Effect of dicationic gemini surfactants 16-s-16 (s = 4, 5, 6) on the ninhydrin-dipeptide (glycyl-tyrosine) reaction

Article abstract of DOI:10.1002/kin.20731

The effect of dicationic gemini surfactants H₃₃C ₁₆(CH₃)₂N⁺-(CH₂) _s-N⁺(CH₃)₂ C₁₆H ₃₃, 2Br^- (s= 4, 5, 6) on the reaction of a dipeptide glycyl-tyrosine (Gly-Tyr) with ninhydrin has been studied spectrophotometrically at 70°C and pH 5.0. The reaction follows first- and fractional-order kinetics, respectively, in [Gly-Tyr] and [ninhydrin]. The gemini surfactant micellar media are comparatively more effective than their single chain-single head counterpart cetyltrimethylammonium bromide (CTAB) micelles. Whereas typical rate constant (k_ψ) increase and leveling-off regions, just like CTAB, are observed with geminis, the latter produces a third region of increasing k_ψ at higher concentrations. This subsequent increase is ascribed to the change in the micellar morphology of the geminis. The pseudophase model of micelles was used to quantitatively analyze the k _ψ - [gemini] data, wherein the micellar-binding constants K _S for [Gly-Tyr] and K_N for ninhydrin were evaluated.

Full text of DOI:10.1002/kin.20731

Effect of Dicationic Gemini
Surfactants 16-s-16 (s = 4,
5, 6) on the
Ninhydrin–Dipeptide
(Glycyl–Tyrosine) Reaction
MOHD. AKRAM, DILEEP KUMAR, KABIR-UD-DIN
Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India
Received 12 August 2011; revised 17 January 2012; accepted 13 March 2012
DOI 10.1002/kin.20731
Published online 23 October 2012 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The effect of dicationic gemini surfactants H33C16(CH3)2N+-(CH2)s -N+(CH3)2
C16H33, 2Br− (s = 4, 5, 6) on the reaction of a dipeptide glycyl–tyrosine (Gly–Tyr) with nin-
hydrin has been studied spectrophotometrically at 70◦C and pH 5.0. The reaction follows first-
and fractional-order kinetics, respectively, in [Gly–Tyr] and [ninhydrin]. The gemini surfactant
micellar media are comparatively more effective than their single chain–single head counter-
part cetyltrimethylammonium bromide (CTAB) micelles. Whereas typical rate constant (kꢀ)
increase and leveling-off regions, just like CTAB, are observed with geminis, the latter produces
a third region of increasing kꢀ at higher concentrations. This subsequent increase is ascribed to
the change in the micellar morphology of the geminis. The pseudophase model of micelles was
used to quantitatively analyze the kꢀ−[gemini] data, wherein the micellar-binding constants
C
K
S for [Gly–Tyr] and KN for ninhydrin were evaluated. ꢀ 2012 Wiley Periodicals, Inc. Int J Chem
Kinet 44: 800–809, 2012
fingerprints [2,3]. The reaction of ninhydrin and amino
acids/peptides starts through the attack of a lone pair
of electrons of nitrogen of the amino group to the car-
bonyl carbon of ninhydrin, which occurs in the loss
of H2O and CO2 and produces purple-colored product
diketohydrindylidenediketohydrindamine (DYDA).
Reactions in micellar media are influenced by hy-
drophobic and electrostatic forces [4]. For a given
reaction, the observed rates depend on the ex-
tent of reagent association with micellar aggregates.
Micelles are considered as models for enzyme action
because they are similar in shape and size, and more im-
portantly, both have a hydrophobic core and polar sur-
faces. All kinds of micellar-mediated organic reactions
INTRODUCTION
Ninhydrin is widely used to analyze and character-
ize amino acids, peptides, and proteins in chemical
and biochemical reactions [1]. Identification of amino
acids/peptides is extremely important in the evaluation
of protein structure as they are structural units and are
also found in numerous natural products. The appli-
cation of ninhydrin for detection/estimation of amino
acids/peptides has a great potential in revealing latent
Correspondence to: Mohd. Akram; e-mail: drmohdakram@
rediffmail.com.
c
ꢀ
2012 Wiley Periodicals, Inc.

EFFECT OF DICATIONIC GEMINI SURFACTANTS ON THE NINHYDRIN–DIPEPTIDE REACTION
801
B
B
B
Ninhydrin
Scheme 1 Structure of surfactants and reagents used in this study.
Glycyl–tyrosine (Gly–Tyr)
(ionic, polar, and neutral) are generally believed to oc-
cur in the Stern layer of a micelle of an ionic surfactant.
Nonfunctionalized micelles, which catalyze reactions,
provide a simple illustration of utilization of the bind-
ing energy between a phase or macromolecule and the
reactants to lower the free energy of activation. The
characteristics of micelles play a very significant role
in catalysis. A small change in the structure of micelles
can bring changes in rigidity and surface properties of
micelles and hence affect the reactants’ activity [5].
