Controlled Formation of Hydrated Micelles by the Intervention of Cyclodextrins

The interaction between surfactants and cyclodextrins (CDs) is well known. Studies have focused mainly on destruction of micelles with CDs to release the encapsulated drugs. However, less emphasis has been given on understanding the formation of micelles with the CD encapsulated surfactants. We have used fluorescence spectroscopy to study the impact of CDs on micelles using a fluorophore that has been tactically designed as a reporter. This molecule has a pyrene moiety on one end and a cationic head group on the other so that the orientation of the compound can be prefixed on micelle formation in aqueous environment. We have observed that the CD encapsulated surfactants can form "hydrated micelles" that allow extensive penetration of water molecules toward the core. The mechanism for such a process involves inclusion of the hydrophobic surfactant tails within the CD core and participation of these inclusion complexes in micelle formation. The process could be controlled by tuning the concentration of CD. The degree of hydration varies as the micelles get more opened up due to the residence of the CDs inside them.

Methodology to study polymers interaction by surface plasmon resonance imaging

The surface plasmon resonance (SPR) technique has been primarily used in the field of biology, in particular for the study of antibody-antigen interactions. Recently, polymers were introduced to form inclusion complexes.

We describe here, a methodology based on surface plasmon resonance imaging to study water-resistant and reversible inclusion complexes using systems which are compatible with a cosmetic use. The purpose of this study is to follow in real time the interaction between two polymers.

To carry out this study:

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A biochip based on a covalent binding of one “host polymer” on a gold-activated surface was developed.

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The binding of the host polymer to a guest polymer was monitored.

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The presence of interactions between the β-cyclodextrins groups of the host polymer and the adamantyl functional groups of the guest polymer and the possibility of dissociating the complex were established.

This technique allowed carrying out parallel assays for optimizing the amount of complexes formed, the host polymer being spotted at five concentrations. It was then possible to study the influence of the concentration in host system for two concentrations of the guest polymer. The concentration in the host polymer yielding the highest immobilization of the guest system was further determined.

Cyclodextrins and surfactants in aqueous solution above the critical micelle concentration: where are the cyclodextrins located?

Cyclodextrins (CDs) are known to bind surfactant molecules below the surfactant critical micelle concentration (CMC); however, interactions of CDs with surfactant micelles (above the CMC) are not well understood. In particular, direct structural evidence of the location of CDs in the different subphases found in micellar solutions is lacking. We have utilized small-angle neutron scattering (SANS) with contrast matching to probe the localization of α-cyclodextrin (α-CD) and 2-hydroxypropyl-β-cyclodextrin (HPβ-CD) in sodium dodecyl sulfate (SDS) micelles in aqueous (D2O) solutions. SANS data from solutions containing either hydrogenated or deuterated surfactants were analyzed by considering three different scenarios pertaining to the localization of cyclodextrin, either all in solution or some in the micelle shell or some in the micelle core, and were simultaneously fitted using the core-shell prolate ellipsoid form factor and the Hansen-Hayter-based structure factor. The scenario that considered a fraction of CD to localize in the micelle core well described the SANS data from both hydrogenated and deuterated SDS-CD-D2O solutions, while the other two scenarios did not. Among the various structural and interaction parameters obtained from this analysis, it emerged that the micelle core consisted of up to ∼10% HPβ-CD or ∼16% α-CD with respect to the total number of molecules (surfactants and CDs) present in the micelle at 25 mM SDS, and up to 14% HPβ-CD or 28% α-CD at 50 mM SDS. This is the first study that provides direct evidence on the location of cyclodextrin in the core of surfactant micelles. An improved understanding of CD interactions with surfactants and lipids would enable better strategies for drug encapsulation and delivery with CDs.

