Molecular Aggregates in Aqueous Solutions of Bile Acid Salts. Molecular Dynamics Simulation Study

Molecular Dynamics Simulation of the Aggregation Patterns in Aqueous Solutions of Bile Salts at Physiological Conditions

Classical molecular dynamics simulations are employed to monitor the aggregation behavior of six bile salts (nonconjugated and glycine- and taurine-conjugated sodium cholate and sodium deoxycholate) with concentration of 10 mM in aqueous solution in the presence of 120 mM NaCl. There are 150 ns trajectories generated to characterize the systems. The largest stable aggregates are analyzed to determine their shape, size, and stabilizing forces. It is found that the aggregation is a hierarchical process and that its kinetics depends both on the number of hydroxyl groups in the steroid part of the molecules and on the type of conjugation. The micelles of all salts are similar in shape-deformed spheres or ellipsoids, which are stabilized by hydrophobic forces, acting between the steroid rings. The differences in the aggregation kinetics of the various conjugates are rationalized by the affinity for hydrogen bond formation for the glycine-modified salts or by the longer time needed to achieve optimum packing for the tauro derivatives. Evidence is provided for the hypothesis from the literature that the entirely hydrophobic core of all aggregates and the enhanced dynamics of the molecules therein should be among the prerequisites for their pronounced solubilization capacity for hydrophobic substances in vivo.

Chemically Locked Bicelles with High Thermal and Kinetic Stability

In situ polymerization of a bicellar mixture composed of a phospholipid and polymerizable surfactants afforded unprecedented stable bicelles. The polymerized composite showed an aligned phase over a wide thermal range (25 to >90 °C) with excellent (2)H quadrupole splitting of the solvent signal, thus implying versatility as an alignment medium for NMR studies. Crosslinking of the surfactants also brought favorable effects on the kinetic stability and alignment morphology of the bicelles. This system could thus offer a new class of scaffolds for biomembrane models.

Fluorescence properties of Schiff base - N,N'-bis(salicylidene) - 1,2-Phenylenediamine in presence of bile acid host

Fluorescence properties of Schiff base - N,N'-bis(salicylidene) - 1,2-phenylenediamine (LH2) is used to study the micelles formed by aggregation of different important bile acids like cholic acid, deoxycholic acid, chenodeoxycholic acid and glycocholic acid by steady state and picosecond time-resolved fluorescence spectroscopy. The fluorescence band intensity was found out to increase with concomitant red shift with gradual addition of different bile acids. Binding constant of the probe with different bile acids as well as critical micelle concentration was obtained from the variation of fluorescence intensity on increasing concentration of bile acids in the medium. The increase in fluorescence quantum yields, fluorescence decay times and substantial decrease in nonradiative decay rate constants in bile acids micellar environment points to the restricted motion of the fluorophore inside the micellar subdomains.

Pyrite in contact with supercritical water: the desolation of steam

The supercritical water and pyrite interface has been studied by DFT calculations. A surprisingly dry surface has been found which points to a new reactivity under extreme conditions which has relevance in the iron–sulfur world prebiotic chemistry of the early Earth.

Thermal stabilization of bicelles by a bile-salt-derived detergent: a combined ³¹P and ²H nuclear magnetic resonance study

The properties of bicelles composed of mixtures of long-chain lipids dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG), stabilized by zwitterionic bile salt analogue 3-[(3-cholamidopropyl)dimethyl-d6-ammonio]-2-hydroxy-1-propanesulfonate (CHAPSO-d6), deuterated at both amino methyls, were investigated by a combination of (31)P and (2)H NMR, focusing on the behavior of CHAPSO as a function of temperature. For compositions of molar ratio q = [DMPC + DMPG]/[CHAPSO] = 3, R = [DMPG]/[DMPC + DMPG] = 0, 0.01 and 0.10 and lipid concentration CL = 25 wt % lipid at temperatures of between 30 and 60 °C, magnetic alignment was readily achieved as assessed via both (31)P NMR of the phospholipids and (2)H NMR of CHAPSO-d6. Increasing temperature yielded higher values for the chemical shift anisotropy of the former and the quadrupole splitting of the latter, consistent with the progressive migration of CHAPSO from edge regions into planar regions of the bicellar assemblies. However, relative to dihexadecyl phosphatidylcholine (DHPC), CHAPSO exhibited lower miscibility with DMPC, although the presence of DMPG enhanced this miscibility. At 65 °C, thermal instability became evident in the appearance of a separate isotropic component in both (31)P and (2)H NMR spectra. This isotropic phase was CHAPSO-enriched but less so as a function of increasing DMPG. These findings indicate that the enhanced thermal stability of CHAPSO- versus DHPC-containing bicelles arises from a combination of the larger surface area that edge CHAPSO is able to mask, mole for mole, and its relative preference for edge regions, plus, possibly, specific interactions with DMPG.