In the present paper, an attempt has been made to
study the effect of surfactant micelles on the reaction
of ninhydrin–peptide to enhance the usefulness of the
method. Conventional surfactant cetyltrimethylammo-
nium bromide (CTAB, having a single hydrocarbon
chain/single polar head group) can be considered as a
monomeric counterpart of the geminis (16-s-16, s =
4, 5, 6). As compared to CTAB, the gemini surfactants
consist of two amphiphilic monomers connected at the
level of head groups. The geminis have comparatively
much lower critical micellar concentration (cmc) as
well as much greater efficiency in reducing the surface
tension of water [6].
Numerous dipeptides are found in nature, perform-
ing a variety of functions, and they can also be made
in laboratory environments. Dipeptides have a number
of commercial and industrial uses in addition to play-
ing an important role in the biology of many species
on the earth. It is well known that peptides are one of
the significant chemical compounds found in the cell
of biotic species. Dipeptides do things such as reduc-
ing fatigue and playing a role as antioxidants. Peptides
such as enkephalins, oxytocin, vasopressin, leutiniz-
ing hormone releasing hormone, opioid peptides, and
elastic sequence play a very important role in biology.
These peptides are susceptible to enzymes.
The dipeptide glycyl–tyrosine (Gly–Tyr) was cho-
sen as the model, and its reaction was studied in
the presence of three cationic gemini surfactants
(Scheme 1). A comparison with the previous results
[7] has also been made.
EXPERIMENTAL
Materials
Ninhydrin (99.0%; Merck, Mumbai, India), acetic
acid (99.0%; Merck), sodium acetate (99.0%;
Merck), Gly–Tyr (99.0%; SRL, Mumbai, In-
dia), 1,6-dibromohexane (>97.0%; Fluka, Stein-
heim, Germany), 1,5-dibromopentane (>98.0%;
Fluka), 1,4-dibromobutane (>98.0%; Fluka), N,N-
dimethylhexadecylamine (>95.0%; Fluka), ethyl ac-
etate (HPLC and spectroscopy grade, 99.0%), and
ethanol absolute (99.8%; Merck, Darmstadt, Germany)
were used as supplied. Water used to prepare the so-
lutions was distilled twice (specific conductance (1–
2) × 10−6 ꢀ−1 cm−1) over alkaline KMnO4 in an all
glass (Pyrex, Mumbai, India) distillation setup. Acetate
buffer of required pH, used as a solvent for preparing
all the stock solutions, was obtained by mixing appro-
priate volumes of 0.2 mol dm−3 acetic acid and sodium
acetate solutions [8]. An ELICO LI-122 (Hyderabad,
India) pH meter in conjunction with a combined glass–
calomel electrode was used for the pH measurements.
International Journal of Chemical Kinetics DOI 10.1002/kin.20731

802
AKRAM, KUMAR, AND KABIR-UD-DIN
tive and quantitative analyses. These results show that
the purple-colored reaction product is the same in the
presence of micelles as in aqueous and CTAB micellar
solutions [7].
Synthesis and Characterization of Gemini
Surfactants
Two main factors are significant in the prepara-
tion of gemini surfactants. One is synthesis, and
the other is purification. These gemini surfactants
were synthesized by refluxing the corresponding
α, ω-dibromoalkanes (s = 4, 5, 6) with N,N-
dimethylhexadecylamine (molar ratio 1:2.1) in dry
ethanol at 80◦C for 48 h. After completion of the reac-
tion, the solvent was removed under vacuum and the
solid thus obtained was recrystallized four to five times
from ethyl acetate to obtain pure geminis. The purity
Kinetic Measurements
A three-necked reaction vessel (fitted with a double-
walled condenser to prevent evaporation) containing
required volumes of Gly–Tyr and surfactant solutions
was kept immersed in an oil bath thermostated at the
desired temperature. For stirring and maintaining an
inert atmosphere, pure nitrogen gas (free from CO2
and O2) was bubbled through the reaction mixture.
The reaction was started by adding a requisite vol-
ume of thermally equilibrated ninhydrin solution. The
progress of the reaction was followed spectrophoto-
metrically by pipetting out aliquots at definite time in-
tervals and measuring the product absorbance (purple
colored) at 570 nm (Fig. 1) with a UV–vis spectropho-
tometer (SHIMADZU-model UV mini 1240, Kyoto,
Japan). The pseudo–first-order rate constants (kꢁ ) were
calculated up to 80% completion of the reaction using
a computer program [9].
1
of the surfactants was ascertained on the basis of H
NMR and C, H, N data. The main features and peaks
were similar, as reported previously [6].
