other bands shifted. The molecule
contained one amide NꢀH group, which showed a stretching vibration at 3209 cmꢀ1 when
in complexed with the acetate anion. The band at 3639 and 3664 cmꢀ1 was due to the
asymmetric stretching of NH2. The amide II (NH bending vibration) also red shifted. In the
expanded spectra (Figure S2) we observed additional bands (3301 cmꢀ1) along with the bands
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observed in the free receptor. These red shifts could be due to involvement of amide protons
in the Hꢀbond formation that provide the stability to the receptor/acetate complex formation.
Comparison of the fingerprint regions of the FTꢀIR spectra also showed very conclusive
results about the complex formation of the receptor with acetate (Figure S3).
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Three principle approaches have been used for detecting anion.15,17,55,56 Paul Beer, the
pioneer of this field developed the concept of anion recognition through hydrogen bonding
between the receptor and the anionic guest.17 This type of anion sensor consists of anionꢀ
binding site containing hydrogen bond donor groups that are attached to a fluorescent group.
When an anion bound to that hydrogen bond donor group, the electronic properties of the
fluorescent group were perturbed resulting in a change in fluorescent properties of the
receptor as a whole leading to anion detection. Another significant strategy used in anion
sensing is the displacement assay.55 This method, developed by Eric Anslyn, involves
complex formation between an indicator and a receptor via nonꢀcovalent interactions. The
whole phenomenon of the anion binding with the receptor and the displacement of the
indicator actually changed the electronic environment around the indicator resulting in
perturbations to its fluorescent properties thus allowing the anion to be detected. The other
effective mechanism for anion sensing using small molecules is chemical reaction that will
generate a new species with different chemical properties.56 Recognition of anions also
follows through electrostatic interactions57 and coordination through metal ions58 etc.
Hydrogen bonding involving NH and OH functional group59,60 provides ample stabilization
in the anion recognition and complex formation. It was found that the phenolic OH ionized
and formed a strong complex in many receptor/anion complexes, however the amide NH
group mostly stabilize the complex via strong Hꢀbond formation and was less susceptible to
deprotonation compared to phenolic OH group present in the receptor.
Our investigation observed hydrogen bond formation61,62 with the amide NH
hydrogen and the acetate anion. In addition, we observed that amine (NH2) hydrogen also
was equally involved and added stability to the complex formation. Our investigation thus
revealed that the amide and amine groups are simultaneously involved in binding the acetate
ion and geometrical arrangement added specificities to bind the acetate anion through
formation of two strong Hꢀbonds.63ꢀ68
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Conclusion
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Recognition and sensing of anions is a critical event in many chemical and biological
processes. In the present study, we have shown effective synthesis of a receptor that
specifically binds acetate anion. We also established its binding details. The molecule is
unique and between the two aromatic amines present in the receptor, only one amine is
involved in the binding and the other one is remained unperturbed. Formation of a ten
member ring in the receptor/ligand complex is rare. DFT analysis clearly showed feasibility
of formation of a ten member ring involving two strong Hꢀbonds between the amine and
amide protons of the receptor molecules with acetate anion. NMR, FTIR analysis supported
this hypothesis and established the acetate sensing property. Other reported acetate sensing
receptors mainly bind to the anions via hydroxyl or amides. This receptor is unique as it
could effectively sense the acetate by forming two unique hydrogen bonds with the amine
and amide protons and formed a ten member cyclic ring structure.
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Supporting information: The supporting information section includes Figure S1—S3,
NMR, IR, HRMS characterization data of the receptor R, all the NMR spectra for titration
experiments, coordinates of the optimized geometries, theoretical IR and Raman spectra of
acetate anion, receptor R and the complex of receptor R with acetate anion. This information
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Correspondence should be addressed to *Biswadip Banerji: phone, (+) 91 33 24995709; fax,
(+) 91 33 24735197; email, biswadip.banerji@gmail.com, address: Department of Organic &
Medicinal Chemistry, CSIRꢀIndian Institute of Chemical Biology; 4, Raja S.C. Mullick Road,
Kolkata, and Country. Indiaꢀ700032
*Nakul Chandra Maiti: phone, +91ꢀ33ꢀ2499ꢀ5940; fax, +91ꢀ33ꢀ2473ꢀ5197; email,
ncmaiti@iicb.res.in, address: Structural Biology and BioInformatics, CSIRꢀIndian Institute of
Chemical Biology; 4, Raja S.C. Mullick Road, Kolkata, and Country. Indiaꢀ700032
Author Contributions
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The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript.
