A chiral (S)-BINOL based fluorescent sensor for the recognition of Fe(III) and cascade discrimination of α-amino acids

Article abstract of DOI:10.1016/j.tetasy.2016.05.002

Minor groove thiosemicarbazone functionalized (S)-BINOL 1 has been synthesized starting from (S)-BINOL; these new thiosemicarbazone functionalized (S)-BINOL molecules introduce new opportunities in the field of chiral recognition and Fe(III) sensing. The developed sensor can serve as a turn off sensor for Fe(III) with high selectivity among metal ions tested. The in situ generated Fe(III) complex of 1 exhibit remarkable fluorescent chiral discrimination toward unmodified α-amino acids via a ligand displacement mechanism.

Full text of DOI:10.1016/j.tetasy.2016.05.002

Tetrahedron: Asymmetry 27 (2016) 492–497
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
A chiral (S)-BINOL based fluorescent sensor for the recognition
of Fe(III) and cascade discrimination of
a-amino acids
⇑
Sathishkumar Munusamy, Sathiyanarayanan Kulathu Iyer
Chemistry Division, School of Advanced Sciences, Vellore Institute of Technology, Vellore 632014, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Minor groove thiosemicarbazone functionalized (S)-BINOL
1 has been synthesized starting from
Received 6 April 2016
Accepted 3 May 2016
Available online 14 May 2016
(S)-BINOL; these new thiosemicarbazone functionalized (S)-BINOL molecules introduce new
opportunities in the field of chiral recognition and Fe(III) sensing. The developed sensor can serve as a
turn off sensor for Fe(III) with high selectivity among metal ions tested. The in situ generated Fe(III)
complex of 1 exhibit remarkable fluorescent chiral discrimination toward unmodified
a-amino acids
via a ligand displacement mechanism.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
membranes (phospholipid head groups). This labile iron, when
exposed to molecular oxygen, enters the redox cycle between Fe2+
and Fe3+ which would in turn assist the formation of an hydroxyl
radical via a Fenton reaction. Since radical species are highly reac-
tive, they are prone to react with sugars, lipids, proteins and nucleic
acids, which would lead to tissue damage. Severe complications
such as b-thalassemia are treated by iron chelation therapy, since
the human biological system does not have natural means to con-
trol iron overload. Considering the crucial role of iron in biological
functions, great importance is attached to rapid and simple analyt-
ical method development.5 The intrinsic paramagnetic properties of
Fe3+ make it an effective quencher of fluorescence of fluorescent
metal chelators. Subsequent effective snatching of Fe3+ by Fe3+
binding analytes (amino acids, etc.) can turn on the fluorescence
lost by chelators. Herein we have designed and synthesized a
BINOL-thiosemicarbazide (minor groove) based fluorescent sensor,
which could serve as a fluorescent turn off sensor for Fe(III). The
in situ generated Fe(III) 1 containing complex was successfully
studied for the fluorescent enantioselective recognition of unmod-
ified amino acids.
Enantioselective recognition is one of the most important fun-
damental processes in the biological field. It plays an important
role in many areas, such as asymmetric synthesis, chiral drug
separation and in understanding the chiral origin of life. Thus, a
simple, high-throughput method for enantiomeric excess mea-
surements would be useful in a number of applications, such as
the synthesis of most natural products and drug development.1
While there are successful traditional methods, such as chiral
HPLC, gas chromatography and NMR chiral shift reagents, optical
methods, such as fluorescence chiral discrimination methods for
chiral discrimination,2 have gained much importance due to their
operational simplicity and utility in high-throughput screening.
Amino acids are fundamental molecules of life and are associated
in many bimolecular processes. Hence, studies on the enantioselec-
tive recognition of amino acids and their derivatives are highly
desirable.3
The crucial role of iron in biological functions is well known. The
facile, redox chemistry and its high affinity for oxygen make it an
important partner in oxygen metabolism and in electron transfer
processes in DNA and RNA. Although it plays a significant role in
many biological functions, when present in excess, it can have a
detrimental effect to the biological system.4 Although iron is
strongly bonded to the enzymes and proteins present in the biolog-
ical system, a minor fraction of iron, known as labile iron, is loosely
bonded to the organic anions, such as phosphates and carboxylates,
polyfunctional ligands such as chelates, surface components of
2. Results and discussion
The synthesis of chiral ligand 1 is outlined in Scheme 1. Com-
pound 2 was synthesized according to the literature procedure.6
As shown in Scheme 1, BINOL-thiosemicarbazone ligand 1 was
synthesized in 4 steps with 55% overall yield from (S)-BINOL. The
UV–Vis spectra of 1 (2 ꢀ 10ꢁ5 M) were recorded in THF solution.