Spectra of Product
All the UV–visible spectra of the solutions were
recorded after completion of the reaction. The ab-
sorbance increases with an increase in the gemini con-
centration (Fig. 1). It can be seen that the absorption
band of the product (DYDA) remains unchanged in the
presence of gemini surfactants with no shift in wave-
length of maximum absorption (λmax = 570 nm). The
absorbance maximum is usually utilized for qualita-
Stoichiometry
As per the well-known general mechanism of the
reaction [10], the reaction of ninhydrin with amino
acids/peptides starts through the attack of a lone pair of
electrons of amino nitrogen (of amino acid/peptide) to
the carbonyl carbon (of ninihydrin) to give the Schiff
base. This Schiff base is unstable and hydrolyses to
give 2-amino-indanedione with the evolution of CO2.
Later, 2-amino-indanedione reacts slowly with another
ninhydrin molecule to yield the product (DYDA). For
determining the stoichiometry, the reaction was car-
ried out under the kinetic conditions with an excess of
ninhydrin. The concentration of evolved CO2 was de-
termined quantitatively by passing it through a standard
Ba(OH)2 solution (wherein a white precipitate was ob-
served). The remaining Ba(OH)2was determined by
titrating the solution against a standard HCl solution
using phenolphthalein as an indicator. It was observed
that 1 mol of the peptide gives 1 mol of CO2.
Figure 1 Spectra of the reaction product of ninhydrin with
Gly–Tyr in the absence and presence of gemini surfactants
in a buffer solution: (A) aqueous medium, (B) 16-6-16,
(C) 16-5-16, and (D) 16-4-16. Conditions: [16-s-16] = 30.0
× 10−5 mol dm−3 (s = 4, 5, 6), [ninhydrin] = 6.0 × 10−3
mol dm−3, [Gly–Tyr] = 3.0 × 10−4 mol dm−3, temperature
= 70◦C, and pH = 5.0.
Determination of cmc by Conductivity
Measurements
The values of cmc of the gemini (16-s-16, s = 4,
5, 6) surfactants under experimental conditions were
measured conductimetrically. First, the conductivity of
solvent was measured and then a small volume of the
International Journal of Chemical Kinetics DOI 10.1002/kin.20731

EFFECT OF DICATIONIC GEMINI SURFACTANTS ON THE NINHYDRIN–DIPEPTIDE REACTION
803
Table I Critical Micellar Concentration (in mM) Values of Gemini Surfactants in the Absence and in the Presence of
Gly–Tyr (3.0 × 10−4 mol dm−3) and Ninhydrin (6.0 × 10−3 mol dm−3
)
16-6-16
16-5-16
16-4-16
Solution
30◦C
70◦C
30◦C
70◦C
30◦C
70◦C
Water
Ninhydrin
Gly–Tyr
Ninhydrin + Gly–Tyr
0.043
0.039
0.040
0.041
0.058
0.055
0.055
0.056
0.034
0.033
0.034
0.033
0.050
0.043
0.049
0.048
0.032
0.031
0.029
0.031
0.044
0.040
0.042
0.041
stock solution of the surfactant was added and the con-
ductivity was noted after each addition. The solvent
correction was applied for determining the specific
conductivity. The break points of nearly two straight-
line segments of the specific conductivity versus con-
centration plots provided the cmc values. The mea-
surements were done by using an ELICO conductivity
bridge type CM 82T (Hyderabad, India) in the ab-
sence and in the presence of reactants (i.e., ninhydrin
and Gly–Tyr). Different conditions were maintained
for carrying out experiments at 30◦C and 70◦C, that is,
water, water + ninhydrin, water + Gly–Tyr, water +
ninhydrin + Gly–Tyr (Table I).
of pH on the reaction rate of the Gly–Tyr–ninhydrin
reaction was studied in the pH range 4.0–6.0, keeping
fixed [ninhydrin] = 6.0 × 10−3 mol dm−3, [Gly–Tyr]
= 3.0 × 10−4 mol dm−3, temperature = 70◦C, and
[gemini] = 30 × 10−5 mol dm−3. It is found that the
value of the rate constant increases up to pH = 5.0 and
then becomes almost constant (Fig. 2). Every elemen-
tary reaction of α-amino acids/peptides and ninhydrin
depends upon the [H+] because the reaction proceeds
through the formation of an intermediate that has the
Schiff base linkage (>C N ). The product (Schiff
base) formation is acid catalyzed, and pH 5.0 is the op-
timum pH. All subsequent kinetic measurements were,
therefore, made at pH = 5.0.
