D.B. Christopher Leslee et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113303
2
+
Fig. 9. Binding constant of BSA and CBT-1+Hg ensemble at λem =368 nm
ex
Fig. 8. Fluorescence response of BSA with incremental addition of CBT-
(λ =278 nm).
2
+
1
+Hg ensemble at λem =368 nm (λex =278 nm).
Transfer (ICT) in probe after appendage of Hg2 ions (Fig. 7).
+
absorption. Also, the secondary amine NH bending vibration at
ꢀ
1
ꢀ 1
1
1
1
476 cm shifted to 1441 cm , and the C = Nstr of CBT-1 shift from
ꢀ 1
ꢀ 1
3.7. Protein binding studies
617 cm
to 1597 cm . In addition, C-Nstr and C-Sstr at 1330 to
ꢀ 1
110 cm also get alters. This observation reveals that the Hg2+ ion
In significance to the optical sensing behavior of ligand CBT-1 under
interacts with nitrogen atom of imine and secondary amine as well as the
physiological condition, it was further subjected to protein binding
sulphur atom of benzothiazole of CBT-1. From the photometric, fluo-
2
+
2
þ
studies to analysis the biological targeting application of Hg ions. The
interaction of the fluorescent organic molecules with proteins is of
greater interest on valuable account of their biological availability,
distribution and transmission. Albumins proteins are most abundant
targets in blood that highly contains tryptophan and tyrosine that
readily interact with sulphur, nitrogen containing ligand and complexes.
rometric, NMR, Mass, and IR studies of the complex CBT-1þHg , the
2
þ
binding mode between ligand CBT-1 and Hg
ion is proposed as
depicted in Scheme 2.
3
.6. Sensing mechanism
2
þ
Here the CBT-1 and its complex CBT-1þHg has been analyzed for the
better understanding of their biological potential through interaction
with protein samples. [45,46]
To investigate role of Intramolecular charge transfer mechanism in
CBT-1 sensing Hg2 ions, the UV–vis spectra of CBT-1 and CBT-1þHg
+
2þ
was recorded in pure solvents of different polarity and spectra are shown
in Fig. 6. The spectra showed a red shift in lower energy band on
increasing polarity of the solvent, similar type of observation was
noticed by Chakraborty et al. [40] and Sowmya et al. [41]. For instance,
from the Fig. 6a, the lower energy band of CBT-1 appears at 381 nm in
hexane, on increasing the polarity the ligand showed no change in ab-
ꢀ 5
The Fig.8 and S10 is showing the interaction of BSA (1 × 10 M)
2
þ
-5
and CBT-1þHg
ensemble (1 × 10 M), from these figures it is
2
þ
ensemble the emission
observed that after addition of CBT-1þHg
band of BSA got blue shifted to 368 nm from 415 nm and gradually in-
crease in intensity when excited at 278 nm. This characteristic changes
2
þ
2
þ
in band revealed the strong interaction of CBT-1þHg
with amino-
sorption wavelength. However, the lower energy band of CBT-1þHg
acids, tryptophan and tyrosine. This intrinsic fluorescence spectral
change of the tryptophan and the tyrosine due to interaction of CBT-1
complex has depicted the conformational changes lead through micro-
environment changes around these two residues. The protein binding
constant has been determined using Benesi-Hildebrand plot (Fig. 9) and
equation (E1) (see supplementary data). The binding constant calculated
from using the slope in the B–H equation found to be KBSA+(CBT-
complex (Fig. 6b) was found at 382 nm in hexane, further increasing the
polarity and in the high polar medium DMSO, the absorption band get
red shifted to 383 nm. This red shift in absorbance with increasing sol-
2
þ
vent polarity indicates that the excited state of CBT-1þHg is more
polar than the ground state relates to the Intramolecular Charge Transfer
process that happens through donation of electron from carbazole to
hydrazine benzothiazole unit. [42–44]
In addition, to support the observed photophysical changes of probe
CBT-1 after binding of Hg2+ ions and the Intramolecular charge transfer
6
-1
1
+Hg2+) = 3.14 × 10
M
(R = 0.99116). The polarity effect was also
considered, from the fluorescence spectra as shown in Fig. S11 was
observed that change in polarity ranging from acetone, ethyl acetate,
DCM, THF, DMF, DMSO with as medium does not apparently affect the
process, the Density functional theory calculations were carried out
using Gaussian 09 program. Initially, the geometries of the probe and
2
+
2
þ
fluorescence of the BSA + CBT-1+Hg ensemble (Fig. S11 see supple-
mentary data).
CBT-1þHg
complex were optimized by B3LYP/6ꢀ 311 G (d,p) and
LANL2DZ basis sets respectively, using the optimized geometries IR
spectra are generated did not show any negative vibrations signifying
the structure is fine (Fig. 7). Further, the TDDFT calculations were
3.8. Three-dimensional fluorescence studies of BSA
2
þ
carried out with the optimized geometries of CBT-1 and CBT-1þ Hg
complex using above said basis sets. The stimulated TDDFT spectra are
clearly matching with the observed experimental data. Frontier molec-
Three-dimensional fluorescence spectral studies have been carried
out for understanding the conformational and microenvironmental
changes of tryptophan and tyrosine in the presence of sensor CBT-1 and
2
þ
ular orbital analysis of CBT-1 and CBT-1þ Hg
complex clearly
its Hg2 complex. The 3D spectrum of BSA in Fig. S12 (see supplementary
data) has five peaks, in which the peak A, B and C are the first, second
and third order Rayleigh scattering peak respectively. [47] The peak 1
represents the emission band of the polypeptide backbone of the BSA
and the peak 2 is the intrinsic fluorescence emission band of tryptophan
+
showing that the operation of internal charge transfer mechanism is
taken place. In probe HOMO is localized on carbazole unit and LUMO is
2
+
extend over on the benzothialzole unit, after formation of Hg complex
thiazole unit with carbazole acts as HOMO and Hg2 behaves as LUMO.
+
It’s clearly confirming that the clear disturbance of Internal Charge
6