There were two absorption peaks at 344 nm and 290 nm. In order
to check the absorption change of 1 over the addition of metal ions
including alkali earth, p-block and transition metal, UV–vis was
⇑
Corresponding author.
E-mail address: sathiya_kuna@hotmail.com (S. Kulathu Iyer).
http://dx.doi.org/10.1016/j.tetasy.2016.05.002
0957-4166/Ó 2016 Elsevier Ltd. All rights reserved.

S. Munusamy, S. Kulathu Iyer / Tetrahedron: Asymmetry 27 (2016) 492–497
493
CHO
OH
OH
chloromethylmethyl
ether
2.4 M n-BuLi
0°C - RT
O
O
O
O
O
O
O
O
DMF
DMF, NaH,0°C
CHO
(S)-BINOL
4
3
acid hydrolysis
H
N
H
N
CHO
NH2
NH2
H2N
N
N
S
S
OH
OH
OH
OH
H+, EtOH
S
CHO
N
H
NH2
1
2
Scheme 1. Synthesis procedure of the sensor 1.
recorded with various metal ions. There was no change in the UV–
vis spectra of 1 over the addition of metal ions such as Ba2+, Bi2+
Inspired by the UV–Vis results of 1 with various metal ions and
its selectivity towards Fe(III), we next investigated the fluorescence
change of ligand 1 (1 ꢀ 10ꢁ5 mol/L in THF) towards Fe3+. The fluo-
rescence spectrum of ligand 1 showed an emission at 394 nm,
which can be attributed to ESIPT.7 A gradual decrease in fluorescent
intensities was observed with the addition of Fe3+ from 0.1 to 1
equivalent (Fig. 1a). The fluorescence intensity of sensor 1 with
Fe3+ revealed that there was a good linear response to Fe3+ (Fig. 1b).
The emission intensity decrease of 1 can be attributed to the
suppression of the excited state intramolecular proton transfer
(ESIPT) of ligand 1 over the addition of Fe3+. The selectivity of flu-
,
Cd2+, Co2+, Fe2+, Hg+, Ni2+, Pd2+, Sr2+, Cu2+, Mn2+ and Zn2+. However,
there was a significant change in the absorption spectra of 1 over
the addition of Fe3+, i.e., the peak at 344 nm completely disappeared
and the intensity of the peak at 290 nm increased (Supporting infor-
mation). An isobestic point appeared at 324 nm. In order to check
the linear change in the absorption of 1 with Fe(III), the absorption
was monitored over a constant increment of Fe(III). As expected, a
linear change in the absorption of 1 occurred with a constant incre-
ment Fe3+ concentration (Supporting information). The UV–Vis
spectra were recorded with concentrations ranging from 0 to
3 ꢀ 10ꢁ5 mol/L in order to check the formation of ground state
intermolecular aggregates. Suitable concentrations for fluorescence
studies should not be greater than 2 ꢀ 10ꢁ5 mol/L in order to avoid
the violation of Lambert–Beer’s Law.
orescence response change of 1 was investigated towards Ba2+
,
Bi2+, Cd2+, Co2+, Fe2+, Hg+, Ni2+, Pd2+, Sr2+, Cu2+, Mn2+ and Zn2+ under
the same experimental conditions. Fortunately, as shown in the
Figure 2, almost no or only a moderate fluorescence change was
observed with the addition of Ba2+, Bi2+, Cd2+, Co2+, Fe2+, Hg+,
Figure 1. (a) Fluorescence spectra of 1 (1 ꢀ 10ꢁ5 M) with an increasing concentration of Fe(III) from 0.1 to 1 equivalents (excitation wavelength = 324 nm). (b) Fluorescence
decrease of 1 vs concentration of Fe(III).

