Journal of Colloid and Interface Science 329 (2009) 160–166

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Journal of Colloid and Interface Science

www.elsevier.com/locate/jcis

Substituent effect on the emission behavior of thiazolidinedione derivativesin cationic and anionic micellar media

Arindam Sarkar, Paltu Banerjee, Subhash Chandra Bhattacharya ∗

Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700032, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 June 2008Accepted 15 September 2008Available online 23 September 2008

Keywords:RotamersHydrogen bondingExciplexCharge density

Thiazolidinedione (TZD) derivatives have been found to possess potent immunostimulatory propertiesas well as antiarthritic, antidiabetic and oncostatic activities. These compounds are free radicalscavengers. Photophysical properties of the compounds have been studied in different aqueous micellarenvironments using steady state and time resolved emission spectroscopy. Appreciable hypsochromicshifts with enhancement in the fluorescence intensities have been observed in the ionic micellar media.The binding constants and energy changes during probe–micelle binding have been evaluated fromrelevant fluorescence data. Polarity of the microenvironment surrounding the probe molecules has beendetermined in the micellar systems.

© 2008 Elsevier Inc. All rights reserved.

1. Introduction

Reactions in micellar aggregates are simple model systems forchemical processes occurring at the interfaces in the living cell[1]. The most important properties of micellar systems are theirability to solubilize a variety of molecules insoluble in bulk (aque-ous) solution and their substantial catalytic effect on many reac-tions [2–5]. The knowledge of the structure of the micelles, thelocal environment, the local concentration and the relative orien-tation of the solubilized molecules is of fundamental importancein understanding the nature of the solubilization and the physi-cal and chemical behavior of the solubilized species [6,7]. Micelles,which are used as membrane biomimetic agents, gain importanceby virtue of their capacity to provide a matrix for arranging thereaction sequentially for efficient interaction, i.e., they help incompartmentalization of the reactants dynamically [8–17]. Watermolecules, which are tightly bound to the surfactant head groupsof micelles, resemble the hydrophilic pockets of enzymes and havehigh viscosities, low mobilities and polarities [18,19].

In this present work, the interaction of three biologically im-portant thiazolidinedione derivatives (TZD) with ionic micelles ofcetyl trimethyl ammonium bromide (CTAB) and sodium dodecylsulfate (SDS) has been studied. CTAB and SDS were chosen as sur-factants due to their potent denaturating behavior and membranelike properties for transport of solutes [20,21]. In spite of its bio-

* Corresponding author.E-mail addresses: sbjuchem@yahoo.com, scbhattacharyya@chemistry.jdvu.ac.in

(S.C. Bhattacharya).

0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2008.09.046

toxicity, CTAB has been used for a range of biomedical applicationsas an antibacterial agent in eye drops [22]. Studies by Singh et al.also demonstrated that the cationic microparticles (CTAB) were po-tent delivery systems for DNA vaccines [23]. DNA-cationic ligands(CTAB) complexation has primary importance due to its signifi-cance in biomedical applications, particularly in therapeutics anddiagnostics [24–27]. CTAB also assists in the refolding of native andrecombinant lysosomes [28]. Another reason for choosing CTABand SDS as surfactants is that the TZD probes possess chargedcenters within them, so an oppositely charged microenvironmentwas required as their medium of study for enhanced electro-static interactions. CTAB and SDS provided the desired oppositelycharged microenvironments, required to study the photophysics ofthe TZD probes. Thiazolidinedione (TZD) derivatives possess potentimmunostimulatory properties as well as antiarthritic, antidiabeticand oncostatic activities [29] and are reducer of plasma glucoselevel in vivo [30]. These compounds suppress the growth of severalcancer cell lines including those in the colon, breast and prostrate[31–37]. They are found to inhibit angiogenesis and are also poten-tial chemo-preventive agents against tongue and gastric carcino-genesis. These compounds are also used as free radical scavengers.Free radicals are formed during normal cellular metabolism. Expo-sure of a healthy cell to free radical is known to damage structuresand consequently interfere with functions of enzymes and criticalmacromolecules. A balance between the formation of free radicalsand their detoxification is essential for normal cellular function.When such a balance is disrupted as a result of excessive gen-eration of free radicals or low level of antioxidants, a cell entersa state of oxidative stress and is damaged [38,39]. Since theseTZD derivatives are newly synthesized, their photophysical study is

A. Sarkar et al. / Journal of Colloid and Interface Science 329 (2009) 160–166 161

Scheme 1. Rotameric forms of “A.”