RESULTS AND DISCUSSION
Effect of [Gly–Tyr] on the Reaction Rate
Because of the use across
a wide spectrum
To confirm the mechanism in gemini micellar medium
(16-s-16 = 30 × 10−5 mol dm−3, s = 4, 5, 6) with
respect to [Gly–Tyr], several kinetic runs were carried
out at 70◦C and pH = 5.0 under the pseudo–first-order
conditions of [ninhydrin] ꢁ [Gly–Tyr]. The ninhy-
of disciplines, the mechanism of ninhydrin–amino
acids/peptides reactions have attracted attention of sci-
entists of different fields [1–3,9]. It is well documented
that the reaction proceeds through the formation of the
Schiff base, which further undergoes decarboxylation
and hydrolysis to yield 2-amino-indanedione (D1) as
a stable intermediate (Scheme 2). The reaction of the
intermediate D1 with another ninhydrin molecule also
involves an addition–elimination-type interaction and
produces DYDA (route (a)). D1 gives ammonia (upon
hydrolysis) and hydrindantin (route (b)). This reaction
is strongly dependent upon pH, the concentration of
reactants, and also the presence of atmospheric oxy-
gen. A yellowish-colored product is formed (instead
of DYDA, which is purple colored) in the presence
of atmospheric oxygen. At pH <5.0, the reaction pro-
ceeds chiefly by route (b) and ammonia is evolved
almost quantitatively with formation of hydrindantin,
whereas route (a) predominates at pH ≥5.0 and, under
these conditions, DYDA is formed.
drin concentration was fixed at 6.0 × 10−3 mol dm−3
,
whereas the [Gly–Tyr] was varied from 2.0 × 10−4 to
4.0 × 10−4 mol dm−3. As the kꢁ values (Table II) were
found independent of the initial concentration of Gly–
Tyr, the order of the reaction with respect to [Gly–Tyr]
is unity, that is,
d[DYDA]
Rate =
= kꢁ[Gly − Tyr]
(1)
dt
Effect of [Ninhydrin] on the Reaction Rate
The dependence of the rate on [ninhydrin] was de-
termined by carrying out the kinetic experiments at
varying [ninhydrin] ((6.0–40.0) × 10−3 mol dm−3) at
constant values of [Gly–Tyr] of 3.0 × 10−4mol dm−3
,
temperature 70◦C, and pH 5.0 (Table II). In all the
runs, the [gemini] was kept fixed at 30 × 10−5 mol
dm−3. The rate constant versus [ninhydrin] profiles
were nonlinear passing through origin (Fig. 3), whereas
Effect of pH on the Reaction Rate
As the solution pH plays a significant role toward the
yield and stability of Ruhemann’s purple [3], the effect
International Journal of Chemical Kinetics DOI 10.1002/kin.20731

804
AKRAM, KUMAR, AND KABIR-UD-DIN
The mechanism
(N)
(Gly– Tyr)
(2-Amino-indanedione,D1)
(Schiff base)
Route (b) (pH < 5.0):
(Hydrindantin)
Scheme 2
International Journal of Chemical Kinetics DOI 10.1002/kin.20731

EFFECT OF DICATIONIC GEMINI SURFACTANTS ON THE NINHYDRIN–DIPEPTIDE REACTION
805
Figure 2 Plots of kꢁversus pH for the reaction of ninhydrin
with Gly–Tyr in the presence of gemini surfactants: (A) 16-
6-16, (B) 16-5-16, and (C) 16-4-16. Conditions: [16-s-16] =
30.0 × 10−5 mol dm−3 (s = 4, 5, 6), [ninhydrin] = 6.0 ×
10−3 mol dm−3, [Gly–Tyr] = 3.0 × 10−4 mol dm−3, and
temperature = 70◦C.
Figure 3 Plots of kꢁversus [ninhydrin] for the reaction of
ninhydrin with Gly–Tyr in the presence of gemini surfactants:
(A) 16-6-16, (B) 16-5-16, and (C) 16-4-16. Conditions: [16-
s-16] = 30.0 × 10−5 mol dm−3 (s = 4, 5, 6), [Gly-Tyr] =
3.0 × 10−4 mol dm−3, temperature = 70◦C, and pH = 5.0.
double-logarithmic plots between kꢁ versus [ninhy-
drin] yielded straight lines with slopes less than unity.
This indicates the order with respect to [ninhydrin] to
be fractional in the micellar media. Besides that, the
linearity of kꢁ−1versus [ninhydrin]−1 plots also repre-
sents the Michaelis–Menten behavior, with a positive
intercept on the y-axis (Fig. 4).
