Nuclear magnetic resonance and x-ray studies on micellar aggregates of sodium deoxycholate

Using Nuclear Magnetic Resonance Spectroscopy to Probe Hydrogels Formed by Sodium Deoxycholate

Hydrogels of bile acids and their salts are promising materials for drug delivery, cellular immobilization, and other applications. However, these hydrogels are poorly understood at the molecular level, and further study is needed to allow improved materials to be created by design. We have used NMR spectroscopy to probe hydrogels formed from mixtures of formic acid and sodium deoxycholate (NaDC), a common bile acid salt. By assaying the ratio of deoxycholate molecules that are immobilized as part of the fibrillar network of the hydrogels and those that can diffuse, we have found that 65% remain free under typical conditions. The network appears to be composed of both the acid and salt forms of deoxycholate, possibly because a degree of charge inhibits excessive aggregation and precipitation of the fibrils. Spin-spin relaxation times provided a molecular-level estimate of the temperature of gel-sol transition (42 °C), which is virtually the same as the value determined by analyzing macroscopic parameters. Saturation transfer difference (STD) NMR spectroscopy established that formic acid, which is present mainly as formate, is not immobilized as part of the gelating network. In contrast, HDO interacts with the network, which presumably has a surface with exposed hydrophilic groups that form hydrogen bonds with water. Moreover, the STD NMR experiments revealed that the network is a dynamic entity, with molecules of deoxycholate associating and dissociating reversibly. This exchange appears to occur preferentially by contact of the hydrophobic edges or faces of free molecules of deoxycholate with those of molecules immobilized as components of the network. In addition, DOSY experiments revealed that gelation has little effect on the diffusion of free NaDC and HDO.

Condensed Supramolecular Helices: The Twisted Sisters of DNA

Condensation of DNA helices into hexagonally packed bundles and toroids represents an intriguing example of functional organization of biological macromolecules at the nanoscale. The condensation models are based on the unique polyelectrolyte features of DNA, however here we could reproduce a DNA‐like condensation with supramolecular helices of small chiral molecules, thereby demonstrating that it is a more general phenomenon. We show that the bile salt sodium deoxycholate can form supramolecular helices upon interaction with oppositely charged polyelectrolytes of homopolymer or block copolymers. At higher order, a controlled hexagonal packing of the helices into DNA‐like bundles and toroids could be accomplished. The results disclose unknown similarities between covalent and supramolecular non‐covalent helical polyelectrolytes, which inspire visionary ideas of constructing supramolecular versions of biological macromolecules. As drug nanocarriers the polymer–bile salt superstructures would get advantage of a complex chirality at molecular and supramolecular levels, whose effect on the nanocarrier assisted drug efficiency is a still unexplored fascinating issue.

Revealing the complex self-assembly behaviour of sodium deoxycholate in aqueous solution

HYPOTHESIS

Sodium deoxycholate is a natural bile salt produced by animals and fulfilling important physiological processes. It is also used as dispersive surfactant and building block for self-assembled architectures in biology and material science. Although long debated, the study of its self-assembly in water is hereto incomplete and the models of the known aggregates are still controversial. This background suggests a complex scenario likely missing of additional mesophases.

EXPERIMENTS

Electron and optical microscopy techniques were crossed with SAXS data for the research.

FINDINGS

Novel rod, sponge, vesicle, lamellae, nanotube phases and reversible transitions among them arise at conditions (concentration, pH, temperature, ionic strength, ionic composition) fitting the physiological working environment of sodium deoxycholate. These findings enlarge the perspective towards different directions. The integration of the previous literature with this work removes any interpretative contradiction since all the structures cover the entire spectrum of phases expected for surfactants, thus being explained according to the Israelachvili's scheme. It is not trivial that a single molecule can show such a high structural variability. This fact highlights a very versatile system. Probably it is not a coincidence that it occurs in a multitasking biomolecule. These results furnish fundamental knowledge to clarify the bile salts' role in vivo.

