Kinetics of interaction of histidine and histidine methyl ester with ninhydrin in micellar media

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:2<103::AID-KIN4>3.0.CO;2-4

Kinetics of the interaction of histidine and histidine methyl ester with ninhydrin under varying concentrations of reactants, anionic (sodium dodecyl sulphate, SDS), cationic (cetyltrimethylammonium bromide, CTAB) and non-ionic (Triton X-100, TX-100) micelles have been carried out. Rate of the reaction was found to be independent of the initial concentration of histidine (and histidine methyl ester) but was dependent on [Ninhydrin]. The SDS micelles had no effect on the rate of the reaction. In the presence of the CTAB micelles a small enhancement in the rate was observed. The rate - [CTAB] profile showed that the increase in [CTAB] increased the rate up to a maximum value and a further increase had a decreasing effect on the rate. The rate was enhanced by TX-100 also but, unlike CTAB micelles, TX-100 possessed a curve without peak for the rate - [TX-100] profile. The following rate equation was obeyed by the reaction in CTAB and TX-100 micelles: 1kΨ = k_w[N]_T + (k_m K_S - k_w) [D_n] M_N^S/1 + K_S [D_n] Values of k_w, k_m, and K_S were evaluated and are reported herein.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:2<103::AID-KIN4>3.0.CO;2-4

Kinetics of Interaction of
Histidine and Histidine
Methyl Ester with Ninhydrin
in Micellar Media
KABIR-UD-DIN,1 MD. Z. A. RAFIQUEE,1 MOHD. AKRAM,1 ZAHEER KHAN2
1Department of Chemistry, Aligarh Muslim University, Aligarh—202 002, India
2Department of Chemistry, Jamia Millia Islamia, New Delhi—110 025, India
Received 20 August 1997; accepted 18 August 1998
ABSTRACT: Kinetics of the interaction of histidine and histidine methyl ester with ninhydrin
under varying concentrations of reactants, anionic (sodium dodecyl sulphate, SDS), cationic
(cetyltrimethylammonium bromide, CTAB) and non-ionic (Triton X-100, TX-100) micelles have
been carried out. Rate of the reaction was found to be independent of the initial concentration
of histidine (and histidine methyl ester) but was dependent on [Ninhydrin]. The SDS micelles
had no effect on the rate of the reaction. In the presence of the CTAB micelles a small en-
hancement in the rate was observed. The rate Ϫ [CTAB] profile showed that the increase in
[CTAB] increased the rate up to a maximum value and a further increase had a decreasing
effect on the rate. The rate was enhanced by TX-100 also but, unlike CTAB micelles, TX-100
possessed a curve without peak for the rate Ϫ [TX-100] profile. The following rate equation
was obeyed by the reaction in CTAB and TX-100 micelles:
S
kw [N]T ϩ (km KS Ϫ kw) [Dn] MN
k⌿
ϭ
1 ϩ KS [Dn]
Values of kw, km, and KS were evaluated and are reported herein. ᭧ 1999 John Wiley & Sons, Inc.
Int J Chem Kinet 31: 103–111, 1999
INTRODUCTION
croenvironments for different parts of the reactant
molecules; that is, a nonpolar hydrophobic core can
provide binding energy for similar groups while the
outer charged shell can interact with the reactant’s po-
lar groups. This inherent microheterogeneity of the
micellar solubilization environment could play an im-
portant role in the catalysis of a reaction. The ionic
micelles enhance the rate of bimolecular reactions by
increasing the concentration of the reactants within the
small volume of its Stern layer. The consideration of
electrostatic and hydrophobic interactions between the
Chemical reactivity in ionic colloidal self-assemblies
(e.g., micelles, microemulsion droplets, and vesicles)
has got importance owing to similarities in action with
the enzymatic reactions. The similarities between the
enzymatic reactions and the catalysis or inhibition by
micelles include shape and size, polar surfaces, and
hydrophobic cores. The micelles provide different mi-
Correspondence to: Kabir-ud-Din
᭧ 1999 John Wiley & Sons, Inc.
CCC 0538-8066/99/020103-09

104
KABIR-UD-DIN ET AL.