Table II Dependence of Rate Constants (kꢀ) on [Gly–Tyr], [ninhydrin], and Temperature at pH 5.0 in the Presence of
Gemini Surfactants (30 × 10−5 mol dm−3
)
105kꢁ (s−1
)
104 [Gly–Tyr] (mol dm−3
)
103[Ninhydrin] (mol dm−3
6
)
Temperature (◦C)
70
16-6-16
16-5-16
16-4-16
2.0
2.5
3.0
3.5
4.0
3.0
10.9
11.1
11.1
11.4
11.6
11.1
18.6
22.5
25.0
27.5
29.0
30.9
31.6
3.7
12.0
12.2
12.3
12.5
12.6
12.3
20.1
26.4
32.2
33.8
36.9
39.2
40.7
3.8
14.4
14.8
14.9
15.1
15.3
14.9
21.9
28.9
35.1
40.3
41.7
43.4
46.2
5.7
6
10
15
20
25
30
35
40
6
70
3.0
60
65
70
75
80
8.4
8.5
9.0
11.1
20.5
29.0
12.3
22.7
32.7
14.9
26.0
37.5
International Journal of Chemical Kinetics DOI 10.1002/kin.20731

806
AKRAM, KUMAR, AND KABIR-UD-DIN
centration (part II, up to 400 × 10−5 mol dm−3) (the
characteristics of parts I and II are as conventional sur-
factant micelles) [7]. In the last, the kꢁ values increase
sharply (part III) (Fig. 5). This is true for all the gemi-
nis, only the extent of the increase in the rate constant
value is different (being dependent upon the spacer
chain length).
Considering part I, the rate constant value should re-
main constant because [geminis] are below their cmc,
but an increase in the observed rate constant values
may be due to the presence of premicelles or prepone-
ment of micellization by reactants [11] (it is also con-
firmed by cmc decreases at reaction conditions (Table
I)). In range II, the values of the rate constant remain
almost constant up to 400 × 10−5 mol dm−3 of gemini
surfactants. The gemini micelles show a much better
catalyzing effect than their single chain analogues (see
inset, Fig. 5) [7]. This can be because of the presence
of a spacer in geminis, which decreases the amount
of water in aggregates, providing different microen-
vironments (less polar), thus causing a rate increase
[12]. Because of the proximity of positive charges in
gemini surfactants, anion binding at the surfaces is in-
creased at the expense of binding of H2O molecules
[13]. The values of kꢁ for all the three geminis at all
concentrations follow the order 16-4-16 > 16-5-16 >
16-6-16 and have the same characteristics (Fig. 5).
Earlier also, the best results were achieved by 16-4-16
gemini among the series 16-s-16 [14]. It is well known
that, to minimize its contact with water, a spacer longer
than the “equilibrium” distance between two NMe2
head groups (the “equilibrium” distance occurs at s
= 4 in 16-s-16 geminis) tends to loop toward the mi-
cellar interior [6]. Increased looping will result in a
N
Figure 4 Plots of 1/kꢁversus 1/[ninhydrin] for the reac-
tion of ninhydrin with Gly–Tyr in the presence of gemini
surfactants: (A) 16-6-16, (B) 16-5-16, and (C) 16-4-16. Con-
ditions: [16-s-16] = 30.0 × 10−5 mol dm−3 (s = 4, 5, 6),
[Gly-Tyr] = 3.0 × 10−4 mol dm−3, temperature = 70 ◦C, and
pH = 5.0.
Effect of Temperature on the Reaction Rate
The rate constant values obtained in the presence of
gemini surfactants ([geminis] = 30 × 10−5 mol dm−3
)
obtained at different temperatures are recorded in
Table II. The activation parameters were evaluated with
the help of the linear least-squares regression tech-
nique. The data obtained were found to obey the Ar-
rhenius and Eyring equations (2) and (3)
kꢁ = A exp(−Ea/RT )
(2)
kꢁ = (kBT /h) exp(ꢂS#/R) exp(−ꢂH#/RT ) (3)
Table III Thermodynamic Parameters, Rate, and
Binding Constant Values for the Reaction of Gly–Tyr and
Ninhydrin in Aqueous and Micellar Media at pH 5.0
where kB, h, ꢂS#, R, ꢂH#, Ea, and A are, respectively,
Boltzmann constant, Planck’s constant, activation en-
tropy, gas constant, activation enthalpy, activation en-
ergy, and frequency factor. The activation parameters
evaluated on the basis of the above equations are given
in Table III.
Parameters and
Constants
16-6-16a 16-5-16a 16-4-16a Aqueousb
E (kJ mol−1
)
102.8
100.0
297.6
94.3
91.5
299.3
86.0
83.2
300.9
135.8
133.0
294.7
a
ꢂH#(kJ mol−1
−ꢂS#
)
(J K−1 mol−1
)
Effect of [Gemini] on the Reaction Rate
105k (s−1 c
)
0.6
2.2
182.0
71.8
1.6
172.0
70.0
–
–
–
m
The results of kinetic experiments carried out at differ-
ent gemini surfactant concentrations at constant [nin-
hydrin] (6.0 × 10−3 mol dm−3), [Gly–Tyr] (3.0 × 10−4
mol dm−3), temperature (70◦C), and pH (5.0) are given
in Table IV. It has been found that the rate constant in-
creases first (part I, [geminis] below their cmc) and
then becomes almost constant up to a definite con-
KS (mol−1 dm3)c 188.0
KN(mol−1dm3)c
66.8
For definition of terms, see Eqs. (2)–(4) and (6).
a[ninhydrin] = 6.0 × 10−3 mol dm−3, [16-s-16] = 30 ×
10−5 mol dm−3
.
b[ninhydrin] = 6.0 × 10−3 mol dm−3
.
cAt 70◦C.