An overview of translational (radio)pharmaceutical research related to certain oncological and non-oncological applications

Translational medicine pursues the conversion of scientific discovery into human health improvement. It aims to establish strategies for diagnosis and treatment of diseases. Cancer treatment is difficult. Radio-pharmaceutical research has played an important role in multiple disciplines, particularly in translational oncology. Based on the natural phenomenon of necrosis avidity, OncoCiDia has emerged as a novel generic approach for treating solid malignancies. Under this systemic dual targeting strategy, a vascular disrupting agent first selectively causes massive tumor necrosis that is followed by iodine-131 labeled-hypericin (¹²³I-Hyp), a necrosis-avid compound that kills the residual cancer cells by crossfire effect of beta radiation. In this review, by emphasizing the potential clinical applicability of OncoCiDia, we summarize our research activities including optimization of radioiodinated hypericin Hyp preparations and recent studies on the biodistribution, dosimetry, pharmacokinetic and, chemical and radiochemical toxicities of the preparations. Myocardial infarction is a global health problem. Although cardiac scintigraphy using radioactive perfusion tracers is used in the assessment of myocardial viability, searching for diagnostic imaging agents with authentic necrosis avidity is pursued. Therefore, a comparative study on the biological profiles of the necrosis avid ¹²³I-Hyp and the commercially available ^99mTc-Sestamibi was conducted and the results are demonstrated. Cholelithiasis or gallstone disease may cause gallbladder inflammation, infection and other severe complications. While studying the mechanisms underlying the necrosis avidity of Hyp and derivatives, their naturally occurring fluorophore property was exploited for targeting cholesterol as a main component of gallstones. The usefulness of Hyp as an optical imaging agent for cholelithiasis was studied and the results are presented. Multiple uses of automatic contrast injectors may reduce costs and save resources. However, cross-contaminations with blood-borne pathogens of infectious diseases may occur. We developed a radioactive method for safety evaluation of a new replaceable patient-delivery system. By mimicking pathogens with a radiotracer, we assessed the feasibility of using the system repeatedly without septic risks. This overview is deemed to be interesting to those involved in the related fields for translational research.

Determination of the critical premicelle concentration, first critical micelle concentration and second critical micelle concentration of surfactants by resonance Rayleigh scattering method without any probe

The purpose of this work is to determine the values of critical premicelle concentration (CPMC), first critical micelle concentration (FCMC) and second critical micelle concentration (SCMC) of surfactants using a common spectrofluorophotometer by recording resonance Rayleigh scattering (RRS) signal without any probe. The plot of the RRS intensities at the maximum scattering wavelength (I(RRS)(max)) versus surfactant concentrations (c) was constructed to obtain the I(RRS)(max)-c curve. From the inflexions in I(RRS)(max)-c curve, the CPMC, FCMC and SCMC values of a surfactant can be obtained sensitively. The FCMC of some anionic, cationic and nonionic surfactants such as sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), Tween-20, and Tween-80 were determined by RRS method and the values are in good agreement with those obtained from conductivity and surface tension measurements and literature values. The CPMC and SCMC of SDS and CTAB were also determined by RRS method respectively and the values conform to literature values too. Furthermore, RRS method can also be used to determine the FCMC of an amphiphilic macromolecule-hemoglobin, whose structure resembles a surfactant. From the experimental results, it is concluded that RRS method can be applied to the simultaneous determination of the CPMC, FCMC and SCMC values in a sensitive, accurate and no probe way.

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 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.

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.

Composition of aqueous solutions containing sodium glycocholate and glycodeoxycholate

Composition and existence range of aggregates formed by sodium glycocholate and glycodeoxycholate contemporary present in aqueous micellar and premicellar solutions were investigated. Solubility measurements of lead (II) glycocholate and glycodeoxycholate give analytical concentration of lead (II) and glycocholate and glycodeoxycholate, respectively. Electromotive force measurements provide the free concentration of hydrogen, sodium and lead (II) ions. Experimental data obtained at 25 degrees C and at three different concentrations of N(CH3)4Cl, used as a constant ionic medium, can be explained by assuming the presence of aggregates of different composition depending on the reagent and ionic medium concentrations. Next to two species containing only glycocholate or glycodeoxycholate, the presence of aggregates formed with the contemporary participation of both bile anions in different ratios was assumed. Species with the hydrogen ion participation are not present in appreciable quantity in the investigated concentration range. As expected, the size of aggregates increases by increasing reagent and ionic medium concentration. Most of the species can be explained with a "core + link" mechanism, where all the glycocholate aggregation numbers are even, while those of glycodeoxycholate are always multiple of three. Analogy and difference with aggregates formed by the two bile anions separately are discussed.

