R. Ghosh et al.
Chemical Physics Letters 769 (2021) 138401
Table 4
The values of chemical shifts δ (ppm) of pure cyclodextrins (HP-β-CyD and β-CyD) and in inclusion complex form.
Type of proton
Spin multiplicity
δ (ppm)
Spin multiplicity
Shift (Δδ)
HP-β-CyD
DMPI/HP-β-CyD complex(IC-1)
H-1
d
5.144–4.969
3.504
5.098
¡0.0415
H-2
dd
3.541–3.483
3.865
d
s
–
H-3
3.908
¡0.043
H-4
3.394
3.325
s
–
H-5
m
3.734
3.702–3.621
3.702–3.621
m
m
0.222
–
H-6
m
3.734
Type of proton
Spin multiplicity
δ (ppm)
Shift (Δδ)
β-CyD
PDI/β-CyD complex(IC-2)
4.940–4.931
H-1
H-2
H-3
H-4
H-5
H-6
d
4.923–4.932
3.506
d
0.017
dd
t
3.565–3.512
m
t
–
3.801–3.848
3.419–3.466
3.706–3.766
3.706–3.766
3.758–3.735
¡0.113
t
3.505–3.460
m
m
m
–
m
m
3.727–3.669
¡0.097
3.727–3.669
–
5.2.1. From scanning electron microscopy (SEM) studies
antibacterial property of the DMPI, so upon inclusion of this part, bio-
logical activity also should be reduced. We get expected result as the IC
of the HP-β-CyD had shown less inhibition of the bacterial growth with
all different concentration compare to the pure DMPI. So from this result
we may conclude that the pure DMPI had more antibacterial effect
compare to IC of the HP-β-CyD. Further, antibacterial property of IC of
β-CyD will be explored in our future study with subsequent comparison
with HP-β-CyD.
All SEM images were taken by conventionally after gold coating.
SEM photographs of HP-β-CyD, DMPI and their inclusion complexes are
shown in Figs. 8a–8d. Typical crystal of DMPI, and HP-β-CyD are found
in many different sizes. Pure DMPI
appears as irregular-shaped crystal particles with large dimensions
(Fig. 8a), HP-β-CyD shows in spherical particle type (Fig. 8b), and β-CyD
crystallizes in polyhedral form [58]. The DMPI/HP-β-CyD inclusion
complex shows as compact and surface-like structure crystal particles
and is pretty unlike from the sizes and shapes of HP-β-CyD and DMPI
(Figs. 8a–8c), which indicate the production of the inclusion complex.
However, the DMPI/β-CyD inclusion complex appears as compact and
homogeneous rod-like structure crystal particles and is also pretty
different from the sizes and shapes of β-CyD and DMPI (Fig. 8d), which
confirm the formation of the inclusion complexes. HP-β-CyD basically a
kind of soft matter upon exposure to highly energetic electronic beam
was deformed from its original structure. It was probably during elec-
tronic exposure static charge was accumulated in the exterior of HP-
β-CyD, was mainly responsible for the structural deformation. Therefore
extra precaution was taken during acquisition of the SEM images of the
same compound. Finally we get better images by maintaining the
voltage within 4 kV and acquire the pictures quickly. The SEM image of
the pure β-CyD is available in another work carried out by our lab on
same chemical by the same instrument (Also published in CPL, Elsevier,
page 7, Fig. 6) [58].
5.4. From differential scanning calorimetry (DSC)
From the Differential scanning calorimetry (DSC), varities of infor-
mation such as crystallization, thermal stability, melting etc. can be
obtained of chemical compounds [62].
The characteristic peaks of guest molecule in the thermogram may be
completely diminished or shifted to the different temperatures due to
the formation of inclusion complexes with the particular host [63].
Thermogram of solid guest, and ICs have been shown in the Fig. 10.
DSC thermogram of DMPI shows a characteristic sharp distinct endo-
thermic peak at 58 ◦C closely related to its melting point while in its ICs
with both β-CyD and HP-β-CyD, comparatively flat and broadened sig-
nals are observed. These broad signals refer that there is a high loss of
the crystallinity of DMPI in its ICs, indicating a strong complexation with
CyDs and the peaks at higher temperature are probably due to the loss of
water molecules adhered with ICs. As from the Fig. 10, it has been
clearly seen that the nature of the peak at 58 ◦C in the thermogram for
IC-2 (I.e. Complex of β-CyD & DMPI) is more flattened compare to that
for the IC-1(i.e. Complex of HP-β-CyD & DMPI) which indicates more
complexation of DMPI with β-CyD rather than with HP-β-CyD in the
solid state.
5.2.2. From EDS SEM
SEM EDS is a qualitative method can be used to determine the
structural features of raw materials, viz. CyDs and guest or the products
prepared by diverse methods of preparation like physical mixture, so-
lution complexation, co-evaporation and others [59,60]. Elemental
composition analysis of all relevant samples was carried out qualita-
tively by using the EDS study and obtained graphical representation are
shown in Fig. S.9.a–d. The elemental quantitative results were observed
for C:O or C: N:O:I or C:O:I atomic ratios, which were fairly close to the
expected bulk ratios indicating good distribution of the different species
in the sample. Likewise Fig. S.9c–d, etc. show peaks corresponding to the
iodine; it is direct evidence of the DMPI along with β-Cyd present in IC-1
& IC-2 both.
5.5. 2D-ROESY SPECTRA analysis
2D-ROESY NMR can confirm about successful inclusion by the
appearance of intermolecular dipolar cross-correlations due to very
close proximity of interacting protons [64,65]. If two protons are situ-
ated within 400 pm in space then they may produce rotating-frame NOE
spectroscopy (ROESY) [66].
This noticeable observations reveal the minimum stoichiometric
ratio of the different elements were maintained in all samples as well as
inclusion complexes.
Inclusion event inside unique structure of β and HP-β-cyclodextrin
cavity can be assertively shown by the appearance of NOE cross-peaks
between the protons of cyclodextrin and the protons of the aromatic
part of cationic surfactant identifying their spatial immediacy [67,68].
To prove successful inclusion here 2D-ROESY spectra of the complexes
of DMPI with β and HP-β-cyclodextrin were recorded, which has
confirmed the important correlation of aromatic protons of DMPI with
H3 and H5 protons of β and HP-β-cyclodextrin (Fig. S.10), establishing
inclusion of aromatic part of the DMPI.
5.3. Antibacterial study
DMPI was shown significant antibacterial effect with gram negative
bacteria E. Coli as expected [Fig. 9a–d] [61].
It may be assumed that pyridinium part was responsible for
10