Size of bile salt micelles: techniques, problems and results

Efficiency of Encapsulation of Thioflavin T (ThT) into Polar and Nonpolar Environments of Different Bile Salt Aggregates: A Femtosecond Fluorescence Study

The binding of Thioflavin T (ThT) with various bile salts, a potential host molecule, has been analyzed by steady-state and time-resolved fluorescence spectroscopy. A comparative study has been executed to investigate the influence of confinement of different bile salts, namely, sodium cholate (NaCh), sodium taurocholate (NaTC), and sodium deoxycholate (NaDC) on binding and excited state torsional motion of ThT molecules. The changes in absorption and emission properties of probe molecules were found to be sensitive to increasing bile salt concentration in aqueous 0.2 (M) NaCl solutions. The photophysics of ThT mainly depends on hydrophobicity, morphology, and size of bile salt aggregates in solution. In the presence of bile salts, the emission intensity and emission lifetime of ThT increase significantly, indicating encapsulation of dye. Moreover, we have also investigated the effect of the ionic strength of the medium by sodium chloride (NaCl) on the spectroscopic properties of ThT in the restricted surroundings of aqueous bile salts. It is observed that the fluorescence lifetime of ThT in bile salts increases significantly in the presence of NaCl. The encapsulation efficiency of ThT in bile salt aggregates has been assessed by iodide (I-) as an external ionic quencher. We found that NaDC aggregates are more efficient in the modulation of photophysical properties of ThT and also provide better protection efficiency to decrease the nonradiative deactivation processes.

Self-assembly of bile salts and their mixed aggregates as building blocks for smart aggregates

The present communication offers a comprehensive overview of the self-assembly of bile salts emphasizing their mixed smart aggregates with a variety of amphiphiles. Using an updated literature survey, we have explored the dissimilar interactions of bile salts with different types of surfactants, phospholipids, ionic liquids, drugs, and a variety of natural and synthetic polymers. While assembling this review, special attention was also provided to the potency of bile salts to alter the size/shape of aggregates formed by several amphiphiles to use these aggregates for solubility improvement of medicinally important compounds, active pharmaceutical ingredients, and also to develop their smart delivery vehicles. A fundamental understanding of bile salt mixed aggregates will enable the development of new strategies for improving the bioavailability of drugs solubilized in newly developed potential hosts and to formulate smart aggregates of desired morphology for specific targeted applications. It enriches our existing knowledge of the distinct interactions exerted in mixed systems of bile salts with variety of amphiphiles. By virtue of this, researchers can get innovative ideas to construct novel nanoaggregates from bile salts by incorporating various amphiphiles that serve as a building block for smart aggregates for their numerous industrial applications.

Solubilization and Interaction Studies of Bile Salts with Surfactants and Drugs: a Review

In this review, bile salt, bile salt-surfactant, and bile salt-drug interactions and their solubilization studies are mainly focused. Usefulness of bile salts in digestion, absorption, and excretion of various compounds and their rare properties in ordering the shape and size of the micelles owing to the presence of hydrophobic and hydrophilic faces are taken into consideration while compiling this review. Bile salts as potential bio-surfactants to solubilize drugs of interest are also highlighted. This review will give an insight into the selection of drugs in different applications as their properties get modified by interaction with bile salts, thus influencing their solution behavior which, in turn, modifies the phase-forming behavior, microemulsion, and clouding phenomenon, besides solubilization. Finally, their future perspectives are taken into consideration to assess their possible uses as bio-surfactants without side effects to human beings.

