Naphthyridine amide-urea conjugate: A case toward selective fluorometric sensing of N-acetyl proline carboxylate

Article abstract of DOI:10.1007/s10847-011-9929-2

A simple neutral naphthyridine-based chemosensor 1, which selectively recognizes the tetrabutylammonium salt of N-acetyl-l-proline over the other N-acetyl-l-amino acid salts studied in CHCl₃ containing 0.1% DMSO, has been designed and synthesiz

Full text of DOI:10.1007/s10847-011-9929-2

J Incl Phenom Macrocycl Chem (2011) 71:243–248
DOI 10.1007/s10847-011-9929-2
SHORT COMMUNICATION
Naphthyridine amide–urea conjugate: a case toward selective
fluorometric sensing of N-acetyl proline carboxylate
•
Kumaresh Ghosh Tanmay Sarkar
Received: 17 May 2010 / Accepted: 19 January 2011 / Published online: 6 February 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A simple neutral naphthyridine-based chemosen-
sor 1, which selectively recognizes the tetrabutylammonium
salt of N-acetyl-L-proline over the other N-acetyl-L-amino acid
salts studied in CHCl3 containing 0.1% DMSO, has been
designed and synthesized. Moreover, the complexation-
induced change in emission characteristics of 1 distinguishes
the amino acid salts examined from their conjugate acids.
Interaction studies were performed by UV–vis, fluorescence
and NMR spectroscopic methods.
these substrates is a field of research in supramolecular
chemistry [13–17]. Conversion of such acids into their
carboxylate salts and their recognition by designed syn-
thetic receptors is of continued interest in our laboratory
[18–21]. In relation to this, we wish to report here a simple
naphthyridine-based fluororeceptor 1, which shows a
preference for the tetrabutylammonium salt of N-acetyl-L-
proline over other N-acetyl-L-amino acid salts examined by
exhibiting complexation-induced greater change in
emission.
Keywords Naphthyridine Á Urea binding site Á
Proline salt recognition Á Fluorometric distinction Á
DFT calculation
O
O
N
N
H
H
HN
Design and synthesis of artificial receptors for a specific
function is of utmost interest in the area of supramolecular
chemistry [1–3]. During last decades, a great deal of
attention has been focused on the recognition of neutral [4],
cationic [5, 6] and anionic substrates [7] by abiotic
receptors. Of the substrates, anions are important because
of their significant roles in biology and environment
[8–11]. Among the different kinds of anions, carboxylates
are important as target species in molecular recognition as
they serve important functions in biological systems [12].
Carboxylic acid functional group is present in amino acids,
keto and hydroxy acids etc. and therefore, recognition of
N
1
N
The receptor 1 was obtained according to the Scheme 1.
Initially, methyl m-aminobenzoate was coupled with
1-naphthylamine in the form of urea linkage to give com-
pound 2 using triphosgene as the reagent that converts
amine into isocyanate functional group in situ. The ester
group of 2 was next hydrolysed in the presence of LiOH in
aqueous methanol to give the acid 3. DCC coupling of
2-amino-7-methyl-1,8-naphthyridine with the acid 3 finally
afforded the desired compound 1 in 56% yield.
Electronic supplementary material The online version of this
article (doi:10.1007/s10847-011-9929-2) contains supplementary
material, which is available to authorized users.
All the compounds were unequivocally characterized by
1H NMR, 13C, FTIR and mass analysis.
Naphthyridine is a unique motif that plays important
role in molecular recognition of various biologically
important substrates [22–24]. Use of this motif as hydrogen
bonding building block is well established in the literature.
K. Ghosh (&) Á T. Sarkar
Department of Chemistry, University of Kalyani,
Kalyani, Nadia 741235, India
e-mail: ghosh_k2003@yahoo.co.in
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LiOH, methanol-water
rt, 4h, 80%
CO2Me
1-Naphthylamine, triphosgene
Et3N, dry CH2Cl2, rt, 5h, 68%
O
O
H2N
CO2Me
N
N
CO2H
N
N
H
H
H
H
2
3
2-amino-7-methyl-1,8-naphthyridine
DCC, DMAP, dry CH2Cl2-DMF, 19h, 56%
1
Scheme 1 Synthesis of receptor 1
During the course of our work in molecular recognition, we
used this motif in the sensing of hydroxy acid such as citric
acid [25]. Inspiring results prompted us to take further this
motif in the construction of new synthetic molecular
architecture 1 for sensing and distinguishing the amino acid
derivatives in organic solvent. In the design, naphthyridine
amide has been considered as (i) hydrogen bonding site for
the binding of carboxylic acid of amino acids and also (ii)
fluorophore. In the present design it is connected to the
naphthalene-based urea motif, which also provides both
hydrogen bond donors and fluorophore unit. However, they
are well disposed in a concave face to provide the recog-
nition surface for tetrabutylammonium salts of amino
acids.