494
S. Munusamy, S. Kulathu Iyer / Tetrahedron: Asymmetry 27 (2016) 492–497
Figure 2. (a) Fluorescence responses of 1 (1.0 ꢀ 10ꢁ5 mol/L, excitation wavelength = 324 nm) to various metal ions (1.0 ꢀ 10ꢁ5 mol/L). (b) Bar representation of fluorescence
responses of 1 (1.0 ꢀ 10ꢁ5 mol/L, excitation wavelength = 324 nm) to various metal ions (1.0 ꢀ 10ꢁ5 mol/L).
Ni2+, Pd2+, Sr2+, Cu2+, Mn2+ and Zn2+. Although Cu2+ and Fe2+ can
cause considerable fluorescent emission quenching, significant flu-
orescence quenching occurred only in the case of Fe3+. Hence, with
this high selectivity and sensitivity of ligand 1 towards Fe3+, ligand
were carried out. Since amino acids can associate with Fe3+ and
displace ligand 1, they may bring back the fluorescence of 1. In
addition to this, since 1 is chiral, there would be difference in the
ligand displacing efficiency between the two enantiomers of amino
acids. We investigated enantioselective recognition of the in situ
generated 1:1 Fe(III)-containing complex sensor towards various
unprotected amino acids. The effect of the enantiomers of alanine
(1 ꢀ 10ꢁ5 mol/L in THF, 50% v/v water) on fluorescence enhance-
ment of the solution of ligand 1 and Fe3+ (1 ꢀ 10ꢁ5 mol/L in THF)
is illustrated in Figure 4. As shown in Figure 4, both enantiomers
1 can be used as fluorescent turn off sensor for Fe3+
.
In order to monitor the conformation change of ligand 1 over
the addition of Fe3+, CD spectra of sensor 1 in the presence and
absence of Fe3+ were recorded in THF solvent. As shown in Figure 3,
the cotton effect of 1 did not change with the addition of Fe3+. This
indicates the conservation of the cisoid conformation of 1. In addi-
tion, the CD spectra of the in situ formed Fe3+ complex showed a
blue shift of approximately 2 nm relative to 1, thus indicating that
the dihedral angel of the binaphthyl unit was slightly enlarged.
This can be attributed to the fact that the Fe3+ had possibly been
captured by 1 and had entered the minor grooves of ligand 1. All
of these results corroborate and reveal the interaction.
could enhance the fluorescence response, whereas
a large increase in the fluorescence intensity; conversely, the
L
-alanine causes
-iso-
D
mer had little effect on the fluorescence intensity of the solution of
ligand 1 and Fe3+ under the same experimental conditions. Fur-
thermore, there was no difference in the absorption wavelength
between the guest-free sensor and the sensor with a guest. The
enantioselective recognition effect of the sensor on the chiral enan-
tiomer of the amino acids can be measured by calculating the
enantiomeric fluorescent difference ratio, ef [ef = (IL ꢁ Io)/(ID ꢁ Io)].
Herein, Io represents fluorescence intensity of the complex sensor
in the absence of a chiral guest, while ID and IL represent the fluo-
Next, in order to explore the cascade chiral recognition of ligand
1 with Fe3+, subsequent chiral recognition evaluations of the in situ
generated ligand 1 containing Fe3+ complex toward amino acids
rescence intensities of sensor in the presence of L-/D-isomers. The
enantiomeric fluorescent difference ratio in the case of alanine is
2.52 at 1:100 molar ratio, which indicates that the in situ prepared
chiral Fe3+ complex of ligand 1 exhibits an enantioselective fluores-
cent response towards L-alanine. We further investigated the fluo-
rescence enhancement of yjr in situ generated complex sensor
with alanine by gradually increasing the concentration. Upon the
addition of L-alanine and D-alanine in the range from 1:10 to
1:100 molar ratios, the fluorescence showed a gradual increase
(Fig. 4d).
Inspired by the preliminary results of alanine, we then investi-
gated the fluorescence chiral discrimination ability of the complex
sensor with various other unprotected amino acids such as proline,
valine, serine and methionine. The selected amino acids were
engaged in titration experiments with the in situ prepared Fe3+
complex of ligand 1. As expected, we obtained a fluorescence
response similar to that of alanine. The respective enantiomeric
fluorescent difference ratio is given in Table 1.
