Synthesis and evaluation of a tri-armed molecular receptor for recognition of mercury and cyanide toxicants

Article abstract of DOI:10.1080/10610278.2016.1175567

A tri-armed-pyrene-linked molecular receptor, 5 has been designed, synthesised and evaluated for ionic recognition. It has been observed that the synthesised molecular receptor can recognise mercury and cyanide ions through a change in colour, UV–Vis and fluorescence intensity. The binding stoichiometry of the receptor and these ionic species has been found to be 1:1 through Job’s plots, Benesi–Hildebrand plots and isothermal titration calorimetry (ITC).

Full text of DOI:10.1080/10610278.2016.1175567

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Synthesis and evaluation of a tri-armed molecular
receptor for recognition of mercury and cyanide
toxicants
Har Mohindra Chawla, Mohammad Shahid, Lakhbeer Singh Arora & Bhawna
Uttam
To cite this article: Har Mohindra Chawla, Mohammad Shahid, Lakhbeer Singh Arora
& Bhawna Uttam (2016): Synthesis and evaluation of a tri-armed molecular receptor
for recognition of mercury and cyanide toxicants, Supramolecular Chemistry, DOI:
10.1080/10610278.2016.1175567
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Date: 11 June 2016, At: 14:52

Supramolecular chemiStry, 2016
http://dx.doi.org/10.1080/10610278.2016.1175567
Synthesis and evaluation of a tri-armed molecular receptor for recognition of
mercury and cyanide toxicants
Har Mohindra Chawla, Mohammad Shahid, Lakhbeer Singh Arora and Bhawna Uttam
Department of chemistry, indian institute of technology Delhi, New Delhi, india
ABSTRACT
ARTICLE HISTORY
received 3 November 2015
accepted 4 april 2016
A tri-armed-pyrene-linked molecular receptor, 5 has been designed, synthesised and evaluated
for ionic recognition. It has been observed that the synthesised molecular receptor can recognise
mercury and cyanide ions through a change in colour, UV–Vis and fluorescence intensity. The
binding stoichiometry of the receptor and these ionic species has been found to be 1:1 through
Job’s plots, Benesi–Hildebrand plots and isothermal titration calorimetry (ITC).
KEYWORDS
tri-armed molecular
receptor; mercury; cyanide;
pyrene; isothermal titration
calorimetry
1. Introduction
quinolone (7) and azobenzene (7). There appears to be no
report on multi-armed receptors for simultaneous recognition
of both mercury and cyanide toxicants.
Mercury ions are extremely poisonous and their action
is accumulative with potentially greater damage to human
health (9–11). Mercury vapour exposure can lead to brain
damage, kidney failure and cognitive and motion disor-
ders (12). Methyl mercury enhances its lipid solubility to
yield extremely toxic and damaging species for the central
nervous system (13).
Likewise, cyanide ions can affect the central nervous
system as well as cardiac, endocrine and metabolic path-
ways. Perhaps the most cited adverse effect of cyanide is its
inhibition of respiration and terminal oxidase (cytochrome
oxidase) activity in the mitochondrial respiratory chain.
Cyanide ion, though extensively used in electroplating,
plastics, gold and silver extraction, tanning and other met-
allurgical operations (14), is also a potent chemical war-
fare agent. There is a need for development of molecular
receptors for fast detection of cyanide releasing toxicants.
We report herein a new tri-armed molecular receptor
(5) which can simultaneously recognise both mercury and
The development of multi-armed artificial receptors for
recognition of toxic ionic species is an important area of
active chemical research (1, 2). Multi-armed (e.g. tripodal)
receptors are more advantageous than monopodal and
bipodal receptors as they offer better chelating opportunities
through apparent convergence of functionalities that can
wrap the target toxicant with rigid arms of the receptor
(3–6). Till date, only a limited number of multi-armed anion
and cation receptors have been reported (3, 7). For example,
tripodal receptors have been examined for the detection
of ammonium ions (8) due to their potential to assume
tetrahedral geometric fit through convergence of functional
groups present in the designed receptor. Likewise multi-
armed receptors for anions have also been reported, e.g. a
tri-armed azo receptor for detection of fluoride (2). Reinhoudt
et al. (3, 7) and Ghosh et al. (7) have designed multi-armed
receptors for phosphate ions. A few multi-armed receptors
have also been developed for the detection of mercury and
other toxicants in solution and the vapour phase (7, 8). Some
of the receptors examined for mercury ions include those
present in the tripodal rhodamine (7), nitrobenzofuran (7),
CONTACT har mohindra chawla
hmchawla@chemistry.iitd.ac.in; mohammad Shahid
shahidchem@gmail.com
© 2016 informa uK limited, trading as taylor & Francis Group