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
Moumita Chatterjee thanks University Grants Commission, India for financial support. Uttam
Pal thanks INSPIRE fellowship program, Department of Science and Technology,
Government of India, India for financial support. The authors would also like to thank the
central instrumentation facilities of CSIRꢀIndian Institute of Chemical Biology for recording
the spectra.
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Sensing in Aqueous Media Using a Novel and Simple Colorimetric Sensor and Its Analytical
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of Molecular Probes Based on a Phenanthroline Derivative: Theory and Experiment.
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Chemosensor Containing a Tripodal Benzaldehydicꢀphenylhydrazone. Chinese J. Chem.
2008, 26, 611ꢀ614.
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(27) Jakusová, K.; Donovalová, J.; Cigáň, M.; Gáplovský, M.; Garaj, V.; Gáplovský, A.
Isatinphenylsemicarbazones as Efficient Colorimetric Sensors for Fluoride and Acetate
Anions – Anions Induce Tautomerism. Spectrochim. Acta A 2014, 123, 421ꢀ429.
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(28) Shi, D.ꢀQ.; Wang, H.ꢀY.; Li, X.ꢀY.; Yang, F.; Shi, J.ꢀW.; Wang, X.ꢀS. Novel N,N′ꢀ
DiacylhydrazineꢀBased Colorimetric Receptors for Selective Sensing of Fluoride and Acetate
Anions. Chinese J. Chem. 2007, 25, 973ꢀ976.
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(37) Maity, S. K.; Bera, S.; Paikar, A.; Pramanik, A.; Haldar, D. Fabrication of Selfꢀ
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Ibrahim, M.; Nada, A.; Kamal, D. E. Density Functional Theory and FTIR
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Mineral Hoganite. Spectrochim. Acta A 2007, 67, 48ꢀ57.
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Study of the Dipeptides AcꢀPheꢀSerꢀNH2 and AcꢀPheꢀCysꢀNH2: Probing the SH...O and
OH...O Hydrogen Bond Interactions. Phys. Chem. Chem. Phys. 2014, 16, 10770ꢀ10778.
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Benzyl Amine) and the Copolymers with Aniline. Study with Cu(II). J. Appl. Polym. Sci.
2004, 92, 31ꢀ36.
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Spectroscopic Investigation of LꢀAlanylꢀLꢀGlutamine Dipeptide. Spectroscopy 2010, 24,
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Infrared Spectra and Hydrogen Bonding in Polycrystalline DLꢀSerine and Deuterated
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Montmorillonite (TBAM) Clay: Thermodynamic and Kinetic Calculations. J. Colloid Interf.
Sci. 2006, 296, 16ꢀ21.
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Studies of HDO Spectra. J. Phys. Chem. B 1999, 103, 3778ꢀ3783.
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Spectroscopy and Hydrogen Bonding in Cytosine Polycrystals. Spectrochim. Acta A 2004, 60,
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Containing OH and NH Recognition Subunits: A Fluoride Ion Selective Colorimetric Sensor.
New J. Chem. 2013, 37, 1591ꢀ1594.
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Hydrogen Bond at High and Low Temperatures. J. Mol. Struct. 1978, 47, 323ꢀ328.
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Hydrogen Bond in Aromatic Amine Complexes of Ethers Ketones, Esters, and Amides. J.
Phys. Chem. 1967, 71, 1421ꢀ1429.
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Constants. Prog. Nucl. Mag. Res. Sp. 2007, 51, 219ꢀ242.
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Ions. Can. J. Chem. 1956, 34, 170ꢀ178.
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Trivedi, M. K.; Branton, A.; Trivedi, D.; Nayak, G.; Bairwa, K.; Jana, S. Fourier
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Merocyanine 540: A Spectroscopic Investigation Supported by Docking. Dyes Pigments
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Anion Recognition by πꢀAcidic Rings. J. Am. Chem. Soc. 2002, 124, 6274ꢀ6276.
