Kinetic exploration supplemented by spectroscopic and molecular docking analysis in search of the optimal conditions for effective degradation of malachite green

Article abstract of DOI:10.1039/c5ra04724b

The degradation of malachite green (MG) by an alkaline hydrolytic process has been explored spectrophotometrically. The kinetics of the reaction have been meticulously studied under the influence of cationic alkyltrimethylammonium bromide (DTAB, TTAB and CTAB) surfactants, α-, β- and γ-cyclodextrins (CDs) and surfactant-β-CD mixed systems applying pseudo-first order conditions at 298 K. The surfactants and cyclodextrins individually catalyze the hydrolytic rate, whereas surfactant-β-CD mixed systems exhibit both an inhibiting and catalytic influence depending on the surfactant concentrations. The kinetic results have been explained precisely based on the pseudo-phase ion exchange (PIE) model of micelles and CD-catalyzed model of CD systems. The surfactants exhibit micellar surface catalysis, while CDs accelerate the rate by forming MG-CD inclusion complexes, thereby facilitating nucleophilic attack of its ionized secondary hydroxyl group on the carbocation center of MG. The encapsulation of MG within the supramolecular host cavity of the CDs has been investigated diligently using a steady-state absorption spectroscopic technique. The result shows 1:1 host-guest complexation with different relative orientations of the guest (MG) inside the hosts. Studies employing density functional theory (DFT) as well as molecular docking analysis provide valuable insight on the insertion mechanism. The results reveal that quantitative analysis can be utilized to predict the optimum conditions for the fastest degradation of MG in ambient environments.

Full text of DOI:10.1039/c5ra04724b

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Kinetic exploration supplemented by spectroscopic
and molecular docking analysis in search of the
optimal conditions for effective degradation of
malachite green†
Cite this: RSC Adv., 2015, 5, 38503
Somnath Dasmandal,a Harasit Kumar Mandal,a Suparna Rudra,a Arjama Kundu,a
b
a
*
Tapas Majumdar and Ambikesh Mahapatra
The degradation of malachite green (MG) by an alkaline hydrolytic process has been explored
spectrophotometrically. The kinetics of the reaction have been meticulously studied under the influence
of cationic alkyltrimethylammonium bromide (DTAB, TTAB and CTAB) surfactants, a-, b- and g-
cyclodextrins (CDs) and surfactant–b-CD mixed systems applying pseudo-first order conditions at 298 K.
The surfactants and cyclodextrins individually catalyze the hydrolytic rate, whereas surfactant–b-CD
mixed systems exhibit both an inhibiting and catalytic influence depending on the surfactant
concentrations. The kinetic results have been explained precisely based on the pseudo-phase ion
exchange (PIE) model of micelles and CD-catalyzed model of CD systems. The surfactants exhibit
micellar surface catalysis, while CDs accelerate the rate by forming MG–CD inclusion complexes,
thereby facilitating nucleophilic attack of its ionized secondary hydroxyl group on the carbocation center
of MG. The encapsulation of MG within the supramolecular host cavity of the CDs has been investigated
diligently using a steady-state absorption spectroscopic technique. The result shows 1 : 1 host–guest
complexation with different relative orientations of the guest (MG) inside the hosts. Studies employing
density functional theory (DFT) as well as molecular docking analysis provide valuable insight on the
insertion mechanism. The results reveal that quantitative analysis can be utilized to predict the optimum
conditions for the fastest degradation of MG in ambient environments.
Received 17th March 2015
Accepted 8th April 2015
DOI: 10.1039/c5ra04724b
www.rsc.org/advances
processes have been found to release more toxic products than
the parent compound that prove fatal for the living creatures.4
1. Introduction
Hydrolysis is one of the prime detoxication mechanisms for
organic compounds as the hydrolysis by-products are normally
less toxic to the environment than the parent compound.5 Thus
in our present study we have extensively studied the kinetics of
alkaline hydrolysis of malachite green in different micro-
heterogeneous environments made up of surfactants, cyclo-
dextrins and their mixed systems.
Microenvironments consisting of surfactants are capable of
both catalyzing and inhibiting the hydrolysis rates depending
on specic interactions between the surfactants and reactant
species, micellar aggregation number, CMC values, the extent of
incorporation of reactants as counter ions in micelles etc.6,7 Not
only surfactants, but also cyclodextrins, by virtue of their cage
like structure, may inuence the rate of hydrolysis reactions.8
Cyclodextrins (CDs) are cyclic oligomers of a-D-glucose linked by
a-1,4 glycoside bonds. Natural cyclodextrins are classied as a-,
b- or g-CD according to whether they have 6, 7 or 8 a-D-gluco-
pyranose residues respectively.9 CDs can form inclusion
complexes by means of non-covalent interactions, including
van der Waals, hydrophobic, electrostatic, hydrogen bonding,
The chemistry of dyes and their application in various scientic
elds have attracted immense attention in modern times.