2
H. M. CHAWLA eT AL.
cyanide toxicants selectively through a change in colour,
UV–Vis and fluorescence spectrum. It induces profound
quenching of fluorescence intensity on interaction with
mercury ions, while it causes enhancement of fluorescence
intensity on interaction with CN− ions. 5–Hg2+ complex
is able to selectivity recognise CN− ions in semi aqueous
medium with a high association constant and favourable
response time. Though binding of receptor seems to be
complex, isothermal calorimetric titrations suggest that
the recognition process involves non-covalent binding of
the receptor and the toxicant.
2. Results and discussion
Figure 2. (colour online) absorption titrations spectra of 5
(10 μam) with hg2+ (0–3.2 equiv) in aqueous-mecN (30%). inset
shows Benesi–hildebrand plot obtained for 1:1 stoichiometry.
1,3,5-Trihydroxy benzene (1) on acylation with ethyl
bromoacetate in the presence of K2CO3 and KI gave
2 in 80% yield (Scheme 1). On stirring with hydrazine
hydrate in ethanol at room temperature, 2 gave 3 as an
off-white solid in 91% yield. Subsequent reaction with
1-pyrenecarboxaldehyde in ethanol – acetic acid yielded
5 as a yellow solid in 60% yield. Full characterisation of 5
was accomplished through spectral and analytical data
(Figures S1–7).
2.1. Interaction of tri-armed-pyrene receptor (5)
with metal ions
2.1.1. UV–visible spectral studies
The ability of 5 for ionic recognition of species was
assessed by UV–visible spectroscopy. For this purpose a
10-μM solution of 5 in 30% aqueous–acetonitrile was pre
-
pared (absorption maxima at 365 (ε = 26,181 M−1 cm−1) and
285 nm (ε = 21,633 M−1 cm−1). The changes in the spectrum
NH2
HN
O
O
N
HN
O
O
O
OH
O
(iii)
O
(i)
(ii)
O
O
O
N
O
N
O
O
O
O
HO
OH
O
O
O
O
O
NH
HN
NH
HN
H2N
NH2
1
2
3
5
Scheme 1. (i) ethyl bromo acetate/K2co3/mecN/∆, (ii) hydrazine hydrate/etoh/stirring/room temperature, (iii) 1-pyrenecarboxaldehyde
(4)/etoh/acoh/∆.
Figure 1. (colour online) (a) interaction of 5 (10 μm) with different metal ions and (b) interference of different metal ions (20 equiv) in the
analysis of mercury ions in aqueous-mecN (30%).

SUPRAMOLeCULAR CHeMISTRy
3
on addition of different metal ions (Na+, K+, Mg2+, Ca2+,
Co2+, Ni2+, Cu2+, Zn2+, Mn2+, Pb2+, Hg2+, Ag+, Cs+, Cd2+, Fe3+
in the form of their nitrate salts) was monitored carefully.
No change was noticed in the UV spectrum except when
mercury ions were added to the solution of 5 when a
bathochromic shift (~15 nm) was observed with appear-
ance of a new absorption maxima at 380 nm with a shoul-
der at 408 nm (Figure 1(a)). In order to understand the
metal ion selectivity, interference studies were carried out
by addition of competitive metal ions (20 equiv) to a solu-
tion of 5 containing Hg2+ ions. A profound selectivity for
the Hg2+ ions was observed over other ionic interferences
(Figure 1(b)).
Gradual addition of Hg2+ ions (0–3.2 equiv) led to a
decrease in absorbance of band at 365 nm with appear-
ance of a new band at 380 nm with a shoulder at 408 nm
and an isosbestic points at 384 and 332 nm. The formation
of tacit isosbestic points indicated that a new species was
formed in the solution (Figure 2). The Job’s plot revealed
stoichiometry to be 1:1 for which association constant was
determined by modified Benesi–Hildebrand method (15)
as 1.45 × 105 M−1 by utilising equation (1),
Figure 3. (colour online) interaction of 5 (10 μm) with various
metal ions (20 equiv) in aqueous-mecN (30%) revealing a
profound quenching on addition of mercury ions. other similar
ions have insignificant quenching effects.
1∕(I − Io) = 1∕(I − If ) + 1∕K(I − If ) [Hg2+]
(1)
where K is the association constant, I is absorbance of free
receptor 5, Io is the observed absorbance of complex, 5–
Hg2+ and If is the absorbance at saturation.
2.1.2. Fluorescence spectral studies
When 5 (10 μM) was excited at 365 nm, it led to an emis-
sion band at 545 nm along with a weak band at 420 nm
in aqueous acetonitrile (30%). The emission band appear-
ing at 420 nm could be attributed to pyrene monomeric
Figure 4. (colour online) Bar diagram illustrates the change in
fluorescence intensity of 5 (blue colour) and a complex, 5 + hg2+
(orange colour) upon addition of tested metal ions (30 equiv) in
aqueous-mecN (30%).
Figure 5. (colour online) emission titration spectra of 5 (10 μm) with hg2+ (0–4 equiv) in aqueous-mecN. inset: (a) Job’s plot and (b)
Benesi–hildebrand plot.