International Journal of Chemical Kinetics DOI 10.1002/kin.20731

EFFECT OF DICATIONIC GEMINI SURFACTANTS ON THE NINHYDRIN–DIPEPTIDE REACTION
807
relatively wet Stern layer that will decrease the value
of kꢁ. Thus, the findings are in agreement with the
earlier results that, on increasing the amount of water,
the reaction environment produces an inhibiting effect
[15–18].
In the case of part III, with increasing [geminis]
(400–3000 × 10−5 mol dm−3), the values of kꢁ in-
crease slowly. At higher [geminis], a fast increment in
the rate of reaction occurs; this is probably associated
with the change in the micellar structure. This is in
conformity with the 1H NMR studies of the surfactants
[12,19]. Thus, at higher [geminis], an increase in the
kꢁ value happens due to changes in the aggregate mor-
phology that provides different reaction environments
(less polar).
The observed catalytic role of gemini micelles
can be explained in terms of the pseudophase model
(Scheme 3).
Although several kinetic equations based on this
general Scheme 3 can be explained on the basis of the
scheme proposed by Menger and Portnoy, the most
successful appears to be that of Bunton [20] and Rom-
sted [21], who suggested an equation, which takes into
account the solubilization of both the reactants into
micelles as well as the mass action model
Figure 5 Plots of kꢁ versus [16-s-16] for the reaction
of ninhydrin with Gly–Tyr: (A) 16-6-16, (B) 16-5-16, (C)
16-4-16, and inset is for variation of [CTAB]. Conditions:
[ninhydrin] = 6.0 × 10−3 mol dm−3, [Gly–Tyr] = 3.0 ×
10−4 mol dm−3, temperature = 70◦C, and pH = 5.0.
Here, kw and km are the second-order rate constants,
referring to the aqueous and micellar pseudophases,
respectively. KS is the binding constant of Gly–Tyr to
kw[N] + (KSkm − kw)MNS[Dn]
kꢁ
=
(4)
1 + KS[Dn]
Table IV Rate Constants for Reaction of Ninhydrin with Gly–Tyr in the Presence of Geminis (30 × 10−5 mol dm−3) at
Constant pH (5.0) and Temperature (70◦C) and Their Comparison with Calculated Values (kꢀcal
)
16-6-16
105kꢁ 105kꢁcal
16-5-16
105kꢁ 105kꢁcal
16-4-16
105[Surfactant]
105kꢁ 105kꢁcal
kꢁ − kꢁcal
kꢁ − kꢁcal
kꢁ − kꢁcal
(mol dm−3
)
(s−1
)
(s−1
)
(s−1
)
(s−1
)
(s−1
)
(s−1
)
kꢁ
kꢁ
kꢁ
5.0
10.0
20.0
30.0
40.0
50.0
60.0
80.0
100.0
250.0
400.0
600.0
1000.0
1500.0
2000.0
2500.0
3000.0
6.7
8.8
9.7
–
–
–
–
8.0
10.5
10.9
12.3
12.6
12.7
12.9
13.4
13.6
14.5
14.7
15.2
16.7
17.3
18.2
18.6
20.0
–
–
–
–
8.2
10.7
12.8
14.9
15.0
15.1
15.3
15.6
15.7
16.0
16.5
16.9
18.4
19.8
20.6
21.3
22.6
–
–
–
–
6.9
10.0
12.3
11.2
11.3
12.5
13.6
15.1
15.4
15.9
–
+0.29
+0.10
−0.10
+0.12
+0.12
+0.03
−0.05
−0.12
−0.10
−0.08
–
9.1
13.6
13.3
14.3
15.1
15.8
16.2
16.9
16.5
17.2
–
+0.17
−0.11
−0.06
−0.13
−0.17
−0.18
−0.19
−0.17
−0.12
−0.13
–
11.1
14.2
16.3
15.7
15.9
17.1
16.4
17.2
16.7
16.8
–
+0.13
+0.05
−0.09
−0.04
−0.04
−0.10
−0.04
−0.08
−0.01
+0.01
–
11.1
11.2
12.7
12.8
12.9
13.0
13.5
14.0
14.7
15.5
16.2
16.9
17.8
19.7
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
International Journal of Chemical Kinetics DOI 10.1002/kin.20731

808
AKRAM, KUMAR, AND KABIR-UD-DIN
Scheme 3
the cationic micelles, and Dn = [surfactant]T – cmc.
MNS is the mole fraction of bound ninhydrin to the
micellar headgroup given by
V
-
[Nm]
MNS =
(5)
Figure 6 Variation of kꢁ versus the spacer chain length (s-
value) on the reaction of ninhydrin and Gly–Tyr. Conditions:
same as in Fig. 1.