Partitioning of prototropic species of an anticancer drug ellipticine in bile salt aggregates of different head groups and hydrophobic skeletons: a photophysical study to probe bile salts as multisite drug carriers

The entrapment of neutral and cationic species of an anticancer drug, namely ellipticine and their dynamic features in different bile salt aggregates have been investigated for the first time using steady state and time-resolved fluorescence spectroscopy. Because ellipticine exists in various prototropic forms under physiological conditions, we performed comparative photophysical and dynamical studies on these prototropic species in different bile salts varying in their head groups and hydrophobic skeletons. We found that the initial interaction between ellipticine and bile salts is governed by the electrostatic forces where cationic ellipticine is anchored to the head groups of bile salts. Bile salts having conjugated head groups are better candidates to bind with the cationic species than those having the non-conjugated ones. The fact implies that binding of cationic species to different bile salts depends on the pK(a) of the corresponding bile acids. The hydrophobic interaction dominates at higher concentrations of bile salts due to formation of aggregates and results in entrapment of neutral ellipticine molecules according to their hydrophobicity indices. Thus bile salts act as multisite drug carriers. The rotational relaxation parameters of cationic ellipticine were found to be dependent on head groups and the number of hydroxyl groups on the hydrophilic surface of bile salts. Cationic ellipticine exhibits a faster rotational relaxation in the tri-hydroxy bile salt aggregates than in di-hydroxy bile salts. We interpreted this observation from the fact that tri-hydroxy bile salts hold a higher number of water molecules in their hydrophilic surface offering a less viscous environment for ellipticine compared to di-hydroxy bile salts. Surprisingly, the neutral ellipticine molecules display almost the same rotational relaxation in all the bile salts. The observation indicates that after intercalation inside the hydrophobic pocket, neutral ellipticine molecules experience similar confinement in all the bile salts.

Modulated photophysics of an anthracene-based fluorophore within bile-salt aggregates: the effect of the ionic strength of the medium on the aggregation behavior

Binding interactions of an anthracene-based fluorescent probe with a series of bile-salt aggregates of varying hydrophobicity, as well as salt induced alterations of the binding behavior have been thoroughly demonstrated.

Photochromism of a spiropyran and a diarylethene in bile salt aggregates in aqueous solution

Bile salt aggregates incorporate aqueous-insoluble photochromic compounds. The photochromism of a spiropyran (1, 1',3',3'-trimethyl-6-nitrospiro[2H-1]-benzopyran-2,2'-indoline) and a diarylethene derivative (2, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluoro-1-cyclopentene) was quantified in different bile salt aggregates. These aggregates act as efficient hosts to solubilize aqueous insoluble photochromic compounds where either both isomers are nonpolar, for example, 2, or compounds where one isomer is hydrophobic and the other is more polar, for example, 1. Methodology was developed to determine molar absorptivity coefficients for solutions containing both isomers and to determine the photoconversion quantum yields under continuous irradiation. The methods were validated by determining parameters in homogeneous solution, which were the same as previously reported. In the case of the colored isomer of 1, the molar extinction coefficient in ethanol at 537 nm ((3.68 ± 0.03) × 10(4) cm(-1) M(-1)) was determined with higher precision. The quantum yields for the photoconversion between the isomers of 2 were shown to be the same in cyclohexane and in the aggregates of sodium cholate (NaCh), deoxycholate (NaDC), and taurocholate (NaTC), showing that bile salt aggregates are not sufficiently rigid to affect the equilibrium between the two possible conformers of the colorless form. In contrast, for 1 the quantum yields for the conversion from the colorless to the colored isomer were higher in bile salts than in ethanol, and the quantum yield was highest in the more hydrophobic aggregates of NaDC, followed by NaCh and then NaTC. The structure of the bile salt had no effect on the quantum yield for the conversion of the colored to the colorless isomer of 1, but these values were higher than in ethanol. For all three bile salts, the absorption maximum for the colored form of 1 suggested that this isomer was located in an environment that is more polar than ethanol.

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Aggregation behavior of pegylated bile acid derivatives

Bile acids are amphiphilic endogenous steroids that act as anionic surfactants in the digestive tract and aggregate in aqueous solutions. Nonionic surfactants were synthesized by grafting poly(ethylene glycol) chains of various lengths (pegylation) to three bile acids (lithocholic, deoxycholic, and cholic acid) using anionic polymerization. The aggregation properties of the derivatives were studied with viscosity measurements and light scattering as well as with steady-state and time-resolved fluorescence techniques, and the aggregates were visualized by transmission electron microscopy to elucidate the effect of pegylation on the aggregation process. The fluorescence results showed a good correlation with the capacity of the bile acid derivatives to solubilize a hydrophobic drug molecule. The solubilization of ibuprofen depends on the length and the number of grafted PEG chains, and the solubilization efficiency increases with fewer PEG chains on the bile acid. The results indicate their potential for use in the design of new bile acid-based drug-delivery systems.