4
H. M. CHAWLA eT AL.
Figure 6. (colour online) Stern–Volmer graphs for 5 (10 μm) with hg2+ metal ions in aqueous-mecN (30%).
unit present in the probe, while the band at 545 nm could
be assigned to the intramolecular excimer emission and
π–π stacking of pyrene units. This was ascertained by the
observation that on dilution of 5 (1–10 μM), the emission
band remained unchanged at 545 nm (Figure S8). Upon
addition of various metal ions (as their nitrate salts),
the emission band at 545 nm showed efficient fluores-
cence quenching only with Hg2+ ions (~88% quenching)
(Figure 3).
A small fluorescence quenching observed when 5 was
interacted with Cu2+ ions (~38% quenching) could be
considered less significant as none of the other transition
metals ions interfered in the detection of mercury in aque-
ous medium. The quenching effects could be attributed
to enhanced spin–orbit coupling and electron transfer
Figure 7. (colour online) reusability of 5, 5–hg2+ on treatment
with eDta in aqueous-mecN (30%).
mechanisms, respectively (16). The other tested metal ions
did not induce any significant change. The interference
experiments established reinforced impressive selectivity
for Hg2+ ions over other metal ions (Figures 4 and S9).
In order to evaluate the association constant, emis-
sion spectroscopic titrations were performed (Figure 5). It
was determined that the emission maxima continuously
decreased in its fluorescence intensity and the spectrum
shifted towards blue region on gradual addition of Hg2+
ions (0–4 equiv) with a band appearing at 531 nm. Analysis
of interaction through Job’s plot, revealed a 1:1 stoichiom-
etry between 5 and Hg2+ ions and association constant was
estimated by utilising equation (1) as 1.13 × 105 M−1. The
detection limit of mercury ion was found to be 0.11 μM
(Figure S10, detailed given in eSI).
where, F0 and F is fluorescence intensity of tri-armed
receptor in the absence and presence of quencher (Hg2+),
respectively. KSV is quenching constant and Q is the
quencher concentration.
The Stern–Volmer plots were linear up to 0.65 equiv
of Hg2+ but it became a non-linear plot with an upward
curvature at 1.2 equiv of mercury. This indicated that both
static (collisional) as well as dynamic (formation of non-
fluorescent ground state complex) quenching might be
governing the interaction of metal ions and the receptor (17).
The reversibility of interaction of receptor and Hg2+ ions
was analysed by adding a strong chelating reagent, disodium
ethylene diamine tetra acetate (eDTA, 50 equiv) to a solution
of plausible 1:1 complex 5–Hg2+ (Figure 7). The quenched
emission band of 5 + Hg2+ was observed to disappear with
the generation of original band at respective wavelength
due to the formation of a strong eDTA-Hg2+ complex in the
medium.This experiment suggested the reversible nature of
complexation between mercury ions and 5 and the obser-
vations could be considered to be a part of a repeat cycle.
The observed fluorescence quenching at 545 nm
was estimated by obtaining the Stern–Volmer (S–V) plot
between the intensity ratio, Io/I and concentration of Hg2+
ions (Figure 6) using Stern–Volmer equation (2) in which
an almost straight line (R2 = 0.9921) was observed up
to 0.65 equiv of Hg2+ ions with a quenching constant of
K
S–V = 164,510 M−1 as calculated from
F0 ∕ F = 1 + KSV [Q]
(2)

