Encapsulation of Prodan in beta-cyclodextrin environments:

A critical study via electronic spectroscopy and molecular mechanics

Anwesha Banerjee, Bidisa Sengupta 1, Sudip Chaudhuri,

Kaushik Basu, Pradeep K. Sengupta *

Biophysics Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata, West Bengal 700064, India

Received 18 November 2005; received in revised form 2 February 2006; accepted 6 February 2006

Available online 29 March 2006

Abstract

We present a detailed study on the binding of the naphthalene based fluorescence probe Prodan with two cyclic oligosachharides namely,

natural beta-cyclodextrin (b-CD) and its synthetic derivative, succinyl-2-hydroxypropyl beta-cyclodextrin (SHPb-CD) using electronic

absorption and fluorescence spectroscopy along with theoretical techniques. The encapsulation of Prodan inside the b-CD cavities leads to

pronounced changes in its emission characteristics, including dramatic blue shifts (27 nm in 10 mM SHPb-CD and 19 nm in 10 mM b-CD) in the

emission maximum accompanied by increase in the emission yield, fluorescence anisotropy and lifetime values. Detailed analyses of the

fluorescence along with relevant absorption spectroscopic data indicate that Prodan readily enters the doughnut-shaped hydrophobic cavities of

the b-CDs and forms 1:1 inclusion complexes, the binding affinity being significantly higher in case of SHPb-CD. Furthermore, docking studies

performed via molecular mechanics methods (MMC) indicate that the dimethylamino group of Prodan is most likely to be oriented towards the

wider rim of the cyclodextrin cavity. Quantum mechanical calculations reveal that incorporation of Prodan into the b-CD cavities, results in the

formation of a N-TICT (dimethylamino twisted intramolecular charge transfer) state.

q 2006 Elsevier B.V. All rights reserved.

Keywords: Prodan; SHPb-cyclodextrin; Fluorescence lifetime and anisotropy; Molecular mechanics; Intramolecular charge transfer (ICT) state

1. Introduction

The organic chromophore Prodan (6-propionyl-2-(dimethy-

lamino)-napthalene) (Scheme 1A) finds widespread appli-

cations in biophysical studies as an exquisitely sensitive

fluorescent probe. In this context much attention has focused,

in particular, on the characterization of microenvironments,

and studying ligand binding effects (including structural

perturbations) in proteins, artificial and natural biomembranes,

etc [1–10]. The extremely sensitive fluorescence properties of

Prodan stem largely from the simultaneous presence of an

electron donor (D) and an electron acceptor (A) group which

facilitates the creation of an intramolecular charge transfer

state (ICT) leading to a large change in the dipole moment

upon photo-excitation, followed by extensive solvent dipolar

0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2006.02.012

* Corresponding author. Tel.: C91 33 2337 0379; fax: C91 33 2337 4637.

E-mail addresses: pradeepk.sengupta@saha.ac.in, pradeepsinp@yahoo.co.

in (P.K. Sengupta).1 Present address: Department of Applied Physics, Chalmers University of

Technology, 412 96 Goteborg, Sweden.

reorganization. In addition, hydrogen bond mediated specific

solvent effects have profound influence on the environmental

sensitivity of Prodan [4]. Thus Prodan can serve as an

important monitor for gaining insight into the molecular

recognition properties of host-guest interactions in organized

assemblies, e.g. liposomes, normal and reverse micelles, and in

encapsulated systems, via judicious choice of appropriate

emission parameters.

Cyclodextrins are interesting microvesicles capable of

encapsulating a wide range of organic and inorganic

compounds [11–25]. They are cyclic oligosaccharides with a

hydrophilic outer surface and a hydrophobic central cavity. In

aqueous solutions they are able to form host-guest inclusion

complexes with a variety of hydrophobic molecules (like

drugs). It is this property of cyclodextrins that is making their

use extremely popular as vehicles for drug delivery. Moreover,

the reduced polarity and restricted geometry of the cavity

provide an interesting microenvironment for studying the

energetics and dynamics of various photophysical processes of

the encapsulated molecules. Beta-cyclodextrins (b-CDs)(Scheme 1B) are the most readily available and widely used

amongst the three naturally occuring cyclodextrins (CDs),

namely alpha (a), beta (b) and gamma (g) CDs, composed of 6,

Journal of Molecular Structure 794 (2006) 181–189

www.elsevier.com/locate/molstruc

Scheme 1.