Effect of the structure of bile salt aggregates on the binding of aromatic guests and the accessibility of anions

The binding of naphthalene (Np), 1-ethylnaphthalene (EtNp), acenaphthene (AcN), and 1-naphthyl-1-ethanol (NpOH) as guests to the aggregates of sodium cholate (NaCh), taurocholate (NaTC), deoxycholate (NaDC), and deoxytaurocholate (NaTDC) was studied with the objective of determining how the structure of the bile salts affects the binding dynamics of guests and quenchers with the bile salt aggregates. Time-resolved and steady-state fluorescence experiments were used to determine the binding efficiency of the guests with the aggregates and were also employed to investigate the quenching of the singlet excited state of the guests by iodide anions. Quenching studies of the triplet excited states using laser flash photolysis were employed to determine the accessibility to the aggregate of nitrite anions, used as quenchers, and the dissociation rate constants of the guests from the bile salt aggregates. The binding efficiency of the guests to NaDC and NaTDC is higher than for NaCh and NaTC, and the protection efficiency is also higher for NaDC and NaTDC, in line with the larger aggregates formed for the latter bile salts. The formation of aggregates is in part driven by the structure of the guest, where an increased protection efficiency and residence time can be achieved by the introduction of short alkyl substituents (AcN or EtNp vs Np). NpOH was shown to be located in a very different environment in all four bile salts when compared to AcN, EtNp, and Np, suggesting that hydrogen bonding plays an important role in the formation of the aggregate around NpOH.

Effect of the guest size and shape on its binding dynamics with sodium cholate aggregates

The binding dynamics of the guests acenaphthene, phenanthrene, fluorene, and acenaphthenol with sodium cholate aggregates were studied using laser flash photolysis and fluorescence. The location of the guests in the bile salt aggregate is determined by the guest's hydrophobicity, where acenaphthene, phenanthrene, and fluorene bind to the primary aggregates, while acenaphthenol binds to the secondary bile salt aggregates. The residence time of the guests in the primary aggregates and the access of ionic species from the aqueous phase to the guest in the aggregate depend on the size and the shape of the guest. These results show that bile salt aggregates are adaptable supramolecular host systems.

Effect of Solvent Polarity and Viscosity on the Guest Binding Dynamics with Bile Salt Aggregates†

Bile salts form supramolecular aggregates with two binding sites with different properties. The guest binding dynamics to the aggregates and guest protection from species in the aqueous phase were investigated using fluorescence and laser flash photolysis experiments. Sodium cholate, deoxycholate and taurodeoxycholate were used as bile salts and acetonitrile or ethylene glycol were added as co-solvents to water in order to alter the binding properties of 1-ethylnaphthalene and 1-naphthyl-1-ethanol with the aggregates. The binding dynamics are faster and protection efficiencies are lower for guests bound to cholate and in the presence of either co-solvent.

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.

Influence of Planarity and Size on Guest Binding with Sodium Cholate Aggregates

Bile salt aggregates are supramolecular structures with two types of binding sites, called primary and secondary sites. The objective of this work was to explore how the nonplanarity and size of guests (biphenyl [BP], 1-1'-binaphthyl [BNP] and dibenz[b,f]oxepin [DBX]) affected their binding affinity and dynamics to sodium cholate (NaC) aggregates. Fluorescence and laser-flash photolysis experiments were performed to obtain information on the binding environment for the guests, the accessibility of quenchers to guests in the aggregate and the dissociation rate constants of the guests from the aggregates. All guests were bound to the more hydrophobic primary aggregate, showing that this site can accommodate nonplanar molecules. However, the structure of the guest affects the structure of the primary aggregates, leading to changes in the accessibility of anions to aggregate-bound guests and to changes for the guest dissociation rate constants from the aggregates.