SUPRAMOLeCULAR CHeMISTRy
5
Figure 8. (colour online) DFt structure of 5 and its probable complex with hg2+
.
Figure 9. (colour online) 1h Nmr titration spectra of 5 (2.23 × 10−2 m) with hg2+ (0–2 equiv) as their nitrate salt in DmSo-d6.
2.2. Mechanism of interaction of 5 and Hg2+
observations suggested that Hg2+ ions may be coordi-
nated to both amide and aldimine functions present in 5.
Studies on binding of 5 with Hg2+ through isothermal
titration calorimetry (ITC):Thermodynamic parameters
for the host–guest complex formation was further
investigated by isothermal titration calorimetry (ITC)
on interaction of 5 and Hg2+ at 25 °C in aqueous-MeCN
(30%, v/v) (8). The binding isotherm again showed a
1:1 stoichiometry for complexation between 5 and
Hg2+ ions (Figure 10) with formation constant as
1.15 × 104 M−1. The enthalpy change ΔH° was meas-
ured as −1.545 × 106 1.704 × 104 cal/mol and entropy
changes ΔS° is −5.16 × 103 cal/mol/deg for the formation
of the 5–Hg2+ complex. The overall binding enthalpy
seems to originate from non-covalent interactions
between 5 and Hg2+ ions with loss of entropy.
Geometry optimisation of 5 and its plausible complex
(5 + Hg2+) was examined through density functional cal-
culations (DFT) employing a basis set B3LyP/6-31G and
B3LyP/LANL2DZ using Gaussian 03 programme, respec-
tively (18). The optimised structure showed that two pyr-
ene moieties of 5 are closer which on interaction with Hg2+
ions get away from each other as shown in Figure 8.
The probable mechanism of interaction of 5 and Hg2+
1
was investigated by H NMR titration of 5 in the pres-
ence of different concentrations of Hg2+ ions (0.2, 0.5, 1
and 2 equiv) as presented in Figure 9. Upon addition of
0.2–2.0 equiv of Hg2+ ions to 5, a significant upfield shift
(∆δ = 0.22 and 0.41 ppm) was observed in the resonances
of amide and aldimine protons, respectively, while res-
onances of pyrene ring protons became broad. These

6
H. M. CHAWLA eT AL.
Figure 10. (colour online) interaction of 5 with hg2+ ions in aqueous-mecN (30%, v/v) at 25 °c.
Figure 11. (colour online) (a) absorption and (b) emission behavior of 5 at different phs.
Figure 12. (colour online) interaction of 5 (10 μm) with different anions (20 equiv) in aqueous-mecN (30%).