Malachite green (MG) is an important dye of commercial and
analytical importance.1 However, the high solubility of the dye
in water makes it a potential water pollutant. Contamination of
water-systems by the dye creates a severe risk to aquatic living
organisms through bioaccumulation and thus by entering the
food chain. Thus the removal of the dye from industrial efflu-
ents in an economic fashion is a leading challenge. A number of
methods have recently been used for dealing with the treatment
of wastewater discharged from textile industries viz., physico-
chemical treatment by biological oxidation, adsorption and
advanced oxidation processes (AOPs), e.g., ozonation, photol-
ysis, electrochemical, sonolysis etc.2,3 Sometimes, these
aDepartment of Chemistry, Jadavpur University, Kolkata 700 032, India. E-mail:
amahapatra@chemistry.jdvu.ac.in; ambikeshju@gmail.com; Fax: +91 33 2414
6223; Tel: +91 33 2457 2770; +91 33 2432 4586
bDepartment of Chemistry, University of Kalyani, Kalyani 741 235, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra04724b
This journal is © The Royal Society of Chemistry 2015
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steric effects etc. with molecules that t into their cavities.10,11 puried water from Milli-Q Ultra has been used for all sample
Because of their ability to form inclusion complexes, the reac- preparation and dilution.
tion rates are generally reduced, i.e. CDs act here as stabilizers,12
but more interest has been placed in the situation where CDs
accelerate the reactions.13 Moreover, surfactant–CD mixed
2.2. Methods
systems have drawn great interest in recent years due to their
numerous applications in commercial formulations.14 These
systems can be used as a model for establishing the effect of
cyclodextrins on phospholipids, an essential part of cell
membranes. The addition of cyclodextrin to a micellar solution
alters its physicochemical properties.15 This is due the forma-
tion of CD–surfactant inclusion complexes where the stoichi-
ometries (CD : surfactant) for the majority of such cases are
found to be 1 : 1 or 1 : 2 or even 2 : 1 depending on the type of
surfactant, tail chain length and also the size of the CD cavity.16
A number of studies have already been reported separately
on the basic hydrolysis of triphenylmethane dyes in the pres-
ence of cationic,17 anionic18 and non-ionic19 surfactants, cyclo-
dextrins20 and their mixed systems.21 Nevertheless, only little
attention has been paid so far to determine the optimal oper-
ating environment for the efficient removal of dyes from
wastewater using a hydrolysis process. According to the litera-
ture, anionic surfactants greatly retard the rate of alkaline
hydrolysis reaction of triphenylmethane dyes.18 On the contrary,
cationic surfactants generally accelerate the rate and their
accelerating effect is much pronounced than for the non-ionic
surfactants.19 Thus in our present study, we have used three
cationic surfactants (DTAB, TTAB, CTAB) of different chain
lengths to monitor their effect on the hydrolysis rate. Again, in
the presence of CDs, the probable mechanistic pathway of their
interaction with MG in an alkaline medium and their efficiency
to degrade the dye are not yet known. Thus three CDs (a-, b-, g-)
of different cavity size have been used herein due to their ability
to interact with MG and thereby inuence the reaction rate.
Besides, the insertion mechanism of MG inside the CD nano-
cavity has been carefully studied employing a steady-state
absorption technique and molecular docking analysis to gain
a better understanding of the experimental ndings. Thus, the
present study may help to nd a system with an optimally
balanced combination of surfactant and CD to degrade mala-
chite green at a much faster rate. The reaction of malachite
green in the presence of an alkali is a one-step reaction22 and
follows pseudo-rst order kinetics. All the experimental results
are discussed with proper correlation to the literature reports.
2.2.1. Experimental. A fresh solution of malachite green
(MG) was usually prepared for day to day work and used only
aer checking its concentration using the molar extinction
coefficient, 3616 (MG) ¼ 1.48 ꢀ 105 Mꢁ1 cmꢁ1
.
23 The absorption
spectra and kinetic measurements were recorded using a Shi-
madzu, UV-1800 Spectrophotometer equipped with a Peltier
temperature controller, TCC-240A, maintaining a constant
temperature of 298 K (ꢂ0.1 K). The kinetics of the alkaline
hydrolysis were followed by measuring the decay in the absor-
bance at 616 nm as a function of time. A representative spectral
change for the hydrolysis reaction is shown in Fig. S1†. The
hydrolysis process has been investigated under pseudo-rst
order reaction conditions using NaOH in large excess. The
hydrolysis of MG was usually followed up to 7–8 half-lives. In all
the kinetic runs, the nal concentration of MG was kept at 1.25
ꢀ 10ꢁ5 M and the ionic strength (m) of the medium was main-
tained at 5.0 mM by adding KNO3 solution. The observed rst
order rate constants (kobs) and their standard deviations were
obtained by tting to the rst order exponential decay of
absorbance (At) versus time (t) plot using Microcal Origin 8.0.
2.2.2. Theoretical methodology. The crystal structures of a-,
b- and g-CDs were obtained from protein complexes (pdb id:
2ZYM, 2Y4S and 2ZYK for a-, b- and g-CD respectively) available
from the Protein Data Bank (PDB). The CDs were then extracted
and all the hydrogen and Gasteiger charges added to prepare
them for docking. The ground state geometry of malachite green
(MG) was optimized employing density functional theory (DFT)