Mixed micelles of sodium cholate and sodium dodecylsulphate 1:1 binary mixture at different temperatures--experimental and theoretical investigations

Micellisation process for sodium dodecyl sulphate and sodium cholate in 1∶1 molar ratio was investigated in a combined approach, including several experimental methods and coarse grained molecular dynamics simulation. The critical micelle concentration (cmc) of mixed micelle was determined by spectrofluorimetric and surface tension measurements in the temperature range of 0–50°C and the values obtained agreed with each other within the statistical error of the measurements. In range of 0–25°C the cmc values obtained are temperature independent while cmc values were increased at higher temperature, which can be explained by the intensive motion of the monomers due to increased temperature. The evidence of existing synergistic effect among different constituent units of the micelle is indicated clearly by the interaction parameter (β_1,2) calculated from cmc values according to Rubingh. As the results of the conductivity measurements showed the negative surface charges of the SDS-NaCA micelle are not neutralized by counterions. Applying a 10 µs long coarse-grained molecular dynamics simulation for system including 30-30 SDS and CA (with appropriate number of Na⁺ cations and water molecules) we obtained semi-quantitative agreement with the experimental results. Spontaneous aggregation of the surfactant molecules was obtained and the key steps of the micelle formation are identified: First a stable SDS core was formed and thereafter due to the entering CA molecules the size of the micelle increased and the SDS content decreased. In addition the size distribution and composition as well as the shape and structure of micelles are also discussed.

Room temperature phosphorescence of 9-bromophenanthrene, and the interaction with various metal ions

The Flow Inject Drop Luminescence Sensor (FIDLS) is featured by advantages such as high precision and work simplification over typical luminescent spectrometers. With the FIDLS system, this study is the first to examine the sodium deoxycholate (NaDC) inducing room temperature phosphorescence (RTP) of 9-bromophenanthrene (BrP). Among the factors that influenced phosphorescence, the injection speed of the FIDLS was optimized at 5.0 mL h(-1). A solvent content of 1.0% or less was selected to avoid RTP quenching. When samples were placed at temperatures higher than room temperature (e.g., 303 K), the standing time of the sample decreased. The minimum detectable level of BrP was 8.0 × 10(-10) mol L(-1), and BrP RTP reached its maximum intensity at a BrP concentration of 1.0 × 10(-5) mol L(-1). The optimal clathrate concentration of NaDC was 4.9 × 10(-3) mol L(-1), which was also the critical micelle concentration. We found that NaDC primary micelles gradually formed, but that secondary micelles formed and decomposed at a considerably faster rate. Fluorescence and absorbance tests demonstrated the coordination reactions of BrP with Cr(6+) and Fe(3+), indicating the potential application of BrP as a fluorescence probe.

Modulation of the photophysical properties of 2,2'-bipyridine-3,3'-diol inside bile salt aggregates: a fluorescence-based study for the molecular recognition of bile salts

2,2'-Bipyridine-3,3'-diol (BP(OH)(2)) has been used as a sensitive excited-state intramolecular proton transfer fluorophore to assess different bile salt aggregates as one of the potential biologically relevant host systems useful for carrying many sparingly water-soluble drug molecules. The formation of inclusion complexes, complex-induced fluorescence behavior, and their binding ability have been investigated from the modulated photophysics of BP(OH)(2) by means of photophysical techniques. The constrained hydrophobic environment provided by the aggregates significantly reduces the water-assisted nonradiative decay channels and lengthens the fluorescence lifetime of the proton-transferred DK tautomer. Both the absorption and fluorescence properties of BP(OH)(2) are found to be sensitive to the change in the structure, size, and hydrophobicity of the aggregates. Fluorescence quenching experiments were performed to gain insight into the differential distribution of the probe molecules between bulk aqueous phase and nanocavities of various aggregates. The observation of longer fluorescence lifetime and rotational relaxation time in NaDC aggregates compared to that in NaCh and NaTC aggregates indicates that the binding structures of NaDC aggregates are more rigid due to its greater hydrophobicity and larger size and therefore provide better protection to the bound guest. It is noteworthy to mention that the hydrophobic microenvironments provided by bile salt aggregates are much stronger than that provided by micelles and cyclodextrins. The accessibility of water to the aggregate-bound guest can significantly be enhanced with the addition of organic cosolvents. However, the efficiency decreases in the order of dimethylformamide, acetonitrile, and methanol.

