Pterin-based highly selective, ratiometric, and sensitive 'naked-eye' sensor for acetate

Article abstract of DOI:10.1016/j.tetlet.2014.03.068

The design and synthesis of a new pterin-based ratiometric and sensitive 'naked-eye' sensor R for highly selective recognition of acetate are reported. The acidic lactam NH and the NH of 2-N-pivaloyl group of receptor R along with 2,4-dinitrophenyl hydrazone group having the other acidic NH moiety lead to the binding of acetate anion in a 1:2 ratio by change of spectroscopic behavior on complexation (UV-vis and ¹H NMR studies) which is also proven by Job plot. The sharp color change from light yellow to violet promises R to be a useful chromogenic ratiometric sensor for acetate amongst other common anions.

Full text of DOI:10.1016/j.tetlet.2014.03.068

Tetrahedron Letters 55 (2014) 2707–2710
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Pterin-based highly selective, ratiometric, and sensitive ‘naked-eye’
sensor for acetate
⇑
Shyamaprosad Goswami , Manas Kumar Das, Abhishek Manna
Bengal Engineering and Science University, Shibpur, Howrah 711103, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The design and synthesis of a new pterin-based ratiometric and sensitive ‘naked-eye’ sensor R for highly
selective recognition of acetate are reported. The acidic lactam NH and the NH of 2-N-pivaloyl group of
receptor R along with 2,4-dinitrophenyl hydrazone group having the other acidic NH moiety lead to the
binding of acetate anion in a 1:2 ratio by change of spectroscopic behavior on complexation (UV–vis and
1H NMR studies) which is also proven by Job plot. The sharp color change from light yellow to violet
promises R to be a useful chromogenic ratiometric sensor for acetate amongst other common anions.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 19 December 2013
Revised 7 March 2014
Accepted 8 March 2014
Available online 21 March 2014
Keywords:
Ratiometric sensor
Acetate sensor
Naked eye
6-Formyl pterin
2,4 DNP
Over the past few years, the recognition and sensing proper-
ties of anions have attracted considerable attention for their
diversity of functions.1–8 Anions also play a fundamental role in
a wide range of chemical, biological, and environmental pro-
cesses. Among the various biologically important anionic ana-
lytes, acetate and dicarboxylate are among the most critical
components of numerous metabolic processes. Without them,
many enzymes and antibodies are unable to function properly.
Therefore, the easily synthesizable anion chemosensor has a
greater priority due to the important role of acetate in biology
and clinical diagnostics.9–12 Acetate is one of the carboxylate an-
ions with the unique trigonal structure which can form strong
hydrogen-bond interaction with hydrogen bond donors. There-
fore, noncovalent hydrogen-bonding interaction is one of the
most useful and effective in this regard. Receptors bearing func-
tional groups such as amides,13 ureas,14 thioureas,15 imidazoli-
um,5 and positively charged groups16 have been widely used to
recognize anions via hydrogen-bonding interactions. A variety
of receptor molecules which selectively recognize acetate anion
are available in the literature,17–19 including many capable of
luminescent sensing. Generally, the recognition of anions is stud-
ied in less polar organic solvents20,21 (e.g., CH2Cl2, DMSO, CH3CN
etc.) and sometimes also in the mixture of protic solvents (e.g.,
H2O, CH3CH2OH). It is also known that the rate of AcOÀ produc-
tion and oxidation usually acts as an indicator of organic decom-
position in marine sediments22 and transmetalation of
tetrapyrroles.23–25
On the other hand, ratiometric probes should enable the mea-
surement at two different wavelengths, providing a built-in recti-
fication for environmental effects and raising the dynamic range
of measurement. This was considered as a high-quality approach
to defeat the major problem of absorption based probes, in which
variations in the environmental sample and probe sharing were
problematic for quantitative measurements. However, so far, ratio-
metric probes for acetate are still very scarce.
In this work, pterin has been chosen as an ideal component of a
chemosensor due to its biological importance,26 good platforms for
sensing,27 and also its ability to form charge transfer complexes.28
Therefore, we have designed and synthesized a chromophore in
our laboratory whose physicochemical properties can be utilized
as
a ratiometric sensor for acetate both qualitatively and
quantitatively.
At first, 2,5,6-triaminopyrimidine hydrochloride is condensed
with monoacetal of tricarbonyl compound according to Grabrel–
Isay condensation.29 Then the crude product under pivaloylation
gives the soluble product which affords 6-formyl pterin with
deprotection of acetal group at 6-position. The receptor R was pre-
pared by applying simple Schiff’s base condensation reaction be-
tween 6-formyl pterin and 2,4-DNP in ethanol which afforded R
as a yellowish precipitate with a good yield (Scheme 1).
Molecular structure and purity level of the receptor R were
established from different spectroscopic studies like 1H NMR and
HRMS analysis (Supplementary data).
⇑
Corresponding author. Fax: +91 3326682916.
E-mail address: spgoswamical@yahoo.com (S. Goswami).
http://dx.doi.org/10.1016/j.tetlet.2014.03.068
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2708
S. Goswami et al. / Tetrahedron Letters 55 (2014) 2707–2710
O
N
19-fold on addition of 5 equiv TBAA after which it gets saturated.
