Anthracene appended pyridinium amide-urea conjugate in selective fluorometric sensing of L-N-acetylvaline salt

Article abstract of DOI:10.3762/bjoc.6.139

A new anthracene labeled pyridinium amide-urea conjugate 1 has been designed and synthesized. The receptor shows a different fluorometric response with L-N-acetylvaline and L-N-acetylalanine salts in CH₃CN in contrast to the other salts of L-N-

Full text of DOI:10.3762/bjoc.6.139

Anthracene appended pyridinium amide–urea
conjugate in selective fluorometric sensing of
L-N-acetylvaline salt
Kumaresh Ghosh*, Tanmay Sarkar and Asoke P. Chattopadhyay
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, 1211–1218.
Department of Chemistry, University of Kalyani, Kalyani-741235,
India, Fax +91-33-25828282
doi:10.3762/bjoc.6.139
Received: 02 August 2010
Accepted: 16 November 2010
Published: 21 December 2010
Email:
Kumaresh Ghosh* - ghosh_k2003@yahoo.co.in
* Corresponding author
Associate Editor: S. C. Zimmerman
Keywords:
© 2010 Ghosh et al; licensee Beilstein-Institut.
License and terms: see end of document.
N-acetyl-L-valine salt; DFT calculation; emission decay; fluorometric
detection; pyridinium amide–urea conjugate
Abstract
A new anthracene labeled pyridinium amide–urea conjugate 1 has been designed and synthesized. The receptor shows a different
fluorometric response with L-N-acetylvaline and L-N-acetylalanine salts in CH3CN in contrast to the other salts of L-N-acetyl
α-amino acids and (S)-α-hydroxy acids studied. Upon complexation of the tetrabutylammonium salt of L-N-acetylvaline, the emis-
sion of 1 increases accompanied by the formation of a new band at higher wavelength and this characteristic change distinguishes it
from other anionic substrates studied. The binding interaction has been studied by 1H NMR, fluorescence and UV titration experi-
ments.
Introduction
The design and synthesis of artificial receptors capable of quenching, enhancement and life time), high sensitivity, low
recognizing α-hydroxy and N-acetyl-α-amino acid carboxylates instrumentation cost, time efficiency, and the possibility of
(i.e., salts of α-amino acids) is an active area of interest in performing real time analysis. However, the development of
supramolecular chemistry due to the biological significance and receptors for these molecules is slow due to their insolubility in
practical importance of α-amino and α-hydroxy acids [1-7]. organic solvents, especially amino acids. For strong complexa-
While α-hydroxycarboxylic acids are useful synthons for many tion of zwitterionic amino acids, synthetic receptors should pos-
organic natural products and drug molecules, α-amino sess complementary binding sites for both ammonium and
carboxylic acids are important as they are the building blocks of carboxylate functionalities. Examples in this domain are known
proteins, enzymes etc., which govern numerous biochemical in the literature [8-15]. To overcome the solubility problem,
processes. In this context, detection and sensing of these mole- α-amino acids are sometimes converted into their N-acetyl
cules by fluorescence spectroscopy offers a variety of advan- derivatives which makes their recognition easier in organic
tages such as different detection modes (fluorescence solvents [16-19], as these are non-competitive solvents and
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Beilstein J. Org. Chem. 2010, 6, 1211–1218.
reported receptors of various structures with different binding
sites. The pyridinium motif, which was first used by Jeong et al.
for carboxylate binding [23], was one of the binding sites in our
designed receptors [24-26]. The pyridinium motif is unique due
to its contribution to the charge–charge interaction and forma-
tion of unconventional hydrogen bonds with the anionic guests
[27]. In order to explore this binding site for a wide range of
substrates, especially for amino acid derivatives, we report here
the design and synthesis of a new fluororeceptor 1 where
anthracene is attached to the binding site through a covalent
CH2 spacer to yield a photo induced electron transfer sensory
system [28]. The receptor 1 shows effective binding of the tetra-
butylammonium salt of L-N-acetylvaline by exhibiting signifi-
cant change in emission.
Complexation induced formation of an exciplex or charge
transfer complex in CH3CN is the key feature in the present
study for the selective detection of a L-N-acetylvaline salt from
other anionic guests. To explain the formation of an exciplex or
charge transfer complex in 1 upon complexation of L-N-acetyl-
valine salt, compound 2 was synthesized and studied (see
Figure 1).
