kinetics and mechanism of interaction of dipeptide (glycyl–glycine) with ninhydrin in aqueous...

Kinetics and Mechanismof Interaction of Dipeptide(Glycyl–Glycine) withNinhydrin in AqueousMicellar MediaMOHD. AKRAM, NEELAM HAZOOR ZAIDI, KABIR-UD-DIN

Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India

Received 14 December 2005; revised 4 March 2006; accepted 21 March 2006

DOI 10.1002/kin.20195Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The rates of reaction between ninhydrin and dipeptide glycyl–glycine (Gly–Gly) havebeen determined by studying the reaction spectrophotometrically at 70◦C and pH 5.0 in aque-ous and in aqueous cationic micelles of cetyltrimethylammonium bromide (CTAB). The reac-tion follows first- and fractional-order kinetics, respectively, in [Gly–Gly] and [ninhydrin]. Theobserved rate constant is affected by [CTAB] changes and the maximum rate enhancementis ca. three-fold. As the kψ − [CTAB] profile shape is characteristic of bimolecular reactionscatalyzed by micelles, the catalysis is explained in terms of the pseudo-phase model of themicelles (proposed by Menger and Portnoy and developed by Bunton and Romsted). The pres-ence of inorganic salts (NaCl, NaBr, Na2SO4) does not reveal any regular effect but the datawith organic salts (NaBenz, NaSal) show an increase in the rate followed by a decrease. Thekinetic data have been used to calculate the micellar binding constants KS for Gly–Gly and KN

for ninhydrin and the respective values are 317 and 69 mol−1 dm3. C© 2006 Wiley Periodicals,Inc. Int J Chem Kinet 38: 643–650, 2006

INTRODUCTION

Micelles in aqueous media have either a polar regionor a region of high-charge density, accompanied by anelectrostatic potential of up to a few hundred millivoltsat the micellar surface and a nonpolar hydrophobicregion in the micellar core. The kinetics in micellarsolutions is governed by electrostatic and hydrophobic

Correspondence to: Kabir-ud-Din; e-mail: [email protected]© 2006 Wiley Periodicals, Inc.

interactions between micelles and reactants, transitioncomplexes, and products. Micelles are considered asmodels for enzyme action because they are similarin shape and size, and more importantly, both havehydrophobic core and polar surfaces. The reactions ofvarious organic compounds are catalyzed or inhibitedby micelles due to exclusive micellar incorporationof the reactants. Various kinetic studies have ex-amined the following types of micellar catalysis:(1) reactions in which the micelles are reagents;(2) reactions in which interactions between the mi-celles and the reacting species affect the kinetics; and

644 AKRAM, ZAIDI, AND KABIR-UD-DIN

(3) reactions in which the micelles carry catalyticallyactive substituents [1]. The studies were undertakento elucidate the factors that influence the rates andcourse of reactions, to gain insight into the exceptionalcatalytic characteristics of enzymatic reactions, and toexplore the usefulness of micellar systems for organicsynthesis [2].

Ninhydrin has established itself as an analytical toolin the fields of chemistry, biochemistry, and forensicscience. The use of ninhydrin for the detection andestimation of amino acids has been the subject of nu-merous investigations because of its potential ability toreveal latent fingerprints [3,4]. It is known that aminoacids [4], on interaction with ninhydrin, produce a pur-ple-colored diketohydrindylidenediketohydrindamine(DYDA). Since the colored product is also formed fromthe primary amino groups of peptides [5], the ninhydrinmethod has been extended to assay proteins as wellas peptides [5]. Whereas the kinetics and mechanismof ninhydrin–amino acid reactions have been studiedextensively in aqueous as well as in different surfactantmicelles [6–10], apparently no such attempt has sofar been made on the ninhydrin–dipeptide reactions.Our present work is the first such attempt wherein thekinetics of glycyl–glycine (Gly–Gly) and ninhydrinhas been described in aqueous and cationic micellesof cetyltrimethylammonium bromide (CTAB).

EXPERIMENTAL

Materials

Gly–Gly (LOBA Chemie, 99%), ninhydrin (Merck,99%), CTAB (BDH, 99%), sodium benzoate (NaBenz;Merck, 99.5%), sodium salicylate (NaSal; CDH,99.5%), sodium bromide (LOBA Chemie, 99%),sodium chloride (BDH, 99.9%), and sodium sulfate(Qualigens, 99%) were used as received. Double-distilled and deionized water was used throughout.Stock solutions of the reactants and CTAB wereprepared in acetic acid–sodium acetate buffer. ThepH measurements were made using a digital ELICOLI-122 pH meter.