From the UV–vis titration data, it is also clear that minimum
M of acetate can be detected by using 10 M of receptor.
Titrations were also carried out with various anions like FÀ, IÀ,
O
NH2
.2HCl
NH2
(i)
HN
H2N
N
N
CH(OMe)2
HN
H2N
2
l
l
N
and C6H5COOÀ as their tetrabutylammonium salts and ADP, ATP,
KDHP, NaNO3, NaNO2, K3PO4, Na2SO3, Na-aspartate, and Na-lactate.
Interestingly there is no observable change detected in the UV
spectrum except with fluoride and benzoate ion, which show very
little interference (Supplementary data). There is a small appear-
ance of a new peak at 573 nm which indicates that the receptor
(R) has an insignificant response toward fluoride ion. In the pres-
ence of benzoate the peak at 411 nm slightly decreases (Fig. 2).
Possibly the basicity difference, that is, weaker basicity of benzoate
over acetate and the steric effects are the main cause of the selec-
tivity of AcOÀ over C6H5COOÀ toward R (Fig. 2).
(1)
NO2
(ii)
NO2
O
NH
H
N
N
CHO
O
N
N
O
HN
(iii)
N
N
O
HN
N
N
(2)
H
N
H
(Receptor)
Scheme 1. Synthetic outlook of the receptor. Reagents and conditions: (i) Na2SO3,
H2O, OHC(CO)CH(OMe)2, rt,12 h. (ii) (a) Pivalic anhydride, DMAP (cat.), 120 °C, 6 h;
(b) TFA, CH2Cl2, rt, overnight. (iii) 2,4-DNP, EtOH, few drops of acetic acid, 80 °C, 2 h.
Interestingly, the very large (absorption shift:
DA = 162 nm) dif-
ference between the two wavelengths not only contributes to the
accurate measurement of the two absorption peaks, but it also re-
sults in a huge ratiometric value. In fact, almost 60-fold enhance-
ment in the ratiometric value (I573/I411, from 0.06 to 4.02) is
achieved with respect to the acetate-free solution in the presence
of 5.0 equiv of acetate (Fig. 3). The change of absorbance with
the concentration of acetate also maintained a linear relationship
0.5
573nm
0.6
0.4
0.3
0.2
0.1
0.0
K1 = (1.98 0.11) x 105
K2 = (2.46 0.31) x 104
R = 0.99
411nm
0.5
0.4
0.3
0.2
0.1
0.0
from 2 lM to 10 lM (Fig. 3).
On the addition of acetate ion, the color changes are most likely
due to the creation of hydrogen bonds and deprotonation of amide
proton of receptor (R) which is illustrated in Scheme 2. These
hydrogen bonds or deprotonations influence the electronic proper-
ties of the chromophore which result in the transformation of color
from light yellow to violet, along with a new charge-transfer com-
munication between the acetate bound NH moieties and the elec-
tron deficient nitro group.31 Furthermore, the K delocalization is
improved due to the strong hydrogen-bonding interaction be-
tween receptor R and acetate ion, which was predictable to reduce
450nm
5.0x10-6 1.0x10-5 1.5x10-5 2.0x10-5
0.0
[G]
350
400
450
500
550
600
650
Wavelength (nm)
Figure 1. UV–vis absorption spectra of R (C = 1 Â 10À5 M) upon titration with TBAA
(tetrabutylammonium acetate) (C = 2 Â 10À4 M) in CH3CN at 25 °C. Inset: nonlinear
1:2 fitting isotherm recorded at 573 nm.
the energy of the
p–
p⁄ transition and therefore accounts for the
appearance of a new absorption band at a higher wavelength, that
is, at 573 nm resulting in the formation of a violet color. A distinct
isosbestic point at 450 nm emerged during the spectral titrations,
which confirmed the formation of the stable complex with a defi-
nite stoichiometric ratio between the receptor and the anion
resulting in a new ICT (internal charge transfer) band that ap-
peared at 573 nm.
The binding and recognition properties of receptor R were stud-
ied by UV–vis and NMR titration methods. The UV–vis spectroscopic
titration of R was carried out in acetonitrile (C = 1 Â 10À5 M) with
acetate and other anions. There is a characteristic band at 411 nm
of the receptor R in the UV–vis spectrum (Fig. 1). Surprisingly, it is
observed that the intensity of the band at 411 nm decreases upon
gradual addition of tetrabutylammonium acetate (TBAA)
(C = 2 Â 10À4 M) and the intensity of a new band at 573 nm in-
creases, forming a distinguished naked-eye color change from light
yellow to violet. The formation of a new complex between R and ace-
tate anion is also indicated by the isosbestic point at 450 nm. Addi-
tionally, the characteristic structured absorption band at 411 nm is
almost completely minimized suggesting that significant electronic
perturbation occurred in the ground state of R.30
The 1:2 stoichiometry for the host–guest complexation was
elaborated by the profile of the intensities of the diminishing band
centered at 411 nm and rising band at 573 nm which was also con-
firmed by Job plot analysis (Fig. 4). The association constant was
estimated
to
be
K1 = 1.98 0.11 Â 105 MÀ1
and
K2 = 2.46 0.31 Â 104 MÀ1 by nonlinear regressive analysis
method32 using absorption titration data. The calculated detection
limit is 0.604
deviation of blank measurements and S is the slope of the calibra-
l
M based on K Â Sb1/S,33 where Sb1 is the standard
The change of color in the presence of acetate ion is attributed
by the probable hydrogen bonding formation between R and ace-
tate ion (Scheme 2). It is also observed that the enhancement of
ratiometric value of absorbance of the R (1 Â 10À5 M) is near about
tion curve (Fig. S1: Supporting information).
We carried out fluorescence titration experiments of the recep-
tor R (1 Â 10À5 M) with the AcOÀ (2 Â 10À4 M) in acetonitrile. The
fluorescence response of the receptor R with AcOÀ was recorded
NO2
NO2
NO2
O2N
O2N
O2N
N
H
N
N
N
H
N
O
N
H3C
O
Hc
O
N
N
O
O
N
O
H
N
N
N
Hb
N
N
N
H
N
N
N
N
H
O
N
Hd
N
H
O
CH3
H
O
CH3
Ha
H
H7
O
O
O
Scheme 2. Probable binding mode of the receptor with acetate.

