Direct Measurement of the Thermodynamics of Chiral Recognition in Bile Salt Micelles

Ternary Mixed Micelle Hexadecyltrimethylammonium Bromide-Dodecyltrimethylammonium Bromide-Sodium Deoxycholate: Gibbs Free Energy of Mixing and Excess Gibbs Energy of Mixing

Pharmaceutical, food, and cosmetic formulations often contain binary or ternary surfactant mixtures with synergistic interactions amongst micellar building blocks. Here, a ternary mixture of the surfactants hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and sodium deoxycholate is examined to see if the molar fractions of the surfactants in the ternary mixed micellar pseudophase are determined by the interaction coefficients between various pairs of the surfactants or by their propensity to self-associate. Critical micelle concentrations (CMC) of the analyzed ternary mixtures are determined experimentally (spectrofluorimetrically using pyrene as the probe molecule). Thermodynamic parameters of ternary mixtures are calculated from CMC values using the Regular Solution protocol. The tendency for monocomponent surfactants to self-associate (lower value of CMC) determines the molar fractions of surfactant in the mixed micelle if there is no issue with the packing of the micelle building units of the ternary mixed micelle. If a more hydrophobic surfactant is incorporated into the mixed micelle, the system (an aqueous solution of surfactants) is then the most thermodynamically stabilized.

Self-Association of the Anion of 7-Oxodeoxycholic Acid (Bile Salt): How Secondary Micelles Are Formed

Bile acid anions are steroidal biosurfactants that form primary micelles due to the hydrophobic effect. At higher concentrations of some bile acid anions, secondary micelles are formed; hydrogen bonds connect primary micelles. Monoketo derivatives of cholic acid, which have reduced membrane toxicity, are important for biopharmaceutical examinations. The main goal is to explain why the processes of formation of primary and secondary micelles are separated from each other, i.e., why secondary micelles do not form parallel to primary micelles. The association of the anion of 7-oxodeoxycholic acid (a monoketo derivative of cholic acid) is observed through the dependence of the spin-lattice relaxation time on total surfactant concentration T₁ = f(C_T). On the function T₁ = f(C_T), two sharp jumps of the spin-lattice relaxation time are obtained, i.e., two critical micellar concentrations (CMC). The aggregation number of the micelle at 50 mM total concentration of 7-oxodeoxycholic acid anions in the aqueous solution is 4.2 ± 0.3, while at the total concentration of 100 mM the aggregation number is 9.0 ± 0.9. The aggregation number of the micelle changes abruptly in the concentration interval of 80-90 mM (the aggregation number determined using fluorescence measurements). By applying Le Chatelier's principle, the new mechanism of formation of secondary micelles is given, and the decoupling of the process of formation of primary and secondary micelles at lower concentrations of monomers (around the first critical micellar concentration) and the coupling of the same processes at higher equilibrium concentrations of monomers (around the second critical micellar concentration) is explained. Stereochemically and thermodynamically, a direct mutual association of primary micelles is less likely, but monomeric units are more likely to be attached to primary micelles, i.e., 7-oxodeoxycholic acid anions.

Determining sequential micellization steps of bile salts with multi-CMC modeling

HYPOTHESIS

Bile salts exhibit complex concentration-dependent micellization in aqueous solution, rooted in a long-standing hypothesis of increasing size in bile aggregation that has historically focused on the measurement of only one CMC detected by a given method, without resolving successive stepwise aggregates. Whether bile aggregation is continuous or discrete, at what concentration does the first aggregate form, and how many aggregation steps occur, all remain as open questions.

EXPERIMENTS

Bile salt critical micelle concentrations (CMCs) were investigated with NMR chemical shift titrations and a multi-CMC phase separation modeling approach developed herein. The proposed strategy is to establish a correspondence of the phase separation and mass action models to treat the first CMC; subsequent micellization steps, involving larger micelles, are then treated as phase separation events.

FINDINGS

The NMR data and the proposed multi-CMC model reveal and resolve multiple closely spaced sequential preliminary, primary, and secondary discrete CMCs in dihydroxy and trihydroxy bile salt systems in basic (pH 12) solutions with a single model of one NMR data set. Complex NMR data are closely explained by the model. Four CMCs are established in deoxycholate below 100 mM (298 K, pH 12): 3.8 ± 0.5 mM, 9.1 ± 0.3 mM, 27 ± 2 mM, and 57 ± 4 mM, while three CMCs were observed in multiple bile systems, also under basic conditions. Global fitting leverages the sensitivity of different protons to different aggregation stages. In resolving these closely spaced CMCs, the method also obtains chemical shifts of these spectroscopically inaccessible (aka dark) states of the distinct micelles.

