Interactions and Molecular Weights of Simple Micelles and Mixed Micelles in Taurocholate and Taurocholate−Lecithin Solutions

Volumetric, Compressibility and Viscometric Approach to Study the Interactional Behaviour of Sodium Cholate and Sodium Deoxycholate in Aqueous Glycyl Glycine

Viscosity, speed of sound (u), and density (ρ) have been measured in aqueous glycyl glycine solution over a temperature range from 293.15 to 313.15 K with a 5 K interlude to evaluate the volumetric and compressibility properties of bio-surfactants, namely sodium cholate (NaC; 1-20 mmol∙kg^-1) and sodium deoxycholate (NaDC; 1-10 mmol∙kg^-1). Density and viscosity findings provide information on both solute-solute and solute-solvent types of interactions. Many other metrics, such as apparent molar adiabatic compression (κS,φ), isentropic compressibility (κS), and apparent molar volume (Vφ), have been calculated from speed of sound and density measurements, utilising experimental data. The results show that the zwitterionic end group in the glycyl glycine strongly interacts with NaDC and NaC, promoting its micellization. Since the addition of glycyl glycine causes the bio-surfactant molecules to lose their hydrophobic hydration, the observed concentration-dependent changes in apparent molar volume and apparent molar adiabatic compression are likely attributable to changes in water-water interactions. Viscous relaxation time (τ) increases significantly with a rise in bio-surfactant concentration and decreases with increasing temperature, which may be because of structural relaxation processes resulting from molecular rearrangement. All of the estimated parameters have been analysed for their trends with regard to the different patterns of intermolecular interaction present in an aqueous glycyl glycine solution and bio-surfactant system.

The mechanism of solifenacin release from a pH-responsive ion-complex oral suspension in the fasted upper gastrointestinal lumen

The main objective of this study was to investigate the mechanism of solifenacin release from a pH-responsive ion-complex oral resinate suspension under conditions simulating the environment in the upper gastrointestinal lumen. A secondary objective was to propose an appropriate in vitro methodology for evaluating the quality of orally administered solifenacin suspensions. The mechanism of solifenacin release from polacrilin potassium resin (Amberlite® IRP88) was investigated using biorelevant media and compendial setups (USP Apparatus 2 and USP Apparatus 4) and using newer, recently validated in vitro methodologies [biorelevant gastrointestinal transfer (BioGIT) system]. We evaluated the impact of particle size and concentration of the resin; thickener concentration (carbomer homopolymer, type B); and the impact of pH, cationic strength, agitation intensity and level of simulation of contents in the upper gastrointestinal lumen. Data suggested that solifenacin release from the resinate was determined by the resin particle size, the medium pH, cationic strength (when the conditions in the upper small intestine are simulated) and the level of simulation of contents in the upper small intestine. The interaction of solifenacin with taurocholic acid/lecithin aggregates was significant, but unlikely to affect the degree of solifenacin absorption, as a BCS Class I compound. Under acidic conditions, solifenacin was dissociated and released from the pH-responsive resin rapidly. Under conditions simulating the contents of the upper small intestine, solifenacin was replaced by cations from the testing media and diffused through the resin matrix. All three in vitro systems with or without a pH gradient are useful in distinguishing solifenacin release characteristics from resinate suspensions with different particle sizes. Because of this drug release mechanism, USP Apparatus 2 with fixed pH media demonstrated equivalent or slightly higher discriminative sensitivity than the other setups and appears to be appropriate for product quality control.

A Model Prediction for Chenodeoxycholate Aggregate Formation

A relationship between the chenodeoxycholate (CDC) monomer concentration and the total concentration of CDC was established using a kinetic dialysis technique. Meanwhile, the sizes of the formed simple CDC micelles were measured by a quasielastic light-scattering (QLS) technique to be nearly constant. The QLS results led to a suggestion for equilibrium models of CDC aggregate formation. According to the established relationship and the suggested models, the best curve-fitting model was selected by a least-squares technique. Furthermore, the model parameters were quantified. Based on the quantified parameters, at a minimum detectable concentration of simple CDC micelles to be ∼0.2 mM, an appropriate model corresponding concentration of CDC monomers was estimated to be ∼3.08 mM. This value is consistent with a minimum monomer CDC concentration of ∼3.13 mM for simple CDC micelle formation estimated according to the present QLS detection and the model prediction. The consistency confirms the model prediction that at a low CDC monomer concentration (<3 mM), the concentration of stable CDC dimers is much higher than that of simple CDC micelles but the contribution of simple CDC micelles to the total CDC concentration cannot be negligible.