in conjugation with B3LYP functional and 6-31G* basis set in
Gaussian 09 program suit.24 The PDB structure of MG was
derived from the optimized structure. MG was considered as a
ligand and CD as a receptor for docking studies. The molecular
docking was carried out applying the Lamarckian Genetic
Algorithm (LGA), inculcated in the docking program Auto Dock
4.2.25 The CDs were laid over a three dimensional grid box (126
3
˚
˚
ꢀ 126 ꢀ 126) A with 0.375 A spacing. The output from Auto
Dock was analyzed using PyMOL soware.26
3. Results and discussion
3.1. Kinetic study
2. Materials and methods
2.1. Materials
3.1.1. Hydrolysis of malachite green in aqueous medium.
Hydrolysis causes the disruption of the extensive conjugation
Malachite green (MG) has been procured from Merck, India as a present in malachite green and thereby results in gradual
chloride salt, [C6H5C(C6H4N(CH3)2)2]Cl and used as received. diminution of the colour intensity. The hydrolysis reaction
The surfactants dodecyltrimethylammonium bromide (DTAB), starts with the attack of the nucleophile, ꢁOH, to the carboca-
tetradecyltrimethylammonium
bromide
(TTAB),
cetyl- tion center and results in the formation of a neutral carbinol
trimethylammonium bromide (CTAB) or in general alkyl- base. The well-known mechanism of the reaction in the
trimethylammonium bromide (ATAB) as well as a, b and g- aqueous phase27 is shown in Scheme 1. The overall order of the
cyclodextrins (CDs) have been purchased in the highest avail- reaction is found to be second i.e. rst order in each of the
able purity from TCI, Japan. All other organic solvents used in reactant species, [MG]0 and [ꢁOH]0.
this study have been obtained from Merck, India. Highly
The observed rate law can be expressed by eqn (1)
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Scheme 1 Alkaline hydrolysis of malachite green (MG).
ꢀ
ꢁ
d½MGꢃ
dt
Fig. 1 Plots of kobs versus concentration of surfactants with different
Rate ¼ ꢁ
¼
k0 þ kw;2½ꢁOHꢃ ½MGꢃ
(1)
0
0
chain lengths for alkaline hydrolysis of MG at 298 K. [MG]0 ¼ 1.25 ꢀ
10ꢁ5 M, [ꢁOH]0 ¼ 1.875 mM. Inset: catalytic factor (k /kw) in different
max
obs
since, [ꢁOH]0 [ [MG]0.
micellar systems.
Therefore,
d½MGꢃ
dt
ꢁ
¼ kobs½MGꢃ
0
the micellar Stern region. The MG is believed to be incorporated
into the micelles by a hydrophobic interaction due to the exis-
tence of the phenyl moieties and ꢁOH is gathered by a
coulombic attraction effect. This observation is in accordance
with our earlier ndings.30 However, the phenomenon of ion-
exchange at the micellar interface plays a vital role in deter-
mining the limiting value of kobs. The gradual addition of
surfactant results in increasing the concentration of coun-
terion, Brꢁ in the medium. These are nonreactive counterions.
Hence, a competition between Brꢁ and ꢁOH in the Stern region
eventually inhibits the reaction rate.31
where,
kobs ¼ k0 + kw,2[ꢁOH]0
(2)
where, kw,2 is the second-order rate constant for the alkaline
hydrolysis reaction. The value of kw,2 determined from the slope
of the plot kobs versus [ꢁOH]0 (by eqn (2)) is 2.508 (ꢂ0.05) Mꢁ1
sꢁ1. The rate constant for hydrolysis of MG in pure water (k0)
obtained from the intercept of the plot is found to be of 9.75 ꢀ
10ꢁ5 sꢁ1
.
The experimental results can be interpreted quantitatively
on the basis of the Pseudo-phase Ion Exchange (PIE) model.32
This model uses the micellar pseudo-phase formalism (Scheme
2) coupled with the assumption of ion-exchange at the ionic
micellar surface33 and yields eqn (3).
3.1.2. Hydrolysis of malachite green in micellar media. The
inuence of concentration of cationic amphiphiles (ATABs) on
the rate of alkaline hydrolysis of malachite green (MG) has been
examined over a wide interval that includes both the region
prior to the CMC and the region aer the CMC. (Fig. 1). Fig. 1
shows that kobs increases only slightly on increasing the
surfactant concentration until the CMC. This is an indication of
weak pre micellar activity.19 But aerwards, a sharp increase in
kobs can be observed due to the presence of micellar aggregates,
and reaches a limiting value, beyond which kobs starts to
decrease considerably. The CMC values have been obtained
kinetically as the minimum surfactant concentration required
to induce a substantial change in the observed rate constant
and found to be 11.3, 1.5 and 0.5 mM for DTAB, TTAB and CTAB
respectively. These data show fair agreement with the literature
values.28 However, the CMCs derived kinetically are found to be
lower than those shown by micellar systems of pure surfactants.
This reduction has been ascribed to the known effect of elec-
trolyte on CMC.
ꢂ
ꢃ
kw;2½ꢁOHꢃ þ KMmGkm ꢁ kw;2 mOH½Dnꢃ
T
kobs
¼
(3)
1 þ KMmG½Dnꢃ
where kw,2 & km are the rate constants in aqueous and micellar
pseudo-phases respectively. KmMG is the association constant
between MG and the micelles. Dn represents the micellized
surfactant ([Dn] ¼ [ATAB]T ꢁ CMC). mOH is the [ꢁOH]m/[Dn]
ratio, where ꢁOHm is the hydroxyl ion at the micellar surface.
The alkaline hydrolysis of MG in cationic micellar media
experiences micellar surface catalysis. An increment of surfac-
tant concentration beyond its CMC leads to an increase in the
number of the micelles in the bulk phase,29 which in turn
increases the concentration of micelle bound MG and ꢁOH in
Scheme 2 Hydrolysis of MG in micellar solution.