The formation of host-guest complexes between surfactants and cyclodextrins

Cyclodextrins are able to act as host molecules in supramolecular chemistry with applications ranging from pharmaceutics to detergency. Among guest molecules surfactants play an important role with both fundamental and practical applications. The formation of cyclodextrin/surfactant host-guest compounds leads to an increase in the critical micelle concentration and in the solubility of surfactants. The possibility of changing the balance between several intermolecular forces, and thus allowing the study of, e.g., dehydration and steric hindrance effects upon association, makes surfactants ideal guest molecules for fundamental studies. Therefore, these systems allow for obtaining a deep insight into the host-guest association mechanism. In this paper, we review the influence on the thermodynamic properties of CD-surfactant association by highlighting the effect of different surfactant architectures (single tail, double-tailed, gemini and bolaform), with special emphasis on cationic surfactants. This is complemented with an assessment of the most common analytical techniques used to follow the association process. The applied methods for computation of the association stoichiometry and stability constants are also reviewed and discussed; this is an important point since there are significant discrepancies and scattered data for similar systems in the literature. In general, the surfactant-cyclodextrin association is treated without reference to the kinetics of the process. However, there are several examples where the kinetics of the process can be investigated, in particular those where volumes of the CD cavity and surfactant (either the tail or in special cases the head group) are similar in magnitude. This will also be critically reviewed.

Modeling of multiple equilibria in the self-aggregation of di-n-decyldimethylammonium chloride/octaethylene glycol monododecyl ether/cyclodextrin ternary systems

The surface tension equations of binary surfactant mixtures (di-n-decyldimethylammonium chloride and octaethylene glycol monododecyl ether) are established by combining the Szyszkowski equation of surfactant solutions, the ideal or nonideal mixing theory, and the phase separation model. For surfactant mixtures, the surface tension at the air-water interface is calculated using nonideal theory due to synergism between the two adsorbed surfactant types. The incorporation of cyclodextrin complexation model to the surface tension equations gives a robust model for the description of the surface tension isotherms of binary, ternary, and more complex systems involving numerous inclusion complexes. The surface tension data obtained experimentally shows excellent agreement with the theoretical model below and above the formation of micelles. The strong synergistic effect observed between the two surfactants is disrupted by the presence of CDs, leading to ideal behavior of ternary systems. Indeed, depending on the nature of the cyclodextrin (i.e., α, β, or γ), which allows a tuning of the cavity size, the binding constants with the surfactants are modified as well as the surface properties due to strong modification of equilibria involved in the ternary mixture.

A spectral displacement study of cyclodextrin/naphthenic acids inclusion complexes

The spectral displacement technique has been used to obtain 1:1 β-cyclodextrin (β-CD)/carboxylate anion equilibrium binding constants (K2) for some complex mixtures of naphthenic acids (NAs) and some examples of single-component NAs in aqueous solution. Three specific examples of single-component NAs were chosen with variable Z values as follows: 2-hexyldecanoic acid (Z = 0; S1), trans-4-pentylcyclohexanecarboxylic acid (Z = –2; S2), and dicyclohexylacetic acid (Z = –4; S3). The estimated K2 values for S1, S2, and S3 are as follows: 1.42 × 103 M–1, 52.2 × 104 M–1, and 13.1 × 104 M–1, respectively. The corresponding K2 values are 2.34 × 104 M–1 and 1.27 × 104 M–1 for commercial (Fluka) and industrial (Syncrude) sourced NAs, respectively. The magnitude of K2 for 1:1 complexes formed between β-CD and S1, S2, or S3 did not correlate with the degree of hydrogen deficiency (Z-series) but there was a correlation with the size of the guest molecules (n) examined in this study. The correlation between complex stability and the relative size of the lipophilic fragments of the guest molecule are related to the importance of the hydrophobic effect for inclusion of such carboxylic acid guest molecules within β-CD.

