Analysis of myoglobin adsorption to Cu(II)-IDA and Ni(II)-IDA functionalized Langmuir monolayers by grazing incidence neutron and X-ray techniques

The adsorption of myoglobin to Langmuir monolayers of a metal-chelating lipid in crystalline phase was studied using neutron and X-ray reflectivity (NR and XR) and grazing incidence X-ray diffraction (GIXD). In this system, adsorption is due to the interaction between chelated divalent copper or nickel ions and the histidine moieties at the outer surface of the protein. The binding interaction of histidine with the Ni-IDA complex is known to be much weaker than that with Cu-IDA. Adsorption was examined under conditions of constant surface area with an initial pressure of 40 mN/m. After approximately 12 h little further change in reflectivity was detected, although the surface pressure continued to slowly increase. For chelated Cu2+ ions, the adsorbed layer structure in the final state was examined for bulk myoglobin concentrations of 0.10 and 10 microM. For the case of 10 microM, the final layer thickness was approximately 43 A. This corresponds well to the two thicker dimensions of myoglobin in the native state (44 A x 44 A x 25 A) and so is consistent with an end-on orientation for this disk-shaped protein at high packing density. However, the final average volume fraction of amino acid segments in the layer was 0.55, which is substantially greater than the value of 0.44 calculated for a completed monolayer from the crystal structure. This suggests an alternative interpretation based on denaturation. GIXD was used to follow the effect of protein binding on the crystalline packing of the lipids and to check for crystallinity within the layer of adsorbed myoglobin. Despite the strong adsorption of myoglobin, very little change was observed in the structure of the DSIDA film. There was no direct evidence in the XR or GIXD for peptide insertion into the lipid tail region. Also, no evidence for in-plane crystallinity within the adsorbed layer of myoglobin was observed. For 0.1 microM bulk myoglobin concentration, the average segment volume fraction was only 0.13 and the layer thickness was < or = 25 A. Adsorption of myoglobin to DSIDA-loaded with Ni2+ was examined at bulk concentrations of 10 and 50 microM. At 10 microM myoglobin, the adsorbed amount was comparable to that obtained for adsorption to Cu2+-loaded DSIDA monolayers at 0.1 M. But interestingly, the adsorbed layer thickness was 38 A, substantially greater than that obtained at low coverage with Cu-IDA. This indicates that either there are different preferred orientations for isolated myoglobin molecules adsorbed to Cu-IDA and Ni-IDA monolayer films or else myoglobin denatures to a different extent in the two cases. Either interpretation can be explained by the very different binding energies for individual interactions in the two cases. At 50 microM myoglobin, the thickness and segement volume fraction in the adsorbed layer for Ni-IDA were comparable to the values obtained with Cu-IDA at 10 microM myoglobin.     

Helix Induction by Dirhodium: Access to Biocompatible Metallopeptides with Defined Secondary Structure

The use of carboxylate side chains to induce peptide helicity upon binding to dirhodium centers is examined through experimental and computational approaches. Dirhodium binding efficiently stabilizes α helicity or induces α helicity in otherwise unstructured peptides for peptides that contain carboxylate side chains with i, i+4 spacing. Helix induction is furthermore possible for sequences with i, i+3 carboxylate spacing, though in this case the length of the side chains is crucial: ligating to longer glutamate side chains is strongly helix inducing, whereas ligating the shorter aspartate side chains destabilizes the helical structure. Further studies demonstrate that a dirhodium metallopeptide complex persists for hours in cellular media and exhibits low toxicity toward mammalian cells, enabling exploitation of these metallopeptides for biological applications.     