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The values of mOH at different [Dn] and at a constant [ꢁOH]T local concentration of the reactant species at the micellar
have been calculated by eqn (4)
surfaces and the lowering of polarity at the micellar pseudo-
phase compared to the bulk aqueous phase.19 The effect of
polarity on the hydrolysis rate is well understood by the Hughes
and Ingold rule: that is, the rate of the formation of a neutral
product from two oppositely charged reactants gets accelerated
as the polarity of the medium decreases.37 The rate acceleration
caused by lowering of polarity in the micellar Stern region has
been discussed in our previous work too.30 Moreover, CTAB
being the highest homologue among the ATABs possesses the
lowest value of CMC. Therefore, the concentration of the
nonreactive counterion, Brꢁ in equilibrium with the CTAB
micellar system is also much less when the micellization
process begins. Thus, CTAB shows the maximum catalytic effect
on the hydrolysis rate among the ATABs.
The characteristic behaviour of catalysis exhibited by all the
cationic micelles (Fig. 1) allows us to compare quantitatively the
catalytic inuence of the surfactants on the rate of alkaline
hydrolysis of malachite green. The catalytic effect is found to
increase towards the higher homologues of ATABs i.e., it follows
the order: DTAB (–C12) < TTAB (–C14) < CTAB (–C16). The
maximum degree of catalytic activity in different micellar
systems is shown in the Fig. 1 inset introducing the parameter,
catalytic factor (kmobasx/kw), and for each value of kombasx/kw, the
"
#
ꢁ
OH
Br
½ꢁOHꢃ þ K ½Br ꢃ
b½ꢁOHꢃ
2
T
T
T
ꢂ
ꢃ
ꢂ
ꢃ
mOH þ mOH
ꢁ b
ꢁ
¼ 0
OH
Br
OH
Br
K
ꢁ 1 ½Dnꢃ
K
ꢁ 1 ½Dnꢃ
(4)
where b is the fraction of surfactant head groups neutralised by
counterions (b ¼ 0.8).34 In order to resolve eqn (4), a value of
OH
K
¼ 15 has been used as it gives the best t of the theoretical
Br
OH
Br
line. Moreover, the value of K
for the most widely studied
systems containing hydroxide ion and bromide ion micelles fall
within the range of 10–20.35 The calculated values of mOH are
P
then used in eqn (3) to evaluate km, KmMG and least squares, di2
(where, di ¼ kobs,i ꢁ kcal,i with kobs,i and kcal,i representing the
observed and calculated rate constant values at the ith concen-
tration of micellized surfactant, [Dn]i) values using the non-
linear least-squares technique. The value of kw (rst-order rate
constant in the aqueous phase) is kept constant and identical to
the value previously obtained in the aqueous phase. The values
of km and KmMG so obtained from eqn (3) for different micellar
systems are presented in Table 1 along with the calculated
OH
Br
values of the rate constants, kcal, at K ¼ 15. A satisfactory t of
observed data to eqn (3) is evident from the plot of Fig. 1, where
a solid line has been drawn by the calculated data points (kcal
for different micellar systems employing the value of K
)
corresponding concentration of surfactant has been presented
OH
Br
¼
max
obs
in terms of CMC factor ([ATAB] for k
to CMC ratios). As can
15.
be seen from Fig. 1 & inset, the catalytic effect exerted by the
CTAB micellar system reaches a maximum degree at [CTAB] of
30.0 mM which is nearly 60 times its CMC under the conditions
of our present study. While TTAB and DTAB exhibit their
maximum catalytic activity at concentrations corresponding to
20 times and 5 times to their respective CMC values.