Scheme 2. Structures of the TZD derivatives.

rare in the literature. Interest in the photophysical studies of thesecompounds in micellar media originates mainly from two aspects:the first one arises from its novel biological applications in phar-maceuticals and the second one arises due to the charged centerspresent within the compounds. Due to the presence of electronrich carbonyl groups around –NH in the thiazolidinedione moiety,a δ+ charged center is formed at the nitrogen, thus making it elec-tron deficient. So it will act as the potential electron-accepting sitewhen mixed with some electron donor like SDS. Similarly the elec-tron rich oxygen centers will facilitate electron transfer processeswith electron acceptors like CTAB, thus providing ample scope forthe formation of a probe–micelle complex. The compounds canalso form hydrogen bonded species in polar protic medium andthe ability of hydrogen bond formation decreases in a non-polarmedium. The –OH group mainly provides the link for hydrogenbond formation. The NH group present in the thiazolidinedionemoiety is also a potential hydrogen bond forming site. The strengthof this site is governed by the substituents in the aryl moiety ofthe TZD. The dependence of the photophysical properties of theprobes on polarity of the medium is clearly indicated by the largehypsochromic shifts observed from polar to non-polar media. Theresults have been analyzed considering probe–micelle interactionsin different dimensions. Molecular structure plays a major role indetermining the shape and wavelength position of the fluores-cence spectra of aromatic molecules. Nonplanar molecules usuallyhave structureless absorption and fluorescence spectra while pla-nar and rigid molecules show absorption and fluorescence spectrawith well-resolved vibrational bands. Very often, transition of amolecule from a nonplanar to a more planar and rigid shape, isaccompanied with an increase in the quantum yield of fluores-cence [40–43]. Going through the structures of the compounds, itis seen that TZD “A” can exist in two rotameric forms A1 and A2(Scheme 1).

The rotation around the marked single bond around “A” en-hances the possibility of these two rotameric forms to exist, thusresulting in a completely different interaction for “A.” A2 may existas intramolecular hydrogen bonded species. A1 can exist as hydro-gen bonded species in polar protic solvents like water. Due to thepresence of bulky methoxy groups in the other two derivatives, ro-tamers are not possible for them.

Scheme 3. Synthesis scheme of the TZD derivatives.

2. Materials and method

The thiazolidinedione derivatives have the structures men-tioned in Scheme 2.

They were synthesized following the procedure mentioned inScheme 3 [38].

These were recrystallized before use. CTAB and SDS used wereof Aldrich products. The photophysics of the thiazolidinedione(TZD: A, B, C) probes in water and micellar media were stud-ied applying absorbance, fluorescence and lifetime measurementtechniques. Absorption spectra of the TZD probes were recordedusing a Pharmaspec UV–Vis 1700 spectrophotometer (Shimadzu),with a matched pair of silica cuvettes (path length 1 cm). Theemission spectra of the probes were recorded using fluorescencespectrophotometer (Fluorolog FII A Spectrofluorimeter, Spex Inc.,NJ, USA) with a slit width of 1.25 mm. Fluorescence lifetimemeasurements were performed using time correlated single pho-ton counting technique and a nanosecond diode laser at 370 nm(IBH, nanoLED-7) as light source. The response time of the instru-ment is 1.1 ns. The decays were analyzed using IBH DAS-6 decayanalysis software. The steady state anisotropy measurements werecarried out using a Perkin Elmer spectrofluorimeter. Geometricaloptimization and ground state dipole moment calculations wereperformed using semi empirical molecular orbital methods at theAustin model 1 (AM 1) level using MOPAC programme.