400
300
200
100
0
In order to understand the recognition behavior of 1
towards the tetrabutylammonium salts of N-acetyl-L-amino
acids such as valine, alanine, proline, phenyl glycine,
UV–vis, fluorescence and 1H NMR studies were per-
formed. In addition, the hydrogen bonding behavior of 1
with pyruvic acid and its salts was studied under similar
condition.
350
400
450
500
550
Wavelength (nm)
Fig. 2 Change in emission of 1 (c = 4.83 9 10-5 M) upon gradual
addition of tetrabutylammonium salt of N-acetyl-L-proline
pyruvic acid
Pyruvate
The receptor solution of 1 in CHCl3 containing 0.1%
DMSO gave emissions at 362 and 452 nm when excited at
290 nm. The emission at 362 nm is attributed to the
naphthalene chromophore and emission at 452 nm is
assigned to the naphthyridine motif. These two emissions
are well resolved and distinct to interpret. Upon addition of
the anionic guests to the receptor solution of 1, the emis-
sion changed to the different extents. Figure 1 shows the
change in emission of 1 upon addition of 1 equivalent
amount of a particular anionic species. In comparison, the
emission of 1 was merely perturbed when corresponding
acid analogue of each carboxylate guest was added (Fig. 1)
to the receptor solution. Among all the guests, salt of
N-acetyl proline showed a marked change in emission. The
complexation induced decrease in emission of 1 in the
presence of N-acetyl-L-proline salt is represented in Fig. 2.
The monomer emissions of both naphthyl and naphthyri-
dine motifs underwent significant change suggesting the
involvement of the bonding sites in complexation of the
guest. The corresponding N-acetyl-L-proline weakly per-
turbed these two emissions although the receptor binding
sites have the complementarities with the guest acid
N-acetyl L-Phenyl glycine
salt ofN-acetyl L-Phenyl glycine
N-acetyl L-Alanine
Salt of N-acetyl L-Alanine
N-acetyl L-Valine
Salt of N-acetyl L-Valine
N-acetyl L-Proline
Salt of N-acetyl L-Proline
Acetate
0.00
0.05
0.10
ΔI/Io
0.15
0.20
Fig. 1 Change in fluorescence ratio of 1 upon addition of one
equivalent of anions (c = 4.83 9 10-5 M) at 362 nm
(supporting information in Supplementary material).
Interestingly, while the change in emission of 1 at 452 nm
was greater upon addition of N-acetyl a-amino carboxylic
acids, the change in emission at 362 nm was much sig-
nificant upon adding N-acetyl a-amino carboxylates. This
observation probably indicated the relative preference of
these two different guests towards the binding domains
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J Incl Phenom Macrocycl Chem (2011) 71:243–248
245
Fig. 3 Different modes of
O
binding of N-acetyl amino
carboxylic acids and N-acetyl
amino carboxylate
O
O
O
O
N
N
N
O
N
O
N
N
N
H
H
N
N
HN
H
H
HN
H
H
HN
OH
O
O
-O
O
N
1
N
N
NH
R
HN
H
R
H
Carboxylate complex
Carboxylic acid complex
250
200
150
100
50
4.0x10-6
3.0x10-6
2.0x10-6
(---)Acetate
(
(
(
(
(
(
(
(
(
(
)N-acetyl L-Alanine salt
)N-acetyl L-Alanine
)N-acetyl L-Valine salt
)N-acetyl L-Valine
)N-acetyl L-Proline salt
)N-acetyl L-Proline
)N-acetyl L-Phenyl glycine salt
)N-acetyl L-Phenyl glycine
)Pyruvate
)Pyruvic acid
1.0x10-6
0.0
0
0
2
4
6
8
10
12
[G]/[H]
Fig. 4 Fluorescence titration curves for 1 (c = 4.83 9 10-5 M) with
the guests in CHCl3 containing 0.1% DMSO at 450 nm
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
XH
Fig. 5 Fluorescence Job plot for 1 with N-acetyl-L-proline salt in
CHCl3 containing 0.1% DMSO ([H] = [G] = 2.66 9 10-5 M)
(urea and naphthyridine amide) of 1. Thus the different
modes of binding of 1 for a-amino acids and its salt are
demonstrated in Fig. 3.