Composition of sodium cholate micellar solutions

To study the composition of sodium cholate solutions, an investigation was carried out at 25 degrees C and in N(CH3)4Cl, as a constant ionic medium, at three different concentrations (W = 0.100; 0.500 and 0.800 mol dm(-3)). Electromotive force measurements of three different galvanic cells, the first involving a glass electrode for hydrogen ions, the second an electrode for sodium ions and the third a lead amalgam electrode, were performed. Independently, lead (II) cholate solubility measurements in the presence of sodium ions were performed, as well. The experimental results obtained from both approaches were explained by assuming the formation of aggregates in cholate and sodium of different composition depending on W and on the cholate concentration. The maximum aggregation found number for cholate was 24 and even aggregation numbers were markedly predominant. Only two species with odd aggregation number were found, but at a low percentage. The assumed species and the relative constants were compared with those found for the other sodium salt of cholanic acids.

Self-Assembled Networks of Ribbons in Molecular Hydrogels of Cationic Deoxycholic Acid Analogues

Aqueous gels derived from three cationic 24-nor 3,12-dihydroxy cholane (DC) derivatives with N-methyl-2-pyrrolidinone (NMP), N-methylmorpholine (NMM), and 1,4-diazabicyclo[2.2.2]octane (DABCO) at the side chain positions have been exhaustively characterized by small-angle neutron-scattering experiments. Although the molecular structures differ slightly by the heterocycle grafted to the steroid core, the derived gels exhibit a range of structural behaviors at the nanoscale that depart from those observed with simple deoxycholate systems. The NMM-DC aggregates are ribbons with a bimolecular thickness of t = 37 A and an anisotropy of the section b/a approximately 0.1. DABCO-DC exhibits a remarkable transition from ribbons (t = 29.5 A, b/a = 0.18) to thicker cylindrical fibers (R approximately 59 A), involving four original ribbons, upon a concentration increase. The NMP-DC system forms thick cylindrical fibers (R approximately 68 A) with steroid molecules organized in a specific morphology. Bilayered or interdigited structures are formed and favored by the presence of multiple polar interaction centers in the DC molecules. Secondary aggregation mechanisms are invoked in the formation of bundles having a lower cross-sectional anisotropic symmetry and exhibiting Bragg peaks corresponding to molecular length periodicities. The relations between the structural information and the rheological properties are discussed.

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.

Selective sensing of Cu(II) at ng ml(-1) level based on phosphorescence quenching of 1-bromo-2-methylnaphthalene sandwiched in sodium deoxycholate dimer

Using sodium deoxycholate as a protective medium, the selective recognition of Cu(II) at ng ml(-1) level is realized through dynamic phosphorescence quenching of 1-bromo-2-methylnaphthalene (BMN) without deoxygenation. The limit of detection is 4.32 ng ml(-1), and the relative standard deviation is 1% at 10 microM, linear up to 1 x 10(-5) M.

Oxygen free radical generating mechanisms in the colon: do the semiquinones of vitamin K play a role in the aetiology of colon cancer?

It is proposed that bile acids (deoxycholic acid), the K vitamins, iron(II) complexes and oxygen interact to induce an oncogenic effect in the colon by the generation of free radicals. In the relatively low oxidising/reducing conditions of the colonic lumen the K vitamins exist in the reduced form; however, if absorbed into the mucosa they have the capacity to be chemically oxidised and to enter into a redox cycle yielding oxygen radicals. The semiquinone radical of K(1) (phylloquinone) has been stabilised in bile acid mixed micelles and investigated by electron paramagnetic resonance spectroscopy and quantum chemical calculations. The estimated half-life of the radical was about 30 min which confirms a remarkably high stability in aqueous micellar solution. A model is presented in which the reduced K vitamins may initiate superoxide radical, O2(-*) generation leading to Fe(II) mediated Fenton reactions in the stem colon cells.

Determination of alkylphenol ethoxylates by micellar electrokinetic chromatography with bile salts

Octyl- and nonylphenol ethoxylates (OPEs and NPEs) with different numbers of ethoxy units (average values: n = 10 and N = 40 for OPEs, and n = 10 for NPEs) were separated by micellar electrokinetic chromatography under positive polarity using an 80 mM borate buffer of pH 8.5 containing sodium deoxycholate (SDC) or sodium cholate (SC). When sodium dodecyl sulfate (SDS) was added to the background electrolyte (BGE) in the absence of the bile salt, a single peak at a migration time longer than that of the EOF was obtained. Substituting the SDS by a bile salt, the homologues were resolved. At the same bile salt concentration, resolution between the homologues was higher with SDC than using SC. Optimum resolution between consecutive homologues was obtained with 50 mM SDC. In the presence of low or moderate amounts of acetonitrile or n-propanol, the background line improved significantly, whereas resolution may increase or decrease slightly. We propose a procedure for the determination of OPEs and NPEs with optimum resolution between the homologues as well as a modified procedure with improved selectivity for the single-run determination of other absorbing nonionic, cationic, and anionic (such as linear alkylbenzene sulfonates) surfactants in industrial and household cleaning products and its application to a variety of samples. The detection limit was ca. 28 microg x mL(-1) of total NPE (n = 10), and peak area repeatabilities at 50 microg x mL(-1) were 1.7% (intraday) and 5.6% (interday).