A small molecular size system giving unexpected surface effects: alpha-Cyclodextrin + sodium dodecyl sulfate in water

Maximum drop volumes (MDV) and the resultant surface tension values (sigma) of alpha-cyclodextrin (alpha-CD) + sodium dodecyl sulfate (SDS) aqueous mixtures have been determined over a broad concentration range of both solutes at 283.15, 293.15, 303.15, 313.15, and 323.15 K. Drops significantly larger than those of pure water (up to approximately 25% larger) were observed at low temperatures for solutions with [alpha-CD]/[SDS] concentration ratios, approximately > 2, producing unexpectedly high surface tension values. Our results indicate that at certain solute concentration ratios and temperatures, the drop volume method provides wrong values for equilibrium surface tensions. This is due to the high viscoelasticity of the surface film whose effect is important even though the injection rate of the drops was slow and the solutes molecular sizes are small.

Inclusion complex of n-octyl beta-D-glucopyranoside and alpha-cyclodextrin in aqueous solutions: thermodynamic and structural characterization

Complex formation between octyl beta-D-glucopyranoside (OG) and alpha-cyclodextrin (alphaCD) was investigated on the basis of three highly accurate and appropriate experimental techniques. First, surface tension measurements showed that alphaCD directly acts on the surfactant monomers in the aqueous phase, leading to progressive depletion of the air-water interface with increasing cyclodextrin contents. Significant shift of OG critical micelle concentration (cmc) was consequently observed: the higher alphaCD concentration, the higher the cmc value. Experiments performed at surfactant and cyclodextrin concentrations in the Gibbs regime of surface tension versus OG content were performed on one hand to establish Job's plot that showed 1:1 stoichiometry of the OG-alphaCD complex and on the other hand to calculate the association constant found equal to (1.85 +/- 0.35) x 10(3) L mol(-1). An inclusion process of the surfactant alkyl residue within the cyclodextrin cavity was confirmed by one-dimensional (1)H NMR, and the structure of the mixed assembly was extensively characterized by two-dimensional NOESY (1)H NMR. OG penetrates alphaCD so that its hydrocarbon chain is embedded inside the cyclodextrin cavity, and its polar head as well as the alpha-methylene group emerges outside the alphaCD secondary face. Solubility behavior of the OG-alphaCD complex in a wide range of host-guest ratios and concentrations was finally examined by turbidity recording and optical microscopy. At very low free cyclodextrin levels in the solution, the complex presented high solubility behavior up to more than 70 mM. By increasing nonassociated alphaCD in the mixture, propensity of the cyclodextrin molecules to crystallize was observed at concentrations far below the 100 mM aqueous solubility of the pure cyclodextrin. The hexagonal shape of the crystals seen in the optical microscopy images suggested they were, partially at least, composed of the solid complex.

Advances in physical chemistry and pharmaceutical applications of cyclodextrins

Cyclodextrins (CDs) attract much attention for industrial applications and academic research. A few experimental methods for determination of the binding constant between CD and a guest molecule were reviewed critically. A hydrophile–hydrophobe matching model for host–guest docking was proposed for estimation of the binding constant and the solution structure of the complex. Rather detailed solution structures of CD complexes were determined by proton NMR spectroscopy, aided by calculations of molecular mechanics and surface areas, and were used to analyze the binding constants. The binding constants of CDs with multi-site guests were analyzed on the basis of their solution structures. The working mechanisms and physicochemical predictions in a few pharmaceutical applications of CDs were proposed on the basis of detailed solution structures and accurate binding constants.

Supramolecular interactions between β-cyclodextrin and hydrophobically end-capped poly(ethylene glycol)s: A quartz crystal microbalance study

In this study, the supramolecular interactions occurring between beta-cyclodextrin-based surfaces and macromolecular chains modified at one end with naphthyl, adamantyl, or phenyladamantyl hydrophobic groups were investigated by means of a quartz crystal microbalance. beta-Cyclodextrin-functionalized gold electrodes were obtained through the amide-coupling reaction between mono-6-deoxy-6-amino-beta-cyclodextrin and 11-mercaptoundecanoic acid self-assembled monolayer allowing the reproducible preparation of densely grafted surfaces with host properties. The interaction data obtained for the three different modified poly(ethylene glycol)s are in good agreement with our previous studies performed by high performance liquid chromatography and surface plasmon resonance. This evidences that the driving force for the supramolecular interaction is based on the inclusion of the hydrophobic terminal group of the chains within the cyclodextrin cavities. The reversibility of the inclusion process was proven through the regeneration of the original host properties of the sensing surfaces using sodium dodecylsulfate as a competitor for the desorption of the poly(ethylene glycol) chains.