The greater nucleophilic character of anionic guests over
their neutral analogues has a clear-cut reflection on this
aspect. The change in emission of 1 was also noted upon
addition of tetrabutylammonium salts of pyruvic and acetic
acids. Between the two, acetate ion interacted strongly. We
believe that this is presumably due to more basic character
and less steric nature of acetate ion.
also showed 1:1 stoichiometries (supporting information in
Supplementary material). For the carboxylic acids, emis-
sion of 1 was barely affected up to the addition of 1
equivalent amount; further addition caused change in
emission although weakly.
Analysis of the emission titration results allowed us to
determine the binding constant values. Non-linear curve
fitting method [27] using Origin 6.1 gave the binding
constant values. The results are summarized in Table 1,
and demonstrate that anionic guests bind with moderate
values. Among the anions, salt of N-acetyl-L-proline
exhibited higher binding value compared to others. For
pyruvate, 1:1 model did not fit well. However, 2:1 model
fits properly. This situation happens due to the cooperative/
non-cooperative action of the amide and urea moieties in
the binding process. Structural aspect of the guest species
may have the contribution in this regard. To our opinion,
initial binding of N-acetyl amino acid salt to the urea motif
(mode A) presumably activates the amide in further
bonding either in the mode B (cooperative mode) or in
mode C (non-cooperative mode) (Fig. 6). Mode B will lead
to higher binding over the modes A and C. Actually which
mode will be dominated in solution that will depend upon
nature of the guest. All the anionic guests except pyruvate
salt (supporting information in Supplementary material),
However, the stoichiometries of the complexes were
determined from the titration curves as well as Job plots.
As can be seen from Fig. 4, receptor 1 shows greater
change in emission with N-acetyl-L-proline salt and acetate.
Other guests indicated small change in emission upon
complexation. For the receptor 1, there is a chance of
formation of 1:1 and 2:1 (guest:host) complexes depending
on the nature of guest as well as the relative orientation of
the urea and amide functional groups of 1. The receptor 1
can initially form 1:1 complex, which in the presence of
excess concentration of guests, may be disrupted to attain a
2:1 (guest to host) stoichiometry. In the present case, the
stoichiometry of the complexes of 1 with the anionic guests
except pyruvate in the excited state was determined as 1:1.
This was confirmed by fluorescence Job plot [26] (sup-
porting information in Supplementary material). Figure 5,
for example shows the Job plot for 1 with N-acetyl-L-pro-
line salt. The Job plots for N-acetyl valine salt and acetate
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J Incl Phenom Macrocycl Chem (2011) 71:243–248
Table 1 Binding constant
values (Ka) determined by
fluorescence method in CHCl3
containing 0.1% DMSO
Guest
K (M-1
)
N-acetyl-L-proline carboxylatea
N-acetyl-L-valine carboxylatea
N-acetyl-L-alanine carboxylatea
N-acetyl-L-phenyl glycine carboxylatea
Pyruvatea
K = (2.13 0.178) 9 104; R = 0.998
K = (3.37 1.04) 9 103; R = 0.974
K = (5.54 0.944) 9 103; R = 0.985
K = (8.41 0.455) 9 103; R = 0.980
K1:1 = nd, K2:1 = (1.46 0.23) 9 104; R = 0.989
K = (8.53 1.18) 9 103; R = 0.989
K = (6.45 1.40) 9 103; R = 0.980
Acetatea
a
Tetrabutylammonium salts
were taken
Pyruvic acid
(A)
(B)
(C)
NH
H
O
R
O
O
O
O-
O
O
N
O
N
N
H
N
H
N
H
H
H
N
HN
HN
N
H
N
N
H
O
O
N
O
O
-O
O
N
-O
H
H
O
O
-O
N
N
H
N
N
H
R
H
R
N
H
R
R = different alkyl substituents
Fig. 6 Possible modes of binding of N-acetyl amino acid salt
Fig. 7 a Change in absorbance
of 1 (c = 4.83 9 10-5 M) upon
(b) 1.2
1.2
(a)
1.0
gradual addition of N-acetyl-L-
proline and b upon gradual
addition of tetrabutylammonium
salt of N-acetyl-L-proline in
CHCl3 containing 0.1% DMSO
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
260
280
300
320
340
360
380
260
280
300
320
340
360
380
Wavelength (nm)
Wavelength (nm)
follow 1:1 binding model giving emphasis on the mode
B. For all the acids except pyruvic acid, the binding con-
stant determination by fluorescence method was difficult
due to minor change in emission titration spectra (sup-
porting information in Supplementary material). Thus the
receptor 1 fluorometrically distinguishes salts of N-acetyl
a-amino acid from their conjugate acids showing marked
change in emission titration spectra as well as binding
constant values.