3
reactants and micelles can account qualitatively for the
kinetic effect on the reactions in micellar media; for
example, cationic micelles may accelerate the rates of
reactions between neutral molecules and anionic nu-
cleophiles while anionic micelles may inhibit such re-
actions.
1.0 mol dmϪ ) were determined conductimetrically at
70ЊC. The values were 6.25 ϫ 10Ϫ (SDS), 8.25 ϫ
3
10Ϫ (CTAB), and 2.8 ϫ 10Ϫ mol dmϪ (TX-100).
4
4
3
The respective values in water are 1.14 ϫ 10Ϫ2 (SDS,
70ЊC). 1.32 ϫ 10Ϫ (CTAB, 55ЊC) and 3.00 ϫ 10Ϫ
3
4
mol dmϪ3 (TX-100, 65ЊC) [5,6].
Histidine is an important amino acid which plays
an active role in biological systems [1,2] and also pro-
vides binding sites for metal ions [3,4]. To understand
the behavior of histidine in enzymatic environments
the reaction between ninhydrin and histidine have
been carried out in aqueous as well as in micellar me-
dia of anionic (SDS), cationic (CTAB), and non-ionic
(TX-100) surfactants. To elaborate the role of binding
of carboxylate group with these micelles, kinetic stud-
ies between histidine methyl ester and ninhydrin have
also been performed. The results are reported in this
paper.
RESULTS AND DISCUSSION
Reaction in the Absence of Micelles
Kinetics of interaction of amino acids with ninhydrin
in aqueous medium has been studied under varying
conditions of temperature, hydrogen ion, and reactant
concentrations, and the role of metal ions thereby has
been well established [7–11]. It is known that amino
acids [12], on interaction with ninhydrin, produce a
purple colored product, called diketohydrindylidene-
diketohydrindamine (DYDA). Different amino acids
(except proline) react with different rates but all pro-
duce the same final product [7–14]. Histidine exists
in zwitterionic form with imidazole and amino groups
having positive charges and carboxylate group bearing
negative charge. The condensation between the car-
bonyl group of ninhydrin and deprotonated amino
group (which exists in equilibrium with its protonated
form) of histidine takes place. The reaction starts
through the attack of lone-pair of electrons of amino
nitrogen to the carbonyl carbon (of ninhydrin) to give
a Schiff’s base (after decarboxylation). This Schiff’s
base is unstable and hydrolyses to give 2-amino in-
danedione (D1) and an aldehyde. D1, in turn, reacts
slowly with another ninhydrin molecule to yield
DYDA. A side product, hydrindantin, may also be ob-
tained depending upon reaction conditions (low pH
and low temperature). If formed, hydrindantin reduces
the yield of DYDA (Scheme-I). The 2-iminoindane-
dione(F), upon hydrolysis, gives ammonia which may
react with hydrindantin to give DYDA (route c). To
our knowledge, detailed studies on kinetics of histi-
dine-ninhydrin interaction in aqueous solutions are yet
to be made. To clarify certain points, experimentswere
carried out, such that reactions of alternate routes (b)
and (c) were suppressed (almost completely) by per-
forming the studies at elevated temperature (Ն70ЊC)
and pH ϭ 5.0 (formation of ammonia is negligible at
pH Ն 5.0) [7]. The results show (Table I) that the rate
constants are independent of the initial concentration
of histidine, indicating the order of the reaction with
respect to histidine to be unity. The observed pseudo-
first-order rate constant vs. [Ninhydrin] plots show a
nonlinear behavior (Figure 1A), thus indicating frac-
tional order in [Ninhydrin].
EXPERIMENTAL
L-Histidine (BDH, England), histidine methyl ester
(Fluka AG), ninhydrin (SRL), sodium dodecyl sul-
phate (SDS, 99%, CPC, USA), cetyltrimethylammon-
ium bromide (CTAB, 99%, Loba Chem), and Triton
X-100 (TX-100, 99%, Fluka AG) were used as re-
ceived. Solutions of L-histidine, ninhydrin, and histi-
dine methyl ester were prepared in acetic acid-sodium
acetate buffer (pH ϭ 5.0). All other chemicals used
were of reagent grade and doubly distilled and deion-
ized water was used throughout.