SUPRAMOLeCULAR CHeMISTRy
7
Figure 13. (colour online) (a) absorption and (b) emission titration spectra of 5 (10 μm) on addition of cN− (0–5 equiv) in aqueous-mecN
(30%). inset shows Benesi–hildebrand plot obtained for 1:1 stoichiometry.
2.3. Spectral behaviour of 5 at different pHs
The absorption and emission spectra of 5 at different
pHs were also studied. It was observed that in the acidic
medium, pH (1–7) a hyperchromic shift was observed. In
the basic medium, a bathochromic shift along with hyper-
chromism was noticed (Figure 10). The fluorescence behav-
iour of 5 when examined as a function of pH under similar
experimental conditions (Figure 11) revealed that relative
fluorescence intensity of 5 increased from pH 1 to 7 while
considerable fluorescence quenching occurred under pH
range of 8–12, respectively. The observed relative emission
intensity was low in the acidic medium (<pH 7) and could
Figure 14. (colour online) interaction studies of 5–hg2+ complex
(10 μm) with various anions (20 equiv) in aqueous-mecN (30%).
be attributed to protonation of carbonyl group, while
under basic conditions (>pH 7) the fluorescence intensity
seemed to get fully quenched possibly because of depro-
tonation of amide functions.
2.4. Anion recognition through 5
A survey of the literature indicates that 5 could also be used
as a receptor for anions as 5 contains a number of amide
groups that could confer appropriate acidity to exhibit
sensitive optical properties. It was observed that the
absorption spectra of 5 could be modulated significantly
when CN− ions were added while no change was noticed
−
−
on addition of other anions (Cl−, Br−, I−, F−, HSO4 , H2PO4 ,
SCN−, AcO− and CN− (as their tetrabutylammonium salts) at
20 equiv concentration. A bathochromic shift (~7 nm) was
noticed with a new absorption peak at 372 nm on addition
of CN− ions (Figure 12(a)). Detailed analysis revealed that
addition of different anions (20 equiv) exhibited fluores-
cence enhancement upon interaction with only CN− ions
in the emission spectrum, whereas related anions like F−,
H2PO4− and AcO− showed efficient fluorescence quenching
in acetonitrile. Tri-armed receptor 5 did not exhibit any
selectivity for anions in acetonitrile solution (Figure S11).
In aqueous–acetonitrile solutions, 5 exhibited a fluo-
rescence enhancement on interaction with only cyanide
Figure 15. (colour online) emission titration spectra of 5–hg2+
complex with cN− (0–4 equiv) in aqueous-mecN (30%). inset
shows Benesi–hildebrand plot.
ions possibly through H-bond interactions with the amide
functions present in 5 (Figure 12(b)) (19).
The binding affinity of 5 for CN− was examined by
absorption and emission spectral titrations (Figure 13).
Gradual addition of CN− (0–5 equiv) to a solution of 5, led
to a decrease in the absorbance at 365 nm with the for-
mation of a new band at 370 nm with an isosbestic point

