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Spectroscopic and Calorimetric Studies of Molecular Recognitions in a Dendrimer-Surfactant Complex Somnath Koley, Manas Ranjan Panda, KIRAN BHARADWAJ, and Subhadip Ghosh Langmuir, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017
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Spectroscopic and Calorimetric Studies of Molecular Recognitions in a Dendrimer-Surfactant Complex
Somnath Koley, Manas Ranjan Panda, Kiran Bharadwaj and Subhadip Ghosh*
School of Chemical Sciences, National Institute of Science Education and Research, HBNI, Khurda-752050, Odisha, India.
ABSTRACT: Molecular recognitions, causing supramolecular complex formation between a hyperbranched polymer molecule (PAMAM dendrimer generation 3) with oppositely charged surfactant sodium dodecyl sulfate (SDS) in aqueous solution, were studied by using various spectroscopic techniques and calorimetric titration of heat change measurements. Spectroscopic measurements were performed using dynamic Stokes shift (DSS), rotational anisotropy decay and translational diffusion of a fluorescent probe molecule coumarin 153 (C153) non-covalently attached to the dendrimer-surfactant complex. All these studies unanimously confirm that the CAC of SDS falls to ~0.8 mM (from its CMC ~ 8 mM) in the presence of ~0.2 mM dendrimer. Further studies of isothermal titration calorimetry (ITC) measurement show that the CAC of SDS in the presence of dendrimer remains invariant to the dendrimer concentration. Complexation reaction between SDS and dendrimer is highly exothermic in nature. A maximum heat release (∆H~ -6.6 kJ/mole of SDS binding) was observed at SDS-to-dendrimer mole ratio of ~3-5; where up to 3 to 5 SDS molecules were encapsulated by one dendrimer molecule to form dendrimer-SDS encapsulation complex. When negatively charged SDS was replaced with a positively charged surfactant dodecyl-trimethylammonium-bromide (DTAB), we found that the DTAB hardly interacted with positively charged dendrimer due to the charge-charge repulsions.
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1. INTRODUCTION Dendrimers, referring to a class of a highly symmetric hyper branched nanometer sized molecular family, have been of great interest to the researchers for their widespread applications, initially confined in drug delivery and nanoparticle synthesis but in most recent days, dendrimers are used for bio-mimicking of proteins, bio-imaging, and gene therapy.1-12 Enormous success of dendrimer in their applications is caused by its high affinity towards stable host-guest complex formation through molecular recognitions, where dendrimer hosts the external molecules (Scheme S1).13-15A significant number of studies in the field of medicinal chemistry have established the utility of dendrimer in solubilizing the hydrophobic drug molecules and thereby transfer the drug molecules to the target locations. While the other studies from a broadly different field of research find dendrimer interior to be an excellent template for nanoparticle synthesis.16-17 A stable dendritic complex formation can be achieved by a judicious choice of dendrimer’s generation size, charge density at the dendrimer rim and surface groups; all these parameters can easily be tailored by various established synthesis routes and modulation of medium pH.18-20Apart from the synthetic modifications, one can potentially tune the binding affinity of dendrimer with a guest molecule by addition of oppositely charged surfactant molecules carrying a long aliphatic chain. Dendrimer-surfactant complex has emerged to be a more efficient binder to the guest molecules compared to the dendrimer alone; as the former one has more number of hydrophobic binding sites due to the presence of hydrocarbon chains of surfactant molecules. Among the recent studies on dendrimer-surfactant complexes, interaction of dendrimer with oppositely charged surfactant has emerged to be the most promising; where ionic, hydrophobic and hydrogen bond interactions all together reinforce the dendrimersurfactant scaffoldings. Indeed the electrostatic attractive force helps the complex formation between dendrimer and oppositely charged surfactant, on the other hand the repulsive force between dendrimer with same charged surfactant thwarts the complexation process. For instance, using fluorescence spectroscopy, Tomalia and his coworkers observed an insignificant amount of interaction between a negatively charged starburst dendrimer (SBD) with negatively charged SDS. However when they replaced SDS with a positively charged surfactant DTAB, they observed the formation of SBD-templated aggregations of surfactant molecules.21 However, unlike SBD, positively charged PAMAM dendrimer interacts strongly with SDS.22 SDS molecules can either go to the dendrimer interior, where surfactant molecules are stabilized by 2 ACS Paragon Plus Environment