Determination of cationic surfactants by capillary zone electrophoresis and micellar electrokinetic chromatography with deoxycholate micelles in the presence of large organic solvent concentrations

Mixtures of the cationic surfactants benzalkonium chloride (BKC) and cetylpyridinium chloride (CPC) were quickly resolved and reproducibly and reliably determined by using background electrolytes (BGEs) containing 80 mM borate, pH 8.5, bile salts and large concentrations of an organic solvent. When the bile salt is present, the separation mechanism changes from capillary zone electrophoresis (CZE) to a mixed micellar electrokinetic chromatography (MEKC)-CZE, with predominant MEKC interactions, which lead to an excellent resolution of all the solutes, including the C12-C18 homologues of BKC and CPC. A BGE containing 50 mM sodium deoxycholate and 30% ethanol for an extreme resolution, or 20% tetrahydrofuran for an adequate resolution within a much shorter analysis time, is recommended. The procedure was applied to the determination of the surfactants in industrial and household formulations, with excellent resolution between the homologues, detection limits of a few microg ml(-1) and reproducibilities below 2%.

Optimization of selectivity and resolution in micellar electrokinetic capillary chromatography with a mixed micellar system of sodium dodecyl sulfate and sodium cholate

Selectivity and resolution were studied for the separation of seven corticosteroids by micellar electrokinetic capillary chromatography (MEKC) using a mixed micellar solution of sodium dodecyl sulfate (SDS) and sodium cholate (SC), buffered with 3-(N-morpholino)propanesulfonic acid (MOPS) or 3-[(1,1-dimethyl-2-hydroxyethyl)amino]-2-hydroxypropane sulfonic acid (AMPSO). The changes in selectivity were compared for the AMPSO-SDS-SC system by varying the pH and the concentrations of AMPSO, SDS and SC. The experimental design started with the central composite design and continued in a sequential manner. The optimum selectivity for the separation of the corticosteroids was calculated from the analyte migration times and the analyte velocities, by using empirical quadratic regression models. Satisfactory regression fits and coefficients of determination for prediction were obtained with cross-validated models. To optimize the resolution, the physical parameters of capillary length and analysis time were varied under the conditions optimal for the selectivity. In both the selectivity and the resolution, optimization the overall optimum was determined by using the desirability function technique. Analysis times were controlled by using 1,3-diaminopropane to influence the electroosmotic flow velocity (veo). The voltage was kept constant, which resulted in higher electric field strength in shorter capillaries. No changes in the selectivity were observed when 1,3-diaminopropane was used to control the electroosmotic flow velocity. Such an optimization technique, where the chemical and physical factors affecting the separation are treated independently, seemed to be effective for finding the best possible resolution for the corticosteroids.

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.

Mixed micelles of short chain alkyl surfactants and bile salts in electrokinetic chromatography: enhanced separation of corticosteroids

The separation of a complex mixture of 17 corticosteroids was investigated by mixed micellar electrokinetic chromatography (MMEKC) employing various bile salts and/or alkylsulfonates. In this study, influence of individual surfactants and mixed micelles of hydrocarbon-bile salt surfactants on retention behavior, selectivity and the size of the elution window is investigated. Retention behavior of corticosteroids in SDS and bile salt micelles is examined using linear solvation energy relationships (LSER). In addition, the effects of type of bile salt surfactant on elution patterns were investigated. It was found that separation patterns are mostly influenced by the number of hydroxyl functional groups on the steroidal backbone of the bile salts, while the type of ionic head group has little, if any, effect on the steroids separation. Comparisons between mixed micellar techniques and the inclusion of conventional modifiers to various single and binary surfactant systems were made. The addition of modifiers such as acetonitrile, urea and beta-cyclodextrin to SDS surfactant systems, as well as mixed bile salt systems of sodium taurocholate and sodium glycodeoxycholate, did not improve the separation of the steroids. On the other hand, the addition of the short-chain alkylsulfonate sodium butanesulfonate to the mixture of taurocholate and glycodeoxycholate greatly improved the separation of the 17 corticosteroids and provided a baseline separation of all solutes. The effects of carbon chain length and concentration of alkylsulfonate on capacity factor, selectivity, efficiency and the size of the elution window were investigated.