An electrochemical study into the interaction between complement-derived peptides and DOPC mono- and bilayers

Electrochemical methods employing the hanging mercury drop electrode were used to study the interaction between variants of the complement-derived antimicrobial peptide CNY21 (CNYITELRRQH ARASHLGLAR) and dioleoyl phosphatidylcholine (DOPC) monolayers. Capacitance potential and impedance measurements showed that the CNY21 analogues investigated interact with DOPC monolayers coating the mercury drop. Increasing the peptide hydrophobicity by substituting the two histidine residues with leucine resulted in a deeper peptide penetration into the hydrophobic region of the DOPC monolayer, indicated by an increase in the dielectric constant of the lipid monolayer (Deltaepsilon = 2.0 after 15 min interaction). Increasing the peptide net charge from +3 to +5 by replacing the histidines by lysines, on the other hand, arrests the peptide in the lipid head group region. Reduction of electroactive ions (Tl+, Pb2+, Cd2+, and Eu3+) at the monolayer-coated electrode was employed to further characterize the types of defects induced by the peptides. All peptides studied permeabilize the monolayer to Tl+ to an appreciable extent, but this effect is more pronounced for the more hydrophobic peptide (CNY21L), which also allows penetration of larger ions and ions of higher valency. The results for the various ions indicate that charge repulsion rather than ion size is the determining factor for cation penetration through peptide-induced defects in the DOPC monolayer. The effects obtained for monolayers were compared to results obtained with bilayers from liposome leakage and circular dichroism studies for unilamellar DOPC vesicles, and in situ ellipsometry for supported DOPC bilayers. Trends in peptide-induced liposome leakage were similar to peptide effects on electrochemical impedance and permeability of electroactive ions for the monolayer system, demonstrating that formation of transmembrane pores alone does not constitute the mechanism of action for the peptides investigated. Instead, our results point to the importance of local packing defects in the lipid membrane in close proximity to the adsorbed peptide molecules.     

Adsorption of myoglobin to Cu(II)-IDA and Ni(II)-IDA functionalized langmuir monolayers: study of the protein layer structure during the adsorption process by neutron and X-ray reflectivity

The structure and orientation of adsorbed myoglobin as directed by metal-histidine complexation at the liquid-film interface was studied as a function of time using neutron and X-ray reflectivity (NR and XR, respectively). In this system, adsorption is due to the interaction between iminodiacetate (IDA)-chelated divalent metal ions Ni(II) and Cu(II) and histidine moieties at the outer surface of the protein. Adsorption was examined under conditions of constant area per lipid molecule at an initial pressure of 40 mN/m. Adsorption occurred over a time period of about 15 h, allowing detailed characterization of the layer structure throughout the process. The layer thickness and the in-plane averaged segment volume fraction were obtained at roughly 40 min intervals by NR. The binding constant of histidine with Cu(II)-IDA is known to be about four times greater than that of histidine with Ni(II)-IDA. The difference in interaction energy led to significant differences in the structure of the adsorbed layer. For Cu(II)-IDA, the thickness of the adsorbed layer at low protein coverage was < or = 20 A and the thickness increased almost linearly with increasing coverage to 42 A. For Ni(II)-IDA, the thickness at low coverage was approximately 38 A and increased gradually with coverage to 47 A. The in-plane averaged segment volume fraction of the adsorbed layer independently confirmed a thinner layer at low coverage for Cu(II)-IDA. These structural differences at the early stages are discussed in terms of either different preferred orientations for isolated chains in the two cases or more extensive conformational changes upon adsorption in the case of Cu(II)-IDA. Subphase dilution experiments provided additional insight, indicating that the adsorbed layer was not in equilibrium with the bulk solution even at low coverages for both IDA-chelated metal ions. We conclude that the weight of the evidence favors the interpretation based on more extensive conformational changes upon adsorption to Cu(II)-IDA.     

Adsorption of amyloid beta (1-40) peptide to phosphatidylethanolamine monolayers.