A. Banerjee et al. / Journal of Molecular Structure 794 (2006) 181–189182

7 and 8 a-D-glucose units, respectively. Although natural

b-CDs have extensively been used as agents for drug delivery,

recently, synthetically derived substituted b-CDs are finding

wider application, because of their enhanced solubility in

water, better drug complexing and tabletting properties.

There are a few earlier reports [26–28] on the interaction of

Prodan with beta-cyclodextrin (natural form) based on steady

state fluorescence spectra and preliminary lifetime studies.

Recognizing the growing potential of SHPb-CD as a drug

carrier, we have undertaken a comparison of the probe

(Prodan) inclusion behaviour to the natural and substituted

(SHPb-CD) varieties of beta-cyclodextrin using electronic

absorption and fluorescence emission (encompassing emission

profiles, lifetime as well as anisotropy data). Furthermore, we

report for the first time, theoretical studies involving molecular

mechanics and quantum chemical calculations, which have

been performed to consolidate our experimental findings. This

provides a detailed understanding of the entire process of

formation of the Prodan-b-CD inclusion complexes and the

photophysics of the probe in such environments.

The present research also focuses attention on the nature of

the emissive state of Prodan encapsulated within the b-CDcavities. We demonstrate in this paper that Prodan exhibits

complete charge transfer (CT) inside the b-CD cavities

(presumably through specific interactions with the hydroxyl

groups of the b-CDs) while in aqueous media it exists in both

charge transfer (CT) as well as a locally excited (LE) state.

Theoretical investigations via quantum mechanical studies

confirm the existence of such a state.

2. Materials and methods

2.1. Experimental section

Prodan was obtained from Molecular probes. b-Cyclo-dextrin (b-CD) and succinyl-(2-hydroxypropyl)-b-cyclodex-trin (SHPb-CD) [Degree of substitution (DS)Z4] were

purchased from Sigma. Solvents used were of spectroscopic

grade and were preliminarily checked for absence of absorbing

and fluorescent impurities. A stock solution of Prodan with a

concentration of 1!10K2 M was prepared in methanol. Stock

solutions of b-cyclodextrin and SHPb-cyclodextrin both with aconcentration of 1!10K2 M were prepared in quartz distilled

water. To prepare each solution for spectroscopic measure-

ments, an aliquot of the Prodan stock solution was transferred

into a glass vial. The methanol was then evaporated. The dried

Fig. 1. A plot of the molar extinction coefficient (3) versus (a) SHPb-CD and (b)

b-CD concentrations recorded at labsZ360 nm.

A. Banerjee et al. / Journal of Molecular Structure 794 (2006) 181–189 183

samples were resuspended in specific volumes of b-cyclo-dextrin and SHPb-cyclodextrin solutions and vortexed. The

concentration of Prodan was maintained at 1!10K5 M in all

the solutions. The samples prepared for spectroscopic

measurements were then equilibrated at room temperature for

60 min. All samples were used immediately after preparation.

In case of emission measurements of the probe in cyclodextrin

solutions, background fluorescence as well as light scattering

were removed by subtraction of the spectra on blank solutions.

Steady state electronic absorption and fluorescence spectra

were recorded with a Cecil 7500 spectrophotometer and

Hitachi F-4010 and Perkin–Elmer LS-55 spectrofluorometers,

respectively. The fluorescence anisotropy (r) values were

obtained using the expression rZ(IVVKGIVH)/(IVVC2GIVH),

where IVV and IVH are the vertically and horizontally polarized

components of probe emission with excitation by vertically

polarized light at the respective wavelength and G is the

sensitivity factor of the detection system [29]. Each intensity

value used in this expression represents the computer averaged

values of 10 successive measurements. Fluorescence lifetime

(t) measurements were carried out with an Edinburgh

Instruments time correlated single photon counting nanose-

cond fluorescence spectrometer. Data analysis was carried out

by a deconvolution method using a non-linear least square

fitting programme and fitted with a multi exponential decay

function, F(t)ZSiAi exp (Kt/ti). The goodness of fit was

estimated by using c2 values. The fluorescence decay

measurements were performed using an exciting line of

wavelength 358 nm, which corresponds to a peak in the

spectral output of the nitrogen flash lamp. All spectral

measurements were carried out at ambient temperature

(298 K). The concentrations of Prodan and the b-CDs were

maintained at 10 mM and 5 mM, respectively.