The lower values of KmMG for CTAB micelles compared to
DTAB may be attributed to the fact that the longer hydrocarbon
chain of CTAB may result in folding and thus provide less space
for binding to MG.36 Hence, the highest value of km in a CTAB
micellar medium (Table 1) helps us to recognize the concurrent
effect on the hydrolysis reaction caused by the increment of
Table 1 Kinetic parameters for alkaline hydrolysis of MG in presence of surfactants at 298 Ka
DTAB
TTAB
CTAB
103[TTAB]0,
M
103[CTAB]0,
M
103[DTAB]0, M
103kobs, sꢁ1
103kcal,b sꢁ1
103kobs, sꢁ1
103kcal, sꢁ1
103kobs, sꢁ1
103kcal, sꢁ1
0.0
4.80
—
5.20
9.43
0.0
1.6
3.9
4.81
—
5.11
0.0
0.67
1.00
2.67
4.0
6.67
10.0
20.0
30.0
40.0
66.7
100.0
133.3
200.0
4.80
—
8.81
11.6
14.5
17.4
23.2
34.8
46.4
58.0
81.2
116.0
150.8
185.6
232.0
4.89 ꢂ 0.1c
5.07 ꢂ 0.1
10.66 ꢂ 0.2
17.07 ꢂ 0.3
20.72 ꢂ 0.3
22.93 ꢂ 0.5
24.52 ꢂ 0.3
22.86 ꢂ 0.6
18.56 ꢂ 0.5
15.99 ꢂ 0.4
14.31 ꢂ 0.1
12.23 ꢂ 0.2
10.92 ꢂ 0.3
4.44 ꢂ 0.1
6.95 ꢂ 0.1
20.64 ꢂ 0.3
26.84 ꢂ 0.2
33.29 ꢂ 0.5
37.33 ꢂ 0.7
39.84 ꢂ 0.6
37.10 ꢂ 0.4
29.73 ꢂ 0.3
24.71 ꢂ 0.4
19.62 ꢂ 0.2
15.81 ꢂ 0.2
13.49 ꢂ 0.1
5.1 ꢂ 0.1
7.3 ꢂ 0.1
17.0 ꢂ 0.2
20.8 ꢂ 0.4
35.8 ꢂ 0.8
43.9 ꢂ 1.1
49.6 ꢂ 1.3
51.7 ꢂ 0.8
49.1 ꢂ 1.0
41.2 ꢂ 0.7
35.9 ꢂ 0.8
30.8 ꢂ 0.9
24.7 ꢂ 0.5
18.13
28.06
32.72
35.13
36.99
37.11
35.61
31.34
25.88
20.81
17.35
14.86
0.447
15.1 ꢂ 0.6
9.1 ꢂ 0.4
14.99
31.48
37.92
44.79
48.67
51.09
49.76
47.61
41.64
35.54
30.88
24.39
13.849
18.3 ꢂ 0.9
9.3 ꢂ 0.4
12.69
17.19
21.50
22.81
22.83
21.48
18.82
16.45
14.53
12.52
10.65
0.098
9.7 ꢂ 0.5
21.5 ꢂ 0.3
7.8
11.7
15.5
23.3
31.1
46.6
77.6
124.2
186.3
248.4
310.5
290.0
P
106 di2
km, sꢁ1
KmMG, Mꢁ1
a
OH
Br
c
[ꢁOH]0 ¼ 1.875 mM. b Calculated from eqn (3) and (4) with b ¼ 0.8, K ¼ 15, kw,2 ¼ 2.508 Mꢁ1 sꢁ1
.
Error limits are standard deviations.
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3.1.3. Inuence of a-, b- and g-cyclodextrins on the reac-
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tion rate. The inuence of cyclodextrins (CDs) on the rate of
alkaline hydrolysis of MG is shown in Fig. 2. As can be observed,
all the CDs catalyze the hydrolysis reaction, but to different
extents. The maximum catalytic inuence of the CDs is pre-
sented in the Fig. 2 inset in terms of catalytic factor (kmobasx/kw) at a
common xed CD concentration (1.0 mM). The experimental
behaviour can be explained by taking into consideration the
mechanistic behaviour as shown in Scheme 3 where CD-
catalyzed and hydrolytic reaction take place through an
MG–CD inclusion complex and in the bulk aqueous phase
respectively. The catalytic effect exerted by CD can be attributed
to the nucleophilic attack by the ionized secondary hydroxyl
group of CD on the MG associated with CD.38 Therefore, it must
be assumed that the geometry of the inclusion complex formed
by CDs of different cavity size is inuential here for the rate
variations found in Fig. 2.
In the presence of CD, binding of MG inside the host (CD)
cavity gives the reaction an intramolecular character which
enhances the rate of the reaction. This concept is quite similar
to enzyme-like catalysis.39 The supramolecular hosts bind MG
in such a way that the electrophilic centre of the carbocationic
dye is positioned close to the deprotonated secondary hydroxyl
groups of CD. The proximity of the two groups in different
MG–CD inclusion complexes will determine the efficiency of
catalytic behaviour of the respective CDs.40
Scheme 3 Hydrolysis of MG in aqueous solution of CD.
CD
MG
K
is the equilibrium binding constant of MG to the cyclo-
dextrin, and kCD is the pseudo-rst order rate constant for the
CD-catalyzed reaction. The value of kw is kept constant and
identical to the value previously obtained in the aqueous phase.
CD
MG
The K and kCD values are obtained by tting the experimental
data to eqn (5) by a non-linear regression analysis and are
presented in Table 2. The kCD values indicate that the rate
acceleration by CDs follows the order: b-CD > g-CD > a-CD i.e.,
b-CD catalyzes the reaction to the most extent among the
CDs.
The mechanism of the CD-catalyzed process is shown in
Scheme 4, where cyclodextrin as its oxyanion reacts with the
carbocationic dye (MG) resulting in a tetrahedral complex. Thus
we have to consider not only the closest approach of the two
reacting centers, but also the geometric change of the MG–CD
complex needed for the entire reaction for a suitable orientation
of the tetrahedral complex so formed.41 A detailed conforma-
tional search regarding different geometrical orientations of
MG inside the various CD cavities have been undertaken in the
following sections viz. 4.1 & 4.2 based on steady-state absorption
and molecular docking studies respectively in order to ascertain
the observed rate variation in different CD systems.
Thus, Table 1 clearly reveals that the alkaline hydrolysis
reaction is catalyzed to the highest extent in the presence of
CTAB micellar medium among the ATABs. Similarly, b-CD
accelerates the reaction most efficiently among the CDs (Table
2). Therefore, a kinetic study of the inuence of surfactant
concentration on the hydrolysis reaction at a xed [CD] would
provide guidance to reach the optimal conditions for the fastest
degradation of MG. In this context, only b-CD has been chosen
as a component of the surfactant–CD mixed system because of
its better catalytic activity than a- or g-CD.