[Dn]
Values of MNS were estimated by considering the equi-
librium
geminis. The formation of micelles in the case of 16-
4-16 is more than 16-5-16 and 16-6-16. This happens
due to a shorter spacer length, which is most likely
because of increasing geometrical constraints in the
formation of aggregates with the decreasing length of
the spacer unit [23,24]. The above argument is also jus-
tified with microviscosity and small angle neutron scat-
tering (SANS) data within geminis. On increasing the
spacer chain length (s-value), the micellar morphology
tends to be less ellipsoidal [14]. Thus, the surfactant
morphology is strongly affected by the spacer chain
length. In the present study, the values of kꢁ obtained
are consistent with the expectation being maximum at
s = 4, beyond which looping of the spacer (to mini-
mize its contact with water) [6,14] will progressively
make the Stern layer more wet with the resultant kꢁ de-
crease. The rate increment in the presence of positively
charged micelles could contribute to the stabilization of
the Schiff base intermediate on a positively charged mi-
cellar surface increasing the concentration of interme-
diate in the Stern layer. The π-electrons present in the
reactant ninhydrin help in its partitioning between the
aqueous and positively charged micelles [25]. There-
fore, both reactants ninhydrin and Gly–Tyr get associ-
ated/incorporated into the aqueous surface of the mi-
celles (i.e., the Stern layer, where most of the ionic
micelle-mediated organic reactions are believed to oc-
cur) [20]. Therefore, the concentration of the reactive
species increases in the micellar reaction zone, catalyz-
ing the reaction and increasing the observed rate (kꢁ).
Nw + Dn ꢀ Nm
[Nm]
KN =
(6)
(7)
[Nw]([Dn] − [Nm])
and the mass balance
[N] = [Nw] + [Nm]
To calculate km and KS kinetically, the cmc values
under kinetic conditions are required which are deter-
mined conductimetrically. For a given value of cmc, the
km and KS were calculated from Eq. (4) using the non-
linear least-squares technique. Such calculations were
carried out at definite presumed values of KN. To con-
firm the validity of the rate equation (4), the rate con-
stants were calculated (kꢁcal) by substituting the values
ofkm andKS in Eq. (4) and comparing with the observed
kꢁ values (Table IV). The agreement is good, which
supports the validity of the proposed mechanism.
Effect of s-Value (Spacer Chain Length)
Under the identical experimental conditions, it is found
that the value of the rate constant kꢁ is more at s =
4 in comparison to s = 5 and s = 6 (Fig. 6). It is
described in the literature that the spacer length and
kind of moiety dictate the conformation of the gemini
molecule [22]. The value of the rate constant follows
the order 16-4-16 > 16-5-16 > 16-6-16 among the
International Journal of Chemical Kinetics DOI 10.1002/kin.20731

EFFECT OF DICATIONIC GEMINI SURFACTANTS ON THE NINHYDRIN–DIPEPTIDE REACTION
809
The activation energies obtained in the case of the
gemini surfactants are according to their efficacy of cat-
alyzing the reaction, that is, 16-4-16 > 16-5-16 > 16-
6-16. Gemini surfactants lower the values of activation
parameters (ꢂH#and ꢂS#) more than aqueous. This
decrement in the parameters occurs not only through
the stabilization of transition state but also through ad-
sorption of substrates on the micellar surface. The large
decrease in ꢂS#shows that the transition state is well
structured in the case of 16-4-16 as compared to 16-5-
16 and 16-6-16. A meaningful mechanistic explanation
of apparent values of activation parameters (ꢂH#and
ꢂS#) is not possible because the values of the rate con-
stant kꢁ do not represent a single elementary step; it
is a complex function of true rate, binding, ionization
constants, etc.
2. Almog, J. Advances in Finger Print Technology; Else-
vier: New York, 1991.
3. Joullie, M. M.; Thompson, T. R.; Nemeroff, N. H.
Tetrahedron 1991, 47, 8791.
4. (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar
and Macromolecular Systems; Academic Press: New
York, 1975; (b) Khan, M. N. Micellar Catalysis, Sur-
factant Science Series; CRC Press: New York, 2006;
Vol. 133.
5. (a) Dunlap, R. B.; Cordes, E. H. J Phys Chem 1969,
73, 361; (b) Cerichelli, G.; Mancini, G.; Luchetti, G.;
Savelli, G.; Bunton, C. A. J Phys Org Chem 1991, 4,
71; (c) Broxton, T. J.; Christie, J. R.; Theodoridis, D. J
Phys Org Chem 1993, 6, 535.
6. Karaborni, S.; Esselink, K.; Hilbers, P. A. J.; Smit, B.;
Karthauser, J.; van Os, N. M.; Zana, R. Science 1994,
266, 254.