Cyclodextrin-hydrophobe complexation in associative polymers

We develop a new rheology-based method to study the complexation of cyclodextrins with hydrophobes in hydrophobically modified associative polymer solutions. The associative polymers have comb-like structure with hydrophobic groups randomly attached to the polymer backbone. Intermolecular interactions between the hydrophobic groups form a transient network resulting in thickening of the polymer solutions. On addition of cyclodextrins (CD) to the solution, the hydrophobes are encapsulated within the hydrophobic cavity of the cyclodextrins. This reduces viscoelastic properties of the polymer solution by several orders of magnitude. We exploit the existence of a dynamic equilibrium between CD adsorbed to the hydrophobes and free CD in the solution, to develop a rheology-based Langmuir-type adsorption isotherm for estimating the binding constant for molecular complexation. The model is based on the assumption that the amount of CD adsorbed is proportional to the reduction in elastic modulus of the polymer solution due to the encapsulation of the network junctions by CD. The effects of temperature on binding constant are studied to estimate the enthalpy and entropy of complexation. Experiments are conducted with both α-and β-CD at different polymer concentrations and temperatures to estimate the relative strength of binding of the CDs. At a given temperature and a polymer concentration, α-CD has a lower binding constant compared to that of β-CD, indicating higher affinity of α-CD to adsorb onto the hydrophobes. Since α-CDs have a smaller ring size, they can snugly fit to the hydrophobes and the association leads to higher enthalpy and entropy change.

On the characterization of host-guest complexes: surface tension, calorimetry, and molecular dynamics of cyclodextrins with a non-ionic surfactant

Three host-guest systems have been characterized using surface tension (sigma), calorimetry, and molecular dynamics simulations (MD). The hosts were three native cyclodextrins (CD) and the guest the non-ionic carbohydrate surfactant octyl-beta-d-glucopyranoside. It is shown that, for any host-guest system, a rough screening of the most probable complex stoichiometries can be obtained in a model free form, using only calorimetric data. The sigma data were analyzed using a model that includes a newly proposed adsorption isotherm. The equilibrium constants for several stoichiometries were simultaneously obtained through fitting the sigma data. For alpha- and beta-CD, the predominant species is 1:1 and to a lesser extent 2:1, disregarding the existence of the 1:2. For gamma-CD, the 1:2 species dominates, the other two being also present. In an attempt to confirm these results, 10 ns MD simulations for each CD were performed using seven different starting conformations. The MD stable conformations agree with the results found from the experimental data. In one case, the spontaneous dissociation-formation of a complex was observed. Analysis of the trajectories indicates that hydrophobic interactions are primarily responsible for the formation and stability of the inclusion complexes. For the 2:1 species, intermolecular H-bonds between CD molecules result in a tight packed structure where their original truncated cone shape is lost in favor of a cylindrical geometry. Together, the results clearly demonstrate that the often used assumption of considering only a 1:1 species is inappropriate.