The influence of the enantiomeric composition of alanine on the
fluorescence intensity of Fe3+ complex of ligand 1 (1 ꢀ 10ꢁ6 M in
THF) was studied for the quantitative analysis of amino acids. In
Figure 3. CD spectra of 1 and 1 with an equivalent amount of Fe3+ in acetonitrile
(c1 = 1 ꢀ 10ꢁ5 mol/L, cFe(III) = 1 ꢀ 10ꢁ5 mol/L).

S. Munusamy, S. Kulathu Iyer / Tetrahedron: Asymmetry 27 (2016) 492–497
495
Figure 4. (a) Fluorescent spectra of complex sensor with increasing concentration fluorescent spectra of complex sensor with
L
-alanine. (b) Fluorescent spectra of complex
sensor with increasing concentration fluorescent spectra of complex sensor with
D-alanine. (c) Combined fluorescence spectra of complex sensor with and without the
addition and -alanine. (d) Fluorescent enhancement of complex sensor versus the increasing concentration
L
D
L
and
D
-alanine (complex sensor: 1 ꢀ 10ꢁ6 mol/L in acetonitrile:
excitation wavelength = 324 nm).
Table 1
with the enantiomeric composition of L- and D-alanine. Thus, by
Amino acids employed in enantioselective sensing studies and their respective
enantiomeric fluorescent difference ratio
using this in situ preparation and by monitoring the fluorescence
response towards the enantiomers of amino acids under the same
conditions will provide an easy means of determining the enan-
tiomeric excess.
The chiral discrimination studies of phenylalanine with the
in situ prepared complex were carried out. There was no significant
emission difference of the complex sensor over the introduction of
Amino acid
(
D
I/Io)max
ef
4.4
4.4
2.0
6.5
0.43
2.52
1.61
2.1
L
L
L
L
L
- and
- and
- and
- and
- and
D
D
D
D
D
-alanine
-valine
-serine
1.9
-methionine
-phenylalanine
0.12
L- and
D-phenylalanine. Due to the presence of a bulky aromatic
group at the
a
-position of phenylalanine, this amino acid was
unable to enter the minor grooves of the sensor. Hence the replace-
Figure 5, curve (a) shows the fluorescence enhancement of com-
plex sensor over the addition of alanine at various enantiomeric
ment of Fe(III) with amino acid was hindered, which led to a sim-
ilar emission enhancement of the complex sensor by both L- and D-
composition of
cence enhancement of complex sensor with the same amount of
-alanine alone. As can be seen from Figure 5, the -isomer exhib-
ited a greater enhancement in the fluorescence intensity than in
the case of both - and -enantiomers. The fluorescence enhance-
ment of the sensor at 394 nm showed an almost linear relationship
D
- and
L
-amino acids. Curve b shows the fluores-
phenylalanine. While ligand 1 was treated with enantiomers of
alanine without the addition of Fe(III), there was no change in
the fluorescence intensity of 1. Even after the excessive addition
of the enantiomers of alanine, the fluorescence response behaviors
of the two enantiomers remained the same, thus indicating the
necessity of Fe(III) for chiral recognition.
L
L
L
D

496
S. Munusamy, S. Kulathu Iyer / Tetrahedron: Asymmetry 27 (2016) 492–497
3:1); [a]
25 = +98 (c 1, THF). 1H NMR (400 MHz, CDCl3). d 8.03–
D
8.00 (d, J = 12 Hz, 2H), 7.95–7.93 (d, J = 8 Hz, 2H), 7.67–7.94 (d,
J = 12H, 2H), 7.43–7.39 (m, 2H), 7.31–7.24 (m, 4H), 5.16–5.14 (d,
J = 8 Hz, 2H), 5.05–5.03 (d, J = 8 Hz, 2H), 3.2 (s, 6H); 13C NMR
(400 MHz, CDCl3) d 152.7, 134.1, 129.9, 129.4, 127.9, 126.3,
125.6, 124.1, 121.3, 117.3, 95.2, 55.8.