7. Akram, M.; Kumar, D.; Kabir-ud-Din J Saudi Chem
Soc. doi: 10.1016/j.jscs.2011.10.019.
CONCLUSION
8. Britton, H. T. S. Hydrogen Ions; Chapman and Hall:
London, 1942; Vol. 1.
Kinetic experiments were carried out on the reaction of
Gly–Tyr (3.0 × 10−4 mol dm−3) and ninhydrin (6.0 ×
10−3 mol dm−3) at 70◦C and pH = 5.0 in the presence
of varying amounts of gemini surfactants (16-s-16, s =
4, 5, 6). A higher value of KS is observed in the present
case of Gly–Tyr in the presence of gemini micelles
(16-s-16, s = 4, 5, 6) as compared to tyrosine [26].
This higher value indicates that the hydrophobicity of
the Gly–Tyr molecule is greater than that of the ty-
rosine molecule. The increased hydrophobicity seems
responsible for a higher concentration of Gly–Tyr in
the Stern layer of micelles (where most of the ionic
micelle-mediated organic reactions are believed to oc-
cur) [20]. The present study may open up and stimulate
a new approach of studying the ninhydrin reaction for
sensitivity. Under the viewpoint of the present reac-
tion situations, a small amount of gemini surfactants
(below their cmc values) was enough to accelerate the
reaction rate than that of pure aqueous or CTAB mi-
cellar medium. It suggests less environmental impact
due to the use of the smaller surfactant quantity to
perform the reaction. Quantitative treatment of kinetic
data seems justified, and the molecule–molecule inter-
action in micellar media could successfully be treated
using the pseudophase model.
9. (a) Kabir-ud-Din; Salem, J. K. J.; Kumar, S.; Rafiquee,
M. Z. A.; Khan, Z. J Colloid Interface Sci 1999,
213, 20; (b) Kabir-ud-Din; Salem, J. K. J.; Kumar, S.;
Rafiquee, M. Z. A.; Khan, Z. Colloids Surf, A 2000,
168, 241.
10. Khan, Z.; Gupta, D.; Khan, A. A. Int J Chem Kinet
1992, 24, 481.
11. Cerichelli, G.; Mancini, G.; Luchetti, G.; Savelli, G.;
Bunton, C. A. Langmuir 1994, 10, 3982.
12. Brinchi, L.; Germani, R.; Gorracci, L.; Savelli, G.;
Bunton, C. A. Langmuir 2002, 18, 7821.
13. Menger, F. M.; Keiper, J. S.; Mbadugha, B. N. A.;
Caran, K. L.; Romsted, L. S. Langmuir 2000, 16, 9095.
14. Kabir-ud-Din; Siddiqui, U. S. Colloid J 2010, 72, 14.
15. Kabir-ud-Din; Salem, J. K. J.; Kumar, S.; Rafiquee, M.
Z. A.; Khan, Z. J Colloid Interface Sci 1999, 215, 9.
16. Kabir-ud-Din; Bano, M.; Khan, I. A. J Surface Sci
Technol 2002, 18, 113.
17. Kabir-ud-Din; Fatma, W. J Surface Sci Technol 2002,
18, 129.
18. Friedman, M. J Am Chem Soc 1967, 89, 4709.
19. (a) Kabir-ud-Din; Fatma, W. J Phys Org Chem 2007,
20, 440; (b) Kabir-ud-Din; Fatma, W.; Khan, Z. A.;
Dar, A. A. J Phys Chem B 2007, 111, 8860.
20. Bunton, C. A. Catal Rev Sci Eng 1979, 20, 1.
21. Romsted, L. S. Micellization, Solubilization, and
Microemulsions; Mittal, K. L., Ed.; Plenum Press: New
York, 1977; Vol. 2.
22. (a) Zana, R.; Talmon, Y. Nature 1993, 362, 228; (b)
Zana, R. J Colloid Interface Sci 2000, 248, 203.
23. Israelachvilli, J. N.; Mitchell, D. J.; Ninham, B. W. J
Chem Soc, Faraday Trans 2 1976, 72, 1525.
24. Mitchell, D. J; Ninham, B. W. J Chem Soc, Faraday
Trans 2 1981, 77, 601.
Dileep Kumar is thankful to the University Grants Com-
mission (UGC) for financial assistance. Kabir-ud-Din also
thanks UGC for awarding the Emeritus Fellowship.
BIBLIOGRAPHY
25. Lindemuth, P. M.; Bertrand, G. L. J Phys Chem 1993,
97, 7769.
1. Friedman, M. J Agric Food Chem 2004, 52, 385, and
references cited therein.
26. Khan, I. A.; Bano, M.; Kabir-ud-Din. J Dispersion Sci
Technol 2010, 31, 177.
International Journal of Chemical Kinetics DOI 10.1002/kin.20731