The aggregation of soluble, nontoxic amyloid beta (Abeta) peptide to beta-sheet containing fibrils is assumed to be a major step in the development of Alzheimer's disease. Interactions of Abeta with neuronal membranes could play a key role in the pathogenesis of the disease. Herein, we study the adsorption of synthetic Abeta peptide to DPPE and DMPE monolayers (dipalmitoyl- and dimyristoylphosphatidylethanolamine). Both lipids exhibit a condensed monolayer state at 20 degrees C and form a similar lattice. However, at low packing densities (at large area per molecule), the length of the acyl chains determines the phase behavior, therefore DPPE is fully condensed whereas DMPE exhibits a liquid-expanded state with a phase transition at approximately 5-6 mNm(-1). Adsorption of Abeta to DPPE and DMPE monolayers at low surface pressure leads to an increase of the surface pressure to approximately 17 mNm(-1). The same was observed during adsorption of the peptide to a pure air-water interface. Grazing incidence X-ray diffraction (GIXD) experiments show no influence of Abeta on the lipid structure. The adsorption kinetics of Abeta to a DMPE monolayer followed by IRRAS (infrared reflection absorption spectroscopy) reveals the phase transition of DMPE molecules from liquid-expanded to condensed states at the same surface pressure as for DMPE on pure water. These facts indicate no specific interactions of the peptide with either lipid. In addition, no adsorption or penetration of the peptide into the lipid monolayers was observed at surface pressures above 30 mNm(-1). IRRAS allows the measurement of the conformation and orientation of the peptide adsorbed to the air-water interface and to a lipid monolayer. In both cases, with lipids at surface pressures below 20 mNm(-1) and at the air-water interface, adsorbed Abeta has a beta-sheet conformation and these beta-sheets are oriented parallel to the interface.     

DNA condensation and interaction with zwitterionic phospholipids mediated by divalent cations

Artificial viruses are considered to be a promising tool in gene therapy. To find lipid-DNA complexes with high transfection efficiency but without toxicity is a fundamental aim. Although cationic lipids are frequently toxic for cells, neutral lipids are completely nontoxic. Zwitterionic lipids do not interact with DNA directly; however, the interaction can be mediated by divalent cations. Langmuir monolayers represent a well-defined model system to study the DNA-lipid complexes at the air/water interface (quasi-2D systems). In this work, isotherms, infrared reflection absorption spectroscopy (IRRAS), X-ray reflectivity (XR), grazing incidence X-ray diffraction (GIXD), and Brewster angle microscopy (BAM) measurements are used to study the interaction of calf thymus DNA with DMPE (1,2-dimyristoyl-phosphatidylethanolamine) monolayers mediated by Ca2+ or Mg2+ ions. DNA adsorption is observed only in the presence of divalent cations. At low lateral pressure, the DNA partially penetrates into the lipid monolayer but is squeezed out at high pressure. The adsorption layer has a thickness of 18-19 A. Additionally, GIXD provides information about a one-dimensional ordering of adsorbed DNA. The periodic distance between DNA strands depends on the type of the divalent cation.     

Effects of monolayer structures on long-range electron transfer in helical peptide monolayer

Self-assembled monolayers of alpha-helical peptides were prepared on gold, and the effects of the monolayer structures (kind of constituent amino acid, molecular orientation, and molecular packing) on long-range electron transfer through the helical peptides were studied. The helical peptides were 16mer peptides having a thiophenyl linker at the N-terminal for immobilization on gold and a redox active ferrocene moiety at the C-terminal as an electron-transfer probe. The peptides were immobilized on gold by a gold-sulfur linkage and the electron transfer from the ferrocene moiety to gold was studied by electrochemical methods. When two types of the peptides, one with the repeating unit of Leu-Aib (Aib represents 2-aminoisobutyric acid) and the other with that of Ala-Aib, were compared, the electron transfer was found one order slower in the Leu-Aib peptide monolayer than that in the Ala-Aib peptide monolayer. The self-assembled monolayers of the Ala-Aib peptide with mixing of three different lengths of the peptides, 8mer, 12mer, and 16mer without a ferrocene moiety, were also prepared. The monolayer regularity in terms of molecular orientation and packing was higher roughly in the order of the monolayers mixed with 16mer > 12mer > no additive > 8mer, but the electron transfer became faster in the opposite order. The logarithms of the standard rate constants showed a nearly linear relationship with the direct distances between the ferrocene moiety and gold (beta = 0.32 A (-1)). Some data deviated from this linear relationship, but the deviations could be explained from the difference in the molecular packing, which was evaluated from the monolayer capacitance. It is thus concluded that an electron is transferred along a few molecules along the surface normal so that the vertical orientation or the increase of the interchain backbone separation slows down the electron transfer. Further, it is demonstrated that a tightly packed monolayer, where vibrational mode is restricted, suppresses the electron transfer. Three models are proposed to account for the observed molecular dynamics effects on the basis of either electron-transfer mechanism of electron tunneling or sequential hopping.     