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Amine Complexes as Receptors. Chem. Soc. Rev. 2013, 42, 1681ꢀ1699.
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and CatecholꢀBased Ruthenium(II) Polypyridyl Complexes as Colorimetric Sensors for
Fluoride Ions. Inorg. Chem. 2007, 46, 5576ꢀ5584.
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Anion Sensor Based on Receptor Having Indoleꢀ and ThioureaꢀBinding Sites. RSC Adv.
2014, 4, 20592ꢀ20598.
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Ratiometric Fluoride Ion Sensors. RSC Adv. 2014, 4, 56466ꢀ56474.
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Figure legends
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7
Figure 1. Molecular model and crystal structure of the receptor. (a) Molecular model of the
receptor , 2ꢀAminoꢀNꢀ(2ꢀaminoꢀbenzyl)ꢀbenzamide, (b) Showing the possible intra and
8
9
R
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inter molecular hydrogen bonding between two unit crystals of receptor R.
Figure 2. UVꢀvis absorption and emission profile of the receptor. (a) Absorption and
fluorescence behavior of receptor R in the UV–Visible range. Absorption spectrum of
receptor
R
(c = 2×10ꢀ5 M) in acetonitrile solvent is shown in blue. Fluorescence emission and
are shown in yellow and green, respectively. Fluorescence
was recorded in the acetonitrile and normalized against its absorption
excitation spectra of receptor
R
spectra of receptor
spectrum.
R
Figure 3. UVꢀvis absorption and emission of the receptor in presence of acetate. (a)
Absorbance spectra of receptor R (c = 2×10ꢀ5 M) in H2O upon titration with
tetrabutylammonium acetate solution (c= 2 ×10ꢀ4 M). (b) Fluorescence spectra of receptor R
(c = 2×10ꢀ5 M) upon titration with tetrabutylammonium acetate (c= 2 ×10ꢀ4 M) in H2O.
Figure 4. Effect of different ions on receptor fluorescence. (a) Bar representation of
changes of fluorescence intensity of receptor R upon titration with the different guest anions
(for acetate anion c = 50µl × 2 × 10ꢀ4 M and other anions c = 200µl × 2 × 10ꢀ4) in acetonitrile.
(b) Bar representation of changes of fluorescence intensity of receptor R upon titration with
the different acetate anion salts (salts concentration = 10µl × 1 × 10ꢀ4 M) in acetonitrile.
Figure 5. Jobs plot diagram of receptor
R with tetra butyl ammonium acetate anion (where
Xh is the mole fraction of acetate and dI Xh indicates the change of the fluorescence).
Figure 6. 1H NMR spectrum of receptor
concentration of tetrabutylammonium acetate. DMSOꢀd6 solvent was used.
R in the absence and presence of different
Figure 7. 1H NMR spectrum of receptor
concentration of tetrabutylammonium bromide. DMSOꢀd6 solvent was used.
R in the absence and presence of different
Figure 8. 1H NMR spectrum of receptor
R in the absence and presence of different
concentration of tetrabutylammonium iodide. DMSOꢀd6 solvent was used.
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Figure 9. Double reciprocal plot for determination of affinity constant (Ka) of the receptor
R
with acetate anion through proton chemical shift with respect to acetate anion concentration.
Here δH0 is the chemical shift of the amide proton (Hc) in the absence of TBOAc, ꢁδH is the
difference in chemical shift in the absence and presence of TBOAc at concentration [G].
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Figure 10. FTꢀIR spectra of the receptor and the complex in solid state. (a) FTꢀIR spectra of
R, (b) FTꢀIR spectra of TBOAc, (c) FTꢀIR spectra of the mixture of receptor
R with TBOAc.
Figure 11. DFT analysis of the complex. (a) Optimized structure of the complex of receptor
with acetate anion, (b) HOMOꢀLUMO band gap of the free receptor and bounded receptor
R
with acetate anion.
Figure 12. Geometrical orientation of the complex. (a) molecular model of the free receptor
R
and the orientation of different functional groups present in the receptor shown in the
crystal structure, (b) Change in orientation of different functional groups of receptor upon
R
binding with acetate anion shown by molecular modeling and optimized structure derived
from DFT calculation.