The reaction vessel, fitted with a double-surface
condenser to prevent evaporation, was kept in a ther-
mostated oil-bath. The reaction was started with the
addition of thermally equilibrated ninhydrin solution
of required volume to the solution containing histidine
or histidine methyl ester, the surfactant, buffer, and
KNO3. Zero time was taken when half of the required
volume of the ninhydrin solution had been added. Pu-
rified nitrogen gas was bubbled through the reaction
mixture for stirring as well as to maintain an inert at-
mosphere. Progress of the reaction was followed spec-
trophotometrically by measuring the absorbance at
570 nm with a Bausch and Lomb spectrophotometer
(Spectronic-20).
In the experiments, pseudo-first-order conditions
were maintained by keeping the ninhydrin in excess
(60ϫ). The pseudo-first-order rate constants were cal-
culated for completion of 80% of the reaction by using
a program run on a VAX-11/780.
The critical micellar concentration (cmc) values of
SDS, CTAB, and TX-100 in the buffer solution
(pH ϭ 5.0) containing the substrate and KNO3 (␮ ϭ

KINETICS OF INTERACTION IN MICELLAR MEDIA
105
On the basis of the observed rate law (d[P]/dt ϭ
kobs [Histidine]T and the mechanism (Scheme 1), the
rate equation (1) is derived:
treatment was carried out by an alternative method us-
ing equation (2). The double-reciprocal plots resulted
in straight
k K [N]T
1/kobs ϭ 1/k ϩ 1/k K [N]T
(2)
kobs
ϭ
(1)
1 ϩ K [N]T
lines (Figure 1B), and thus confirmed the validity of
the proposed mechanism. From the intercepts and
where [N]T ϭ total ninhydrin concentration. The data

106
KABIR-UD-DIN ET AL.
Table I Values of Rate Constants at Varying
mediate (i.e., the Schiff’s base) in the development of
purple color and also to stabilize the DYDA, the ef-
fects of anionic, cationic and non-ionic micelles on the
histidine- and histidine methyl ester-ninhydrin inter-
action under similar kinetic conditions were studied.
Concentrations of Histidine and Ninhydrin at [Hϩ] ϭ
5
1.0 ϫ 10Ϫ mol dmϪ3, temperature ϭ 70ЊC
104 [Histidine]T
103 [Ninhydrin]T
104kobs
Ϫ3
Ϫ3
Ϫ1
mol dm
mol dm
s
1.0
1.5
2.0
2.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
6.0
6.0
6.0
1.67
1.68
1.69
1.68
3.04
4.46
5.39
6.11
6.56
6.80
7.10
Reaction in the Presence of Anionic SDS
Micelles
6.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
The rate constants for the interaction of ninhydrin with
histidine obtained at varying concentrations of SDS
micelles were the same while those with histidine
methyl ester were slightly higher as compared to the
aqueous medium values (Table II).
From the mechanism of the reaction (Scheme 1),
we can see that SDS (as it is an anionic micelle having
negatively charged surface) cannot bind either with an
electron-cloud-rich ninhydrin molecule or with a his-
tidine bearing, negatively charged carboxylate group
and amino nitrogen having a lone pair of electrons
towards its surface. Therefore, it can be safely said that
the presence of SDS micelles will not affect the rate
of the reaction. The slight increase in rate for histidine
slopes, the respective values of k and K were evalu-
ated, which are: 1.77 ϫ 10Ϫ3 sϪ , 12.67 (70ЊC);
1
3.64 ϫ 10Ϫ3 sϪ , 16.48 (80ЊC); 5.00 ϫ 10Ϫ3 sϪ ,
1
1
23.81 (90ЊC).
In order to elaborate the role played by the inter-
Figure 1 (A) Plots of kobs vs. [Ninhydrin]T for the interaction of ninhydrin with histidine at
Ϫ
4
Ϫ
ϩ
Ϫ5
(a) 70ЊC, (b) 80ЊC, and (c) 90ЊC. [Histidine] ϭ 1.0 ϫ 10 mol dm 3, [H ] ϭ 1.0 ϫ 10
mol dm 3. (B) Double-reciprocal plots of 1/kobs vs. 1/[Ninhydrin]T. [Histidine] ϭ 1.0 ϫ
Ϫ
Ϫ4
Ϫ
ϩ
Ϫ
Ϫ3
10 mol dm 3, [H ] ϭ 1.0 ϫ 10 5 mol dm
.