According to the Scheme 3, for a substrate that experiences
an uncatalyzed reaction in a given medium as well as a catalyzed
reaction by the formation of a 1 : 1 substrate–CD complex, the
expected variation of kobs with CD concentrations can be given
by eqn (5).38
kw þ kCDKMCDG½CDꢃ
0
kobs
¼
(5)
CD
1 þ K ½CDꢃ
MG
0
3.1.4. Inuence of ATAB–b-CD mixed systems on the
reaction rate. The inuence of surfactant concentration on kobs
for alkaline hydrolysis of MG in the presence of a constant
concentration of b-CD, [b-CD]0 ¼ 1.0 mM is shown in Fig. 3. As
we increase the surfactant concentration, kobs initially decreases
until it reaches a minimum value. This fact is attributed to the
complexation of surfactant monomer with b-CD displacing MG
to the bulk phase. The CD catalytic route thereby is lost and
consequent reduction of the rate is observed. Dorrego and
coworkers reported a similar observation with the base hydro-
lysis of m-nitrophenylacetate in mixed systems made up of
alkyltrimethylammonium micelles and a- or b-cyclodextrins.28
Aer the minima of the curve kobs versus [ATAB], kobs exhibits
the same kind of [ATAB] dependence as we have observed in the
Fig. 2 Plots of kobs versus [CD]0 for the reaction of MG and ꢁOH in
aqueous medium at 298 K. [MG]0 ¼ 1.25 ꢀ 10ꢁ5 M, [ꢁOH]0 ¼ 1.875
max
obs
mM. Inset: catalytic factor (k /kw) in different cyclodextrin systems at
fixed [CD] ¼ 1.0 mM.
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Table 2 Kinetic parameters for alkaline hydrolysis of MG in the presence of CDs at 298 Ka
a-CD
b-CD
g-CD
103[CD]0, M
103kobs, sꢁ1
103kcal,b sꢁ1
103kobs, sꢁ1
103kcal, sꢁ1
103kobs, sꢁ1
103kcal, sꢁ1
0.0
4.82
4.80
5.96
6.41
6.64
6.79
6.89
6.99
7.11
7.15
7.20
7.22
7.25
0.308
7.4 ꢂ 0.10
4050 ꢂ 90
4.81
4.80
7.89
8.73
9.12
9.35
9.50
9.66
9.83
9.87
9.94
9.97
10.01
0.652
10.3 ꢂ 0.11
6700 ꢂ 110
4.77
4.80
6.29
6.84
7.12
7.29
7.40
7.53
7.67
7.71
7.77
7.79
7.83
0.244
8.0 ꢂ 0.08
4370 ꢂ 80
0.20
0.40
0.60
0.80
1.00
1.33
2.00
2.33
3.00
3.33
5.88 ꢂ 0.04c
6.58 ꢂ 0.06
6.82 ꢂ 0.02
6.93 ꢂ 0.08
7.01 ꢂ 0.04
7.07 ꢂ 0.10
7.19 ꢂ 0.05
7.22 ꢂ 0.05
7.21 ꢂ 0.3
7.23 ꢂ 0.03
7.29 ꢂ 0.04
7.85 ꢂ 0.05
8.73 ꢂ 0.10
9.46 ꢂ 0.11
9.59 ꢂ 0.08
9.73 ꢂ 0.04
9.57 ꢂ 0.07
9.62 ꢂ 0.07
9.86 ꢂ 0.04
9.88 ꢂ 0.06
10.06 ꢂ 0.03
9.97 ꢂ 0.04
6.52 ꢂ 0.08
6.79 ꢂ 0.07
6.94 ꢂ 0.09
7.11 ꢂ 0.08
7.38 ꢂ 0.08
7.57 ꢂ 0.4
7.79 ꢂ 0.05
7.74 ꢂ 0.03
7.81 ꢂ 0.7
7.69 ꢂ 0.5
7.74 ꢂ 0.04
4.00
P
106 di2
103kCD, sꢁ1
KCMDG, Mꢁ1
a
c
[ꢁOH]0 ¼ 1.875 mM. b Calculated from eqn (5), kw ¼ 4.8 ꢀ 10ꢁ3 sꢁ1
.
Error limits are standard deviations.
complexation of surfactant monomers with b-CD.16 At [ATAB] >
CMC, the observed catalytic effect is due to the micellar catalysis
by ATAB micelles and it is similar to what we have found in
section 3.1.2. The catalytic efficiency of the mixed systems can
be identied by the catalytic factor, i.e. the ratio of the observed
rate constant at the maximum, kmobasx, to that at micellization
concentration, kmobisn. The catalytic factor, kombasx/kombisn, of the
various mixed systems has been found to be 7.5, 7.58 and 7.72
for DTAB, TTAB and CTAB respectively. The value of kmobasx/kombisn is
quite independent on the nature of the surfactants. This result
contrasts with the values of kmobasx/kw ¼ 5.11, 8.51 and 11.39 for
the same surfactants in pure micellar systems. This loss of
catalytic efficiency of the surfactants in the mixed systems is due
to the complexation between the surfactant monomer and b-
CD. This is because the complexation displaces the CMC to
higher values, which in turn increases the concentration of non-
reactive counterions (Brꢁ) at the point of micellization. More-
over, the percentage of uncomplexed b-CD in equilibrium with
the micellar systems increases together with the length of the
hydrocarbon chain of the surfactants.28 The increase in
concentration of uncomplexed b-CD causes a shi in the
minimums of the curves kobs versus [ATAB] to a higher value of
Scheme 4 Mechanism of hydrolysis reaction in CD medium.
kobs as the chain length of the surfactants increases. Both of the
min
above incidents reduce the value of kmobasx/k
.
obs
Fig. 3 Influence of ATAB concentration on kobs for alkaline hydrolysis
of MG in the presence of b-CD. [b-CD]0 ¼ 1.0 mM, [ꢁOH]0 ¼ 1.875 mM,
T ¼ 298 K. Inset: maximum rate constant, kmobasx, in CTAB–b-CD mixed
and CTAB micellar systems.
