Acetate selective fluorescent turn-on sensors derived using Vitamin B6 cofactor pyridoxal-5-phosphate

Article abstract of DOI:10.1016/j.saa.2015.12.024

Two new Schiff base receptors have been synthesized by condensation of pyridoxal-5-phosphate with 2-aminophenol (L¹) or aniline (L²). In DMSO, the receptors showed both chromogenic and 'turn-on' fluorescence responses selectively in the presence of AcO^- and F^-. However, in mixed DMSO-H₂O medium, the receptors showed AcO^- selective 'turn-on' fluorescence without any interference from other tested anions including F^-. The detection limit for AcO^- was found to be 7.37 μM and 22.9 μM using the receptors L¹ and L², respectively.

Full text of DOI:10.1016/j.saa.2015.12.024

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 110–115
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Acetate selective fluorescent turn-on sensors derived using vitamin B6
cofactor pyridoxal-5-phosphate
Darshna Sharma a,1, Aman Kuba a,1, Rini Thomas a,1, S.K. Ashok Kumar b, Anil Kuwar c,
Heung-Jin Choi d, Suban K. Sahoo a,d,
⁎
a
Department of Applied Chemistry, SV National Institute Technology, Surat, Gujarat, India
School of Advanced Sciences, VIT University, Vellore, Tamil Nadu, India
School of Chemical Sciences, North Maharashtra University, Jalgaon, Maharashtra, India
Department of Applied Chemistry, Kyungpook National University, Daegu, South Korea
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new Schiff base receptors have been synthesized by condensation of pyridoxal-5-phosphate with 2-
aminophenol (L1) or aniline (L2). In DMSO, the receptors showed both chromogenic and ‘turn-on’ fluorescence
responses selectively in the presence of AcO− and F−. However, in mixed DMSO–H2O medium, the receptors
Received 22 September 2015
Received in revised form 3 December 2015
Accepted 20 December 2015
Available online 28 December 2015
showed AcO− selective ‘turn-on’ fluorescence without any interference from other tested anions including F−
.
The detection limit for AcO− was found to be 7.37 μM and 22.9 μM using the receptors L1 and L2, respectively.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Fluorescent sensor
Acetate
Schiff base
Vitamin B6 cofactor derivative
1. Introduction
receptors have gained wide applications in the detection of acetate
anion. Acetate anion is a vital component of numerous metabolic pro-
Anions play important role in various biological, environmental and
industrial processes [1–5]. Therefore, anion recognition and sensing
have gained an immense interest among the scientific community in
the recent years. Because of the simplicity and low cost of chemosensors
based on colorimetric (UV–Vis) and fluorescence changes, to date, sev-
eral anion selective chemosensors have been reported [6–19]. Most
common chemosensors contained polar-NH protons of urea or thiourea,
amide, pyrrole and imidazolium moieties for the recognition of target
anion [1–5]. Synthetic of such sensors required multi-steps reactions,
tedious purification work and sometimes exhibit delayed response for
anion recognition event. In order to avoid such complexity, Schiff
bases containing polar-OH/NH groups are applied for the sensing of
anions because of the simple preparation without any need of further
purification, high yield and can be structurally modified easily according
to choice [20]. Schiff bases can induced immediate optical changes for
the selective detection of target anion mainly due to the formation of
hydrogen bonded host-guest complex, deprotonation of polar-OH/NH
groups and less commonly chemodosimeter type reaction with anions.
The synthetic simplicity and fascinating optical properties of Schiff base
cesses and plays specific behaviours in the activity of enzymes, protein
synthesis, transport of hormones and DNA regulation [21–23]. The
acetate production and oxidation rate have been used as an indicator
of organic decomposition in marine sediments and transmetalation of
tetrapyrroles [24–26].
In this article, as a part of our ongoing research on ions sensing using
Schiff base derivatives of vitamin B6 cofactors like pyridoxal (PL) and
pyridoxal-5-phosphate (PLP), we have introduced two new Schiff base
ligands by the reaction of PLP with 2-aminophenol (L1) or aniline (L2)
(Scheme 1). In mixed DMSO–H2O medium, the receptors showed high-
ly selective ‘turn-on’ fluorescence in the presence of acetate among the
other tested anions.
2. Experimental
2.1. General
The reagents and chemicals were purchased commercially either
from SpectroChem Pvt. Ltd. or Sigma-Aldrich. Solvents such as DMSO
and methanol were purchased from Merck, India and were used with-
out any further purification. The anions used for the sensing studies
were in the form of their tetra-n-butylammonium (TBA) salts.
⁎
Corresponding author at: Department of Applied Chemistry, SV National Institute
Technology, Surat, Gujarat, India.
All experiments were carried out at 298 K, unless otherwise
E-mail addresses: suban_sahoo@rediffmail.com, sks@ashd.svnit.ac.in (S.K. Sahoo).
Contributed equally.
mentioned. 1H NMR spectra were recorded on a BRUKER AVANCE II
1
http://dx.doi.org/10.1016/j.saa.2015.12.024
1386-1425/© 2015 Elsevier B.V. All rights reserved.