Structure and phase behavior of archaeal lipid monolayers

We report X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXD) measurements of archaeal bipolar tetraether lipid monolayers at the air-water interface. Specifically, Langmuir films made of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius grown at three different temperatures, i.e., 68, 76, and 81 °C, were examined. The dependence of the structure and packing properties of PLFE monolayers on surface pressure were analyzed in a temperature range between 10 and 50 °C at different pH values. Additionally, the interaction of PLFE monolayers (using lipids derived from cells grown at 76 °C) with the ion channel peptide gramicidin was investigated as a function of surface pressure. A total monolayer thickness of approximately 30 ? was found for all monolayers, hinting at a U-shaped conformation of the molecules with both head groups in contact with the interface. The monolayer thickness increased with rising film pressure and decreased with increasing temperature. At 10 and 20 °C, large, highly crystalline domains were observed by GIXD, whereas at higher temperatures no distinct crystallinity could be observed. For lipids derived from cells grown at higher temperatures, a slightly more rigid structure in the lipid dibiphytanyl chains was observed. A change in the pH of the subphase had an influence only on the structure of the lipid head groups. The addition of gramicidin to an PLFE monolayer led to a more disordered state as observed by XRR. In GIXD measurements, no major changes in lateral organization could be observed, except for a decrease of the size of crystalline domains, indicating that gramicidin resides mainly in the disordered areas of the monolayer and causes local membrane perturbation, only.     

Folding pentapeptides into left and right handed alpha helices

Left or right handed alpha helicity can be induced in a pentapeptide (ANGYG) by appending left or right handed helical cycles as chiral templates. This sequence corresponds to a rare left handed helix found in the protein alanine racemase. Circular dichroism spectra reveal that pentapeptide ANGYG has no detectable structure in aq phosphate buffer, that it is an ambidextrous peptide in that it can be directed to fold into either a left handed or right handed alpha helix in water, with greater propensity for the uncommon left handed than the normal right handed conformation. A helix-inducing cyclic peptide at both ends of this peptide was more effective at inducing alpha helicity than a single cyclic peptide at one end. The alpha helical cyclic peptides provide novel tools for folding short peptides into thermodynamically unstable helices in water, and for studying factors that control chirality and helix induction.     

Phase transition of alkylsilane monolayers studied by temperature-dependent grazing incidence X-ray diffraction

The phase transition of organosilane monolayers on Si-wafer substrate surfaces prepared from octadecyltrichlorosilane (OTS) or docosyltrichlorosilane (DOTS) was investigated on the basis of grazing incidence X-ray diffraction (GIXD) at various temperatures. The OTS monolayer was prepared by a chemisorption method. The DOTS monolayer was prepared by a water-cast method (DOTS). The GIXD measurement clarified that the OTS monolayer also changed from hexagonal phase to amorphous state above a melting point of otadecyl groups. The GIXD measurements also clarified that the molecular aggregation state of the DOTS monolayer changes from an anisotropic phase to an isotropic phase with an increase in temperature. An estimated linear thermal expansion coefficient of the lattice lengths of a and b of the DOTS monolayer in the rectangular crystalline state assigned a similar value to those of bulk polyethylene with an orthorhombic crystalline lattice. The setting angle of the ab plane of the rectangular DOTS monolayer also showed similar behavior to that of the ab plane of bulk polyethylene.     

Poly(trifluoroethylene) adsorption and heterogeneous photochlorination reactions

Heterogeneous (gas-solid) photochlorination reactions of poly(trifluoroethylene) (PF 3E) films were studied as a function of reaction time and light intensity. The rate of chlorination was found to be faster in high-intensity light when compared to the reaction in ambient light. PF 3E irreversibly adsorbed to oxidized silicon and covalently attached amine monolayers supported on silicon, producing hydrophobic thin films in the thickness range of 8-40 A. Adsorption conditions such as polymer concentration and solvent composition were investigated. Radical grafting of maleic anhydride to the polymer backbone resulted in increased adsorption on oxidized silicon.     