2.2. Theoretical studies

2.2.1. Molecular mechanics studies

Molecular Mechanics (MMC) was used to investigate the

process of inclusion of Prodan into the b-CD cavity and the

stability of the inclusion complexes formed were judged from

their energy of formation values. Since, the exact crystal

structure and the substitution pattern in SHPb-CD is still

unknown, theoretical studies were conducted on the parent b-CD and Prodan only. As the basic closed structure of b-CD is

maintained in SHPb-CD, and the degree of substitution is low

(DSZ4), the type of complex formed in SHPb-CD is assumed

to be similar to b-CD.HYPERCHEM 7.5 [30] was used to build the structure of

Prodan and b-CD molecules. b-CD was built on screen starting

from an a-D-glucose optimized monomer provided in the

HYPERCHEM 7.5 database. The structures of Prodan-b-CDcomplexes were considered in gaseous state and the molecular

mechanics program MMC implemented in the software

package was used to minimize them. No cut-offs were used

and geometry optimization was carried out to an energy

convergence of 0.01 kcal/A per mol with the Polak–Ribiere

conjugate gradient algorithm.

Docking calculations were performed in vacuo to locate the

low energy structures for the Prodan–b-CD complexes [31–

33]. Two principal relative arrangements of Prodan and b-CDwere considered. In one case the Prodan molecule was allowed

to approach the b-CD cavity from the wider rim which contains

the secondary hydroxyl groups while in the other it was made

to approach the b-CD cavity from the primary hydroxyl rim

(Ref. Fig. 7A and B, respectively). The initial Prodan

configuration was parallel to the b-CD hydroxyl rim at a

distance of about 2 A from the rim. Prodan was then manually

rotated by steps of 458 and an optimization was set to start after

each rotation. Each of these stochastically generated low

energy structures were then grouped so as to identify the path

of entry of Prodan into the b-CD cavity and the nature of

complex formed with b-CD.Solvation studies were performed with MMC and OPLS

force fields with 260 water molecules in a periodic box of

dimensions 20 A!20 A!20 A. Only the lowest energy

structures obtained from the studies in vacuum were chosen

for this purpose. The results were then analysed to yield further

insight into the conformation of Prodan in a solvated complex

and the forces driving the complexation process.

A. Banerjee et al. / Journal of Molecular Structure 794 (2006) 181–189184

2.2.2. Quantum mechanical investigations

The ground and excited state geometries of Prodan were

obtained by a semi-empirical optimization using the respective

AM1 Hamiltonians for the Prodan molecule in aqueous

solution and in the 1:1 complex with b-CD. The dihedral

angles of the dimethyl amino and propionyl groups were

considered in detail in order to assess the influence of the

constrained environment of the CD cavity on these geometrical

parameters of the Prodan molecule.

3. Results and discussion

3.1. Binding of Prodan with the b-CDs

The binding of Prodan with b-CDs has been investigated byusing electronic absorption and fluorescence spectroscopic

methods. Fig. 1 displays the molar extinction coefficients of

Prodan as a function of added SHPb-CD and b-CDconcentration. The value of the molar extinction coefficient

monotonically increases on addition of both SHPb-CD and b-CD upto a certain range of concentration. Such changes

are indicative of the penetration of Prodan into the hydrophobic

Fig. 2. Fluorescence emission spectra of Prodan (10 mM) in presence of various c

variations in fluorescence intensity and emission maxima with SHPb-CD (:) and

b-CD and SHPb-CD cavities resulting in the formation of

Prodan–b-CD and Prodan–SHPb-CD inclusion complexes.

Fig. 2a and b represent the fluorescence emission profiles of

Prodan at various concentrations of SHPb-CD and b-CDrespectively. Addition of b-CDs results in a dramatic increase

in the fluorescence intensity (Ref. Fig. 2c) accompanied by a

gradual blue shift in the emission maximum (lem) of Prodan. In

SHPb-CD the lem shows a large blue shift of 27 nm (from

521 nm (in water) to 494 nm (in 10 mM SHPb-CD)) while in

b-CD the corresponding blue shift is ca. 19 nm (Ref. Fig. 2d).

The fluorescence emission data thus suggests that Prodan

enters the hydrophobic cavities of the b-CDs where dipolar

relaxation processes are inhibited leading to the observed

changes.

For a 1:1 complex formation between fluorescent guest

molecule and b-CDs the binding constant can be obtained fromthe fluorescence data by the modified Benesi–Hildebrand

equation [34]

1=ðFKF0ÞZ 1=ðFNKF0ÞK½bKCD�C1=ðFNKF0Þ (1)

where F0 is the fluorescence intensity of the guest in the

absence of b-CD, F is the fluorescence intensity at a particular

concentration of b-CD, FN is the fluorescence intensity of the

oncentrations of (a) SHPb-CD and (b) b-CD respectively. The corresponding

b-CD (C) concentrations are displayed in 2(c) and (d). (lexcZ360 nm).