Figure 1: Synthesized compounds 1 and 2.
guests exhibit minimal solubility. Hamilton et al. introduced Results and Discussion
simple pyridine amide-based receptors for the effective recogni- Compounds 1 and 2 were synthesized according to Scheme 1.
tion of N-acetyl-α-amino acids by multiple hydrogen bonding Initially, 3-nitrobenzoyl chloride was reacted with 3-aminopyri-
interactions [20]. During our work on selective recognition of dine to give the amide 3. Reduction of the –NO2 group in 3 with
different anionic species including carboxylates [21,22], we SnCl2 in EtOAc afforded the amine 4, which was further
Scheme 1: Syntheses of 1 and 2.
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Beilstein J. Org. Chem. 2010, 6, 1211–1218.
reacted with 4-nitrophenyl isocyanate (obtained from
4-nitroaniline by reaction with triphosgene in dry THF) in dry
THF to give urea derivative 5. Subsequent reaction of 5 with
9-chloromethylanthracene under refluxing conditions in dry
CH3CN gave the chloride salt 6. Anion exchange of the salt 6
with NH4PF6 afforded the desired receptor 1 as a white solid.
Compound 2 was obtained from the intermediate amine 4 after
performing a series of reactions such as amide formation, alkyl-
ation on pyridine ring nitrogen followed by anion exchange
with NH4PF6 (Scheme 1b). Compounds 1 and 2 were character-
ized by 1H NMR, 13C NMR, FTIR and mass spectrometry.
Figure 2: Change in 1H NMR of (A) 1 (400 MHz, CDCl3 containing
0.4% d6-DMSO; c = 2.80 × 10−3 M) and in the presence of equivalent
amount of (B) L-N-acetylvaline, (C) L-N-acetylproline salts.
The solution phase binding interaction of the tetrabutylammoni-
um salts of L-N-acetylalanine, L-N-acetylvaline, L-N-acetylpro-
line and L-N-acetylphenylglycine, (S)-mandelic and pyruvic
acids was investigated by 1H NMR, UV–vis and fluorescence the change in emission of 1 at 492 nm in the presence of one
techniques. Initially, we recorded the 1H NMR of 1 in the pres- equivalent of each guest in CH3CN. The increase in emission of
ence and the absence of the guests in CDCl3 containing 0.4% 1 at 492 nm in the presence of tetrabutylammonium salts of
d6-DMSO (due to insolubility of 1 in pure CDCl3). In the pres- L-N-acetylvaline and L-N-acetylalanine is moderate and distin-
ence of equivalent amounts of all the guests, urea (ΔδNHb = guishable from the other guests. It is worth noting that the
0.93–1.90, ΔδNHc = 0.87–1.96) and amide protons (ΔδNHa = appearance of the new emission at 492 nm is more significant in
0.46–0.79) underwent downfield shifts suggesting their involve- the presence of L-N-acetylvaline salt than with L-N-acetylala-
ment in the binding process. In addition, the pyridinium ortho nine. This is attributed to the formation of a charge transfer
proton (Ho) in 1 was also downfield shifted (Δδ = 0.83–1.06) complex between the excited state of anthracene and the elec-
which indicated its involvement in the formation of strong tron deficient nitrophenyl urea during the interaction process.
hydrogen bonding with the carboxylate moiety of the guests. In We believe that this characteristic feature with L-N-acetylva-
contrast, the pyridinium para proton (Hp) showed a small line is due to its structural feature that controls the distance
downfield shift (Δδ = 0.02–0.48). This small change in chem- between donor and acceptor.
ical shift of the para proton is due to either a change in bond
length of the intramolecular hydrogen bond between the amide
carbonyl oxygen and para hydrogen of pyridinium motif, or
involvement in the formation of a hydrogen bond with the guest
in solution. On addition of an equivalent amount of tetrabutyl-
ammonium salts of acetate and pyruvate to the solution of 1,
precipitation occurred. A similar finding was observed in the
presence of an equivalent amount of the alanine salt. Figure 2
shows the change in 1H NMR spectrum of 1 in presence of an
equivalent amount of tetrabutylammonium salts of L-N-acetyl-
valine, L-N-acetylproline and (S)-mandelic acid.