8
H. M. CHAWLA eT AL.
at 382 nm. Similarly in the emission spectra, addition of
CN− ions (0–5 equiv) led to increase in fluorescence inten-
sity of 5. Job’s plot analysis revealed a 1:1 stoichiometry
for the complexation of 5 with CN− (19). The association
constant estimated from non-linear fitting of absorption
and emission titration data using equation (1) was found
to be Kassos = 1.8 × 104 and 1.5 × 104 M−1, respectively (inset
of Figure 13).
Hg2+ and enhanced by CN− ions in an aqueous–acetonitrile
solvent. The Job’s plot, Stern–Volmer plots, Benesi-
Hildebrand treatment and isothermal titration calorimetry
data show a 1:1 stoichiometry between 5 and Hg2+ ions.
Preliminary DFT calculations support this conclusion. The
5–Hg2+ complex can be further utilised for the detection
of CN− ions through metal ion displacement mechanism.
The probable mechanism of interaction of 5 and CN−
was investigated by 1H NMR titration. Though addition of
CN− ions (1 equiv) to the molecular receptor 5 in deuter-
ated dimethyl sulfoxide, led to the broadening of all signals
but a significant downfield shift of NH signal at 1 equiv and
total disappearance of amide protons at 2 equiv addition
of cyanide indicates strong interaction of cyanide with
amide protons as expected (Figure S12).
4. Experimental
4.1. Synthesis of compound 2
Compound 2 was synthesised by a minor modification of
the previously reported procedure (19, 20). In the present
case, to a solution of 1, 3, 5-trihydroxy benzene (0.25 g,
1.98 mmol), K2CO3 (1.09 g, 7.89 mmol) and KI (a pinch)
in anhydrous acetonitrile, ethyl bromoacetate (0.76 ml,
4.59 mmol) was added. The reaction mixture was refluxed
overnight and the progress of the reaction was monitored
by thin layer chromatography. The reaction mixture was
filtered and the solvent was evaporated under reduced
pressure. The crude product obtained was dissolved in
chloroform and washed with water thrice (3 × 20 ml). The
organic layer was dried over anhydrous Na2SO4. The sol-
vent was evaporated under vacuum to yield 2 as a gummy
solid (yield: 80%). 1H NMR (300 MHz, CDCl3): δ (ppm) 6.184
(s, 3H, phenyl), 4.71 (s, 6H, –OCH2CO), 4.47 (m, 6H, COCH2),
1.34 (t, 9H, –CH3).
2.5. Ion recognition properties of 5–Hg2+ complex
The optical behaviour of 5–Hg2+ complex (10 μM) was
examined in the presence of different anions in aque-
ous-MeCN solution (30%). Upon addition of different ani-
−
ons (10 equiv) such as, F−, Cl−, Br−, I−, SCN−, H2PO4 , CN−,
AcO−, HSO4− (as their tetrabutylammonium/sodium salts)
to a solution of 5–Hg2+, significant changes in the optical
properties were observed on interaction with CN− ions
only. A complete restoration of the emission spectra of 5
was observed (Figure 14).
The binding affinity of a complex, 5–Hg2+ with CN−
ions was estimated by performing emission titration
experiments (Figure 15). On sequential addition of
CN− (0–4 equiv) to a solution of 5–Hg2+, the intensity
of emission band centred at 530 nm got enhanced. It
was considered significant to observe that the colour
of the solution could be distinguished by naked eye.
Fluorescence titration could be utilised to estimate bind-
ing constant of CN− ions for a complex, 5–Hg2+ using
equation (1) as 1.61 × 105 M−1. The detection limit of 5–
Hg2+ complex for CN− ion was estimated to be 0.7 μM,
which is lower than the maximum permissible level of
cyanide in drinking water (1.9 ppm).
4.2. Synthesis of compound 3
To a solution of compound 2 (0.50 g, 1.6 mmol) in ethanol,
hydrated hydrazine (0.52 g, 16.3 mmol, 99%) was added
and the reaction mixture was stirred at room temperature
for 2 h. The precipitate obtained was filtered and washed
with cold water and dried to get 3 as off white solid. (yield:
91%). Mp. 236–240 °C. 1H NMR (300 MHz, CDCl3): δ (ppm)
9.22 (s, 3H, –CONH), 6.18 (s, 3H, phenyl), 4.47 (s, 6H, –
OCH2CO), 3.59 (s, 6H, –NH2); FT-IR (KBr, cm−1): 3322 cm−1
(N–H), 2917 cm−1 (–CH2 asym. stretch, Alkyl), 2869 cm−1
(CH2, sym. stretch), 1669 cm−1 (–CONH), 1611 cm−1 (C=C,
aromatic), 1477 cm−1 (C–C stretch in ring).
Though final resolution of speculative nature of binding
mechanism is possible through X-ray diffraction analysis,
single crystals suitable for X-ray analysis, however, could
not be grown at this point of time.
4.3. Synthesis of compound 5
To a suspension of 3 (0.10 g, 0.29 mmol) in ethanol contain-
ing acetic acid (0.1 ml), 1-pyrenecarboxaldehyde, 4 (0.21 g,
0.91 mmol) was added and the reaction mixture was
refluxed overnight. The precipitate obtained was filtered
and washed with cold ethanol to yield 5 as a yellow solid
(yield: 60%). Mp. 275–280 °C (charring); 1H NMR (300 MHz,
DMSO-d6): δ (ppm) 11.74 (s, 3H, NH), 9.31 (s, 3H, CH=N), 8.46
(m, 27H), 6.17 (s, 3H, phenyl), 4.53 (s, 6H, COCH2); 13C NMR
(75 MHz, CDCl3): δ (ppm) 168.8, 166.8, 159.9, 136.3, 131.4,
130.6, 128.9, 128.6, 127.6, 126.3, 125.4, 125.3, 124.2, 123.0,
3. Conclusion
A new tri-armed receptor 5 has been synthesised and
evaluated for recognition of cations and anions. It has
been observed that 5 exhibits an intramolecular excimer
fluorescence due to π–π interaction of pyrene ring. The
intensity of fluorescence of receptor 5 is quenched by

SUPRAMOLeCULAR CHeMISTRy
9
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3324 (N–H), 2920 (C–H, Alkane), 1672 (CO–NH), 1610 (C=C),
1182 (C–N), 1080 (C–O–C).
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Supplemental material
Supplemental data for this article can be accessed online
here: http://dx.doi.org/10.1080/10610278.2016.1175567.
Acknowledgements
The authors thank SeRB, a unit of the Department of Science
and Technology (DST), New Delhi for financial support vide
sanction No. SB/FT/CS-010/2013 and financial assistance from
MoeF, MoRD and MoFPI (Govt. of India) for the project on fru-
wash technology is gratefully acknowledged. The authors thank
Professor Shashank Deep and Mr Vinay Kumar for their help in
securing isothermal titration calorimetry data.
Disclosure statement
No potential conflict of interest was reported by the authors.
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