Oxidation of Sodium Deoxycholate Catalyzed by Gold Nanoparticles and Chiral Recognition Performances of Bile Salt Micelles

Au nanoparticles (NPs) were prepared by UV light irradiation of a mixed solution of HAuCl4 and sodium deoxycholate (NaDC) under alkaline condition, in which NaDC served as both reducing agent and capping agent. The reaction was monitored by circular dichroism (CD) spectra, and it was found that the formed gold NPs could catalyze the oxidation of NaDC. A CD signal at ~283 nm in the UV region was observed for the oxidation product of NaDC. The intensity of the CD signal of the oxidation product was enhanced gradually with the reaction time. Electrospray ionization (ESI) mass spectra and nuclear magnetic resonance (NMR) spectra were carried out to determine the chemical composition of the oxidation product, revealing that NaDC was selectively oxidized to sodium 3-keto-12-hydroxy-cholanate (3-KHC). The chiral discrimination abilities of the micelles of NaDC and its oxidation product, 3-KHC, were investigated by using chiral model molecules R,S-1,1'-Binaphthyl-2,2'-diyl hydrogenphosphate (R,S-BNDHP). Compared with NaDC, the micelles of 3-KHC displayed higher binding ability to the chiral model molecules. In addition, the difference in binding affinity of 3-KHC micelles towards R,S-isomer was observed, and S-isomer was shown to preferentially bind to the micelles.

Self-association of sodium isoursodeoxycholate and sodium isohenodeoxycholate in water

Bile salts (BS) form hydrophobic Small's primary micelles at concentrations above the critical micelle concentration (CMC), while at concentrations above 3CMC they form secondary micelles (by the association of primary micelles via H-bonds). In this paper the self-associations of the anions of isohenodeoxycholic acid (3-epimer of henodeoxycholic acid, ICD) and the anions of isoursodeoxycholic acid (3-epimer of ursodeoxycholic acid, IUD) are examined, since the thermodynamic parameters of their self-association have not yet been published. Forming of IUD aggregates with two or three building units is slightly more favorable via α sides of steroid skeletons, regarding hydrophobicity, while regarding steric repulsive interactions it is more favorable to associate via β sides. Due to this, IUD in the vicinity of the CMC can form primary micelles by association of IUD particles both from the convex side and from the concave side of the steroid ring system. Therefore, IUD is significantly more prone to initial micellization than bile salt derivatives whose steroidal skeletons contain equatorially oriented OH groups.

Stepwise Aggregation of Cholate and Deoxycholate Dictates the Formation and Loss of Surface-Available Chirally Selective Binding Sites

Bile salts are facially amphiphilic, naturally occurring chemicals that aggregate to perform numerous biochemical processes. Because of their unique intermolecular properties, bile salts have also been employed as functional materials in medicine and separation science (e.g., drug delivery, chiral solubilization, purification of single-walled carbon nanotubes). Bile micelle formation is structurally complex, and it remains a topic of considerable study. Here, the exposed functionalities on the surface of cholate and deoxycholate micelles are shown to vary from one another and with the micelle aggregation state. Collectively, data from NMR and capillary electrophoresis reveal preliminary, primary, and secondary stepwise aggregation of the salts of cholic (CA) and deoxycholic (DC) acid in basic conditions (pH 12, 298 K), and address how the surface availability of chirally selective binding sites is dependent on these sequential stages of aggregation. Prior work has demonstrated sequential CA aggregation (pH 12, 298 K) including a preliminary CMC at ca. 7 mM (no chiral selection), followed by a primary CMC at ca. 14 mM that allows chiral selection of binaphthyl enantiomers. In this work, DC is also shown to form stepwise preliminary and primary aggregates (ca. 3 mM DC and 9 mM DC, respectively, pH 12, 298 K) but the preliminary 3 mM DC aggregate is capable of chirally selective solubilization of the binaphthyl enantiomers. Higher-order, secondary bile aggregates of each of CA and DC show significantly degraded chiral selectivity. Diffusion NMR reveals that secondary micelles of CA exclude the BNDHP guests, while secondary micelles of DC accommodate guests, but with a loss of chiral selectivity. These data lead to the hypothesis that secondary aggregates of DC have an exposed binding site, possibly the 7α-edge of a bile dimeric unit, while secondary CA micelles do not present binding edges to the solution, potentially instead exposing the three alcohol groups on the hydrophilic α-face to the solution.