D. Sharma et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 110–115
111
Scheme 1. Synthesis of receptors L1 and L2.
400 MHz NMR in DMSO-d6 using tetramethylsilane (TMS) as an internal
2.3. Synthesis of L2
standard. The IR spectra were recorded on a Perkin-Elmer IR spectro-
photometer using KBr pellet. The mass spectra were recorded on a
Waters Q-TOF micromass (LC-MS). All Elemental analysis data were ob-
tained by using the CE Instrument Corporation EA 1108. Melting points
were measured on a digital melting point apparatus VMP-DS “VEEGO”
and is uncorrected. UV–Vis spectra were recorded on a VARIAN CARY
50 Spectrophotometer in the wavelength range of 200–700 nm with a
quartz cuvette of path length 1 cm. Fluorescence spectra were recorded
with an Agilent Cary Eclipse fluorescence spectrophotometer.
For all the spectroscopic experiments, the stock solutions of the re-
ceptors (1.0 × 10−4 M) and the anions (TBA salts, 1.0 × 10−3 M) were
prepared in DMSO. These solutions were used for various experiments
after appropriate dilutions. For spectroscopic titrations, required
amount of the receptors (2 mL, 1.0 × 10−5 M/5.0 × 10−5 M) was
taken directly into cuvette and the spectra were recorded after each
incremental addition of anion by using micropipette (10 μL/50 μL,
1.0 × 10−3 M).
Receptor L2 was prepared by stirring aniline (0.075 g, 0.0008 mol)
and pyridoxal-5-phosphate (0.2 g, 0.0008 mol) in 25 mL methanol.
The yellow colour precipitates were filtered and dried. Yield: 70%;
M.P.: 187 °C; FTIR (KBr, υmax, cm−1): 3424, 3089, 3079, 2910, 2659,
1751, 1685, 1605, 1581, 1524, 1484, 1449, 1386, 1329, 1240, 1180,
1080, 1041, 1023, 985, 949, 923, 833, 797, 776, 750, 714, 686, 649,
615, 571, 529, 497. 1H NMR (400 MHz, DMSO-d6, δ ppm): 14.16 (1H,
s, −OH), 9.25 (1H, s, −CH_N), 8.05 (1H, s, Ar–H), 7.58–7.39 (5H, Ar–
H), 5.19 (2H, d, −CH2), 2.08 (3H, s, −CH3); LC-MS (m/z) for
[C14H15N2O5P + H+]: calculated 323.26 and found 323.00; Anal. Calc
for C14H15N2O5P: C 52.18, H 4.69, N 8.69. found: C 51.98, H 4.63, N 8.63.
3. Results and discussion
The receptors L1 and L2 were synthesized by one step Schiff base