As we are concerned about the fastest degradation of mala-
chite green, that is the value of the maximum rate constant,
kmobasx, then it can undoubtedly be seen that the CTAB–b-CD
mixed system is the most efficient environment in this scenario
(Fig. 3 inset). Fig. 3 inset shows a comparison of the kmobasx value
absence of CD, that is it increases rapidly up to a limiting value
and falls gradually thereaer. The concentration of surfactant at
that minimum value of kobs has been taken as the onset of
micellization and found to be 17.5, 3.6 and 1.5 mM for DTAB,
TTAB and CTAB respectively. The CMCs have been found to be
displaced to higher values than those of the respective surfac-
tants in the absence of CD. This fact is ascribed solely to the
between CTAB–b-CD mixed and CTAB micellar systems and it is
max
observed that k
acquires a higher value in the CTAB–b-CD
obs
mixed system than in pure CTAB micellar system. Hence, the
desired optimum conditions for the most effective degradation
of malachite green is found to be [CTAB]0 ¼ 30 mM at [b-CD]0 ¼
1.0 mM.
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The length of each of the phenyl rings bearing the –NMe2
4. Characterization of MG–CD
inclusion complex
˚
(dimethylaminobenzene) group (6.12 A) and that of the benzene
˚
moiety (3.97 A) of MG are compatible with the height of the CD
cavity. Thus, during the process of inclusion, it is possible for
the dimethylaminobenzene part of MG to penetrate the CD
cavity, keeping the benzene moiety exposed to the bulk water, or
vice versa. If MG orients in the latter pattern, there is no scope
for change in the spectral behaviour, this is because in this
situation the extended conjugated part of MG containing the
auxochrome, –NMe2, remains outside the CD cavity. Therefore,
a considerable change in the absorption of MG can only be
produced if the dimethylaminobenzene part is inside the CD
cavity.
4.1. Absorption studies
The change in absorption spectra of MG upon the quantitative
addition of CD implies a signicant interaction between them.
Fig. S2† shows that the absorbance of MG at 616 nm decreases
gradually with CD concentration. The stoichiometry of binding
between MG and CDs has been ascertained using Benesi–Hil-
debrand (B–H) equation (eqn. (6)):42
1
1
1
¼
þ
(6)
DA A0 ꢁ A0 KðA0 ꢁ A0Þ½CDꢃ
0
where A0 is the initial absorbance of the free guest (MG), DA is
the change in absorbance of the inclusion complex (MG–CD), A0
is the absorbance at the highest concentration of CD. The
experimental results show a linear plot for 1/(DA) versus 1/[CD]0
throughout the entire concentration range for all the CDs. This
clearly indicates 1 : 1 MG–CD complexation.43 Fig. 4 shows the
B–H plots for MG–b-CD, MG–a-CD and MG–g-CD inclusion
complexes. The association constant (K) of the host–guest
inclusion complexes have been evaluated from the intercept
and slope of these plots, and found as 4.89 (ꢂ0.07) ꢀ 102, 21.04
(ꢂ0.04) ꢀ 102 and 5.42 (ꢂ0.03) ꢀ 102 Mꢁ1 for a-CD, b-CD and g-
CD respectively.
The above results indicate that MG links to only one CD
molecule. To substantiate the discussion, the molecular
dimensions of MG have been calculated theoretically using the
DFT method (Scheme 5) with B3LYP functional and 6-31G*
basis set, and the data indicates that the complete incorpora-
tion of MG inside any of the CD cavities is not possible. It is well
The minute change in the absorption prole of the MG–a-CD
system indicates the weak interaction between a-CD and the
˚
dimethylaminobenzene moiety (6.12 A). It is presumably due to
the dimensional incongruity, thereby producing the small
value, 4.89 (ꢂ0.07) ꢀ 102 Mꢁ1 of association constant (K) for
MG–a-CD inclusion complex. However, the considerable dimi-
nution in absorbance of MG upon the quantitative addition of
b-CD (up to 6 mM) is probably due to excellent dimensional
compatibility between the dimethylaminobenzene moiety (6.12
˚
˚
A) and the b-CD cavity (6.2–7.8 A), indicating effective interac-
tion between them (Scheme 5). This is reected by the large
value of K, 21.04 (ꢂ0.04) ꢀ 102 Mꢁ1 for the MG–b-CD inclusion
complex. The signicant change in the absorption prole of the
MG–g-CD system clearly supports the inclusion of the dime-
thylaminobenzene part inside the g-CD core. But the much
˚
larger g-CD cavity (7.9–9.5 A) compels us to predict the two types
of inclusion complexes, where the dimethylaminobenzene
˚
moiety (6.12 A) may enter inside the g-CD core either alone or
˚
together with the benzene moiety (3.97 A). It is evident that both
reported that CD molecules acquire
a
truncated, right-
forms of the MG–g-CD complex are capable of inducing a
considerable change in the absorption behaviour of MG in g-CD
medium.