KINETICS OF INTERACTION IN MICELLAR MEDIA
107
Table II Values of the Observed Pseudo-First-Order
Rate Constants, k , for the Interaction of Ninhydrin
␺
with Histidine/Histidine Methyl Ester Showing
Dependence upon [SDS].
5
[Ninhydrin] ϭ 6.0 ϫ 10Ϫ3 mol dmϪ3, [Hϩ] ϭ 1.0 ϫ 10Ϫ
mol dmϪ3, temperature ϭ 70ЊC
Ϫ1
104 k⌿/s
102 [SDS]
Ϫ3
mol dm
Histidine
Histidine Methyl Ester
0.0
0.5
1.0
2.0
5.0
1.67 Ϯ 0.62
1.74 Ϯ 0.54
1.70 Ϯ 0.63
1.68 Ϯ 0.58
1.68 Ϯ 0.44
1.68 Ϯ 0.57
0.53 Ϯ 0.04
0.62 Ϯ 0.04
1.02 Ϯ 0.05
1.02 Ϯ 0.05
1.02 Ϯ 0.05
1.02 Ϯ 0.05
10.0
Figure 2 Plots of k⌿ vs. [CTAB] for the interaction of
ninhydrin with histidine (᭹)/histidine methyl ester (᭡) at
Ϫ
4
Ϫ3
70ЊC. [Histidine] ϭ 1.0 ϫ 10
mol dm
,
[Histidine
methyl ester-ninhydrin reaction (Table II) may be due
to the catalytic effect of SDS on the rate of hydrolysis
of ester group to produce carboxylate group. This is
in conformity with several studies made in the pres-
ence of an ionic micelle which, in most cases, have
catalytic effect on the rate of hydrolysis of esters [15].
Ϫ4
Ϫ
Methyl Ester] ϭ 1.0 ϫ 10
mol dm 3, [Ninhydrin] ϭ
6.0 ϫ 10 3, [H ] ϭ 1.0 ϫ 10 5 mol dm
Ϫ
ϩ
Ϫ
Ϫ3
.
in [CTAB] leads to a slow decrease in the observed
rate. Detailed studies were carried out in the presence
of 2 ϫ 10Ϫ2 mol dmϪ3 CTAB. All the results are given
in Table III. The values of the rate constants were in-
dependent of the initial concentration of histidine/his-
tidine methyl ester but were dependent on the ninhy-
drin concentration. Typical results are shown in Fig-
ure 2.
Reaction in the Presence of Cationic CTAB
Micelles
The pseudo-first-order rate constants for the develop-
ment of purple color by the interaction of ninhydrin
with histidine or histidine methyl ester reached a max-
Ionic and polar reagents bind close to the micellar
surface and reactions take place in this region. Nin-
2
3
imum value at 2 ϫ 10Ϫ mol dmϪ . Further increase
Table III The Dependence of Pseudo-First-Order Rate Constants on [CTAB] for Histidine/Histidine Methyl Ester-
Ninhydrin Reaction.