condensation method (Scheme 1), and characterized by various spec-
troscopic (IR and 1H NMR) and LC-MS data. Then, the anion sensing
ability of the receptors was investigated by monitoring the absorbance
and fluorescence behaviour upon addition of various anions, such as
2.2. Synthesis of L1
Receptor L1 was prepared by stirring 2-aminophenol (0.087 g,
0.0008 mol) and pyridoxal-5-phosphate (0.2 g, 0.0008 mol) in 25 mL
methanol. The yellow colour precipitates were filtered and dried.
Yield: 75%; M.P.: 210 °C; FTIR (KBr, υmax, cm−1): 2949, 2904, 2829,
2825, 2719, 2599, 2401, 2359, 2325, 2142, 2065, 1892, 1868, 1804,
1775, 1747, 1713, 1700, 1650, 1601, 1541, 1433, 1381, 1336, 1304,
1272, 1242, 1216, 1178, 1017, 988, 946, 886, 864, 828, 778, 761, 707,
640, 532, 519, 485, 449. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 15.01
(s, 1H, −OH), 9.29 (s, 1H, −CH_N), 8.00 (s, 1H, Ar–H), 7.58 (d, 1H,
−OH), 7.57–6.91 (m, 4H), 5.17 (d, 1H, −CH2); LC-MS (m/z,) for
[C14H15N2O6P + H+]: calculated 339.26 and found 339.14; Anal. Calc
for C14H15N2O6P: C 49.71, H 4.47, N 8.28. found: C 49.65, H 4.48, N 8.20.
F
−, Cl−, Br−, I−, HSO4−, H2PO−4 and AcO− in DMSO and mixed DMSO–
H2O medium.
The UV–Vis absorption spectrum of L1 (1.0 × 10−5 M, in DMSO)
showed a strong absorption at 360 nm due to π–π* transitions. Upon ad-
dition of F− and AcO− (Fig. 1a), a new charge transfer band with absorp-
tion maxima at 450 nm was observed which indicates the formation of
an anion-receptor complex through intramolecular hydrogen bonding
and/or the anion induced deprotonation of the receptor. The appear-
ance of new band also indicates the internal charge transfer (ICT) oc-
curred between the receptor and the anions added. Under similar
conditions, the receptor L2 alone showed absorption bands at 300 nm
and 350 nm. The receptor L2 solution resulted red-shift in the
Fig. 1. UV–Vis spectral changes of (a) L1 and (b) L2 ([L] = 1.0 × 10−5 M) upon addition of equivalent amount of different anions in DMSO. Inset shows the naked-eye detectable colour
changes of the receptors. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