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the hydrophobic interactions with the internal wall of dendrimer, or surfactant molecules may get attached to the outer rim of dendrimer molecule through an electrostatic interaction.22-25 In a recent work by Xu and his coworkers, using 1H NMR, PFG-NMR (of diffusion measurement) and NOE studies, they observed that the oppositely charged PAMAM-dendrimer and SDS form miscellaneous complexes depending on the concentrations of the interacting molecules, and SDS-to-dendrimer mole ratio.22,26At low SDS-to-dendrimer mole ratio (~4-8; similar to the concentration ratio we used here), SDS monomers enter into the dendrimer’s internal cavities and get stabilized through hydrophobic and hydrogen bond interactions. Within this dendrimerSDS encapsulation complex, water molecules in the internal cavities of dendrimer are largely replaced by the surfactant molecules and thereby hydrophilic pockets of the dendrimer turn into active binding sites for a hydrophobic drug molecule. Encapsulation of SDS by dendrimer is entropy as well as enthalpy-driven process; as binding of SDS with dendrimer is exothermic in nature and also causes releasing of water molecules from dendrimer cavities.27 SDS encapsulation gets saturated at a relatively high SDS-to-dendrimer mole ratio (~8-16), where all the cavities of a dendrimer molecule are already occupied by SDS molecules. Above this ratio, ion-dipole interactions lead to the formation of various types of binding architectures. These architectures consist of noncooperative and cooperative bindings of SDS mono-layer with dendrimer (at SDS-to-dendrimer mole ration ~128), binding of SDS bilayer with dendrimer surface (at SDS-to-dendrimer mole ration ~256) and so forth. Although the onset of SDS globular micelle formation happens at intermediate mole rations (~56-128), yet these micelles interact with dendrimers only when SDS-micelle concentration is significantly high (at a mole ratio >516~1024). In a SDS micelle rich aqueous dendrimer solution the translational diffusion of dendrimer is greatly retarded due to giant structures formations, where one SDS micelle remain attached with two dendrimer molecules.13,22 Like SDS, bio-active surfactant bile salt, has also been found to be localized at the interior pockets of a dendrimer molecule at a low bile salt concentration.23-26 Using SANS and fluorescence quenching experiments, Holzwarth, WynJones and their coworkers observed that SDS strongly interacts with positively charged poly(1,4diaminobutane) dendrimer, while a neutral micelle hexa-ethyleneglycol-monododecyl-ether (C12EO6) was found to interact mildly with charged dendrimer.28 In a recent work, Tam and his coworkers used dynamic light scattering (DLS), ITC, transmission electronic microscopy (TEM) and electrophoretic mobility to study the supramolecular aggregations formed by amine and 3 ACS Paragon Plus Environment
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hydroxyl terminated PAMAM dendrimers with SDS molecules.15 They observed at pH≤ 2, when all amine centres of an amine terminated dendrimer are protonated, dendrimer-SDS complex gains maximum stability through electrostatic interactions. When they compared between amine terminated dendrimer with a hydroxyl terminated dendrimer, they observed former one binds more number of SDS molecules compared to the latter one. Motivated by the phenomenal success in applications of dendrimer molecules in medicinal chemistry and material sciences, nowadays researchers have started exploring its utility in modern biology.29-31 Biological applications of dendrimer are
mostly governed by how efficiently and selectively dendrimer conjugates with a target biomolecule.14,23,32 Fortunately these desired properties for biological applications are inherited within the dendrimer molecules and one can easily customize these properties to the requisite level by several ways.32 Wang et al, by using NMR titration and NOESY experiments, showed that the binding of amino acids with poly-propylene-imine (PPI) dendrimer is strongly controlled by the nature of the amino acid side chains . They found that for tryptophan, hydrophobic interaction within the dendrimer cavity prevails over the ionic interactions during the complexation process.32 This is in analogy to the binding of biologically active surfactant bile salt or SDS with dendrimer molecule; bile salt and SDS are known to bind at dendrimer pockets through hydrophobic and hydrogen bond interactions at low surfactant concentrations.32 In the present work we have provided an insight to the molecular interaction, responsible for dendrimer-surfactant complex formation, by using several spectroscopic techniques and heat change measurements. The observations from all different techniques are highly congruent in a qualitative sense; one dendrimer molecule may encapsulate up to ~3-5 SDS molecules utilizing its four binding pockets. Our DLS study shows the structure of dendrimer-SDS encapsulation complex is highly monodispersed in nature.. We also observed a higher binding capability of dendrimer-SDS complex, as compared to the dendrimer or SDS monomer alone towards the binding with a hydrophobic guest molecule. We restricted our study at low SDS-to-dendrimer mole ratios (~3-5), as previous reports suggest that at this range of concentration ratio, only monodispersed dendrimer-SDS encapsulation complexes are formed.13,22,26