Chemometric studies of lysozyme upon interaction with sodium dodecyl sulfate and β-cyclodextrin

The interaction of hen egg-white lysozyme with sodium n-dodecyl sulfate (SDS) as an anionic surfactant was investigated by UV-vis spectrophotometry at different pHs at 25 degrees C using HCl/glycine and NaOH/glycine for acidic and basic pH ranges, respectively. Analysis of the spectral data using chemometric method gave the evidence for the existence of intermediate components during the cited interaction. Results also indicated a connection between turbidity of the protein solution upon interaction with SDS and distribution of our newly found intermediates. As intermediates are important in aggregation of proteins, beta-cyclodextrin was employed as an anti-aggregation agent and the results obtained for the lysozyme-SDS-beta-cyclodextrin ternary system were compared with those obtained in the absence of beta-cyclodextrin on distribution and mole fraction of intermediates with. It is also shown that as the distribution of intermediates broadens in a range of SDS concentrations, the turbidity and aggregation state of solution are reduced.

Inclusion Complexes between β-Cyclodextrin and a Gemini Surfactant in Aqueous Solution: An NMR Study

(1)H NMR spectra, diffusion-ordered NMR (DOSY), and 2D rotating-frame Overhauser enhancement spectroscopy (ROESY) experiments for aqueous solutions at 298 K containing the gemini surfactant, bis (dodecyl dimethylammonium)diethyl ether dibromide (12-EO(1)-12), in the absence and presence of beta-cyclodextrin (beta-CD) were used to characterize the surfactant and to determine the effects of the complexation in the micellization. For the binary system, the critical micelle concentration (cmc), the aggregation number, the stepwise micellization constant, and the size of the monomer have been obtained by studying the dependence of the chemical shifts and the self-diffusion coefficients with the concentration of surfactant. For the ternary system, the analysis of the (1)H NMR spectra and the self-diffusion coefficients reveal the formation of complexes of 1:1 and 2:1 stoichiometry (beta-CD:gemini), with a calculated stability constant for the second binding step higher than that of the first. The values of the hydrodynamic radii of the complexes were obtained from the calculated diffusion coefficients. The presence of beta-CD modifies the cmc in an extension that indicates mainly the formation of a 2:1 complex. The analysis of the chemical shifts of the surfactant indicates the nonparticipation of the complexes into the micelles. ROE enhancements depend substantially on the amount of the macrocycle added and therefore on the stoichiometry; at low concentrations of beta-CD, one of the hydrocarbon chains binds favorably with the cavity whereas the other interacts with the outer face. By contrast, at higher concentrations of beta-CD, the two hydrocarbon tails are included in two different macrocycles.

Study on the Supramolecular Multirecognition Mechanism of β-Naphthol/β-Cyclodextrin/Anionic Surfactant in a Tolnaftate Hydrolysis System

Based on the fact that tolnaftate degrade to beta-naphthol sodium (RONa) at 5.00 mol/L NaOH solution and RO(-) was protonated to ROH after being acidified and adjusted to the pH 4.50 by acetic acid-sodium acetate buffer solution, we studied and discussed the mechanism of the supramolecular multirecognition interaction among the anionic surfactants sodium lauryl sulfate (SLS), beta-cyclodextrin (beta-CD), and beta-naphthol (ROH) by means of fluorescence spectrum, surface tension of the solution, infrared spectrograms, and (1)HNMR spectroscopy. The apparent formation constant of the ternary inclusion complex was determined to be (5.48 +/- 0.13) x 10(3) L(2)/mol(2). The thermodynamic parameters (DeltaG degrees, DeltaH degrees, DeltaS degrees ) for the formation of the inclusion complexes were obtained from the van't Hoff equation. It was indicated that the multiple and synergistic protection effect of SLS and beta-CD on the excited singlet state ROH played very important roles in the enhancement of the fluorescence of ROH. Results showed that, at room temperature, the naphthalene ring of ROH and the hydrophobic hydrocarbon chain of SLS were included into the cavity of beta-CD to form a ROH/SLS/beta-CD ternary inclusion complex with stoichiometry of 1:1:1, which provided effective protection for the excited state of ROH and increased the fluorescent intensity of ROH obviously.