Fig. 3. Double-reciprocal plots using fluorescence data for complexation of

Prodan with (a) SHP-b-CD and (b) b-CD, respectively.

Fig. 4. The variation in the fluorescence anisotropy (r) of Prodan on addition of

(a) SHP-b-CD and (b) b-CD, respectively. (lexcZ360 nm; lemZ520 nm).

A. Banerjee et al. / Journal of Molecular Structure 794 (2006) 181–189 185

guest-b-CD complexes (i.e. the fluorescence intensity when all

the guest molecules are complexed) and K is the binding

constant for the 1:1 complex.

A plot of 1/(FKF0) vs 1/[SHPb-CD] and 1/[b-CD]concentrations displayed in Fig. 3a and b, respectively show

good linearity. This indicates formation of inclusion complexes

between the hosts (SHPb-CD and b-CD) and the guest

(Prodan) with a stoichiometry of 1:1 (Prodan:SHPb-CD or

Prodan:b-CD). In case of Prodan–b-CD complex the binding

constant was found to be 862.2 MK1. Analysis of the steady

state fluorescence data for Prodan–SHPb-CD complexes yield

a significantly higher binding constant of KZ1387 MK1.

Fluorescence anisotropy (r) measurements were also

performed since this parameter serves as a sensitive indicator

for monitoring ligand binding to macromolecular systems

[35,36], biomembranes [37], biomembrane mimetic organized

assemblies [38,39], etc. The anisotropy values of fluorophores

are very low in fluid solution where the fluorophore molecules

can freely rotate and increase in motionally constrained

environments [29]. Here, the ‘r’ value was found to gradually

increase with the concentration of the b-CDs reaching a

maximum of 0.073 in case of SHPb-CD and 0.048 in case of b-CD (Ref. Fig. 4). This is consistent with the picture that the

Prodan molecules are firmly incorporated into the b-CDcavities, SHPb-CD offering a relatively more restricted

environment for binding compared to the unsubstituted b-CD.

3.2. Emissive states of Prodan in the inclusion complexes

with b-CDs

Previous studies [8,40–42] have reported the existence of a

locally excited (LE) state of Prodan apart from the

intramolecular charge transfer (ICT) state. In the LE state,

the excitation is localized on the naphthalene ring, so that the

molecule is not very polar while in the ICT state complete

charge transfer from the amino group to the carbonyl group

occurs which requires twisting of the dimethylamino group to

allow the nitrogen electrons to be in conjugation with the

naphthalene ring.

In agreement with previous workers [8], the fluorescence

spectra of Prodan in water with excitation at 313 nm shows the

existence of a low intensity short wavelength band atw425 nm

Fig. 5. Excitation wavelength-dependent fluorescence spectra of Prodan in

water (—), methanol (- - - -), b-CD ($$$$) and benzene (-$-$-) environments:

(a) lexcZ313 nm (b) lexcZ360 nm.

Fig. 6. The fluorescence decay profiles of Prodan obtained in (a) SHP-b-CD (b)

b-CD solutions (lexcZ358 nm, lemZ520 nm). The lamp profiles are shown by

unconnected points (&) and the solid curves (—) represent the computer best

fits for the experimental points (:).

A. Banerjee et al. / Journal of Molecular Structure 794 (2006) 181–189186

(attributed to fluorescence from the LE state) together with a

long wavelength emission band atw520 nm (arising from ICT

state). Excitation wavelength dependence of the fluorescence

spectra of Prodan recorded in water and in the b-CDs,respectively, shows the complete disappearance of the short

wavelength band w425 nm on incorporation into the b-CDcavities (Fig. 5). The emission band at w520 nm, however,

shows a significant blue shift on encapsulation of the probe in

b-CDs, as indicated in the previous section. These results

indicate that the microenvironment of the b-CDs favor the

formation of the ICT state in preference to the LE state.

Comparison of these results with those in homogenous solvents

of different polarity suggest that b-CDs facilitate the formation

of the ICT state probably through specific H-bonding

interactions between the cyclodextrin hydroxyl groups and

Prodan.