Aggregation behaviors of PEO-PPO-ph-PPO-PEO and PPO-PEO-ph-PEO-PPO at an air/water interface: experimental study and molecular dynamics simulation

The block polyethers PEO-PPO-ph-PPO-PEO (BPE) and PPO-PEO-ph-PEO-PPO (BEP) are synthesized by anionic polymerization using bisphenol A as initiator. Compared with Pluronic P123, the aggregation behaviors of BPE and BEP at an air/water interface are investigated by the surface tension and dilational viscoelasticity. The molecular construction can influence the efficiency and effectiveness of block polyethers in decreasing surface tension. BPE has the most efficient ability to decrease surface tension of water among the three block polyethers. The maximum surface excess concentration (Γ(max)) of BPE is larger than that of BEP or P123. Moreover, the dilational modulus of BPE is almost the same as that of P123, but much larger than that of BEP. The molecular dynamics simulation provides the conformational variations of block polyethers at the air/water interface.

Aggregation behavior of ibuprofen, cholic acid and dodecylphosphocholine micelles

Non-steroidal anti-inflammatory drugs (NSAIDs) are frequently used to treat chronic pain and inflammation. However, prolonged use of NSAIDs has been known to result in Gastrointestinal (GI) ulceration/bleeding, with a bile-mediated mechanism underlying their toxicity to the lower gut. Bile acids (BAs) and phosphatidylcholines (PCs), the major components of bile, form mixed micelles to reduce the membrane disruptive actions of monomeric BAs and simple BA micelles. NSAIDs are suspected to alter the BA/PC balance in the bile, but the molecular interactions of NSAID-BA or NSAID-BA-PC remain undetermined. In this work, we used a series of all-atom molecular dynamics simulations of cholic acid (CA), ibuprofen (IBU) and dodecylphosphocholine (DPC) mixtures to study the spontaneous aggregation of CA and IBU as well as their adsorption on a DPC micelle. We found that the size of CA-IBU mixed micelles varies with their molar ratio in a non-linear manner, and that micelles of different sizes adopt similar shapes but differ in composition and internal interactions. These observations are supported by NMR chemical shift changes, NMR ROESY crosspeaks between IBU and CA, and dynamic light scattering experiments. Smaller CA-IBU aggregates were formed in the presence of a DPC micelle due to the segregation of CA and IBU away from each other by the DPC micelle. While the larger CA-IBU aggregates arising from higher IBU concentrations might be responsible for NSAID-induced intestinal toxicity, the absence of larger CA-IBU aggregates in the presence of DPC micelles may explain the observed attenuation of NSAID toxicity by PCs.

Relative acidity scale of glycine- and taurine-conjugated bile acids through ESI-MS measurements

The most important bile acids, in the form of glycine and taurine conjugates, have been ordered in terms of relative acidity scale. The measurements have been carried out using mass spectrometric techniques. The group of taurine conjugates confirm the superior acidity over the glycine derivatives. Rationale of the differences found in gas-phase and comparison with the data reported in solution-phase are discussed with the support of theoretical calculations. The study has been completed with the acidity sequence of mixed oxo-hydroxy bile acids.

Effect of amino acids on aggregation behaviors of sodium deoxycholate at air/water surface: surface tension and oscillating bubble studies

The aggregation behaviors of sodium deoxycholate (NaDC) at the air/water surface were investigated via surface tension and oscillating bubble measurements in the absence and presence of three alkaline amino acids, namely, L-Lysine (L-Lys), L-Arginine (L-Arg), and L-Histidine (L-His). The results of surface tension measurements show that NaDC has a lower ability to reduce the surface tension of water, because NaDC molecules orient at the surface in an oblique direction and tend to aggregate together, which is approved by molecular dynamics (MD) simulation. L-Lys is the most efficient of the three amino acids in reducing the critical aggregation concentration (cac) of NaDC in aqueous solution. The influence of amino acids on the dilational rheological properties of NaDC was studied using the drop shape analysis method in the frequency range from 0.02 to 0.5 Hz. The results reveal that the absolute modulus passes through a maximum value with increasing NaDC concentration. The addition of amino acids increases the absolute modulus of NaDC, and the maximum value is observed at much lower concentration. From the perspective of structures of amino acids, the performance of L-Arg is similar to that of L-His, and both of them bring out a smaller effect on the absolute modulus than that of L-Lys. From the above results, it may be presumed that electrostatic and hydrophobic effects are important impetus during the interaction between amino acids and NaDC at the air/water surface. Hydrogen bonding is so ubiquitous in the system that the difference of hydrogen bonding between NaDC and amino acid is ignored.