112
D. Sharma et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 110–115
anions showed slightly higher anion binding ability than L2. Further,
the analysis of the BH plots supports the formation of receptor-anion
interaction in 1:1 stoichiometry, which was further confirmed from
the Job's plots (Fig. S5).
Considering the similar spectral changes of L1 and L2, we have
recorded the 1H NMR spectra of L1 in the absence and presence of
different equivalents of TBAF in DMSO-d6 to give clear evidence on the
host-guest interaction occurred between the receptor and anions
(Fig. 2). The receptor L1 showed singlet due to the pyridoxal–OH,
imine–H and aromatic–OH protons at 15.01 ppm, 9.29 ppm and
7.58 ppm, respectively. Addition of F− from 0 to 3 equivalents resulted
in the downfield shift and broadening of the OH protons signal.
Simultaneously, the signal due to imine–H proton was also shifted to
downfield region. At higher equivalents of F−, the intensity of the
pyridoxal–OH proton signal was weakened may be due to the possible
elongation of O–H bond on interaction with F−. Other peaks of L1
were shifted to upfield region due to through-bond effect, but no
peaks disappeared even in the excess addition of F−. Thus, the NMR
results supporting the participation of pyridoxal–OH, imine–H and
aromatic–OH protons in the anion recognition events by forming multi-
Fig. 2. The 1H NMR titration of receptor L1 in the presence of different equivalents of TBAF
(from top: 3, 2, 1 and 0 equivalents) in DMSO-d6.
absorption band with the appearance of a new absorption band centred
at 425 nm upon addition of F− and AcO− (Fig. 1b). The addition of other
anions Cl−, Br−, I−, H2PO4− and HSO−4 did not result in obvious spectral
changes of L1 and L2 even in abundance. The appearance of new charge
transfer absorption band resulted an instantaneous colour change of L1
and L2 (inset Fig. 1), which help in the naked-eye detection of F− and
AcO−. However, no obvious colour changes of L1 and L2 were observed
in the presence of other tested anions such as H2PO−4 , HSO4−, Cl−, Br−
and I−. These results clearly supported the colorimetric selectivity of
L1 and L2 towards F− and AcO− in DMSO.
ple intermolecular hydrogen bonds with F−
.
Considering the 1H NMR results, the probable 3D structure of recep-
tors and their complexes with anions (F− and AcO−) was calculated by
the DFT method at B3LYP/6-31G(d,p) level in the gas phase. All calcula-
tions were performed using the computer program Gaussian 09 W [28].
The enolimine form of receptors was found to be energetically more sta-
ble than the ketoenamine form (L1 is stable by 1.95 kcal/mol whereas L2
by 1.96 kcal/mol). Both the receptors showed the presence of the intra-
molecular hydrogen bond which caused an appreciable downfield shift
of pyridoxal–OH signal in 1H NMR. As shown in Fig. 3, apparent confor-
mational rotation of receptors was observed upon interaction with F−
and AcO− because of the participation of imine–H and –OH protons in
the recognition of anions through multiple intermolecular hydrogen
bonds.
The anion binding abilities of receptors L1 (Fig. S1–2) and L2 (Fig. S3–
4) (2 mL, 1.0 × 10−5 M) were next investigated by recording the UV–Vis
spectra of the receptors with the incremental addition of anions (F− and
AcO−) in DMSO. The shift of the receptor absorption bands with the ap-
pearance of the new charge transfer band indicates the formation of a
new complex species with unique spectroscopic properties as a result
of interaction between the receptor and the anion added. The spectral
data were analysed to estimate the binding constants (K) by applying
Benesi–Hildebrand equation [27]. The selectivity trend of anion binding
affinity of L1 was determined to be F− (6.94 × 104 M−1) N AcO−
(3.52 × 104 M−1). The similar binding affinity of L2 was determined,
i.e. F− (3.39 × 104 M−1) N AcO− (1.90 × 104 M−1). Receptor L1 having
more number of hydrogen bond donor groups for the recognition of
The anion binding behaviour of the receptors was next investigated
by fluorescence methods (Fig. 4). The receptors L1 and L2
(5.0 × 10−5 M) showed a weak emission band centred at 520 nm
(λexc = 450 nm) and 475 nm (λexc = 415 nm) in DMSO, respectively.
Fig. 3. DFT computed optimized structure: (a) L1, (b) L1·AcO−, (c) L1·F−, (d) L2, (e) L2·AcO− and (f) L2·F−
.