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2 RESULTS AND DISCUSSION 2.1 Evidence of Binding of Fluorescent Dye Coumarin 153 with Dendrimer-SDS Complex and its Structure. We employed solvent evaporation method to prepare aqueous dendrimer-SDS complex (see experimental section for details of sample preparation). Coumarin 153 (C153) is almost insoluble in water (OD~ 0.018 at λabsmax of C153). On addition of SDS to the aqueous C153 solution at pre-CMC level (~1-2 mM), the solubility of C153 in water does not improve much. However a similar addition of dendrimer (~0.2 mM) causes drastic improvement of solubility (OD~0.25 at absorption peak) of C153 in water.33 This is due to the encapsulation of C153 by the pockets of the dendrimer molecules.33 A further increase in solubility of C153 in water is observed when SDS is added (at 1-2 mM concentration) to the aqueous solution of ~0.2 mM dendrimer; a maximum OD of~0.42 at the absorption peak of C153 can be reached within this solution (Figure 1). At these concentration ratios (~5-10) of SDS-to-dendrimer, structurally uniformed encapsulation complexes are formed where SDS monomers are completely encapsulated by dendrimer molecules.13,22,26 It may be noteworthy that one dendrimer molecule possesses four pockets and each pocket can easily accommodates ~1-2 SDS monomer.
Figure 1. Absorption spectra of saturated C153 in (i) water (black), (ii) ~1 mM aqueous SDS solution (magenta), (iii) ~2 mM aqueous SDS solution (cyan), (iv) ~0.2 mM aqueous dendrimer solution (green), (v) aqueous solution of dendrimer (~0.2 mM) in the presence of ~1 mM SDS (red) and (vi) aqueous solution of dendrimer (~0.2 mM) in the presence of ~ 2 mM SDS (blue), respectively. Generation 3 (G3) PAMAM dendrimer was used for the study presented in this figure. 5 ACS Paragon Plus Environment
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Like absorption, fluorescence spectroscopy study also reveals the existence of strong interactions between the oppositely charged SDS and dendrimer molecules. Fluorescence intensity of C153 from aqueous dendrimer solution is found to be extremely weak [fluorescence quantum yield (ɸf)~0.12], which weakens further (to ɸf~0.08) on addition of ~0.2 mM dendrimer to the aqueous solution of C153 solution.33-34 C153 is
highly soluble in aqueous dendrimer solution, but
sparingly soluble in neat water. Our earlier report suggests that water soluble PAMAM dendrimer encapsulates the C153 molecules utilizing its water filled peripheral pockets that causes retaining of water like spectral properties (low ɸf, emission maximum position, rotational correlation time and solvation time etc) of C153 even after being encapsulated by a dendrimer molecule.33 The most interesting observation was found when ~1-2 mM SDS was added to the aqueous dendrimer (~0.2 mM) solution; ϕf of C153 in SDS-dendrimer solution was dramatically increased to ~0.28 from ~0.08 in aqueous dendrimer solution (Figure 2 and 3). This observation is well under our intuitive expectation; C153 resides within the water rich cavity of a dendrimer molecule as described in our earlier report33 and the cavity water is replaced by SDS when SDS enters into a dendrimer cavity. This results a change of environment at the dendrimer cavity from hydrophilic (in the absence of SDS) to hydrophobic (in the presence of SDS). C153 exhibits higher fluorescence intensity in dendrimer-SDS complex, as C153 is known to have a higher fluorescence quantum yield in a hydrophobic environment.34 An environment change of C153 at dendrimer interior on addition of SDS is further inferred from the blue shifting of its emission peak position, from ~550 nm in aqueous dendrimer solution in absence of SDS, to ~540 nm in the presence of SDS.
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(iv) (iii)
(ii) (vi) (v) (i)
Figure 2. Emission spectra of C153 in (i) neat water (black solid line), (ii) ~0.2 mM aqueous dendrimer solution (red), (iii) aqueous solution of ~0.2 mM dendrimer and ~1mM SDS (blue) and (iv) aqueous solution of ~0.2 mM dendrimer and 2mM SDS (magenta), respectively. The dashed blue (v) and magenta (vi) curves are the emission spectra of C153 from ~1 mM and ~2 mM aqueous solution of SDS in the absence of dendrimer. In all the cases concentrations of C153 were kept to its saturation level and excited at the corresponding absorption peaks. Generation 3 (G3) PAMAM dendrimer was used for the study presented in this figure. Vertical dashed line (black) shows shift of emission energy of C153 towards blue side on addition of SDS in the aqueous dendrimer solution.