Kinetic Measurements

For each set of kinetic experiments, the requisite vol-umes of Gly–Gly, buffer, and CTAB solutions weretaken in a three-necked reaction vessel (also fitted witha double-surface water condenser), which was thenkept in an oil bath at the experimental temperature.The reaction was started by adding a requisite volumeof thermally equilibrated ninhydrin solution; zero-time

was taken when half of the ninhydrin solution had beenadded. Pure N2 gas (free from O2 and CO2) was bubbledthrough the reaction mixture for stirring as well as tomaintain an inert atmosphere. Pseudo-first-order con-ditions were maintained in all kinetic runs by using ex-cess of ninhydrin over Gly–Gly concentration. The ab-sorbance of the product DYDA was measured at 570 nm(λmax) [4] at definite time intervals with a Bausch &Lomb Spectronic-20 spectrophotometer. Other detailsregarding pH measurements and kinetic methodologywere the same as described elsewhere [6,8].

Determination of cmc by ConductivityMeasurements

The critical micellar concentration (cmc) values ofthe CTAB solutions under the experimental conditionswere determined conductimetrically. The conductivitymeasurements were made by Philips conductivity me-ter model PR 9500, using platinized electrodes. Thebreak points of nearly two straight-line portions of thespecific conductivity versus concentration plots pro-vided the cmc values [11]. Experiments were carriedout under different conditions, i.e., solvent being water,water + Gly–Gly, water + ninhydrin, or water + Gly–Gly + ninhydrin and the respective cmc values are(× 104) 9.5, 9.4, 9.4, and 9.4 (at 30◦C); 14.2, 13.5,14.1, and 13.0 mol dm−3 (at 70◦C).

Viscosity Measurements

Using Ubbelohde viscometer the viscosity measure-ments were made at 70 ± 0.1◦C. The method of viscos-ity measurements was the same as reported elsewhere[12]. The whole range of concentrations of CTAB andsalts could not be examined due to large increase inviscosity, particularly with organic salts.

RESULTS AND DISCUSSION

Spectra of the Product

It is known [13,14] that the same purple-colored prod-uct (DYDA) is formed by the reaction between ninhy-drin and peptides as produced by the reaction betweenninhydrin with different amino acids. Spectra of theproduct formed by the reaction between Gly–Gly andninhydrin in the buffer solution have been taken in theabsence and presence of CTAB micelles (Fig. 1). Wesee that the absorbance increases with CTAB micelleswith no shift in λmax, i.e., the wavelength of maximumabsorbance remains the same in both aqueous and mi-cellar media. This indicates the purple-colored product

KINETICS AND MECHANISM OF INTERACTION OF DIPEPTIDE WITH NINHYDRIN 645

Figure 1 Spectra of reaction product of ninhydrin

(6.0 × 10−3 mol dm−3) with Gly–Gly (1.5 × 10−4 mol

dm−3) in the presence of 20 × 10−3 mol dm−3 CTAB (A) and

in the absence of surfactant (B) in buffer solution (pH 5.0) at

70◦C.

of Gly–Gly reaction with ninhydrin to be the same asin aqueous medium.

Dependence of Reaction Rateon Gly–Gly Concentration

To see the dependence on [Gly–Gly], the reactionwas carried out under pseudo-first-order conditions of

Table I Dependence of Pseudo-First-Order Rate Constants (kobs or kψ ) on [Gly–Gly], [Ninhydrin] and Temperature atpH 5.0

[Gly–Gly] × 104 (mol dm−3) [ninhydrin] × 103 (mol dm−3) Temperature (◦C) kobsa × 105 (s−1) k�

b × 105 (s−1)

1.0 6 70 6.5 18.0

1.5 7.4 18.1

2.0 6.4 18.1

2.5 6.7 18.5

3.0 6.7 18.4

3.5 6.8 18.8

1.5 6 70 7.4 18.1

10 15.9 25.7

15 28.3 45.8

20 32.5 55.7

25 41.7 75.2

30 46.1 81.3

35 50.9 89.5

40 61.1 92.8

1.5 6 60 2.3 7.5

65 3.7 11.3

70 7.4 18.1

75 8.7 23.5

80 11.2 33.8

a In the absence of surfactant.b In the presence of [CTAB] = 20.0 × 10−3 mol dm−3.