Once it was realized that both urea and pyridinium sites of 1 are
Figure 3: Change in fluorescence ratio of 1 upon addition of one
equivalent of anions (c = 4.31 × 10−5 M) at 492 nm.
involved in hydrogen bonding with the guests studied, we
recorded the emission spectra of 1 in CH3CN. Receptor 1
showed an intense emission at 415 nm when excited at 370 nm
in CH3CN. Upon gradual addition of the guests to a solution of Figure 4a and Figure 4b show the change in emission of 1 (c =
1 (c = 4.31 × 10−5 M) in CH3CN, the emission at 415 nm was 4.31 × 10−5 M) upon increasing the quantity of tetrabutylammo-
changed differently. For all guests, except the salts of L-N- nium salts of L-N-acetylvaline and L-N-acetylalanine, respect-
acetylvaline and L-N-acetylalanine, emission of 1 at 415 nm ively. The expected charge transfer in 1 upon complexation of
decreased gradually (Supporting Information File 1). However, valine salt was further established by performing similar fluo-
in the case of the L-N-acetylvaline salt, a broad emission at rescence titration experiment with the receptor 2, where the
492 nm with moderate intensity was observed. Figure 3 displays electron deficient urea motif is absent. In this case, no broad
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Beilstein J. Org. Chem. 2010, 6, 1211–1218.
Figure 4: Change in emission of 1 (c = 4.31 × 10−5 M) upon gradual addition of tetrabutylammonium salts of (a) L-N-acetylvaline and (b) L-N-acety-
lalanine.
band at 492 nm was observed when the titration was carried out regulated in different ways. We presume that receptor 1 may
by gradual addition of the valine salt (Supporting Information follow any equilibrium-binding mode A, B or C with valine,
File 1). This was also the case for the alanine salt. The other alanine and phenylglycine salts in solution as shown in
salts merely perturbed the emission of 2 and thus indicate the Figure 5. This is also true for the mandelate, pyruvate and
positive role of the urea motif in 1 for effective complexation of proline salts. Relevance of the suggested modes in Figure 5 was
anionic substrates. This is in accordance with Hamilton’s obser- followed from the change in chemical shift of the key protons
vation [20].
of 1 in the 1H NMR upon complexation (see Figure 2) as well
as from Hamilton’s observation on related systems [3]. In the
The different modes of emission (enhancing and quenching) of interaction process, the stoichiometry of the complexes was 1:1
1 in the presence of the different guests studied is believed to be as confirmed by Job plots (Supporting Information File 1). In
due to the structural difference and hydrogen bonding abilities addition, the break in the titration curves at [G]/[H] = 1 in
of the guests for which the PET process occurring between the Figure 6 also supports this stoichiometry. A closer look at the
amide-urea binding site and the excited state of anthracene is curves for L-N-acetylvaline and L-N-acetylalanine salts in
Figure 5: Suggested modes of binding of the amino acid salts into the open cleft of 1.
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Beilstein J. Org. Chem. 2010, 6, 1211–1218.
Figure 6 shows that there are breaks at [G]/[H] = 1 within the 2.59 ns is real. However, in the presence of 1 equiv of L-N-
range of 3 equivalents of the added guest. Further addition acetylvaline salt, the decay profile followed a tri-exponential
causes a steady increase in emission and reaches saturation only fitting that indicated three emitting species with life times τ1 =
when 20 to 30 equivalent amounts of the guest are added 1 ns (0.02%), τ2 = 4.23 ns (0.02%) and τ3 = 0.62 ps (99.96%)
(Supporting Information File 1). This result suggests that in the (Figure 7). Among these, the fast decay component 0.62 ps
presence of a large excess of the L-N-acetylvaline and L-N- could be attributed to the formation of a hydrogen-bonded
acetylalanine salts that complexes with higher order stoichiome- short-lived species where presumably intramolecular charge
tries are being formed in solution. The almost linear nature of transfer between anthracene and nitrophenyl urea as repre-
the titration curves for all other guests except L-N-acetylvaline sented in Figure 4a, takes place. This component coexists with
and L-N-acetylalanine salts at 492 nm (Figure 6) corroborated the other components contributing large preexponential factor
the weak interaction.
to the total fluorescence. This finding was not observed with
L-N-acetylalanine or L-N-acetylproline salts (Table 1). Neither
was this observed when the emission decay of 2 was monitored
at 418 nm in the presence of L-N-acetylvaline salt upon excita-
tion at 370 nm. This observation is thus noteworthy since it
distinguishes the L-N-acetylvaline salt in the present study form
the others with receptor 1.
Figure 6: Fluorescence titration curves for 1 (c = 4.31 × 10−5 M) at
492 nm.