Table 1

Fluorescence decay parameters of Prodan in SHPb-CD and b-CD

Sample t1 (ns) A1 t2 (ns) A2 c2

Prodan in SHPb-CD 1.012 25.71 3.307 74.29 0.457

Prodan in b-CD 2.083 77.24 4.211 22.76 0.834

3.3. Fluorescence lifetime measurements

Preliminary lifetime measurements of Prodan in b-CD and

SHPb-CD were also carried out. Examination of the

fluorescence lifetime data reveals bi-exponential behaviour in

both b-CD and SHPb-CD environments indicating the

presence of at least two excited state species of Prodan when

complexed to b-CDs (Fig. 6a and b). The decay parameters

along with the corresponding c2 values are given in Table 1.

These values are significantly higher than those observed in

water (t1w0.53 ns and t2w1.86 ns) [8], which indicate

considerable change in the microenvironment of Prodan upon

incorporation into the apolar b-CD cavities.

3.4. Theoretical studies

3.4.1. Docking calculations

The energy change accompanying the formation of the 1:1

Prodan-b-CD complex (shown in Eq. (1)) can be calculated as

Fig. 7. Axial views of the two initial orientations (A and B) of the Prodan and b-CD molecules used in the calculations. (C) Total energy of formation values (DE) of

the inclusion complexes obtained from docking studies.

Fig. 8. Axial views of the inclusion complexes: (A) complex A and (B)

complex B.

A. Banerjee et al. / Journal of Molecular Structure 794 (2006) 181–189 187

DE ZEcomplexKEProdanKECD (2)

where Ecomplex, EProdan, ECD are the total energy of the

complex, the free Prodan (guest) and the free b-CD (host). The

energy of formation can alternatively be decomposed into three

components: changes taking place in the energy of the guest

(Prodan) on complexation (DEProdan), changes taking place in

the energy of the host (DECD) and the mutual interactions

between the guest and the host (DEprod-CD) [31]. These provide

quantitative measures of the interaction forces driving the

complexation process.

The most stable complex among all the configurations

corresponds to the greatest negative value of DE. The

energy of formation values of the complexes for various

orientations of the guest molecule (Prodan) when docked

on the secondary and primary rims of b-CD (host) are

plotted in Fig. 7C. Fig. 8 shows the two possible types of

inclusion complexes formed viz Complex A (with the

dimethylamino group of Prodan towards the secondary

rim) and Complex B (with the propionyl group of Prodan

towards secondary rim). The energy of formation values of

the two possible types of inclusion complexes formed and

the changes in the energies of the host (b-CD) and guest

(Prodan) accompanying such complexation process are

listed in Table 2. The energy of formation values and the

changes in the energies of the host (b-CD) and the guest

(Prodan) indicate favorable formation and increased

Table 2

Energy of formation values and the energy changes (changes in Prodan, b-CD and their mutual interaction) accompanying the formation of the Prodan-b-CD

inclusion complexes, as obtained by molecular mechanics calculations in vacuo

Energy values (in kcal/mol) Changes in energy (kcal/mol)

Complex

Ecomplex

(b) complex B

Free Prodan

EProdan

Free b-CD

ECD

Energy of

formation of

complex, DE

Prodan in

complex

b-CD in

complex

Prodan

DEProdan

b-CD

DECD

ProdanKb-CD

interactions

DEprod-CD

(a) 62.035 0.124 89.724 K27.812 K0.157 87.113 K0.281 K2.611 K24.921

(b) 65.678 0.124 89.724 K24.17 0.898 86.394 0.774 K3.330 K21.696

A. Banerjee et al. / Journal of Molecular Structure 794 (2006) 181–189188

stability of complex A. Moreover, it can be seen that the

mutual host–guest interactions (DEprod-CD) contribute

greatly to DE and are crucial in determining complex

stability. Thus from the above results, it appears that

amongst the two inclusion complexes the one in which the

dimethylamino group points towards the wider rim i.e.

complex A (corresponding to the 908 orientation of Prodan

in Fig. 8) of the b-CD cavity is the most favored one.

These results are in agreement with the proposal made by

Hassan et al. [27] based on their experimental findings.

Fig. 9 displays the entry of Prodan into the b-CD cavities

by the path of least resistance, i.e. lowest energy barrier.

From the results of the solvation studies (Ref. Table 3)

it is evident that the complexation process is driven by a

combination of forces, namely van der Waals, electrostatic,

dihedral angle bending and bond angle bending forces. The

solvation energies calculated for the two orientations of Prodan

in the complex also indicate that complex A (dimethylamino

towards secondary ring) is the most favorable. The selectivity

of Prodan toward this particular conformation could pre-

sumably facilitate specific interactions taking place between

the host and the guest.