Tuning the cooperativity of the helix-coil transition by aqueous reverse micelles

We show in this letter that the thermodynamic properties of helical peptides can be tuned by varying the degrees of backbone hydration. The latter was achieved by solubilizing peptides in the water pool of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles with different water contents or w0 values. Far-UV circular dichroism measurements on a series of alanine-rich peptides indicate that the helicity of shorter peptides is significantly increased in AOT reverse micelles at low w0 values, as compared to the corresponding helical content in buffer. This result therefore corroborates the previous simulation studies suggesting that desolvation of backbone CO and NH groups increases the stability of monomeric helices. In addition, it was found that the thermal unfolding transition of these peptides can either be very noncooperative or very cooperative, depending on w0 and peptide chain length. A simple model, which considers the heterogeneous distribution of the water molecules inside the polar core of AOT reverse micelles as well as the geometric confinement effect exerted on the peptide by the reverse micelles, was used to interpret these results.     

Playing with peptides: how to build a supramolecular peptide nanostructure by exploiting helix···helix macrodipole interactions

A novel method to build bicomponent peptide self-assembled monolayers (SAMs) has been developed, by exploiting helix···helix macrodipole interactions. In this work, a peptide-based self-assembled monolayer composed of two helical peptides was immobilized on a gold surface. Specifically, a pyrene-containing octapeptide, devoid of any sulfur atom (A8Pyr), and a hexapeptide, functionalized at the N-terminus with (S,R) lipoic acid, for binding to gold substrates (SSA4WA) via a Au-S linkage, have been employed. Both peptides investigated attain a helical structure, because they are almost exclusively formed by strongly folding inducer C(α)-tetrasubstituted α-amino acids. We demonstrate that the two peptides generate a stable supramolecular nanostructure (a densely packed bicomponent peptide monolayer), where A8Pyr is incorporated into the SSA4WA palisade by exploiting helix···helix macrodipole interactions. The presence of both peptides on the gold surface was investigated by spectroscopic and electrochemical techniques, while the morphology of the monolayer was analyzed by ultra high-vacuum scanning tunnelling microscopy. The composition of the bicomponent SAM on the surface was studied by a combination of electrochemical and spectroscopic techniques. In particular, the amount of Au-S linkages from the sulfur-containing peptides was quantified from reductive desorption of the peptide-based SAM, while the amount of A8Pyr was estimated by fluorescence spectroscopy. The antiparallel orientation of the A8Pyr and SSA4WA peptide chains minimizes the interaction energy between the helix dipoles, suggesting that this kind of electrostatic phenomenon is the driving force that stabilizes the bicomponent SAM.     

Silica nanoparticle size influences the structure and enzymatic activity of adsorbed lysozyme

Adsorption of chicken egg lysozyme on silica nanoparticles of various diameters has been studied. Special attention has been paid to the effect of nanoparticle size on the structure and function of the adsorbed protein molecules. Both adsorption patterns and protein structure and function are strongly dependent on the size of the nanoparticles. Formation of molecular complexes is observed for adsorption onto 4-nm silica. True adsorptive behavior is evident on 20- and 100-nm particles, with the former resulting in monolayer adsorption and the latter yielding multilayer adsorption. A decrease in the solution pH results in a decrease in lysozyme adsorption. A change of protein structure upon adsorption is observed, as characterized by a loss in alpha-helix content, and this is strongly dependent on the size of the nanoparticle and the solution pH. Generally, greater loss of alpha helicity was observed for the lysozyme adsorbed onto larger nanoparticles under otherwise similar conditions. The activity of lysozyme adsorbed onto silica nanoparticles is lower than that of the free protein, and the fraction of activity lost correlates well with the decrease in alpha-helix content. These results indicate that the size of the nanoparticle, perhaps because of the contributions of surface curvature, influences adsorbed protein structure and function.     