Molecular Motions of .ALPHA.-Cyclodextrin on a Dodecyl Chain Studied by Molecular Dynamics Simulations

Motions of an alpha-cyclodextrin (alpha-CD) molecule on a dodecyl chain adopting the all-trans conformation were investigated in the presence of water by molecular dynamics simulations with CVFF force fields, where the trimethylammonium group of dodecyltrimethylammonium bromide (DTAB) is protruded outside the secondary hydroxyl rim of alpha-CD (the secondary-in structure). The alpha-CD molecule shuttled rapidly on the chain without decomplexation. This rapid motion is consistent with the NMR data. The plane formed by 6 O4 atoms of alpha-CD is most populated between the C6 and C7 atoms of DTAB. This structure is very close to that estimated by NMR. The alpha-CD molecule underwent a restricted rotation in a range of 60 degrees with regard to the plane of the dodecyl chain: this plane at the most population is middle between the two diagonal lines of the normal hexagon formed by 6 O4 atoms of alpha-CD. The published NMR data were reanalyzed in terms of the rotation angle, and a slightly better structure was obtained. The distortion of the alpha-CD cavity from the normal hexagon was decreased upon complex formation with DTAB. The deviation of the center of alpha-CD from the center of the dodecyl chain predicted by molecular dynamics simulations is consistent with the NMR data. The secondary-in structure is energetically more stable than the primary-in structure, as calculated by molecular mechanics with CVFF and Amber force fields. This result is consistent with the NMR data. Molecular dynamics simulations were also carried out for the primary-in structure. Some of the results are close to those of the secondary-in structure.

Determination of the molecular complexation constant between alprostadil and alpha-cyclodextrin by conductometry: implications for a freeze-dried formulation

The binding constant between alprostadil (PGE1) and alpha-cyclodextrin (alpha-CD) was determined at four temperatures using conductance measurements. Alpha-cyclodextrin is an excipient material in Caverject dual chamber syringe (DCS) that was added to enhance stability. The binding constant was used to calculate the amount of PGE1 free upon reconstitution and injection, since only the free drug is clinically active. The conductivity measurement is based on a decrease in specific conductance as alprostadil is titrated with alpha-CD. The change in conductivity was plotted versus free ligand concentration (alpha-CD) to generate a binding curve. As the value of the binding constant proved to be dependent on substrate concentration, it is really a pseudo binding constant. A value of 742+/-60 M(-1) was obtained for a 0.5 mM solution of alprostadil at 27 degrees C and a value of 550+/-52 M(-1) at 37 degrees C. These results compare favorably to values previously obtained by NMR and capillary electrophoresis. Calculation of the fraction PGE1 free upon reconstitution and injection show it to approach the desired outcome of one. Hence, the amount of drug delivered by Caverject DCS is nominally equivalent to that delivered by Caverject S. Po., a predecessor product that contains no alpha-cyclodextrin.

Highly sensitive and selective room-temperature phosphorescence determination of thiabendazole by the supramolecular interaction of thiabendazole/β-cyclodextrin/triton X-100

A strong and stable room temperature phosphorescence (RTP) signal (lambda(ex)/lambda(em) = 298/481 nm) resulting from a 1:1:1 beta-cyclodextrin (beta-CD)/thiabendazole (TBZ)/triton X-100 (TX-100) supramolecular ternary inclusion complex was induced by KI as a heavy atom perturber. Based on the heavy-atom induced RTP, a new phosphorescence method for TBZ determination was established. The analytical curve of TBZ gave a linear range of 20-820 ng mL(-1) with a detection limit and relative standard deviation of 2.1 ng mL(-1) and 1.9%, respectively. The interference of 46 coexisting substances was studied. Compared with the method using a chemical oxygen scavenger, this method is simpler as deoxygenation of the solution is not required. The detection limit and the heavy-atom concentration of the proposed method were decreased about 8 and 4 times, respectively. The lifetime of the phosphorescence was prolonged 9 times and the pH range was greatly broadened. The proposed method has been successfully applied to the determination of TBZ in tap water, lake water and pineapples.