Figure 3. Plots of fluorescence quantum yields (ϕf) of C153 as a function of surfactant concentration (SDS or DTAB) in (i) aqueous dendrimer-SDS solution (red filled circle), (ii) aqueous dendrimer-DTAB solution (blue filled circle), (iii) aqueous SDS solution (red hollow circle) and (iv) aqueous DTAB solution (blue hollow circle). Dendrimer concentrations were 7 ACS Paragon Plus Environment
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fixed at ~0.2 mM in all the cases. Complex or aggregation formation causes the enhancement of ϕf. This figure shows in the aqueous dendrimer solution. CAC of SDS drastically falls to ~0.8 mM (red dotted line) from its CMC ~ 8 mM (red dashed line) in water. DTAB interacts mildly with dendrimer that causes a little shifting of its CAC (to ~13 mM, blue dotted line) in the presence of dendrimer from its CMC (~ 16 mM, blue dashed line) in neat water. Generation 3 (G3) PAMAM dendrimer was used for the study presented in this figure. Rotational anisotropy study of C153 non-covalently attached to the dendrimer-SDS complex, which reports the
rotational diffusion time of the excited C153 dipole through molecular
rotation, reveals qualitatively a similar fact to what we observed in steady state spectral studies of absorption and emission energies (as discussed earlier). Details of rotational anisotropy technique can be found within the experimental section of SI and in our earlier reports.35-38 C153 within the water filled pockets of dendrimer displays a fast rotational correlation time of ~ 150 ps (single exponential in nature), which is comparable to the rotational correlation time (~ 100 ps; single exponential in nature) of C153 in bulk water.33,39 This fact indicates that the C153 molecule resides at the middle of the dendrimer pocket without getting attached to the internal surface of dendrimer. Multiple water layers separate C153 dipole from dendrimer wall that causes a free bulk water like rotation of C153 within the dendrimer cavities.33 However on addition of SDS (~1-2 mM), to the ~0.2 mM aqueous dendrimer solution, average rotational correlation time of C153 increases dramatically to ~3.4 ns (bi-exponential in nature, Figure 4). Rotational anisotropy decay of C153 in dendrimer-SDS complex is conclusively fitted with the following bi-exponential fitting equation r(τ)=r0{a1exp(-t/τfast) + a2exp (-t/τslow)}
(1)
Where r0 is the anisotropy at time t=0. Two decay components (τfast and τslow) arise due two uncoupled rotational kinetics; (i) overall rotation of the dendrimer-SDS scaffold (assuming C153 is strongly attached to the scaffold) and (ii) partial depolarization due to the wobbling type motion of the excited dipole, where one end of C153 dipole is attached to the internal surface of dendrimer molecule. This type of assumption for interpreting the biexponential nature of anisotropy decay is widely accepted in several reports for micellar aggregates, polymer-micelle aggregates and dendrimers.33,40-42 Slow component (τslow; equation 1) of the rotational anisotropy decay is directly correlated to the overall size of the dendrimer-SDS complex through following Stokes-Einstein equation.
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τ slow
4π rh3η = 3kT
(2)
Where rh is the hydrodynamic radius of the dendrimer-SDS complex, which was calculated to be ~ 1.6-1.7 nm at pre-miceller concentrations of SDS (using τslow ~ 4.8-4.4 ns, η = 1.05-1.15 cP and T= 298K). The size of dendrimer-SDS complex obtained from the fluorescence anisotropy study is highly congruent with the reported size of dendrimer alone.33 We therefore conclude that the SDS molecules are completely encapsulated by dendrimer molecules at low SDS concentration regime. Slowing down of the rotational time of C153 from a fast bulk-water like rotation in dendrimer alone to a much slower rotation in dendrimer-SDS complex, suggesting that unlike dendrimer alone, C153 is firmly attached with dendrimer-SDS complex. Complete depolarization of C153 in dendrimer-SDS complex is possible only through the overall rotation of the complex.
Figure 4. Time evolution of rotational correlation functions [r(t)] of C153 in aqueous solution in the presence of (i) ~0.2 mM dendrimer (red ∆), (ii) ~1mM SDS (blue ∆) and (iii) ~0.2 mM dendrimer + ~1 mM SDS (green ∆). The black lines are representing the best fittings to the experimental curves. Generation 3 (G3) PAMAM dendrimer was used for the study presented in this figure. Samples were excited at 445 nm and emissions were collected at ~10 nm blue wavelengths from corresponding emission peak positions.