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Tables
Table 1. Chemical shift of receptor (R) protons in the absence and after addition of
5
6
7
tetrabutylammonium acetate (TBOAc)
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9
Proton
type
Only
R + 0.5
R + 1 eq R + 2 eq R + 3 eq R + 5 eq R + 7 eq
receptor eq*
R (ppm) TBOAc
(ppm)
TBOAc TBOAc
TBOAc
(ppm)
TBOAc
(ppm)
TBOAc
(ppm)
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60
(ppm)
5.16
(ppm)
5.18
Ha
5.13
5.15
5.19
5.24
5.24 or
diminish
ed
Hc
Hb
8.63
6.41
8.70
6.41
4.25
8.76
6.42
4.25
8.83
6.43
4.25
8.88
6.44
4.25
9.06
6.45
4.245
9.12
6.44
Methylene 4.26
protons
4.24
Ar1
Ar2
Ar3
Ar4
Ar5
Ar6
Ar7
Ar8
7.50
7.14
7.02
6.95
6.69
6.61
6.50
6.50
7.51
7.13
7.02
6.95
6.69
6.61
6.50
6.50
7.525
7.13
7.03
6.94
6.69
6.61
6.50
6.50
7.535
7.12
7.03
6.94
6.68
6.60
6.49
6.49
7.54
7.12
7.03
6.93
6.68
6.60
6.48
6.48
7.58
7.11
7.05
6.92
6.68
6.60
6.47
6.47
7.59
7.10
7.05
6.92
6.68
6.60
6.46
6.46
*eq = equivalent
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Table 2. FTꢀIR bands of the receptor
R and tetrabutylammonium acetate in solid state
Receptor R
Tetrabutylammonium acetate
vibration mode
IR
vibration mode
IR
7
8
9
frequency
(cmꢀ1)
3456
frequency
(cmꢀ1)
3420
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asymmetric stretching of
aromatic NH2 groups
moisture
3354
3340
symmetric stretching of
aromatic NH2 groups
2961
2875
1565
CꢀH asymmetric stretching
vibrations of the CH2 and CH3
group
amide NH symmetric
stretching
CꢀH symmetric stretching
vibrations of the CH2 and CH3
group
3031
2941
aromatic CꢀH stretching
C=O asymmetric stretching
CꢀH asymmetric bending
vibrations of the CH2 and CH3
group
CꢀH stretching vibrations of 1472
the methylene group
1620
1582
amide C=O stretching mode 1410
aromatic skeletal vibrations 1166
CꢀO symmetrical stretching
twisting mode of vibrations of the
CH2 and CH3 group
1527
CꢀH bending vibration of
the ꢀCH2ꢀ group
1109
rocking mode of vibration of CH3
1455
1319
1261
1157
aromatic C=C stretching
CꢀH bending deformation
CꢀN stretching
1063
1029
923
rocking mode of vibration of CH2
asymmetric CꢀCꢀN vibration
CꢀC stretching
twisting mode of vibrations 889
of the CH2 group
asymmetric CꢀCꢀC vibration
753
679
NꢀH wagging
738
in plane bending of CH2 group, Cꢀ
NꢀC, CꢀCꢀN deformation
COOꢀ deformation rocking
aromatic CꢀH Bending
652
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Table 3. Infraꢀred/Raman frequency of the receptor
R
and tetrabutylammonium acetate when
free and in complex as computed by DFT using B3LYP/6ꢀ311+G(2d,p) basis set.
6
7
8
9
Acetate
Free
Receptor R
Free
Modes
In complex
638.19
Modes
In complex
1658.04
3209.82
3251.06
3421.78
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O=C=O
863.97
C=O stretching 1669.25
NH stretching 3634.59
scissoring
O=C=O
1346.41
1397.59
1400.95
NH2
3466.50
3521.05
symmetric
stretching
O=C=O
symmetric
stretching
NH2
1636.60
1597.66
1621.14
3639.98
3664.88
3621.49
3633.54
asymmetric
stretching
asymmetric
stretching
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Figure 1
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