Complexation thermodynamics of α -cyclodextrin with ionic surfactants in water

The role of hydrophobic interactions in the molten globule state of globular protein modulated by surfactants

In order to highlight the role of hydrophobic interactions in the molten globule (MG) state of globular protein modulated by surfactants, the interactions of bovine α-lactalbumin (α-LA) with alkyl trimethylammonium bromides (CnTAB, n = 10, 12, 14, and 16) have been studied by experimental and theoretical techniques. Isothermal titration calorimetry (ITC) showed that the enthalpy changes (ΔH) and area of the enthalpogram increased with increasing the chain length of CnTAB. The result of fluorescence, circular dichroism (CD) and 1H nuclear magnetic resonance (NMR) spectrum suggested that C10TAB and C12TAB unfolded α-LA partially, C14TAB reconstructed protein with a native-like secondary structure content, and C16TAB induced an MG state α-LA. The SAXS results confirmed that the tertiary structure of α-LA was disrupted by C16TAB forming an MG state complex with a micelle-like structure even at the surfactants concentrations below CMC. As indicated by MD results, the β-domain and unstructured region(s) were involved in the MG state α-LA modulated by CnTAB. This work not only provides molecular insights into the role of hydrophobic interactions in the MG state of a globular protein but also helps understand the mechanism of preparing α-LA based biomacromolecule modulated by hydrophobic interactions.

High Interfacial Viscoelasticity of Aqueous Mixed Dodecyltrimethylammonium Bromide-Sodium Dodecyl Sulfate Surfactants Forming Inclusion Complexes with α-Cyclodextrin

Mixtures of anionic-cationic surfactants have shown high synergistic effects in the bulk solution and at the liquid/air interface. These studies have been limited to a reduced concentration range, where there is no formation of aggregates or precipitates. The addition of host molecules, such as cyclodextrins, to these systems reduces the effects of precipitation by forming inclusion complexes and also modifies the values of other surfactant properties, like the Krafft temperature and the critical aggregation concentration (CAC). We studied the interfacial synergistic effects promoted by electrostatic interactions, using the Rosen model to calculate an interaction parameter for mixtures of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in the presence of α-cyclodextrin (αCD), in aqueous solutions. We measured the CAC of SDS-DTAB-αCD mixtures using a pendant drop tensiometer, with the αCD concentration fixed at 10 mM and at 283.15 K. We performed rheological measurements on the mixtures where the surfactant total concentration is fixed below the measured CAC, varying the αCD concentration and temperature. We found that the dilatational modulus shows a clear correlation with the interaction parameter. It appears that the attractive interactions within the film are those due to the inclusion complexes formed by two αCD and one surfactant molecule, which according to the previous studies, is the dominant species in both the bulk and liquid/air interface. The synergistic effect observed here for SDS-DTAB surfactant mixtures with αCD can be applied to systems and processes (drop emission, drug delivery methods, stabilization of viral capsids and bacterial membranes, and emulsification) where interfacial processes require specific viscoelastic properties.

Dynamical shapes of droplets of cyclodextrin-surfactant solutions

We present a series of experiments with droplets of aqueous cyclodextrin-surfactant solutions, in which the volume is reduced after the equilibrium spherical shape is reached. The final shape of the drop after this perturbation is found to be dependent on the concentration of inclusion complexes in the bulk of the solution. These inclusion complexes are formed by two cyclodextrin molecules and one surfactat molecule. We propose a model to describe these dynamical processes. Dipole–dipole interactions on the surface of the drop trigger a competition between water surface tension and dipole–dipole interaction energies. The results of the model reproduce the spherical and rod-like shapes found in the experiments.

Dipole-dipole interactions control the interfacial rheological response of cyclodextrin/surfactant solutions

A recent surface rheological study has shown that aqueous solutions of α-cyclodextrin (αCD) with anionic surfactants (S) display a remarkable viscoelasticity at the liquid/air interface, which has not been observed in similar systems. The dilatational modulus is various orders of magnitude larger than those for the binary mixtures αCD + water and S + water. The rheological response has been qualitatively related to the bulk distribution of species, the 2 : 1 inclusion complexes (αCD2 : S) playing a fundamental role. In this work, we have developed a model that considers dipole-dipole interactions between 2 : 1 inclusion complexes ordered on the liquid/air interface. When the model is applied to the specific experimental conditions, the dependencies on concentration and temperature of the dilatational modulus and the surface tension were found to be in excellent agreement with the data, indicating clearly that dipole-dipole interactions determine and control the rheological behavior of the interface.