Effect of sodium salts of 3α,12α-dihydroxy-7-oxo-5β-cholanoic and 3,7,12-trioxo-5β-cholanoic acids on verapamil hydrochloride in biophysical-chemical model experiments

It is known that certain bile acids have a promotive effect on the action of some drugs. Special attention is paid to bile acids having oxo groups instead of OH groups in the steroid skeleton of their molecule, since these derivatives have a lower hemolytic potential (membrane toxicity). This study examined the effects of sodium salts of 3?,12?-dihydroxy-7-oxo-5?-cholanoic acid (7-oC) and 3,7,12-trioxo-5?- cholanoic acid (3,7,12-toC) on the adsorption of verapamil hydrochloride on activated carbon (model of the cell membrane). The interaction was followed by measuring the effect of verapamil on the functional dependence between the spin-lattice relaxation time T1 (protons of the C18 angular group of the bile acid molecule) and the bile acid concentration in deuterated chloroform (model of the cell membrane lipid phase). Whether a depot effect of verapamil exists when 7-oC and 3,7,12-toC (in the form of methyl esters) are present in chloroform was also investigated. It was found that 7-oC exhibited a significant effect in the experiments with verapamil, whereas 3,7,12-toC showed no difference of the measured parameters with respect to the control. This indicates that bile acid molecules should have OH groups bound to the steroid nucleus, in order to exhibit an effect on the monitored physico-chemical parameters of verapamil.

Cholesterol solubilization by oxo derivatives of selected bile acids and their membranotoxicity

This study is concerned with the effect of the structure of bile acids on the solubilization of cholesterol (cholesterol solubilizing capacity (CChm) and the equilibrium micellary solubilization of cholesterol (xChm)). It was found that the replacement of the hydroxy group in the bile acid molecule with an oxo group results in a decrease of the solubilization power of cholesterol. The examined bile acids form two linear groups at the plane of critical micellar concentration (CMC) and solubilization power of cholesterol (CChm or xChm). The group I is formed by bile acids with lower CMCs and higher cholesterol solubilites (deoxycholic (1), chenodeoxycholic (2), hyodeoxycholic (3), cholic (8), 12-oxolithocholic (4), and 7-oxolithocholic (5) acids). On the other hand, the group II is formed by bile acids with higher CMCs and lower cholesterol solubilites (7-oxodeoxycholic (9), 12-oxochenodeoxycholic (10), 12α-hydroxy-3,7-dioxocholanic (11), 7,12-dioxolithocholic (12), 3,7-dioxocholanic (6), and 3,12-dioxocholanic (7) acids). The common conformational characteristics of the bile acid molecules was determined (orientation of OH or oxo grups considered to the mean plane of the steroid skeleton). They form joint groups in the plane of CMC – (CChm or xChm), and the linear function (CChm or xChm) = f(CMC). The osmotic resistance of erythrocytes determines the membranotoxicity of bile acids. 12-Oxolithocholic acid represents the best compromise with regard to the solubilization of cholesterol and membranotoxicity.

Evaluation and optimization of removal of an acid-insoluble surfactant for shotgun analysis of membrane proteome

Due to its compatibility with protease activity at high concentration, sodium deoxycholate (SDC) can be used to effectively improve the solubilization and enzymolysis of membrane proteins and has received increasing attention in the field of membrane proteome analysis in recent years. SDC can be removed from digests by means of acidification followed by centrifugation (i.e. acid precipitation, AP) or extraction with ethyl acetate (i.e. phase transfer, PT) so as not to interfere with the downstream analyses like LC-MS/MS. In this study, the two strategies were systematically evaluated, compared and optimized. The results of the study demonstrated that both of the AP and PT strategies led to a certain amount of tryptic peptides being lost, and in PT strategy even more peptides were lost during SDC removal process. However, the lost peptides could be mostly recovered by washing the pellet and solid content produced during AP and PT, respectively. By recovering the lost peptides, the identification efficiency of proteins, especially transmembrane and low abundance ones, was significantly improved. Comparatively, after optimization by recovering the lost peptides, AP strategy was superior to PT strategy because the former not only could achieve the comparable identification efficiency with the latter but also was more economical, safer and easier to operate than the latter.