The time resolved emissions for 1 and 2 upon excitation at
370 nm were then studied to have an insight into the interaction
process. The emission decay profile of 1 monitored at 420 nm
could be fitted bi-exponential with two constants τ1 = 0.46 ps
(100%), τ2 = 2.59 ns (0%). The faster decay component (0.46
ps) is either due to a very short-lived species or an artifact, or
Figure 7: Fluorescence decays (at λmax = 420 nm) of receptor 1 upon
the addition of 1 equiv of L-N-acetylvaline salt ([H] = 4.71 × 10−5 M, [G]
= 9.42 × 10−4 M) in CH3CN.
for tunneling of extra energy to the bulk by a non-radiative Concurrently, the ground state binding properties of 1 in
pathway [29,30]. The emitting species with a life time of CH3CN were evaluated by UV–vis studies. The absorption
Table 1: Fluorescence decay times (τ), and preexponential factors for 1 and 2 in CH3CN.
Receptor in presence and absence of guest
Fluorescence decay time τ (preexponential factor)
1
τ1 = 0.46 ps ( 100%), τ2 = 2.59 ns ( 0%) ; (χ2 = 1.43)
τ1 = 1 ns (0.02%), τ2 = 4.23 ns (0.02%), τ3 = 0.62 ps (99.96%); (χ2 = 1.08)
τ1 = 5.0 ps (58.83%), τ2 = 2.75 ns (41.17%); (χ2 = 1.03)
τ1 = 2.5 ps (85.56%), τ2 = 2.76 ns (14.44%); (χ2 = 1.15)
τ1 = 0.58 ps (99.98%), τ2 = 3.92 ns (0.02%); (χ2 = 1.54)
τ1 = 0.63 ps (99.95%), τ2 = 3.87 ns (0.05%); (χ2 = 1.72)
τ1 = 2.52 ps (65.38%), τ2 = 3.84 ns (34.62%); (χ2 = 1.69)
τ1 = 0.49 ps (100%), τ2 = 4.18 ns (0%); (χ2 = 1.36)
1 + 1 equiv L-N-acetylvaline salt
1 + 1 equiv L-N-acetylalanine salt
1 + 1 equiv L-N-acetylproline salt
2
2 + 1 equiv L-N-acetylvaline salt
2 + 1 equiv L-N-acetylalanine salt
2 + 1 equiv L-N-acetylproline salt
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Beilstein J. Org. Chem. 2010, 6, 1211–1218.
spectrum of 1 (c = 4.31 × 10−5 M) in the absence of anions
showed a structured band with maximum intensity at 341 nm.
Upon titration with the salt of L-N-acetylvaline the ground state
of 1 was affected significantly and the absorption was weakly
red shifted because of recognition of the anion and a distinctive
isosbestic point was observed at 340 nm (Figure 8). Similar
observations were noted with the other guests (Supporting
Information File 1). The stoichiometry of the interaction
process in the ground state was also 1:1 as established from the
break in the titration curves as well as Job plots [31]
(Supporting Information File 1). In the titration curves, down-
ward running of the titration curves in few cases indicates that
as a greater excess of the guest anion is added, the 1:1
host–guest complex is assumed to be disrupted and anions
begin to bind individually to the pyridinium amide and urea
binding sites, rather than in a cooperative manner. Thus, in prin-
ciple, equilibrium complexes of multiple stoichiometries can be
generated in solution. This behavior depends on the anions [32]
Figure 8: Change in absorbance of 1 (c = 4.31 × 10−5 M) upon gradual
addition of tetrabutylammonium salts of L-N-acetylvaline.
and also on the concentrations of the host and guest used at CH3CN by fluorescence (Table 2) and the values were found to
which the titration experiments are monitored.
be less compared to the values for 1. This is in accordance with
Hamilton’s observation [3] and indicates the role of the urea
Analysis of the emission data provided the association constants motif in the binding event.
(Ka), reported in Table 2 [33]. For determining binding constant
values for L-N-acetylvaline and L-N-acetylalanine salts, we In order to realize the conformational behavior and the reactiv-
considered the emission data up to the addition of 13 equiva- ity of 1 towards hydrogen bonding with the anionic guests as
lents of the guests added, since in the presence of large excess noted in Table 2, we carried out detailed DFT calculations. The
of such guests complex stoichiometries were noted. From Gaussian-03 package [34] and GAMESS-US suite [35] (version
Table 2 it can be seen that receptor 1 exhibits higher binding April 11, 2008) was used for the calculations and the MO
constant values for L-N-acetylvaline in the series. At this figures were obtained using the MaSK software [36]. DFT
moment, we believe that it is due to structural aspects as well as calculations were performed in the gas phase on the two
to the hydrogen bonding capacity of the valine salt. Further- different conformations of 1 using the 6-311G** basis set [37]
more, to establish the hydrogen bonding influence of the and the popular b3LYP functional [38,39]. Similar calculations
acylamino and hydroxy groups of the amino and hydroxy acids were also carried out on all the guests given in Table 2. For all
respectively, we determined the binding constant for the acetate the compounds, parameters such as global electrophilicity (ω),
anion. This was found to be slightly less when compared to all global electronegativity (χ), global hardness (η), dipole moment
the other guests. Pyruvate with carbonyl at the α-position did (µ) and the energies of HOMO and LUMO are also reported
not produce any marked change in emission of 1. We also deter- (Table S1; Supporting Information File 1) from their DFT opti-
mined the binding constants for 2 with the same anions in mized structures [34-36]. The orientation of the binding sites in
Table 2: Association constants (Ka) in CH3CN from fluorescence measurements.