˚
cylindrical, cone-shaped structure with a height of 7.8 A and
central cavities with diameters of the narrower and wider rim of
˚
˚
˚
a-, b- and g-CD are 4.5–5.7 A, 6.2–7.8 A and 7.9–9.5 A
respectively.44,45
Keeping in mind the spatial distribution of the three phenyl
rings and the size of MG (height and width), it is clear that only a
part of MG can enter inside the CD cavity as shown in Scheme 5.
4.2. Molecular docking studies
Molecular docking is frequently used to predict the orientation
of a guest molecule relative to a receptor when bound to each
other.46,47 Molecular docking analysis is accomplished herein to
predict the probable location of MG within the complex archi-
tecture of cyclodextrins. The convergence of the conformational
search employing the molecular docking model indicates the
reliability of the experimental results for extracting structural
orientations.
According to the docking model, the relative orientation of
MG inside the a-CD nanocavity (Fig. 5a) allows us to identify a
vertical mode of insertion of the benzene moiety with a binding
energy of ꢁ5.38 kcal molꢁ1. Another docked conformation (not
shown) with dimethylaminobenzene as the inserting moiety
(ꢁ2.7 kcal molꢁ1) is also found to form. It is evident that the
latter docked form is energetically less contributing to the
MG–a-CD inclusion complex. Hence the minute change in the
absorption behaviour of MG in a-CD medium is well justied by
the latter docked form (Fig. S2a†). In b-CD, the dimethylami-
nobenzene moiety of MG inserts in a somewhat tilted fashion
Fig.
4 Benesi–Hildebrand plot for the 1 : 1 MG–CD inclusion
complexes in aqueous media.
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Scheme 5 Inclusion of MG inside the CD cavity.
producing a binding energy of ꢁ5.44 kcal molꢁ1 (Fig. 5b). The CD fairly well supports the effective interaction between MG
high negative binding energy with a torsional energy of 1.49 kcal and b-CD (Fig. S2b†). However, the other docked form, albeit
molꢁ1 clearly reveals that MG snugly ts into the host cavity. energetically less favourable (ꢁ4.98 kcal molꢁ1), is also found to
The large diminution in absorbance of MG in the presence of b- be formed by the benzene moiety as inserting group in the same
Fig. 5 Molecular docking of 1 : 1 complex for (a) MG–a-CD, (b) MG–b-CD, (c) MG–g-CD (tilted) and (d) MG–g-CD (horizontal). MG and CDs are
depicted in the ball stick and capped stick model respectively (distances given in the pictures are in A˚).
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tilted fashion (Fig. S3†). In the case of g-CD, the large cavity catalytic activity of the respective CDs. A mixed system
˚
volume (7.9–9.5 A) indeed strongly prohibits the benzene composed of CTAB and b-CD eventually provided the required
˚
moiety (3.97 A) inserting alone inside the host. The docked results for this study exhibiting the maximum value of kobs
conformations in Fig. 5c and d show that MG orients itself in among all the ATAB–b-CD mixed systems. The conditions for
the two most probable arrangements inside the g-CD core. One the fastest degradation of MG by an alkaline hydrolysis process
exhibits a bent vertical insertion of the dimethylaminobenzene have been achieved at [CTAB]0 ¼ 30 mM in presence of [b-
moiety (Fig. 5c), and another shows the horizontal insertion of CD]0 ¼ 1.0 mM at normal temperature and pressure. Thus, the
the same together with the benzene moiety (Fig. 5d) inside the present study explicitly points towards an efficient route for the
empty pocket of the g-CD. The considerable change in the decontamination of malachite green from industrial wastewater
absorption prole of the MG–g-CD system is justied once in a comparatively greener way.
again by the two docked forms (Fig. S2c†). Between the two
docked forms, the latter has a binding energy of ꢁ5.79 kcal
Acknowledgements
molꢁ1, seems more favourable to be formed than the rst one
(ꢁ4.99 kcal molꢁ1) with the same torsional energy (1.49 kcal SD sincerely acknowledges University Grants Commission
molꢁ1). As we have stated previously that the proximity of the (UGC), New Delhi, for a junior research fellowship and AK
attacking group (here, the deprotonated secondary hydroxyl acknowledges the Council of Scientic and Industrial Research
group of CD) to the electrophilic center of the carbocationic dye (CSIR), New Delhi, for a senior research fellowship. TM grate-
plays a vital role over the rate variation in different CD envi- fully acknowledges UGC, Govt. of India for funding with BSR-
ronments. The distance between the carbocation center and the Startup grant (no. F.20-35/2013 (BSR) dated 09.12.2013) and
˚
˚
closest secondary hydroxyl is 5.268 A in b-CD, but only 4.466 A UGC SAP program to Dept. of Chemistry, University of Kalyani.
in a-CD (Fig. 5a and b). Therefore, nucleophilic attack by the
ionized secondary hydroxyl group seems to be taking place with
greater ease in the MG–a-CD inclusion complex. But the lower
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g-CD core (Fig. 5c and d) producing a long distance between the
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