162 A. Sarkar et al. / Journal of Colloid and Interface Science 329 (2009) 160–166

Fig. 1. Fluorescence spectra of “A” in CTAB. (Inset) Fluorescence spectra of “A” in SDS. [A] = 17.2×10−6 M. [CTAB] = 0 (1) to 4.6×10−3 M (11). [SDS] = 0 (1) to 28.5×10−3 M(16).

Fig. 2. Fluorescence spectra of “B” in CTAB. (Inset) Fluorescence spectra of “C” in CTAB. [B] = 18.7 × 10−6 M. [C] = 19.4 × 10−6 M. [CTAB] = 0 (1) to 2.9 × 10−3 M (10).

3. Results and discussions

3.1. Absorbance and steady state fluorescence

The absorption and fluorescence spectra of A, B, and C havebeen studied in micellar solution of cationic CTAB and anionic SDS.The concentration of the probes was kept below 20 μM to avoidany concentration effect or self aggregation of the probe molecules.The absorbance of the compounds (due to π → π∗ transition) de-creases in surfactant solutions before cmc and then increases after

cmc. However, the variation of absorbance with concentration ofthe surfactants is not prominent. The effect of micelles on theemission spectra of the probes is more prominent compared tothat in absorbance. With increase in concentration of surfactantin solution, the fluorescence intensity of the probes is enhanced(Figs. 1, 2). For “A,” the fluorescence maxima shifted from 481 nmin water to 421 nm in CTAB. For “B,” the shift is from 475 nm inwater to 433 nm in CTAB and for “C” it is from 494 nm in wa-ter to 456 nm in CTAB. A new peak is observed at 564 nm for“A,” in micellar medium of CTAB, which is more intense than the

A. Sarkar et al. / Journal of Colloid and Interface Science 329 (2009) 160–166 163

Fig. 3. (a) Plot of F/F0 vs concentration of CTAB. (b) Plot of F/F0 vs concentrationof SDS.

one at 421 nm. The same trend has been observed with SDS also.Compound “A” shows two peaks at 464 nm and 568 nm, the lat-ter being broad and less intense. For “B” the peak is at 466 nmand for “C” it is at 478 nm. The magnitude of the shift in fluores-cence maxima of the probes is less in SDS than in CTAB. A singlepeak at 355 nm has been observed for “A,” when excitation spec-tra were monitored at 421 nm and 564 nm. So formation of thesetwo peaks for “A” is completely due to an excited state interactionof “A” with CTAB and SDS. The enhancement of fluorescence in-tensity, on addition of surfactants, may be due to the impositionof some steric restriction on the compounds. As the compoundsshow a blue shift in micellar media, so from the photophysicalstudies of these compounds in solvents of different polarity, itmay be assumed that the compounds are in a less polar environ-ment compared to water. Therefore the location of the probes islikely to be in the micellar interface. From the above results it isfound that the fluorescence enhancement is significant at all sur-factant concentrations above cmc. Using the concentration of thecompounds of the order of 20 μM, a nearly 10-fold increase inthe fluorescence intensity of “A” in comparison to “B” and “C” inCTAB has been observed (Fig. 3a). In SDS the increase is 2-fold(Fig. 3b). This suggests that the enhanced interaction of the probeswith the cationic surfactant CTAB is probably due to the presenceof more negatively charged centers than positively charged centerswithin the probes. Comparing the shifts in emission wavelengthmaxima of the probes in CTAB and SDS with respect to aqueous

Table 1Emission wavelength maximum (λfls

max/nm) of the probes in aqueous and micellarmedia.

Probes Water CTAB SDS

A 481 421 (SW) 464 (SW)564 (LW) 568 (LW)

B 475 433 466C 494 456 478

Table 2Binding constants K a (dm3 mol−1).