Performance of amino-silylated fused-silica capillaries for the separation of enkephalin-related peptides by capillary zone electrophoresis and micellar electrokinetic chromatography

Enkephalin-related peptides were separated at low pH in a capillary with covalently bonded aminopropyl groups. The peptides are electrostatically repelled from the capillary surface and much higher efficiencies and faster separations were achieved compared to separations using uncoated capillaries. At low pH the amino groups are protonated, which results in reversed electroosmosis. The influence of voltage and ionic strength on the mobility and the separation efficiency was studied. The repeatability of migration times within one day was very good with relative standard deviations of 0.3-0.7%. Increasing the pH decreased the electroosmosis, eventually turning towards the cathode in the pH range 5-6; the separation performance, however, was lower at higher pH. Neutral and anionic micellar agents were added to the background electrolyte at different concentrations; the enkephalins had weak association with the neutral micellar agents but were distributed to the anionic taurodeoxycholic acid (TDC) micelles, giving rise to changes in separation selectivities. Very high efficiencies were obtained for peptides with a low distribution to the TDC micelles, while the efficiencies were impaired for those with a strong association with the micelles, which may indicate a slow mass transfer in the association process.

Mixed micellar electrokinetic chromatography of corticosteroids

Mixed micellar electrokinetic chromatography (MMEKC) using mixtures of bile salt surfactants and/or sodium dodecyl sulfate (SDS) was employed to separate a group of corticosteroids. Resolution of electrokinetic chromatography (EKC) can be greatly enhanced by the use of mixed micellar systems due to the fact that the composition of the mixed micelles has a great influence on retention, selectivity and the size of the elution window. By combining surfactants with different structural properties, solute-micelle interactions were manipulated in order to elicit specific separations. Various combinations of bile salts and/or SDS at different mole fractions as well as total micelle concentrations were used in order to enhance the resolution of corticosteroid separations. Large changes in retention and selectivity were observed that often resulted in frequent variations in elution order. In addition, the composition of mixed micellar systems had a great influence on the elution window in EKC, as measured by the ratio of tmc/teo. Addition of SDS to the mixtures of bile salt micelles resulted in significant extension of the elution window and subsequently improvement in resolution. A separation of seventeen corticosteroids was achieved. Finally, MMEKC was applied in order to separate the steroidal components of a mixture of three anti-inflammatory creams.

On the interaction of polypeptides with bile salts or bilirubin-IX alpha

Aqueous solutions formed by polypeptides, simple models of proteins, and bile salts (sodium cholate and deoxycholate, NaC and NaDC, respectively) or bilirubin-IX alpha (BR) have been studied by CD measurements. They could mimic more complicated biliary systems, thus supplying a possible interpretation of the behavior of some amino acid residues in the biliary proteins. The aggregation of NaDC and NaC in water can be monitored by CD measurements. Bile salts, in submicellar and micellar form, stabilize poly(L-Lys) (PLL) in alpha-helical conformation. The alpha-helix content increases with increasing bile salt concentration and ionic strength. NaDC seems to be a slightly better stabilizing agent of the alpha-helix conformation than NaC. Models characterized by hydrogen bonds between bile salts and PLL are proposed, also resorting to previous data available on the systems formed by NaDC and poly(L-Leu-L-Leu-L-Lys) (PLLL) or poly(L-Leu-L-Leu-L-Asp) (PLLA). Binding of BR to PLL, poly(D-Lys), poly(L-Glu), PLLL, and PLLA in water has been investigated by CD spectra in order to clarify the nature of the association complexes and the mechanism of the BR enantioselective complexation. Potential energy calculations provide binding models capable of explaining the enantioselective ability of the PLL and PLLL alpha-helices toward the left- and right-handed enantiomer of BR, respectively. BR is bound to -NH2 groups of PLL and PLLL lying on a right- and left-handed spiral, respectively. These results, together with those formerly obtained for some bile salts-BR systems, indicate that the selectivity originates from a binding that involves large regions of the BR molecule and gives rise, very probably, to moderate conformational changes from the "ridge tile" structure observed in the crystals. In some cases van der Waals forces can play a crucial role in the chiral recognition of bilirubin. Moreover, possible interaction models of BR with human serum albumin are proposed on the basis of a recent x-ray crystal structure of the protein.