S. Goswami et al. / Tetrahedron Letters 55 (2014) 2707–2710
2709
0.5
0.4
0.3
0.2
0.1
0.0
Receptor + AcO-
Receptor
+ other anion
350
400
450
500
550
600
650
Wavelength (nm)
Figure 2. UV–vis absorption spectra of R upon titration with all the tested anions in
CH3CN with the naked-eye color change of R in the presence of acetate.
Figure 5. Partial 1H NMR spectrum of R in DMSO-d6 with addition of different equiv
of TBAA.
4
0.4
R
2= 0.997
at 573 nm
3
2
1
0
0.3
0.2
0.1
R2= 0.998
at 411 nm
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
[Acetate]
Figure 3. The bar plot of ratiometric response of R in the presence of all the tested
anions (left) and plot of absorbance versus [AcOÀ] at two different wavelengths
(right).
(A)
(B)
(C)
Figure 6. Photographs of test strips of R toward various concentrations of acetate
(C Â 10À3 mol/L) C = (A) 0, (B) 2, (C) 10.
1.0
0.67
0.8
0.6
0.4
0.2
0.0
(0.417 ppm and 0.137 ppm) and complex 1:2 (0.551 ppm and
0.296 ppm), respectively. Besides, the protons of the 2,4-dinitro-
substituted aromatic ring of R also undergo upfield shift on com-
plexation with acetate in 1:1 and 1:2 complexes.
Many sensors for AcOÀ detection could only be performed in
solution, which would limit their applications. To examine the
realistic function of chemosensor R, test strips were prepared by
immersing TLC plates into a solution of R (2 Â 10À3 M) and then
drying in air. As shown in Figure 6, when acetate solution was
added on the test kits in different concentrations, the obvious color
changes from greenish yellow to brown were observed. Thus, the
test strips could straightforwardly sense acetate. Development of
such dipsticks is helpful as immediate qualitative information is
obtained without resorting to the instrumental analysis.
0.2
0.4
0.6
Xh
0.8
1.0
Figure 4. Job plot diagram of receptor R for anion tetrabutylammonium acetate
(where Xh is the mole fraction of host and dI indicates the change of the
absorbance).
Thus we have been able to develop a unique pterin-based
hydrazone receptor specific for chromogenic recognition of acetate
over many other anions. The electron withdrawing pterin moiety
having in one side, the lactam NH and the pivaloylamide N–H,
binds one acetate moiety and the other side having 2,4-DNP hydra-
zone moiety binds the other acetate ion making the sensor R a
bidentate one for acetate (1:2). Test strips (TLC sticks) of R also
demonstrated its effective solid surface application as a sensor
for acetate.
with an excitation at 411 nm (Fig. S3: Supporting information) and
there is no observable change in the emission spectrum of the
receptor after the addition of excess amount of acetate salt. How-
ever, the slight increase of fluorescence intensity suggests that the
ICT process operates due to the occurrence of a tautomeric equilib-
rium during the anion recognition method (Scheme 2).
Now we describe the 1H NMR titration of receptor R with TBAA
in DMSO-d6 (Fig. 5) to recognize the nature of interaction between
the specific hydrogens of R and the acetate ion. It is found that the
amides and NAH-(AC@NHA) protons (Ha, Hb, and Hc) of R are
gradually dispersed with the gradual addition of acetate anion
indicating the cyclic intermolecular H-bonding interaction as
shown in Scheme 2. Additionally, the –CH– of hydrazone moiety
and H7 of R are distinguishably shifted upfield in complex 1:1
Acknowledgements
The authors thank DST and CSIR (Govt. of India) for financial
supports. We are thankful to Dr. Kumaresh Ghosh for his kind help
to determine the binding constant. M.K.D. and A.M. acknowledge
CSIR for providing fellowship.

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