Medium TZD-A TZD-B TZD-C

CTAB 4.33 × 105 3.89 × 105 1.35 × 105

SDS 1.37 × 104 7.42 × 103 3.73 × 103

a Error limits of K values are within 8%.

medium (Table 1); it has been observed that the probes show alarger hypsochromic shift in CTAB than SDS. This also indicatesthat the probes interact to a greater extent with CTAB and reside ina less polar medium compared to SDS. The magnitude of blue shiftis also in the order of A > B > C in CTAB micellar medium, whichreveals the effect of aryl substituents in the probes on their modeand extent of interaction with the ionic micelles. The effect of ureaon the probes, surrounded by micellar environment, has also beenstudied. When urea (1 to 8 mM) was added in the micellar solu-tions of the probes, the nature of the fluorescence spectra of theprobes remained unchanged and the fluorescence intensities wereidentical. This indicates that urea was unable to loosen the strongelectrostatic binding between the charged probes and the chargedmicellar surfaces of CTAB and SDS. The strong binding between theprobes and the micelles is also evident from the high binding con-stant values of the probes in different micellar media, which hasbeen discussed in the next section.

3.2. Determination of probe–micelle binding constants

The emission spectral data were used to calculate the bindingconstant K (Table 2), in the excited state for the substrate micelleinteraction. The interface of the micelles being charged, due to theionic nature of the surfactants, help the probes to orient towardsthe micellar interface by electrostatic attraction, since the probesalso posses charged centers in them.

The fluorescence lifetime of the substrate molecules (discussedin the next section) are in the nanosecond range, so the micellesmay be considered as individual compartment and multiple equi-libria model may be considered for micellization of the solute [44,45]. For a given concentration of the solute and the micelle, the so-lute will be distributed between the aqueous phase (Sw) and themicellar phase (Sm) according to the model

Sw + M = Sm. (1)

The binding constant (K ) for solute–micelle interaction is K =[Sm]/([Sw][M]) or K [M] = [Sm]/[Sw] where [Sm] and [Sw] repre-sent the concentration of the solutes in the micellar phase and theaqueous phase respectively, [M] is the effective micellar concentra-tion. For a given value of [M], K [M] assumes the role of partitioncoefficient for the solute in two phases. Assuming that the ob-served fluorescence (F ) in the solution is the sum of the fractionalfluorescence of aqueous phase (Fw) and the micellar phase (Fm),the expression (2) can be deduced considering equilibrium (1).

The binding constant K of the probes with the micelles hasbeen determined using the relation given below [46].

(F − Fw)−1 = (Fm − Fw)−1[1 + (K [M])−1]

, (2)

164 A. Sarkar et al. / Journal of Colloid and Interface Science 329 (2009) 160–166

Fig. 4. Plot of (Fm − Fw)(F − Fw)−1 vs 1/[M] for “C” in CTAB. (Inset) Same plot for “C” in SDS. [C] = 19.4 × 10−6 M.

where F , Fw and Fm represent the fluorescence intensity of theprobes in micellar solution, water and micellar solution showingmaximum fluorescence intensity respectively.

M = ([Surfactant] − cmc)/aggregation number. K values weredetermined from the slope of the plot of (Fm − Fw) (F − Fw)−1 vs1/[M] (Fig. 4). The binding constant values decrease with increas-ing substituents of the probes in the phenyl moiety. The bind-ing interaction is greater in case of cationic micelles. In otherwords, the probes are fairly bound with the micelles in the orderA > B > C.

The values of, KA/KB is 1.1, KB/KC is 2.88 and KA/KC is 3.20in case of CTAB. For SDS the values are 1.84, 1.98 and 3.67 re-spectively. Interaction of SDS with the probes mainly occurs at thepositively charged NH site in the thiazolidinedione moiety. Withincrease in the electron donating substituents like –OMe in thephenyl moiety, the electron deficiency at the –NH site is reduced.Thus a decrease in the positive charge leads to lesser interactionwith SDS resulting in low binding constant values. In case of CTAB,bulky –OMe groups prevent CTAB molecules from approaching theelectron rich oxygen sites of the carbonyl groups in the thiazo-lidinedione moiety, thus reducing the binding ability of CTAB withthe probes B and C. Similar results were obtained when photo-physics of “A” was studied in cationic cetyl pyridinium chloride,CpCl (K = 1.25 × 105 dm3 mol−1) and anionic dioctyl sodium sul-fosuccinate, AOT (K = 2.76 × 103 dm3 mol−1) micellar media. This,once again validates the fact that the probes interact with thecationic surfactants to a greater extent than the anionic surfactants.