Guesta
Receptor 1
Ka [M−1]e
Receptor 2
Ka [M−1]e
L-N-acetylvaline
L-N-acetylalanine
L-N-acetylproline
L-N-acetylphenylglycine
acetate
1.87 x 104; R = 0.998c
2.60 x 103; R = 0.995d
1.38 x 103; R = 0.993c
1.31 x 103; R = 0.989c
2.20 x 103; R = 0.997c
1.60 x 103; R = 0.995c
—b
1.30 x 103; R = 0.988c
6.50 x 102; R = 0.993c
—b
—b
1.70 x 103; R = 0.998c
(S)-mandelate
pyruvate
—b
—b
atetrabutylammonium salts were used; bnot determined due to irregular change; cdetermined at 414 nm; ddetermined at 492 nm; eerror ≤ 10%.
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Beilstein J. Org. Chem. 2010, 6, 1211–1218.
both 1A and 1B are displayed in Figure S15 (Supporting Infor- exhibits a value of 0.5384, which is significantly greater than
mation File 1). Of the two possible conformations of receptor 1 the value obtained for urea site (0.0021) and thus binding of the
(Figure S15), 1B is found to be more stable than 1A by 2.82 carboxylate part of all the guests will take place preferentially at
kcal/mol. Therefore, we may reasonably expect that in solution the pyridinium site instead of urea (see structures B/C in
both conformers may remain in equilibrium. However, con- Figure 5).
formation 1A may form a more stable hydrogen-bonded com-
plex with guests, owing to formation of a greater number of In conclusion, we have designed and synthesized a new fluo-
hydrogen bonds with the guests. For example, Figure 9 demon- roreceptor 1, which is able to bind α-acylamino as well as
strates the hydrogen-bonded complex of 1A with N-acetyl-L- hydroxy acids with moderate binding constant values. The
valine carboxylate, where both the urea and pyridinium motifs receptor clearly distinguishes L-N-acetylvaline and L-N-acety-
are cooperatively involved in bonding.
lalanine salts from the other anionic substrates studied by ex-
hibiting different modes of emission. The appearance of charge
transfer band and three decay components in time resolved
spectroscopy upon complexation of L-N-acetylvaline salt are
the distinct features in the present study to distinguish it from
the other anions examined. Further progress in this direction is
underway in our laboratory.
Supporting Information
Synthesis and characterization data for 1 and 2, general
procedure for fluorescence and UV titrations, change in
absorption and fluorescence spectra, Job plots of receptor 1
with few selected guests, change in emission of 2 in the
presence of few selected anions, UV titration curves for 1,
selected binding constant curve for 1, change in emission of
1 upon dilution with solvent, titration curves for 1 with
valine and alanine salts, DFT optimized geometry of 1 and
MO energies, global hardness, global electronegativity,
global electropositivity, dipole moment etc. are available.
Figure 9: DFT optimized geometry of the complex of 1 with L-N-acetyl-
valine carboxylate salt [a = 1.93 Å, b = 1.99 Å, c = 1.64 Å, d = 1.97 Å,
e = 2.60 Å and f = 2.69 Å].
Supporting Information File 1
Detailed experimental data for 1 and 2.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-139-S1.pdf]
However, interaction between the electrophile and the nucle-
ophile can be quantified in terms of the electrophilicity index
(ω) of individual species involved in the process. Although ω of
structure 1A is found to be only slightly different from that of
1B (Supporting Information File 1), it is obvious that structure 1
has an affinity for anionic guests. As can be seen from Table S1 Acknowledgements
(Supporting Information File 1), guests such as the salts of We thank CSIR, Government of India for financial support. TS
valine, phenylglycine and mandelic acid exhibit the lowest thanks CSIR, New Delhi, India for a fellowship. We also thank
values of ω among all anionic guests considered. This indicates DST, Government of India for providing facilities in the depart-
that these guests have higher nucleophilic character compared ment under FIST program.
to the other guests studied. Therefore, receptor 1, in principle,
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