Fig. 9. The proposed three step path of entry of

Table 3

Calculated solvation energy of the complex (DEsolv) and the contributions of bond

electrostatic forces to the complexation process

Energy of complex

only (kcal/mol)

(a) complex A

(b) complex B

Components of the en

Bond Angle Dihedral

(a) K20.312 2.655 33.502 71.971

(b) K7.156 3.781 34.754 73.662

3.4.2. Nature of the electronic excited states: influence

of encapsulation

Table 4 lists the geometric parameters of Prodan in water

and in b-CD as obtained from quantum mechanical (AM1)

calculations. It is evident that in water (in both the ground (S0)

and excited (S1) electronic states) Prodan occurs primarily in a

propanoyl twisted (awK35.88) form although some dimethy-

lamino twisted (N-TICT) character is also present (dwC13.198). On encapsulation into b-CD, however, the geometry of

Prodan is completely altered. In this case, while the propanoyl

group is almost planar (awK7.838) it is the dimethylamino

group which is twisted to a significant degree (dwK148.838).

The restricted rotation of the propanoyl group suggests specific

interactions of the propanoyl group of Prodan with the primary

hydroxyls of b-CD.

4. Concluding remarks

We report that Prodan readily enters the doughnut-shaped

hydrophobic cavity of the b-CDs forming a 1:1 host-guest

complexes and undergoes distinct changes in the photophysical

characteristics on encapsulation. The binding affinity and

the Prodan molecule into the b-CD cavities.

stretching, bond angle bending, dihedral angle deformation, van der Waals and

ergy (in kcal/mol) Solvation energy

DEsolv (kcal/mol)van der Waals Electrostatic

K102.163 K26.277 K9.043

K90.093 K29.260 4.113

Table 4

Geometric parameters of Prodan in water and in b-CD calculated by AM1

method

Ground state (S0) of

PRODAN

Excited state (S1) of

PRODAN

In water In b-CD In water In b-CD

Bond lengths (in A)

(i) N8–C11 1.38 1.42 1.37 1.40

(ii) C11–C12 1.41 1.39 1.45 1.45

(iii) C12–C13 1.41 1.41 1.38 1.37

(iv) C13–C14 1.42 1.42 1.45 1.45

(v) C14–C17 1.42 1.42 1.38 1.37

(vi) C18–C5 1.47 1.48 1.46 1.46

Bond angles (in degrees)

(i) C6–N8–C11 119.02 112.48 120.01 114.77

(ii) C19–C18–C5 116.78 117.99 118.14 119.21

(iii) O4–C5–C18 121.79 119.38 121.80 119.44

(iv) C6–N8–C7 119.58 114.78 119.02 117.22

Dihedral angles (in degrees)

(i)d (C7–N8–C11–C9) C14.44 K148.83 C13.19 K147.05

(ii)a (O4–C5–C18–C19) K38.51 K7.83 K35.80 K7.64

A. Banerjee et al. / Journal of Molecular Structure 794 (2006) 181–189 189

hydrophobicity of the binding site of Prodan in SHPb-CD is

found to be higher than in case of the unsubstituted b-CD.Molecular docking studies suggest that Prodan forms an

inclusion complex with the dimethylamino group preferably

oriented towards the larger rims of b-cavities. These studies

also indicate that van der Waals forces along with electrostatic

and dihedral angle interactions contribute greatly to the process

of complexation.

In summary the present research provides a comprehensive

picture on the interactions of the hydrophobic fluorescence

probe Prodan with two different cyclodextrins highlighting the

following aspects: (a) the binding mode and stoichiometry of

the Prodan-CD inclusion complexes, (b) the forces involved in

the complexation process, and (c) the influence of the hydrogen

bond mediated switching between the intramolecular charge

transfer (ICT) and locally excited (LE) states of the Prodan

molecule in the CD environments. The insights obtained

from these findings should be of considerable importance, in

general, for the understanding of the influence of such

constrained environments on the structure, stability, photo-

physical properties and other related aspects of such

encapsulated molecules.

Acknowledgements

One of us (AB) would like to acknowledge the Council for

Scientific and Industrial Research (CSIR), India for Senior

Research fellowship (CSIR grant award no. 9/489 (44)/2002-

EMR-I). We would like to thank Prof. S. Basak of the

Chemical Sciences Division, Saha Institute of Nuclear Physics

for use of the time correlated single photon counting

nanosecond fluorescence spectrometer.

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