Analysis of an old controversy: The compensation temperature for micellization of surfactants

The actual significance of the so-called compensation temperature T_c for micellization of surfactants is reviewed. It is demonstrated that it is possible to obtain as many T_c values as the number of temperature intervals in which the dependencies of enthalpy and entropy changes with temperature are analyzed. The value of each T_c will be the central value T_o of each temperature interval. These two facts suggest that T_c is simply such experimental T_o. Thus any physical interpretation derived from T_c is unfounded.

Structure-Property Relationships in Sodium Muricholate Derivative (Bile Salts) Micellization: The Effect of Conformation of Steroid Skeleton on Hydrophobicity and Micelle Formation-Pattern Recognition and Potential Membranoprotective Properties

It is known that β-muricholic acid anions prevent membrane toxicity of hydrophobic bile acids, which are being used in therapy for solubilization of the cholesterol type bile stone. Better knowledge of these derivative micelles is very important for understanding their physiological and pharmacological effects. β-Axial (a) oriented hydroxyl group from the steroid skeleton decreases the hydrophobic surface of the convex side of the steroid skeleton. Therefore, the critical micellization concentration (CMC) for steroid surfactants with β-a-OH group should increase, but in the case of OH groups of different orientations forming H-bonds in the hydrophobic phase of the micelle, it has the opposite effect; the CMC decreses, and aggregation is more favored. The set of muricholic acids (MCs) is composed by α-MC, β-MC, γ-MC, and ω-MC, where α-MC and β-MC have β-axial-OH groups. The aggregation numbers (n) are determined using the Moroi-Matsuoka-Sugioka thermodynamic method. CMC, enthalpy of demicellization, and ΔC_p are determined by isothermal titration calorimetry (ITC). This report pioneers in the study of MC derivatives micellization. Micelles of β-MC and γ-MC belong to the linear congeneric group (LCG) and their micelles above 85 mM have constant aggregation numbers n = 4-5. Micelles of α-MC and ω-MC are outliers in relation to the LCG, their aggregation number constantly increases; at 85 mM n = 6.8 (α-MC) and 6.5 (ω-MC). In micelles of derivatives β-MC and γ-MC, there is a low probability for the existence of hydrogen bonds. A micelle of α-MC probably has hydrogen bonds in its hydrophobic domain.

Contrasting effects of pH on the modulation of the structural integrity of hemoglobin induced by sodium deoxycholate

Bile salt-mediated conformational modification of hemoglobin (Hb) was examined at three different pHs i.e., 3.2, 7.4 and 9.0. The added bile salt, sodium deoxycholate (NaDC), decreases the α-helicity in Hb (α-helix: 71.3% → 61.7% in the presence of 9.6 mM NaDC, and 83.2% → 66.2% in the presence of 14 mM NaDC, at pH 7.4 and 9.0, respectively), while a reverse pattern of modification in the Circular Dichroism (CD) spectra of Hb is found at pH 3.2. The acid-induced denatured Hb (pH 3.2) regains its structural integrity by changing conformation from a random coil to an α-helix rich secondary structure upon addition of NaDC (α-helix: 10.4% → 53.4%, β-sheet: 31.0% → 18.5% and random coil: 58.6% → 28.1%, in the presence of 0.65 mM NaDC). Also, a step-wise binding interaction pattern of Hb with NaDC was revealed at pH 7.4 and 9.0 upon variation of steady-state fluorescence intensity and average lifetime of Hb. From the fluorescence lifetime decay pattern, the decrement of energy transfer from Trp to a heme group was found upon the addition of NaDC at pH 7.4 and 9.0. However, at pH 3.2, the modification of the time-resolved fluorescence decay behavior of Hb within NaDC is typically reversed, where the energy transfer from Trp to heme is restored to some extent. Thermodynamic analysis suggests that the Hb-NaDC binding interaction is characterized by a dominant entropic contribution interpreted on the basis of release of ordered water molecules to the bulk aqueous phase.