The stoichiometry of the complexes in the ground state was
also ascertained from the Job plot and analysis of the
results revealed 1:1 (guest:host) stoichiometry in each case
[26] (supporting information in Supplementary material).
However, we did not measure the binding potencies of 1
with the guests in the ground state due to minor change in
UV–vis titration spectra.
The involvement of the binding sites of 1 in complex-
ation of the anionic and neutral guests was established by
1H NMR in CDCl3 containing 3% d6-DMSO. In the pres-
ence of equivalent amount of salt of N-acetyl-L-proline the
urea and amide protons altogether underwent downfield
chemical shift and thereby suggested their involvement in
complexation through hydrogen bonding more preferably
through the mode B. The similar experiment was done in
the presence of the equivalent amount of the other salts.
The interaction of 1 with the same guests in the ground
state was also realized by UV–vis titration experiments. In
all cases, the absorption of 1 decreased to the small extents
when guest solutions were progressively added to the
receptor solution. This was also true for the acid analogues.
For example, the change in absorption of 1 upon titration
with N-acetyl-L-proline and its salt is represented in Fig. 7.
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J Incl Phenom Macrocycl Chem (2011) 71:243–248
247
Comparison of the results corroborated greater downfield
shifting of the interacting protons in 1 only in the presence
of the salt of N-acetyl-L-proline. The changes in chemical
shift of the urea and amide protons of 1 in the presence of
equivalent amount of the acid analogues were small in
comparison to their salts and thus indicated weak interac-
complex with N-acetyl-L-proline salt. Mulliken charges on
the different atoms of the binding sites have been shown in
Fig. 9. In the complex, all the hydrogen bond donors are
intimately involved in the formation of hydrogen bonds
and one hydrogen of –CH3 group forms bifurcated hydro-
gen bonds with the naphthyridine ring nitrogens. Further-
more, MM2 calculations [31] on the complexes of 1 with
N-acetyl-L-valine, alanine, phenylglycine salts were per-
formed and the hydrogen bonding schemes were realized
(supporting information in Supplementary material).
1
tions. Figure 8, represents the H NMR changes of 1 upon
addition of equivalent amount of the guests such as
N-acetyl-L-valine, N-acetyl -L-valine salt and N-acetyl-L-
proline salt.
The receptor 1 and its 1:1 complex with the salt of
N-acetyl proline in the mode B as shown in Fig. 6 was
optimized by DFT method using 6-31G* basis set [28] and
the popular b3LYP functional [29, 30]. Figure 9 represents
the DFT optimized geometries of the receptor 1 and its
In conclusion, we have designed and synthesized an
easy-to-make naphthyridine-based fluororeceptor 1, which
selectively recognizes the tetrabutylammonium salt of
N-acetyl-L-proline over the other amino acid salts exam-
ined in the present study. Inspite of hydrogen bonding
(d)
(c)
O
O
N
H
N
H
b
c
HN
a
H
d
N
(b)
(a)
1
N
Fig. 8 Changes in 1H NMR of (a) 1 (400 MHz, CDCl3 containing 3% d6-DMSO; c = 5.16 9 10-3 M) and in the presence of equivalent
amount of (b) L-N-acetyl valine (c) L-N-acetyl valine salt (d) L-N-acetyl proline salts
Fig. 9 DFT optimized geometries of (a) receptor 1 (HF = -1465.808 au, Dipole moment = 0.9121 D) and its complex with N-acetyl-L-proline
salt (HF = -2019.149 au, Dipole moment = -5.0401 D)
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J Incl Phenom Macrocycl Chem (2011) 71:243–248
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the urea and amide motifs in a compact manner. Calcula-
tion performed at the DFT level of theory explained the
cooperativity in the binding process. The greater quenching
of emission and higher binding constant value as deter-
mined altogether underline the selective sensing of
N-acetyl-L-proline salt. Further study along this direction is
in progress in our laboratory.
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Acknowledgments We thank CSIR, New Delhi, India for financial
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