Probing the binding dynamics to sodium cholate aggregates using naphthalene derivatives as guests

The binding dynamics with bile salt aggregates for a series of naphthalene derivatives of different polarities was studied using fluorescence and laser flash photolysis. Fluorescence was employed to determine the nature of the binding site for each guest and the accessibility of the bound guest to quenchers. Laser flash photolysis was employed to study the mobility of the triplet states of the naphthalenes between the sodium cholate aggregates and the aqueous phase. Primary aggregates, which provide an environment protected from quenchers in the aqueous phase, bind 1- and 2-ethylnaphthalene as guests. The complexation dynamics with this type of aggregate is slow. 1- and 2-Naphthyl-1-ethanol, and 1- and 2-acetonaphthone bind to the secondary aggregates, which provide moderate protection from quenching and faster binding dynamics. The addition of salts lowered the cholate concentration at which primary aggregates were formed, but did not influence the formation of secondary aggregates.

Modulation with acetonitrile of the dynamics of guest binding to the two distinct binding sites of cholate aggregates

Bile salt aggregates are supramolecular systems containing two different binding sites. The effect of the addition of acetonitrile on the specificity and dynamics of guest binding to the two binding sites of cholate aggregates was studied. The protection of guests included in the aggregate from interaction with ions in the aqueous phase was evaluated from quenching of the singlet and triplet excited states of guest molecules bound to the cholate aggregates. The dynamics of guest binding to the primary and secondary binding sites of the cholate aggregates were determined at increasing acetonitrile mole fractions. The structure of the aggregates was not significantly altered provided the cholate concentrations were higher than 20 mM and the acetonitrile mole fraction did not exceed 0.033 (9.1% v/v). These results show that acetonitrile can be used to modulate the solubility of guests in the aggregates and to manipulate the residence time of guests in the primary and secondary binding sites.

Effect of Sublytic Concentrations of Sodium Cholate on Phospholipase C Hydrolysis of Phospholipid Bilayers

Phospholipase C activity has been assayed with phosphatidylcholine as substrate in the presence of sodium cholate at concentrations well below those producing lipid solubilization. With short-chain phosphatidylcholine, which exists in monomeric form in aqueous solution, cholate has little or no effect. However, when the substrate is egg phosphatidylcholine in the form of bilayers, small cholate concentrations (below 1 mM, corresponding to an effective surfactant:lipid ratio below 0.05) increase the maximum enzyme rates by about threefold, while decreasing drastically the latency periods of enzyme activity. Previous studies from this laboratory have associated the phospholipase enhancing activity of a variety of amphiphiles to their ability to facilitate the formation of inverted hexagonal phospholipid structures, yet sodium cholate has the opposite effect, stabilizing the lamellar versus the inverted hexagonal phase. This suggests that cholate is activating phospholipase C through a hitherto undescribed mechanism. Sodium cholate concentrations above 1 mM decrease further the enzyme lag time, but they are less effective in enhancing enzyme rates. These observations may be pertinent in the analysis of biochemical data with purified lipases, as well as in physiological studies of biliary function. Copyright 1999 Academic Press.

Separation of chiral compounds by capillary electrophoresis

This review presents the different chiral selectors used in capillary electrophoresis (CE) for the separation of enantiomers. The use of charged cyclodextrins, crown ethers, polysaccharides, proteins, natural and synthetic micelles, macrocyclic antibiotics and ergot alkaloids is discussed in detail. Neutral native and derivatized cyclodextrins are not treated because several review articles have already been published on this topic. Recent developments like the application of two chiral selectors in the same background electrolyte are highlighted.