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Further insights to this dendrimer-SDS complex and water dynamics inside it are obtained from the ITC, Fluorescence Correlation Spectroscopy (FCS) and dynamics Stokes shift studies which are discussed in the succeeding paragraphs. 2.2 Isothermal Titration Calorimetry (ITC) Study of Dendrimer-SDS Complex Formation Heat change associated with the dendrimer-SDS complex formation was measured through ITC study. In this experiment, injector of the ITC instrument was loaded with titrant SDS with ~100 times higher concentration compared to the concentration of titrand dendrimer (~0.1-0.3 mM), placed within the sample cell. Total ~40 µL of titrant was injected into ~200 µL of titrand through 40 successive injections. Concentration of SDS (~10-40 mM) within the injector was higher than its CMC (~8 mM).27 Therefore, after every injection, SDS micelles first break into monomers before it form complexes with dendrimer molecules. In this titration, there will be successive heat changes for two different processes: (1) demicellization of SDS immediately after injection, as final concentration of SDS after dilution in the sample cell falls below the CMC and (2) formation of dendrimer-SDS encapsulation complexes. Heat of demicellization (∆Hdemic~2.02 kJ/mole-of-SDS; Figure 5) of SDS was estimated through a similar titration, where SDS micelles were injected into water in the absence of dendrimer. Our estimated value here is highly congruent to the earlier report (∆Hdemic~1.9 kJ/mole-of-SDS) by Blume and his coworkers on heat of demicellization of SDS micelles.27 After demicellization, associations of SDS monomers with dendrimers are found to be highly exothermic in nature. A maximum heat change (~-6.6 kJ/mole-of-SDS encapsulation) due to a complete encapsulation of SDS molecules by dendrimer molecules can be observed when ~3-5 SDS monomers bind to a single dendrimer molecule (Figure 5). In this regime of SDS-to-dendrimer mole ratio (~3-5), ITC curve shows a plateau and exhibiting a maximum heat change. On further increasing of mole ration (SDS-todendrimer> 5) causes a lower magnitude of heat release. It is worth mentioning here that Tam and his coworkers obtained a higher magnitude of heat change (up to ~10-12 kJ/mole-of-SDS binding) for similar kind of complex formation.15,43 The higher heat change in their study is a result of the lower pH (5), where SDS molecules already occupied the internal space of dendrimer, anionic head group of excess SDS molecules interacts with protonated amine centers at the dendrimer rims through electrostatic interactions. This type of interaction leading to the formation of non-cooperative and cooperative binding complexes (Figure 5).13 Within these complexes, hydrophobic chain of SDS is plunged into water, which causes a lesser stabilization energy as compared to the stabilization through encapsulation complex formation at low SDS-to-dendrimer mole ratios (~35).
2.3
Dynamics Stokes Shift Study of C153 in Dendrimer-SDS Complex
Time resolved emission spectra (TRES) of C153, attached to a dendrimer–SDS encapsulation complex in aqueous solution, were constructed using the recipe provided by Maroncelli and Fleming.44 The observed TRES were found to be shifted continuously towards the low energy side within the detectable time window (from 0.08 ns to 10 ns) of our TCSPC setup (IRF~70 ps). Such a shifting of emission energy of a solvatochromic dye molecule with time is caused by the reorganization of the solvent molecules around the excited probe dipole and thereby stabilizes the excited state energy of the probe molecule. This phenomenon is widely known as solvation dynamics.35-38,45 Reorganization time of water molecules around the solute dipole can be obtained by exponential fitting to the decay of experimentally measured solvent correlation function C(t). C(t)[= (νt-ν∞)/(ν0-ν∞)] is a function of energies of complete unsolvated (ν0; at t=0), complete solvated (ν∞; at t=∞) and partially solvated (ν∞; at t=t) states of the excited state solute dipole. A details of solvation dynamics phenomenon, procedure of constructing TRES and thereby construction of C(t) can be found in several literatures.35-38,44-45 Solvation dynamics in neat water is completed in an ultrafast time scale (~1 ps), due to the free movements of the water molecules around the solute dipole.46-47 However, solvation dynamics of water bound to a micelle, cyclodextrin, and at the surface of a bio-molecule are found to be orders of magnitude slower compared to the bulk water.48-51 Although solvation dynamics in micelles, cyclodextrins or bio-supramolecular assemblies are greatly retarded, the solvation dynamics at dendrimer 12 ACS Paragon Plus Environment
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interior proceed with a much faster timescale; even faster than the time resolution (