4
3
[Histidine] ϭ 1.0 ϫ 10Ϫ mol dmϪ3, [Histidine Methyl Ester] ϭ 1.0 ϫ 10Ϫ4 mol dmϪ3, [Hϩ] ϭ 1.0 ϫ 10Ϫ5 mol dmϪ
,
temperature ϭ 70ЊC
Ϫ1
104 k⌿/s
Ϫ3
103[Ninhydrin]/mol dm
6.0
10.0
6.0
10.0
103 [CTAB]
mol dm
Ϫ3
Histidine
Histidine Methyl Ester
0.0
0.5
1.0
1.67 Ϯ 0.04
2.13 Ϯ 0.12
2.49 Ϯ 0.08
3.85 Ϯ 0.15
4.68 Ϯ 0.09
4.94 Ϯ 0.11
4.71 Ϯ 0.10
4.48 Ϯ 0.12
4.22 Ϯ 0.30
3.41 Ϯ 0.12
2.55 Ϯ 0.21
3.06 Ϯ 0.18
3.81 Ϯ 0.22
5.62 Ϯ 0.31
7.14 Ϯ 0.41
7.65 Ϯ 0.46
7.39 Ϯ 0.38
7.23 Ϯ 0.42
6.89 Ϯ 0.33
6.32 Ϯ 0.29
0.53 Ϯ 0.04
0.86 Ϯ 0.07
0.99 Ϯ 0.08
2.01 Ϯ 0.12
2.32 Ϯ 0.13
2.45 Ϯ 0.11
2.39 Ϯ 0.08
2.28 Ϯ 0.09
2.17 Ϯ 0.11
1.85 Ϯ 0.09
0.96 Ϯ 0.06
1.27 Ϯ 0.11
1.66 Ϯ 0.12
2.74 Ϯ 0.08
3.61 Ϯ 0.14
3.84 Ϯ 0.13
3.75 Ϯ 0.11
3.68 Ϯ 0.09
3.45 Ϯ 0.12
3.22 Ϯ 0.14
5.0
10.0
20.0
30.0
40.0
50.0
100.0

108
KABIR-UD-DIN ET AL.
Table IV Parameters that Best Fit the Kinetic Results
for the Interaction of Ninhydrin with Histidine/
Histidine Methyl Ester in the Presence of CTAB
Micelles [see equation (9)]
hydrin is a polar molecule with a ␲-electron cloud
around it, which has the likelihood of coming closer
to the cationic micellar surface [16]. Histidine, being
in the zwitterionic form with its negatively charged
carboxylate and protonated amino groups, binds and
gets accumulated in the Stern layer of the cationic
CTAB micelles (the effect will be slightly diminished
in the histidine methyl ester due to the presence of an
ester group). Thus, the CTAB micelles help in bring-
ing the reactants in the Stern layer and the reaction
takes place in this region. This reaction of ninhydrin
with histidine/histidine methyl ester in the presence of
a surfactant can be explained by considering the
pseudo-phase kinetic model with micelles and water
regarded as distinct reaction regions [17,18].
The mechanism of the reaction is presented in
Scheme 2
Parameters
Histidine Histidine Methyl Ester
KS
4 2
102 km (s
)
3.33 Ϯ 0.81a
3.36 Ϯ 0.82b
3.34 Ϯ 0.92a
3.39 Ϯ 0.86b
4.67 Ϯ 1.29a
4.74 Ϯ 1.21b
Ϫ1
103k2 (mol 1dm3s 1) 4.65 Ϯ 1.13a
4.71 Ϯ 1.15b
m
Ϫ
Ϫ
1
KN ϭ 100, 103 kw ϭ 27.60 dm3 molϪ sϪ1 and 10.12 dm3
1
1
molϪ sϪ for histidine and histidine methyl ester, respectively.