[ninhydrin] � [Gly–Gly] in the range of 1.0 × 10−4 to3.5 × 10−4 mol dm−3 of [Gly–Gly] at constant [ninhy-drin] of 6.0 × 10−3 mol dm−3, temperature (70◦C) andpH (5.0). It has already been established that the op-timum pH of the ninhydrin reaction with amino acidsand peptides is 5.0 [4,13,14]. Therefore, a constant pH(5.0) was maintained throughout the kinetic studies.The kobs values are recorded in Table I. Similar studieswere performed in CTAB micelles. As the values ofrate constants (kobs and k� ) were found to be indepen-dent of the initial concentration of Gly–Gly, the orderof reaction with respect to [Gly–Gly] is unity in boththe media.

Dependence of Reaction Rateon Ninhydrin Concentration

The effect of ninhydrin concentration was deter-mined by carrying out the kinetic experimentswith different concentrations of ninhydrin, keep-ing [Gly–Gly] (1.5 × 10−4 mol dm−3), temperature(70◦C), and pH (5.0) constant (Table I). Experi-ments were also performed in the presence of CTAB(20 × 10−3 mol dm−3) micelles. The plots of rateconstants versus [ninhydrin] were curved passingthrough the origin that indicate the order to befractional with respect to [ninhydrin] in both themedia.


Dependence of Reaction Rateon Temperature

A series of kinetic runs were carried out at differenttemperatures (60–80◦C) with fixed reactant concen-trations both in the absence and presence of CTABmicelles (Table I). The observed data were found tofit the Arrhenius and Eyring equations. The activationparameters were calculated using linear least squaresregression technique.

Reaction in the Absence of Micelles

The general mechanism for the reaction between nin-hydrin and amino acids is well known [4]. Aminoacids on interaction with ninhydrin produce a purple-colored product, called diketohydrindylidenediketohy-

Scheme 1

drindamine (DYDA). Different amino acids (exceptproline) react with different rates but all produce thesame final product [4,15–17]. The amount of the re-action products depends upon temperature, pH, andreactant concentrations. In the present case, the con-densation between carbonyl group of ninhydrin andamino group of Gly–Gly takes place. The reaction startsthrough the attack of lone-pair of electrons of aminonitrogen (of Gly–Gly) to the carbonyl carbon (of ninhy-drin) to give Schiff base A. This Schiff base is unstableand hydrolyzes to give 2-amino-indanedione, B, whichreacts slowly with another ninhydrin molecule to yieldthe product P (DYDA).

On the basis of the observed rate lawd[P]/dt = kobs[Gly–Gly]T and the proposed mech-anism (Scheme 1), the following rate equation isderived:


kobs = kK [Nin]T

1 + K [Nin]T

(1)

where [Nin]T is the total concentration of ninhydrin.The data treatment was carried out by an alternativemethod using Eq. (2).

1

kobs

= 1

k+ 1

kK [Nin]T

(2)

The double-reciprocal plot between 1/kobs and 1/[Nin]T

should give a straight line with a positive slope (= 1/kK)and an intercept (= 1/k). Indeed, it is found so (Fig. 2),and thus confirms the validity of the proposed mech-anism. From the intercept and slope, the respec-tive values of k and K were evaluated, which are2.5 × 10−3 s−1 and 5.18 mol−1 dm3, respectively.

Reaction in the Presence ofSurfactant Micelles

The effect of [CTAB] upon the rate of reactionwas studied at constant [ninhydrin] (6.0 × 10−3 moldm−3), [Gly–Gly] (1.5 × 10−4 mol dm−3), and pH5.0 at 70◦C. The rate constant (k� ) increased from7.4 × 10−5 to 20.3 × 10−5 s−1 (ca. three-fold) with in-crease in [CTAB] from 0 to 70 × 10−3 mol dm−3. Thek� −[CTAB] profile shape (Fig. 3) is perfectly generalbeing a common characteristic of bimolecular reactionscatalyzed by micelles [18].

The same first- and fractional-order kinetics withrespect to [Gly–Gly] and [ninhydrin], respectively, wasfollowed in both aqueous and micellar media. Also,

Figure 2 Plots of 1/k versus 1/[ninhydrin] for the in-

teraction of Gly–Gly with ninhydrin in the presence (A)

and absence of surfactant (B). Reaction condition: [Gly–

Gly] = 1.5 × 10−4 mol dm−3, [CTAB] = 20.0 × 10−3 mol

dm−3, pH 5.0, and temperature = 70◦C.