Thermodynamic Studies of the Adsorbed Films and Micelles of Sodium Taurodeoxycholate

Surface tension of aqueous solutions was measured for sodium taurodeoxycholate, as a typical example of the bile salt compounds, in the temperature range 20 to 35 degreesC at 2.5 degreesC intervals and concentration range 0 to 7 mmol kg-1. We examined thermodynamic quantities obtainable from the surface tension measurements according to the thermodynamic relations given by K. Motomura [J. Colloid Interface Sci. 64, 348 (1978)]. Sodium taurodeoxycholate was strongly adsorbed and formed the saturated adsorbed film at low concentrations. However, the gaseous/expanded phase transition does not take place in the film. The thermodynamic quantities associated with adsorption did not change as markedly at the critical micelle concentration as those observed for typical surfactants. It was suggested that molecular interactions between sodium taurodeoxycholate molecules in aqueous solutions and adsorbed films are too weak to induce critical changes in the thermodynamic quantities. Copyright 1997 Academic Press.

Effects of bile salt structure on chiral separations with mixed micelles of bile salts and polyoxyethylene ethers using micellar electrokinetic capillary chromatography

The chiral resolving abilities of micellar solutions of four different bile salts alone and in mixtures with polyoxyethylene-4-dodecyl ether (C12E4) and methanol were investigated using MECC. The four bile salts investigated were the unconjugated sodium salts of cholic, deoxycholic, chenodeoxycholic and ursodeoxycholic acids. The test solutes included verapamil, norverapamil, gallopamil, bi-2-naphthol, atenolol and BAYK8644. The relative hydrophobicities of the micellar aggregates formed in solutions of binary mixtures of each bile salt with C12E4 were investigated by fluorescence spectroscopy using pyrene as a probe molecule. The observed enantiomeric resolution for the test compounds using these binary mixtures as MECC pseudo-stationary phases was determined. Correlations between micellar hydrophobicity for these solutions and chiral resolution of these test solutes are presented. The addition of C12E4 with or without methanol to solutions of sodium cholate and sodium deoxycholate enhanced the chiral resolution observed for compounds containing a longer hydrocarbon chain separating some of the major functional groups from the chiral center. The pure bile salt solutions generally provided better chiral resolution for the compounds where the major functional groups, such as aromatic rings, were closer to the chiral center.

Gel filtration chromatographic study on the self-association of surfactants and related compounds

After a brief survey of gel filtration chromatography (GFC) and the self-association equilibrium, it is shown that GFC is a unique tool for investigating the self-association of many substances, such as surfactants, chlorpromazine hydrochloride (CPZ), Methylene Blue (MB) and a sulfobetaine derivative (CHAPS) of cholic acid. The hydrodynamic radius of an aggregate can be estimated with the gel of large pore size, such as Sephadex G-200. This method is utilized for the study of the sphere-rod transition of surfactant micelles. A precise value of monomer concentration can be determined from the centroid elution volume of a frontal chromatogram on Sephadex G-10. This important parameter in self-associating systems allows us to determine the micellar aggregation number as a function of the total surfactant concentration. The derivative chromatogram can be used to detect slight changes in self-association. From these results, it is found that nonionic surfactants form premicelles including dimer and that multiple equilibrium model for micelle formation is more appropriate than mass-action model. The Tanford theory with some modifications appears to fit the concentration dependence of the aggregation properties of surfactants forming small globular micelles and allows us to estimate the stepwise aggregation constant and the micelle size distribution function. The pronounced cooperativity in the self-association of surfactants forms the basis of mass action model and the concept of the cmc, though they are actually approximations even for surfactants. They would be more inadequate, though often used, for the self-association of drugs and bile salts. The stepwise aggregation constant of MB is almost independent of aggregation numbers, which may be expected from the stacking mode of aggregation. The main aggregates of CPZ in 154 mM sodium chloride solution are dimer and 38-mer and its dimerization constant is smaller than that of MB and larger than those of surfactants. The stepwise aggregation constant of CHAPS has the maximum at hexamer and oscillates, depending on odd or even aggregation numbers. Thus, the aggregation pattern of a substance is closely related with the chemical structure of the hydrophobic group of its molecule. The definition of cmc is discussed together with its significance.