3
3
a [Ninhydrin] ϭ 6.0 ϫ 10Ϫ mol dmϪ
2
3
b [Ninhydrin] ϭ 1.0 ϫ 10Ϫ mol dmϪ
Values of MNS were estimated by considering the equi-
librium
KN
KN
NW ϩ Dn
ϩ
NDn
ϩ
Nw ϩ Dn ERF NDn
KS
[NDn]
KN ϭ
(6)
(7)
SW ϩ Dn
SDn
[Nw]([Dn] Ϫ [NDn])
kWЈ
kmЈ
and the mass balance
[N]T ϭ [Nw] ϩ [NDn]
Products
Scheme 2
Upon solving equations (6) and (7), a quadratic equa-
tion (8) results which was solved for [NDn] with the
help of a computer program where KN was an adjust-
able parameter. MNS was then calculated with the help
of equation (5). Equation (3) takes the form of equa-
tion (9) as developed by Rodenas et al. [18], when
values of kЈw and kЈm are substituted from equation (4)
where Sw is the substrate (histidine/histidine methyl
ester) in water and SDn is the subtrate complexed with
the micellized detergent Dn ([Dn] ϭ [surfactant] Ϫ
cmc) with an association constant KS, kЈw, and kmЈ are
the first-order rate constants for the reaction taking
place in aqueous and micellar pseudo-phases. The
pseudo-first-order rate constant, k⌿, for overall
reaction, consistent with the Scheme-II, is given
by [18]
KN[NDn]2 Ϫ (1 ϩ KN[Dn] ϩ KN[N]T)[NDn]
ϩ KN[Dn][N]T ϭ 0 (8)
kЈw ϩ kЈmKS[Dn]
S
kw[N]T ϩ (KSkm Ϫ kw) MN [Dn]
k⌿ ϭ
(3)
k⌿ ϭ
(9)
1 ϩ KS[Dn]
1 ϩ KS[Dn]
The respective pseudo-first-order rate constants in
aqueous (kЈw) and micellar (kЈm) pseudo-phases are de-
fined as
The value of KN for the binding of ninhydrin with
CTAB micelles was assumed as 100 (see our earlier
paper [11].) In order to find km and KS, the nonlinear
least squares technique was used for equation (9). This
process gave a value of least squares; that is, di2
S
kЈw ϭ kw [Nw], kmЈ ϭ km MN
(4)
͸
where di ϭ kobsi Ϫ kcali at ith KS. The calculation was
repeated for different values of KS ranging from 0.2
MNS is the molarity of ninhydrin bound to the micellar
head groups, given as
to 20 and the best value was the one for which di2
͸
was minimum. The KS, thus obtained, was used to
obtain the value of km. These values are given in
Table IV.
S
MN ϭ [NDn]/[Dn]
(5)

KINETICS OF INTERACTION IN MICELLAR MEDIA
109
Table V Values of Pseudo-First-Order Rate Constants for Ninhydrin-Histidine/Histidine Methyl Ester Reaction
Showing Dependence upon [TX-100].
[Histidine] ϭ 1.0 ϫ 10Ϫ4 mol dmϪ3, [Histidine Methyl Ester] ϭ 1.0 ϫ 10Ϫ4 mol dmϪ3, [Hϩ] ϭ 1.0 ϫ 10Ϫ5 mol dmϪ
3
,
temperature ϭ 70ЊC
Ϫ1
104 k⌿/s
Ϫ3
103[Ninhydrin]/mol dm
6.0
10.0
6.0
10.0
102[TX-100]
mol dm
Ϫ3
Histidine
Histidine Methyl Ester
0.0
1.6
4.0
6.0
8.0
10.0
12.0
16.0
1.67 Ϯ 0.04
2.82 Ϯ 0.08
6.02 Ϯ 0.12
10.12 Ϯ 0.31
13.41 Ϯ 0.34
16.37 Ϯ 0.42
17.21 Ϯ 0.39
17.64 Ϯ 0.47
2.55 Ϯ 0.21
5.17 Ϯ 0.11
9.32 Ϯ 0.22
13.46 Ϯ 0.28
18.38 Ϯ 0.37
22.51 Ϯ 0.38
25.19 Ϯ 0.47
26.43 Ϯ 0.44
0.53 Ϯ 0.04
1.09 Ϯ 0.05
1.94 Ϯ 0.08
2.87 Ϯ 0.11
4.12 Ϯ 0.09
5.62 Ϯ 0.18
6.71 Ϯ 0.22
7.22 Ϯ 0.22
0.94 Ϯ 0.06
2.13 Ϯ 0.07
3.74 Ϯ 0.09
5.38 Ϯ 0.13
7.26 Ϯ 0.14
8.96 Ϯ 0.22
10.27 Ϯ 0.14
11.63 Ϯ 0.23
Reaction in the Presence of Non-ionic
Triton X-100 Micelles
dependent on the concentrations of ninhydrin and TX-
100. The results are summarized in Table V. The ki-
netic studies carried out at two different [Ninhydrin]
under varying [TX-100] are shown graphically in Fig-
ure 3. Unlike the effect of CTAB micelles, the TX-
100 micelles gave no peaked behavior for rate vs. [sur-
factant] profile. At low [surfactant], the rate did show
linear dependency but at higher concnetrations it be-
came independent of [TX-100].