3.3. Fluorescence decay

The fluorescence decay behavior of the systems has been stud-ied in micellar solutions of CTAB and SDS (Fig. 5). In all the casesfor the compounds A, B and C, biexponential fitting was observedirrespective of environment. The lifetime values were calculatedusing the expression,

I(t) =n∑

Aie−t/τ

i,

i=1

where I(t) is the intensity of the fluorescence at time t , Ai is thepre-exponential factor for the fraction of the fluorescence inten-sity, τi is the fluorescence lifetime of the emitting species and nis the total number of emitting species. The average fluorescencelifetimes were calculated using the relation; τ = (a1τ1 + a2τ2)/

(a1 + a2).With increase in surfactant concentration, the population of the

long time component decreases and that in the short time compo-nent increases with faster decay. The average lifetime of the probesincreases in SDS and decreases in CTAB. Chakrabarty et al. reportedan increase in the lifetime value of a positively charged probe(norharmane), when bound by oppositely charged anionic micellesof SnDS [47]. From our earlier work, it has been observed that,when negatively charged probe 1-anthracene sulfonate is associ-ated with positively charged CTAB, the lifetime value decreases dueto an exciplex formation [48]. The probes have negatively chargedcenters on the carbonyl oxygen and positively charged center onthe nitrogen atom. Since the probes have a greater extent of neg-ative charge, so they interact with CTAB to a greater extent thanwith SDS. For this reason, average lifetime value of the probesdecreases in CTAB and increases in SDS (Table 3). Three distinctregions are observed in a micelle; a nonpolar core formed by thehydrocarbon tail of the surfactants, a compact Stern layer hav-ing the head groups, and a wider Gouy–Chapman layer containingthe counter ions. A probe molecule may bind either to the headgroup region or to the nonpolar core of the micelles. Dependingon the nature of the solute, the solute molecules reside in any oneof these sites. The Stern layer for ionic micelles consists of po-lar head groups and largely structured water molecules. Since thelifetime of the probes differs in two types of micelles, so the probemolecules do not reside in the inner core. They are associated withthe charged head groups in the Stern layer through electrostatic at-traction.

3.4. Steady state fluorescence anisotropy

Fluorescence anisotropy depends upon the rotational diffusionof the fluorophore and the rotational diffusion changes with chang-

A. Sarkar et al. / Journal of Colloid and Interface Science 329 (2009) 160–166 165

Fig. 5. Fluorescence decay curve of “A”: (a) prompt, (b) in water. (Inset) (c) in 2.5 mM CTAB and (d) in 12.4 mM SDS. [A] = 17.5 × 10−6 M.

Table 3Average lifetime values (ns).

Probes Water CTAB SDS

A 0.94 0.30 2.24B 0.41 0.28 0.64C 0.40 0.06 0.56

ing viscosity of the medium as well as shape and size of thediffusing species. For the compound “A” having two rotamers inthe excited state, fluorescence anisotropy changes depending uponthe emission wavelengths of the rotamers [49]. The fluorescenceanisotropy of “A” in aqueous solution changes in micellar mediumof CTAB and SDS at both wavelength maxima. The fluorescenceanisotropy of “A” in CTAB shows two values: 0.141 at 564 nm and0.105 at 421 nm. In SDS the values are, 0.127 at 464 nm and 0.087at 568 nm. These two values were attributed to the two rotamersof “A” in the excited state.