Cyclodextrin/surfactant inclusion complexes: An integrated view of their thermodynamic and structural properties

Cyclodextrins (CDs) play an important role in self-assembly systems of amphiphiles. The structure of CDs provides distinguished physicochemical properties, including the ability to form host-guest complexes. The complexation affects the properties of guest molecules and can produce supramolecular aggregates with desirable characteristics for fundamental and practical applications. Surfactants are particularly attractive host molecules due to their wide variety, availability, responsiveness to different stimuli, and high relevance in different fields, e.g. medical, cosmetic, pharmaceutical, and food industries. The tendency of organization in higher-order supramolecular aggregates arises the interest in applying such versatile complexes in the development of novel materials. In this review, we provide a comprehensive overview of the thermodynamics aspects of surfactants and CDs inclusion complexes formation in aqueous environment, emphasizing the assessment of the interactions, thermodynamic driving forces, and structural aspects. Also, the most common analytical techniques used to gather deep insight into the aspects of CDs complexes are discussed and the perspectives for the surfactant-cyclodextrin complexes are pointed out.

Contrasting Thermodynamics Governs the Interaction of 3-Hydroxyflavone with the N-Isoform and B-Isoform of Human Serum Albumin

Herein we report the interaction of 3-hydroxyflavone (3HF) with various isomeric forms of Human Serum Albumin (HSA), namely, the N-isoform (or native HSA at pH 7.4) and the B-isoform (at pH 9.2). Spectroscopic signatures of 3HF reveal that the interaction of 3HF with the N-isoform of HSA results in significant lowering of absorbance of the neutral species (λ_abs ∼ 345 nm) with concomitant increase of the anionic species (λ_abs ∼ 416 nm) whereas interaction with the B-isoform of HSA leads to selective enhancement of absorbance of the anionic species. The fluorescence profile of 3HF displays marked increase of intensity of the proton transferred tautomer (λ_em ∼ 538 nm) as well as the anionic species (λ_em ∼ 501 nm) for both the forms of the protein. However, analyses of the associated thermodynamics through temperature-dependent isothermal titration calorimetric (ITC) indicate that the interaction of 3HF with the N-isoform of HSA is more enthalpic in the lower temperature limit while the entropy contribution predominates in the higher temperature limit. Consequently, the 3HF-HSA (N-isoform at pH 7.4) interaction reveals an unusual thermodynamic signature of a positive heat capacity change (ΔC_p = 3.84 kJ mol^-1K^-1) suggesting the instrumental role of hydrophobic hydration. On the contrary, the 3HF-HSA (B-isoform at pH 9.2) interaction shows qualitatively reverse effect. Consequently, the interaction is found to be characterized by an enthalpy-dominated hydrophobic effect (negative heat capacity change, ΔC_p = -1.15 kJ mol^-1K^-1) which is rationalized on the basis of the nonclassical hydrophobic effect.

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.

Highly viscoelastic films at the water/air interface: α-Cyclodextrin with anionic surfactants

This work showcases the remarkable viscoelasticity of films consisting of α-cyclodextrin (α-CD) and anionic surfactants (S) at the water/air interface, the magnitude of which has not been observed in similar systems. The anionic surfactants employed are sodium salts of a homologous series of n-alkylsulfates (n = 8-14) and of dodecylsulfonate. Our hypothesis was that the very high viscoelasticity can be systematically related to the bulk and interfacial properties of the system. Through resolution of the bulk distribution of species using isothermal titration calorimetry, the high dilatational modulus is related to (α-CD)₂:S₁ inclusion complexes in the bulk with respect to both the bulk composition and temperature. Direct interfacial characterization of α-CD and sodium dodecylsulfate films at 283.15 K using ellipsometry and neutron reflectometry reveals that the most viscoelastic films consist of a highly ordered monolayer of 2:1 complexes with a minimum amount of any other component. The orientation of the complexes in the films and their driving force for adsorption are discussed in the context of results from molecular dynamics simulations. These findings open up clear potential for the design of new functional materials or molecular sensors based on films with specific mechanical, electrical, thermal, chemical, optical or even magnetic properties.

Association and sequestered dissociation of an anticancer drug from liposome membrane: Role of hydrophobic hydration

Herein, the interaction of a potent anticancer drug (Sanguinarine, SG) with dimyristoyl-l-α-phosphatidylglycerol (DMPG) liposome membrane has been investigated at physiological pH. The spectroscopic fluorescence decay results demonstrate a modification of the photophysics of SG within DMPG-encapsulated state leading to preferential stabilization of the iminium ion over the alkanolamine form. This suggests a key role of electrostatic force underlying the interaction. The complex dependence of the thermodynamic parameters on temperature yields a unique finding of a positive heat capacity change (ΔC_p) indicating the signature of hydrophobic hydration. The study also demonstrates the application of β-cyclodextrin (βCD) as a prospective host system resulting in release of the DMPG-bound drug. A calorimetric exploration of the DMPG-βCD interaction reveals an intrinsically complex thermodynamics of the process leading to ΔC_p > 0 and thus marking the instrumental role of hydrophobic hydration which follows that the DMPG-βCD interaction is accompanied with burial of polar molecular surfaces. A systematic investigation of the diffusion of the drug within various microheterogeneous environments by Fluorescence Correlation Spectroscopy (FCS) categorically reinforces our arguments.