TX-100 is a non-ionic polar surfactant and provides
an ambient environment for the binding of histidine/
histidine methyl ester and ninhydrin. The reaction in
the presence of TX-100 too followed Scheme 2 and
the rate equation (9). Values of km and KS, evaluated
with the help of equations (4–9), are given in Ta-
ble VI.
The effect of TX-100 micelles on the reaction of his-
tidine/histidine methyl ester with ninhydrin at varying
concentrations of histidine/histidine methyl ester,
ninhydrin, and TX-100 were studied at 70ЊC and
5
3
[Hϩ] ϭ 1.0 ϫ 10Ϫ mol dmϪ . It was found that the
rate constants were independent of the initial concen-
trations of histidine (and histidine methyl ester) but
Table VI Parameters that Best Fit the Kinetic Results
for the Interaction of Ninhydrin with Histidine/Histidine
Methyl Ester in the Presence of Triton X-100 Micelles
[see equation (9)].
Parameters
Histidine Histidine Methyle Ester
KS
11
7
102 km (s
)
4.67 Ϯ 0.94a
4.74 Ϯ 0.89b
2.15 Ϯ 0.92a
2.17 Ϯ 0.93b
1.46 Ϯ 0.62a
1.48 Ϯ 0.63b
Ϫ1
102k2 (mol 1dm3s 1) 3.18 Ϯ 0.63a
3.22 Ϯ 0.60b
m
Ϫ
Ϫ
1
1
1
KN ϭ 50, 103 kw ϭ 27.60 dm3 molϪ sϪ and 10.12 dm3 molϪ
sϪ for histidine and histidine methyl ester, respectively.
1
Figure 3 Plots of k⌿ vs. [TX-100] for the interaction of
ninhydrin with histidine (᭹)/histidine methyl ester (᭡) at
70ЊC. Reaction conditions same as in Figure 2.
a [Ninhydrin] ϭ 6.0 ϫ 10Ϫ mol dmϪ
3
3
b [Ninhydrin] ϭ 1.0 ϫ 10Ϫ mol dmϪ
2
3

110
KABIR-UD-DIN ET AL.
General
The values of k2m in the micelles of CTAB and TX-
100 are quoted in Tables IV and VI. The second-order
rate constants for histidine-ninhydrin reaction in
CTAB and TX-100 micelles are, respectively, 5.93
and 0.87 times than the second-order rate constants in
water (kw/k2 ). The kw/k2 values for histidine methyl
ester-ninhydrin in CTAB and TX-100 micelles are
2.17 and 0.69, respectively. Thus, the second-order
rate constants (for both histidine and histidine methyl
ester) in the CTAB micellar pseudo-phase are lower
than the second-order rate constants in aqueous phase,
whereas a reverse case is observed in TX-100: but the
highest reaction rate was obtained in micellar media.
The values of cmc of surfactants are sensitive to the
nature of the reactants and also depend upon reaction
conditions. Therefore, the “kinetic” cmc is often taken
as an adjustable parameter, with the proviso that it
must be lower than that in water. In our study the
variation in values of the kinetic cmc did not signifi-
cantly affect the results and, therefore, all the calcu-
lations for km and KS were made using the cmc values
given in the experimental part.
Histidine and histidine methyl ester are highly sol-
uble in water and due to their hydrophilic nature the
values of KS are quite low [19]. The higher values of
KS for histidine rather than histidine methyl ester with
micelles of both CTAB and TX-100 may be because
histidine is more polar and hence more suitable for
solubilization in the Stern layer of CTAB/outer shell
of TX-100 micelles. As regards higher values of KS in
TX-100 rather than in CTAB, the positive charge on
imidazole moiety of A (cf. Scheme 1) seems to play a
role, as there will be less objection for solubilization
in the non-ionic TX-100 than in the cationic CTAB.
The rate constants in micellar pseudo-phase, km
(whose unit is reciprocal seconds), cannot be com-
pared directly with the second-order rate constant in
m
m
One of us (MZAR) is thankful to CSIR, New Delhi, for
the award of RA. The authors would also like
to thank referees for valuable suggestions and com-
ments.
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1
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KINETICS OF INTERACTION IN MICELLAR MEDIA
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