Molecular level properties of the free water surface and different organic liquid/water interfaces, as seen from ITIM analysis of computer simulation results

Molecular dynamics simulations of the interface of water with four different apolar phases, namely water vapour, liquid carbon tetrachloride, liquid dichloromethane (DCM) and liquid dichloroethane (DCE) are performed on the canonical ensemble at 298 K. The resulting configurations are analysed using the novel method of identification of the truly interfacial molecules (ITIM). Properties of the first three molecular layers of the liquid phases (e.g. width, spacing, roughness, extent of the in-layer hydrogen bonding network) as well as of the molecules constituting these layers (e.g., dynamics, orientation) are investigated in detail. In the analyses, particular attention is paid to the effect of the polarity of the non-aqueous phase and to the length scale of the effect of the vicinity of the interface on the various properties of the molecules. The obtained results show that increasing polarity of the non-aqueous phase leads to the narrowing of the interface, in spite of the fact that, at the same time, the truly interfacial layer of water gets somewhat broader. The influence of the nearby interface is found to extend only to the first molecular layer in many respects. This result is attributed to the larger space available for the truly interfacial than for the non-interfacial molecules (as the shapes of the two liquid surfaces are largely independent of each other, resulting in the presence of voids between the two phases), and to the fact that the hydrogen bonding interaction of the truly interfacial water molecules with other waters is hindered in the direction of the interface.

Molecular dynamics simulations of glycocholate-oleic acid mixed micelle assembly

We have applied a molecular dynamics (MD) method to investigate the aggregation behavior and physicochemical properties of bile salt as well as bile salt/fatty acid mixed micelles. Local atomic density profiles from the center of the micelles confirm that the self-assembly of the trihydroxy bile salt, glycocholate, is largely driven by hydrophobic aggregation of the nonpolar beta-faces of the steroid backbones. Additional association occurs between neighboring monomers through hydrogen-bonding interactions. The average micellar aggregation number for glycocholate at 37 degrees C with a background salt concentration of 150 mM is shown to be 8.5 molecules per micelle, while the critical micelle concentration (cmc) is 3.1 mM. The good agreement of these results with experimental values illustrates that a MD approach is useful to study mixed micelles of bile salts and fatty acids, critical to the understanding of oral lipid-based formulations. The aggregation behavior and colloidal structure of such micelles are simulated and presented in this article.

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.

Percolation Threshold of Water in Ideal Binary Mixture

Percolation transitions of water and solute molecules in an ideal completely miscible aqueous solution are studied by computer simulations. Three concentration ranges with different kinds of water and solute clustering can be distinguished. An infinite water network, present in pure liquid water, breaks at some solute concentration. Accordingly, the infinite network of solute molecules, present in pure liquid solute, breaks upon addition of water. At ambient and high temperatures, there is a concentration range were both components are below their respective percolation thresholds and there are no water or solute infinite networks. The existence of such concentration range is a characteristic of a completely miscible binary mixture. Upon supercooling, the threshold concentrations decrease and simultaneous existence of the infinite networks of water and of solute becomes possible. The presence of two inter-penetrating infinite networks of molecules may be one of the conditions required for the liquid-liquid transition of one-component isotropic fluid.

Percolation threshold parameters of fluids

Extensive Monte Carlo simulations on three qualitatively different model supercritical fluids (square-well fluid, Lennard-Jonesium, and primitive water) have been performed to examine percolation threshold parameters for continuum (correlated) models and their relation to general results valid for random lattice models; random-site percolation simple-cubic lattice has therefore been considered as well. Two different bond criteria, the configurational and self-bound ones, defining a cluster have been used. In addition to the percolation threshold occupation probability pc and the percolation threshold fluid density rhoc, the correlation length exponent nu and the wrapping probability at the percolation threshold Rw,c have also been evaluated. It is found that parameters nu and Rw,c exhibit not only strong temperature dependence but also, unlike the case of lattice systems, dependence on the nature of the system considered and the employed definition of the cluster.

Counterion binding in the aqueous solutions of bile acid salts, as studied by computer simulation methods

We investigate the structural and dynamical properties of counterion binding in sodium cholate and sodium deoxycholate micelles at three different concentration, namely, 30, 90, and 300 mM, by means of molecular dynamics simulations at the atomistic level. The obtained results can resolve a long-standing, apparent contradiction between different experiments that reported discordant values for the degree of counterion binding. Namely, our results suggest that certain experimental techniques, such as freezing point depression, are only sensitive to the contact counterions, and hence, the degree of contact binding of the counterions is measured. On the other hand, in experiments employing, e.g., electrode potential or nuclear magnetic resonance measurements, the solvent-separated counterions also contribute to the signal detected, and hence, the counterions that are measured as bound ones do include the solvent-separated counterions as well.