4.3. (S)-3,30-Diformyl-2,20-bis(methoxymethoxy)-1,10-binaphtha-
lene 3
Compound 4 (1.23 g, 3.3 mmol) was taken in two neck RB flask
and dissolved in 50 ml of THF. To this N2 was purged and the tem-
perature was reduced to 0 °C. Next, n-butyllithium (2.5 M in hex-
ane, 4.5 mL, 11 mmol) was added to the reaction mixture via
syringe. After the addition of n-butyllithium, the reaction mixture
was stirred for 3 h at room temperature. The reaction mixture was
turned into grey suspension. After this, the reaction mixture was
cooled to 0 °C and DMF (0.92 ml, 12 mmol) was added. The reac-
tion mixture was allowed to stir at room temperature overnight.
Next, saturated NH4Cl (25 ml) was added to the reaction mixture
to quench the reaction. The reaction mixture was extracted with
ethyl acetate (100 ml). The organic layer was washed with brine
and dried over MgSO4. The solvent was removed under reduced
pressure and the residue was purified by column chromatography
on silica gel using hexane/ethyl acetate (4:1) to give the title com-
pound. Isolated yield, 55%. Rf = 0.17 (hexane/ethyl acetate 4:1). 1H
NMR (400 MHz, CDCl3). d 10.67 (s, 1H), 8.57 (s, 1H), 8.04–7.98
(m, 2H), 7.90–7.87 (d, J = 12 Hz, 1H), 7.61–7.59 (d, J = 8 Hz, 1H),
7.47–7.43 (t, J = 8 Hz, 1H), 7.39–7.33 (q, J = 8 Hz, 2H), 7.30–7.20
(m, 2H), 7.18–7.16 (d, J = 8 Hz, 1H), 5.15–5.14 (d, H = 4 Hz, 1H),
5.04–5.03 (d, J = 4 Hz, 1H), 4.75–4.74 (d, J = 4 Hz, 1H), 4.64–4.63
(d, J = 4 Hz, 1H), 3.15 (s, 3H), 2.99 (s, 3H); 13C NMR (400 MHz,
CDCl3) d 191.2, 152.9, 131.0, 133.7, 131.0, 130.3, 130.2, 129.6,
129.0, 128.0, 126.9, 126.8, 126.0, 125.9, 125.2, 124.3, 119.4,
116.3, 100.2, 94.9, 57.1, 56.0.
Figure 5. (a) Fluorescence enhancement of complex sensor (1 ꢀ 10ꢁ6 mol/L) in the
presence of various enantiomeric compositions of alanine. (b) Fluorescence
enhancement of complex sensor (1 ꢀ 10ꢁ6 mol/L) in the presence of enantiomer-
ically pure L-alanine.
3. Conclusion
In conclusion, a new (S)-BINOL based chiral ligand 1 has been
designed and synthesized, with excellent fluorescence turn-off
sensitivity towards Fe(III). This ligand exhibited good selectivity
among a series of various group metals tested. Furthermore, the
in situ generated Fe(III) complex of 1 showed considerable fluores-
cence enhancement responses with remarkable enantioselectivi-
ties towards unmodified
mechanism8 can be attributed to the recognition of Fe(III) and
unmodified -amino acids.
a-amino acids. The ligand displacement
a
4.4. Synthesis of (S)-3,30-diformyl-2,20-dihydroxy-1,10-binaph-
4. Experimental
4.1. General
thalene 2
Compound 3 (1 g, 2.5 mmol) was dissolved in THF (10 ml). The
temperature of the reaction mixture was reduced to 0 °C, and then
12 M HCl (10 ml) was added over 5 min with stirring. After the
addition of the acid, the reaction mixture was allowed to stir at
room temperature for approximately 3 h. After completion of the
reaction, the reaction mixture was extracted with ethylacetate.
The organic layer was washed with satd. NaHCO3, brine and dried
over MgSO4. The organic layer was allowed to evaporate at room
temperature, after which a yellow color solid that settled was dried
and recrystallized (0.95 g, 99%). Rf = 0.35 (hexane/ethyl acetate
Commercially available solvents and regents were purchased
from Sigma Aldrich and Avra synthesis. Nuclear magnetic
resonance spectra were recorded on a Bruker Ascent 400 MHz
spectrometer. Chemical shift values are reported in d in parts per
million using trimethyl silane (TMS) as the standard. Absorbtion
spectra were recorded on a Shimadzu 3600 spectrophotometer.