Steroidal Surfactants: Detection of Premicellar Aggregation, Secondary Aggregation Changes in Micelles, and Hosting of a Highly Charged Negative Substance

CHAPSO and CHAPS are zwitterionic surfactants derived from bile salts which are usually employed in protein purification and for the preparation of liposomes and bicelles. Despite their spread use, there are significant discrepancies on the critical concentrations that determine their aggregation behavior. In this work, we study the interaction between these surfactants with the negative fluorescent dye pyranine (HPTS) by absorbance, fluorescence, and infrared spectrometry to establish their concentration-dependent aggregation. For the studied surfactants, we detect three critical concentrations showing their concentration-dependent presence as a monomeric form, premicellar aggregates, micelles, and a second type of micelle in aqueous medium. The nature of the interaction of HPTS with the surfactants was studied using analogues of their tails and the negative bile salt taurocholate (TC) as reference for the sterol ring. The results indicate that the chemical groups involved are the hydroxyl groups of the polar face of the sterol ring and the sulfonate groups of the dye. This interaction causes not only the incorporation of the negative dye in CHAPSO and CHAPS micelles but also its association with their premicellar aggregates. Surprisingly, this hosting behavior for a negative charged molecule was also detected for the negative bile salt TC, bypassing, in this way, the electrostatic repulsion between the guest and the host.

Micelle formation of bile salts and zwitterionic derivative as studied by two-dimensional NMR spectroscopy

The self-association of sodium taurodeoxycholate (NaTDC) and a zwitterionic derivative of cholic acid (CHAPS) in deuterium oxide was investigated by one- and two-dimensional nuclear magnetic resonance spectroscopy (NMR) spectroscopy. Analysis of the concentration dependence of the chemical shifts of several protons suggested that NaTDC and CHAPS form nonamers and heptamers, respectively, as well as dimer. The equilibrium constants of dimerization and the micellar aggregation numbers are close to the literature values. From the intensities of intermolecular cross-peaks in the nuclear Overhauser effect spectroscopy (NOESY) and rotating frame nuclear Overhauser effect spectroscopy (ROESY) spectra of NaTDC and CHAPS micellar solutions, partial structures of their micelles were estimated. The CHAPS micelle consists mainly of the back-to-back association, similarly to taurocholate (NaTC). However, the NaTDC micelle consists of the back-to-face association, because the face of NaTDC is rather hydrophobic. Furthermore, the back of bile molecules forms a convex plane and the face forms a concave plane. The back-to-face structure of NaTDC will be stabilized by a close contact between these planes. The chemical shift changes of several protons of CHAPS and NaTC in the micellar state are close to each other, but are different from those of NaTDC. This finding is consistent with the difference in their micellar structures.

Thermodynamics of demicellization of mixed micelles composed of sodium oleate and bile salts

Isothermal titration calorimetry (ITC) was used to determine the critical micelle concentration (cmc) and the thermodynamic parameters associated with the demicellization of sodium oleate (NaO) and mixed micelles composed of the bile salt (BS) sodium cholate (NaC) or sodium deoxycholate (NaDC), respectively, and NaO at a molar ratio of 5:2. The influence of the ionic strength (pure water and 0.1 M NaCl at pH 7.5) as well as that of the temperature (10-70 degrees C) were analyzed. For NaO, two cmc's were detected, indicating a two-step aggregation process, whereas only one cmc was observed for the two BSs. A single aggregation mechanism is also evident for the demicellization of mixed micelles (BS/NaO 5:2). Increasing the ionic strength induces the well-known decrease of the cmc. The cmc shows a minimum at room temperature. The cmc(mix) of the mixed micelles was analyzed using models assuming an ideal or nonideal mixing behavior of both detergents. The thermodynamic parameters describing the enthalpy (deltaHdemic), entropy (deltaSdemic), and Gibbs energy change (deltaGdemic), as well as the change in heat capacity (deltaCp,demic) for demicellization, were obtained from one ITC experiment. From the temperature dependence of deltaHdemic, the change of the hydrophobic surface area of the detergents from the micellar into the aqueous phase was derived. In all cases, the deltaCp,demic values are positive. In addition, the temperature dependence of the size of the formed aggregates was studied by dynamic light scattering (DLS). DLS indicated two populations of aggregates in the mixed system, small primary micelles (0.5-2 nm), and larger aggregates with a hydrodynamic radius in the range of 50-150 nm.