D. Sharma et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 110–115
113
Fig. 4. Fluorescence spectral changes of L1 (a, c) and L2 (b, d) ([L] = 5.0 × 10−5 M) upon addition of different anions (2.5 × 10−4 M) in DMSO (a, b) and mixed DMSO–H2O medium (b, c).
(For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
Fig. 5. Fluorescence titration of L1 (5.0 × 10−5 M) upon incremental addition of AcO− (50 μL–450 μL, 1 × 10−3 M) in DMSO containing 5% H2O. Inset shows the [AcO−] vs fluorescence
intensity at 525 nm.

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The fluorescence of receptor L1 was enhanced selectively upon incre-
mental addition of AcO− (Fig. 4a). No significant fluorescence changes
were observed with other tested anions, except F− ions showed a
weak change in the fluorescence profile of L1. The high selectivity of L1
towards the triangular shaped (Y-shaped) AcO− might be due to the
shape complementarity to form multiple hydrogen bonds with the
two –OH groups. In contrast, the receptor L2 showed a red-shifted and
enhanced fluorescence at 510 nm in the presence of both F− and
AcO− (Fig. 4b). Upon addition of H2PO−4 also caused a weak fluorescent
enhancement of L2 but no significant changes were observed with other
examined anions.
titrations of L1 (Fig. 5) and L2 (Fig. 6) were performed with incremental
addition of AcO− ions in DMSO containing 5% H2O and 25% H2O, respec-
tively. Upon successive addition of AcO− ions, the fluorescence of L1 was
enhanced continuously at 525 nm whereas L2 at 510 nm. Receptor L1
showed a good linearity range (5 μM to 25 μM) for the detection of
AcO− with the estimated LOD (3α/slope) of 7.37 × 10−6 M. However,
the LOD of L2 for AcO− detection was found to be 2.29 × 10−5 M. The
receptor L1 with multiple hydrogen bond donor groups best fit with
the Y-shaped AcO− to form the host-guest complex in compared to L2
and therefore, the receptor L1 showed better AcO− detecting ability
than the L2.
Encourage from the higher selectivity of L1 towards AcO−, the anion
sensing ability of the receptors was examined in the mixed DMSO–H2O
medium. Fluoride ion is highly solvated in water medium due to its high
hydration energy in comparison to AcO−. Considering this fact, we be-
lieve that the receptors L1 and L2 can be developed as an AcO− selective
fluorescent ‘turn-on’ sensors in the mixed DMSO–H2O medium. The
anion selectivity of the receptors L1 and L2 was examined in DMSO con-
taining a varying % of H2O. When tested in mixed solvent DMSO–H2O
(95:5, v/v), the receptor L1 showed AcO− anions selective fluorescence
enhancement at 525 nm (Fig. 4c). Other anions including F− showed
either quenching or no change in the fluorescence profile of L1. Interest-
ingly, the anion selectivity of L2 in mixed solvent (DMSO:H2O, 3:1, v/v)
was also drastically changed. As shown in Fig. 4d, the fluorescence
enhancement was observed selectively in the presence of AcO−. No
noticeable fluorescence changes of L2 were observed with other tested
anions including H2PO−4 , except F− showed a red-shift but without
any significant enhancement in the fluorescence intensity of L2. The
above results clearly indicate the fluorescence ‘turn-on’ selectivity of
L1 and L2 towards AcO− in mixed DMSO–H2O medium.
4. Conclusions
In conclusion, we have introduced two new easy-to-prepare fluores-
cent ‘turn-on’ chemosensors derived using vitamin B6 cofactor
pyridoxal-5-phosphate for the selective detection of AcO− in mixed
DMSO–H2O medium. The developed sensors can detect AcO− without
any noticeable interference from other tested anions and showed a de-
tection limit down to micromolar range. From this study, we believe
that pyridoxal-5-phosphate will be used to develop many more sensors
for the selective sensing of bioactive anions.
Acknowledgement
This work was made possible by a grant from the DST SERB, New
Delhi (SR/S1/IC-54/2012). The authors wish to thank the Director, S.V.
National Institute of Technology, Surat for providing necessary facilities
and scholarship.
The fluorescence anion sensing ability of receptors L1 (Fig. S6) and L2
(Fig. S7) was investigated under competitive environment which clear-
ly inferred that the fluorescence enhancement induced by AcO−
Appendix A. Supplementary data
showed negligible interference in the presence of other anions viz. F−
,
Cl−, Br−, I−, HSO4−, and H2PO−4 in mixed DMSO–H2O medium. Further,
in order to determine the limit of detection (LOD), the fluorescence
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.saa.2015.12.024.
Fig. 6. Fluorescence titration of L2 (5 × 10−5 M) on subsequent addition of AcO− (50 μL–500 μL, 1 × 10−3 M) in (DMSO: H2O, 3:1). Inset shows the [AcO−] vs fluorescence intensity at
510 nm.

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