Figure 3 Effect of [surfactant] on the reaction rate

for the interaction of ninhydrin with Gly–Gly. Reac-tion condition: [Gly–Gly] = 1.5 × 10−4 mol dm−3, [nin-

hydrin] = 6.0 × 10−3 mol dm−3, pH 5.0, and tempera-

ture = 70◦C.

the absorption band of the product remains unchangedin the presence of CTAB micelles (Fig. 1). Thus, weconclude that the reaction mechanism remains the samein the presence of cationic micelles as that in aqueousmedium.

The rate increase for many reactions upon addi-tion of surfactants has been explained on the basis ofScheme 2, proposed by Menger and Portnoy [19] anddeveloped by Bunton [20] and Romsted [21].

Scheme 2

Although several kinetic equations based on thisgeneral Scheme 2 have been developed, the most suc-cessful appears to be that of Romsted, who suggestedEq. (3), which takes into account the solubilization ofboth the reactants into micelles as well as mass actionmodel

k� = kw[Nin]T + (KSkm − kw)MSN[Dn]

1 + KS[D]T

(3)

Here, kw and km are the second-order rate constants,referring to aqueous and micellar pseudo-phases,


respectively, KS is the binding constant of the Gly–Glyto the cationic micelles, and [Dn] = [CTAB] − cmc.MS

N is the mole ratio of bound ninhydrin to the micellarheadgroup, given by

MSN = [NDn]

[Dn](4)

Values of MSN were estimated by considering the equi-

librium

Nw + Dn

KN⇀↽ NDn

KN = [NDn]

Nw([Dn] − [NDn])(5)

and the mass balance

[Nin]T = [Nw] + [NDn] (6)

In order to determine km and KS kinetically, we needthe cmc values under kinetic conditions and we deter-mined it conductimetrically. For a given value of cmc,the km and KS were calculated from Eq. (3). Usingthe nonlinear least squares technique, such calculationswere carried out at different presumed values of KN.The best value was considered to be the one for whichthe value of

∑d2

i (where di = k� obsi − k� cali ) turnedout to be a minimum. These values are recorded inTable II. Fitting of the calculated data (KS , km, and KN)to Eq. (3) was evidenced from the calculated values ofrate constants, k� cal.

Because of the different properties of micellarpseudo-phase it is not possible to precisely locate theexact site of the reaction but at least localization of thereactants can be considered. Most of the ionic micelle-mediated reactions are believed to occur either insidethe Stern layer [18] or at the interface between mi-cellar surface and bulk water solvent [2,22]. The mi-cellar surface can attract or repel ionic species due toelectrostatic interactions whereas hydrophobic inter-

Table II Thermodynamic Parameters, Rate and Binding Constant Values for the Reaction of Gly–Gly and Ninhydrin atpH 5.0

Parameters and Constants Aqueous CTAB

Ea (kJ mol−1) 85.3 77.0

�H �=(kJ mol−1) 82.5 74.2

−�S �=(J K−1 mol−1) 97.9 103.9

km × 104 (s−1)a – 2.0

kw × 105 (mol−1 dm3 s−1)a – 7.4

KS (mol−1 dm3)a – 317

KN (mol−1 dm3)a – 69

a At 70◦C.

actions can bring about the incorporation of the re-actants into micelles. The observed catalysis is dueto the increased concentration of both ninhydrin andGly–Gly in the Stern layer of micelle. Besides this, mi-celles also exert a medium effect influencing reactivity(the effect arises from a combination of cage, preori-entation, microviscocity, polarity, and charge effects[23]).

In order to calculate the dissociation constant ofthe micellized surfactant back to its components (KD)and the index of cooperativity (n), the Piszkiewiczmodel [24], analogous to the Hill model applied forthe enzyme-catalyzed reactions, was used. In micel-lar systems, the value of n reflects the average numberof surfactant molecules associated with each substratemolecule. The Piszkiewicz model relates n and KD andits contribution to the rate is given by

k� = k ′m[Dn]n + k ′

w KD

KD + [Dn]n(7)

On rearrangement, Eq. (7) gives

log

(k� − k ′

w

k ′m − k�

)= n log [Dn] − log KD (8)

According to Eq. (8), a plot of log((k� − k ′w)/

(k ′m − k� )) versus log [Dn] should be a straight line with

a positive slope (= n). Such a plot has been realized inthe CTAB catalysis of the present study. The KD andn are 2.19 × 10−3 and 1.02, respectively. A value of ngreater than unity indicates positive cooperativity, i.e.,the binding of the first molecule of the substrate makesit easier for subsequent molecules to bind. The advan-tage of Eq. (8) is that it does not require the knowledgeof cmc of surfactant used.