3.5. Quantum chemical calculations and intramolecular rotation

The ground state (S0) geometries of the molecules have beenoptimized using the AM 1 method. Subsequently AM 1-SCI (singlyexcited electron configuration) has been performed to get theground state energy (Eg), dipole moments in the ground state andthe transition energy (�Ei→ j ) to different excited electronic states.For CI calculations, we have considered only the single electronictransitions between all the configurations within a predefined en-ergy window (13–14 eV) (depending on the molecular system)from the ground state. To find the relative stability of the differ-ent rotational conformers (rotamers), the torsional angle betweenthe benzene ring (containing the –OH group) and the thiazolidine-dione ring has been preset to different values followed by a fulloptimization of all other geometrical parameters. From AM 1-SCIcalculation it has been found that the energy difference (�E S0→S1)of “B” and “C” having bulky methoxy group is slightly less than “A.”So for “B” and “C,” it is expected that there is no greater shift influorescence wavelength maxima than “A” and the assumption isin accordance with the experimental observation. “A” has two ro-

tational isomers. The energy difference between the two rotamersis 27 kcal/mol, as calculated using geometry optimization methodfor both the rotamers. As a result of this, “A” exhibits two flu-orescence maxima; one as a SW emission and another as an LWemission. In polar protic media like water, interconversion betweenthe rotameric forms of “A” is very fast, as a result of which alow intensity broad band is observed. This is also due to hydrogenbonding with water. In micellar media, “A” exhibits two distinctpeaks. The LW emission (more intense) is probably due to the ro-tamer A1, because there are two free electron rich carbonyl groupswith which CTAB can interact to a greater extent. In A2, one of thecarbonyl groups get involved in intramolecular hydrogen bond for-mation, thus resulting in a SW and a less intensed emission band.Hence, in CTAB, A1 predominates over A2. The energy difference[E(λabs

max) − E(λflsmax)] between the excited states and ground states

of these two rotamers of “A” in CTAB is ≈31 kcal/mol (A1) and≈15 kcal/mol (A2). In SDS, the values are 32 kcal/mol (A1) and21 kcal/mol (A2). In this case, the anionic SDS interacts with posi-tively charged –NH center in the thiazolidinedione moiety. So, it isseen that a change in the micellar medium affects the energeticsof the rotamer A2 whereas that of A1 remain unchanged. In SDS,A2 predominates over A1 (the SW emission has a higher inten-sity than the LW emission) because A2 will interact with anionicSDS to a greater extent since it has less negatively charged centers(one of the electron rich carbonyl group is blocked via intramolec-ular hydrogen bonding) and hence will face lesser repulsion fromSDS. Thus it is observed that the predominance and stability of onerotamer of “A” over the other, is being governed by the nature andcharge of the corresponding micellar media.

4. Summary

The present work deals with the modulation of the photo-physics of the TZD probes by charged micellar assemblies. Theunique excited state behavior of TZD “A” over “B” and “C” in ionicmicellar media of CTAB and SDS has been attributed to the exis-tence of two rotameric forms of “A.” The difference in the spectro-scopic properties of the three derivatives arises due to the incor-poration of bulky methoxy groups in the aryl moiety. The binding

166 A. Sarkar et al. / Journal of Colloid and Interface Science 329 (2009) 160–166

constants of the probes with the micelles are in accordance to theirmolecular structure. The values decrease with increase in methoxysubstituents in the phenyl moiety of the probes. The choice of ionicmicelles, cationic or anionic, will govern the stability and forma-tion of the rotamer of TZD “A,” having greater antibiotic activity.CTAB and SDS being potent denaturating agents interact with thedrug molecules and prevent them from the further attack of addeddenaturating agents. Electrostatic interactions play a major role inthe binding of TZD’s with the charged micelles. The effect of hy-drophobicity of the micelles on these probes has been taken upfor future works. This work thus gives an idea about the photo-physical behavior of these biologically active and pharmaceuticallyimportant probes in ionic micellar media and throws light on theirstructural orientations and substituent effects. This study will alsohelp in opening up new avenues for the designing of suitablecharged organized media for targeted delivery of these bio-activeTZD probes.

Acknowledgment

One of the authors (A.S.) thanks UGC for providing a JRF.

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