Emission spectra were recorded on a Perkin Elmer LS-55 lumines-
cence spectrometer.
4:1); mp 285 °C;
[a]
25 = +249.5 (c 0.8, CH2Cl2). 1H NMR
D
4.2. Synthesis of 2-(methoxymethoxy)-1-(2-(methoxymethoxy)-
naphthalen-1-yl)naphthalene 4
(400 MHz, DMSO-D6). d 10.58 (s, 2H), 10.18 (s, 2H), 8.34 (s, 2H),
8.00–7.98 (m, 2H), 7.43–7.37 (m, 4H), 7.25 (s, 1H), 7.21–7.19 (m,
2H); 13C NMR (400 MHz, DMSO-D6) d 196.7, 153.6, 138.5, 137.4,
130.7, 130.0, 127.6, 124.8, 124.5, 122.1, 166.3.
Stirred suspension of sodium hydride (60% suspension in min-
eral oil, 1.18 g, 29.5 mmol) in dimethylformamide (DMF) (20 ml)
was added to a solution of (S)-BINOL (1.2 g, 4.2 mmol) in DMF at
0 °C. After stirring the reaction mixture for 10 min at 0 °C, methoxy
methyl chloride (1.25 ml, 16.65 mmol) was added and the reaction
mixture was allowed to stir at 0 °C for 1 h. The reaction was contin-
ued at room temperature for 3 h. The reaction mixture was
quenched with water and extracted with diethyl ether (60 ml).
The extracted organic layer was washed with saturated NaHCO3
solution, brine, and dried over magnesium sulfate. The title com-
pound, with diethyl ether as the solvent, was allowed to recrystal-
lize at room temperature. Colorless crystals of the title compound
appeared with 95% isolated yield. Rf = 0.39 (hexane/ethyl acetate
4.5. Synthesis of 1
A mixture of 2 (2 mmol), ammonium acetate (4 mmol), benzil
(4 mmol) and iodine (10 mol %) in 10 ml of ethanol was added to
an oven dried RB flask. Thereaction mixture was refluxed for approx-
imately 4 h. Completion of the reaction was monitored by TLC. After
completion of the reaction, iodine was removed by the addition of
saturated solution of Na2S2O3. The crude product obtained was puri-
fied by column chromatography using 20% ethylacetate in hexane as
eluent to yield 1. The structure of the product was confirmed by their
spectral analysis (1H, 13C and mass spectroscopic analysis) (0.28 g,

S. Munusamy, S. Kulathu Iyer / Tetrahedron: Asymmetry 27 (2016) 492–497
497
20%). Rf = 0.21 (hexane/ethyl acetate 4:1); mp 285 °C; [a]
25 = +230.0
D
2. (a) Akdeniz; Mosca, L.; Minami, T.; Anzenbacher, P., Jr. Chem. Commun. 2015,
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(c 0.8, CH2Cl2); 1H NMR (400 MHz, DMSO-D6) d 13.42 (s, 2H), 13.13 (s,
2H), 8.76 (s, 2H), 7.87–7.85 (d, J = 8 Hz, 2H), 7.55 (s, 4H), 7.46–7.43
(m, 10H), 7.30–7.27 (t, J = 4 Hz, 2H), 7.22–7.15 (m, 8H), 7.03–7.01
(d, J = 8 Hz, 2H); 13C NMR (400 MHz, DMSO-D6) d 151.1, 145.7,
134.3, 133.7, 133.3, 130.1, 128.8, 128.4, 128.2, 127.1, 127.0, 126.8,
126.6, 124.3, 123.3, 116.9, 114.9. HRMS (EI) calcd for C50H34N4O2
(M+) 722.2682, found 722. 2685.
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Sathishkumar Munusamy thanks CSIR for providing Senior
Research Fellowship. The DST-FIST NMR facility at VIT University
and VIT management are duly acknowledged. Authors would like
to thank Dr. R. Srinivasan, SSL, and VIT University for his support
for English corrections.
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