Activation parameters such as activation energy(Ea), enthalpy of activation (�H �=), and entropy of ac-tivation (�S �=) are given in Table II. Comparing the val-ues we find that the presence of CTAB micelles lowers


Figure 4 Effect of [inorganic salts] on the reaction

rate for the interaction of ninhydrin with Gly–Gly in

the presence of surfactant. Reaction condition: [Gly–

Gly] = 1.5 × 10−4 mol dm−3, [ninhydrin] = 6.0 × 10−3 mol

dm−3, [CTAB] = 20.0 × 10−3 mol dm−3, pH 5.0, and tem-

perature = 70◦C. NaBr (A), NaCl (B), Na2SO4 (C).

the �H �= with a substantial negative �S �=. This low-ering occurs not only through the adsorption of boththe reactants on the micellar surface but also throughstabilization of the transition state. The fitting of theobserved k� at different temperatures to the equationwas examined and it was found that the Eyring equa-tion is applicable to the micellar media and the sensi-tivity of micelle structure to temperature is kineticallyunimportant. A meaningful mechanistic explanation of

Figure 5 Effect of [organic salts] on solution viscosity

(C, D) and on the reaction rate (A, B) for the interaction

of ninhydrin with Gly–Gly in the presence of surfactant. Re-action condition: same as in Fig. 4. NaBenz (A, C), NaSal

(B, D).

the apparent values of �H �= and �S �= is not possiblebecause the k� values do not represent a single elemen-tary kinetic step; it is a complex function of true rate,binding, and ionization constants.

Salt Effect

The effect of added salts on the rate was also ex-plored in the presence of [CTAB] (20.0 × 10−3 moldm−3), [ninhydrin] (6.0 × 10−3 mol dm−3), [Gly–Gly](1.5 × 10−4 mol dm−3), and pH (5.0) at 70◦C. Salts, asadditives, in micellar systems acquire a special placedue to their ability to induce structural changes, whichmay, in turn, modify the substrate–surfactant interac-tions [25]. At sufficiently elevated salt concentrations,micelles grow above their size in pure water. In thepresence of strongly binding counterions, such as sal-icylate, CTAB forms greatly extended thread-like mi-cellar pseudo-lattice [26]. The salt effect on micellarcatalysis should be considered in the light of its com-petition with the substrate molecule that interacts withthe micelle electrostatically and hydrophobically.

Figure 4 shows no regular pattern in the presenceof inorganic salts. Most probably the change in thereactivity is a combined effect of the change in themicroenvironment of reagents resulting from a changein their location in micelles, in the ionic force, etc.However, hydrophobic salts (NaBenz and NaSal) givemarked rate enhancement at low-salt concentrations,passing through a maximum as the [salt] was increased(Fig. 5). The addition of these salts means that weare adding ionic species having hydrophobic character.Such anions will be solubilized in the micelle palisadelayer with the acidic group exposed close to the head-group region [27,28]. Therefore, in addition to neutral-ization of micellar positive charge, they will restrict sol-ubilization sites to hydrophobic substrates. Thus, theycatalyze the reaction initially by virtue of increasedconcentration of reactants in the Stern layer. The de-creased rate observed at relatively higher concentra-tions of added organic salts is a consequence of theadsorption of hydrophobic anions at the micellar sur-face (adsorption of these anions at higher [salt] is wellknown [29]) and the exclusion of substrates (i.e., re-stricted solubilization). The progressive withdrawal ofthe substrate from the reaction site (micellar surface)would slow down the rate, as was indeed observed.Another factor that could inhibit the rate is the pos-sible micellar growth at higher [salt] as reflected byviscosity data (Fig. 5). In our case the change in mor-phology from spheroidal micelles to rod shaped (asinferred by viscosity increase [30]) would have cer-tain changes on the characteristics of the micelle. In


rod-shaped micelles the counterions bind more tightlyand will, therefore, suppress the ionic interactions atthe micellar surface with a concomitant decrease in therate.

CONCLUSION

The present work is the first attempt in which the ki-netics of Gly–Gly and ninhydrin has been studied inaqueous and cationic micelles of CTAB. The study isalso important from the viewpoint that various suitablesurfactants (e.g., Gemini surfactants [31,32] or mixedsurfactants [33]) could be tried to study the ninhydrinreaction for enhanced sensitivity. Finally, we can con-clude that the interaction of Gly–Gly with ninhydrinin micellar media could successfully be treated usingthe pseudo-phase and Piszkiewicz models. Quantitativetreatment of the kinetic data seems justified as k� andk� cal were in close agreement within the experimentalerrors.

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