The crystal structures of the calcium-bound con-G and con-T[K7gamma] dimeric peptides demonstrate a metal-dependent helix-forming motif

Short peptides that have the ability to form stable alpha-helices in solution are rare, and a number of strategies have been used to produce them, including the use of metal chelation to stabilize folding of the backbone. However, no example exists of a structurally well-defined helix stabilized exclusively through metal ion chelation. Conantokins (con)-G and -T are short peptides that are potent antagonists of N-methyl-D-aspartate receptor channels. While con-G exhibits no helicity alone, it undergoes a structural transition to a helical conformation in the presence of a variety of multivalent cations, especially Mg2+ and Ca2+. This complexation also results in antiparallel dimerization of two peptide helices in the presence of Ca2+, but not Mg2+. A con-T variant, con-T[K7gamma], displays very similar behavior. We have solved the crystal structures of both Ca2+/con-G and Ca2+/con-T [K7gamma] at atomic resolution. These structures clearly show the nature of the metal-dependent dimerization and helix formation and surprisingly also show that the con-G dimer interface is completely different from the con-T[K7gamma] interface, even though the metal chelation is similar in the two peptides. This represents a new paradigm in helix stabilization completely independent of the hydrophobic effect, which we define as the "metallo-zipper."     

Nonfibrous beta-structured aggregation of an Abeta model peptide (Ad-2alpha) on GM1/DPPC mixed monolayer surfaces

Adsorption and aggregation of transformed peptides and proteins onto the cell membrane surface is commonly associated with forms of amyloidosis such as Alzheimer's disease and prion disease. To address dynamic features of these pathological phenomena molecularly, the in situ Ad-2alpha model peptide deposition on glycolipid-containing monolayers was studied by using a 9 MHz quartz-crystal microbalance (QCM). The Ad-2alpha peptide has two amphiphilic alpha-helix segments, each modified with a 1-adamantanecarbonyl group at the N-terminal as a hydrophobic defect. The peptide folds in a 2alpha-helix structure in the bulk solution. In the presence of mixed monolayers of glycolipids (GM1, asialo-GM1, GM3, or LacCer) and/or dipalmitoyl phosphatidylcholine (DPPC) laminated on the QCM plate, the peptide deposition and the conformational change to beta-structure on the monolayers were accelerated. The adsorption kinetics and the amount of Ad-2alpha were dependent on the sort and contents of the glycolipid in the DPPC matrix. Although the Ad-2alpha peptide adsorbs onto most of the glycolipid membranes as monolayer coverage, it adsorbed largely onto the GM1/DPPC (30/70 mol%) mixed monolayer with characteristic kinetic behaviors. The accumulation of beta-structured nonfibrous aggregations was confirmed by AFM and fluorescence microscopy with Thioflavin T (ThT).     

Dependence of the molecular aggregation state of octadecylsiloxane monolayers on preparation methods

The molecular aggregation state of octadecylsiloxane monolayers on Si-wafer substrate surfaces prepared from octadecyltrimethoxysilane (OTMS) or octadecyltrichlorosilane (OTS) was investigated on the basis of grazing incidence X-ray diffraction (GIXD), Fourier transform infrared spectroscopy (FT-IR), contact angle measurement, field emission scanning electron microscopy (FE-SEM), and scanning force microscopy (SFM). The OTMS monolayer was prepared by using the chemical vapor adsorption (CVA) method, and the OTS monolayers, which were used as reference samples, were prepared either by chemisorption (OTS-S) or by the water-cast method (OTS-W). The GIXD, FT-IR, lateral force microscopic (LFM) measurements, and FE-SEM observation revealed that the alkyl chains in the OTMS monolayers prepared using the CVA method are in an amorphous state at room temperature. According to the LFM measurement, the transition temperature from the hexagonal crystalline phase to the amorphous phase was found to be ca. 333 K for the OTS-S monolayer prepared by the chemisorption method. However, the phase transition was not observed in the OTMS monolayer prepared by the CVA method. Also, the atomic force microscopic (AFM) observation and the contact angle measurement showed that the OTMS monolayer prepared by the CVA method has a uniform surface when compared to the OTS monolayers. These results indicated that organosilane compounds in the monolayer prepared by the CVA method were immobilized on the Si-wafer substrate surface in an amorphous state, which was quite different from the hexagonal crystalline state obtained by the chemisorption and water-cast methods.     

Enantioselective esterification of prochiral phosphonate pendants of a polyphenylacetylene assisted by macromolecular helicity: storage of a dynamic macromolecular helicity memory

A poly(phenylacetylene) bearing a phosphonic acid monoethyl ester as the pendant forms a one-handed helical structure induced by an optically active amine, and this helicity can be "memorized"after the amine is replaced by achiral diamines. The helicity memory lasts for an extremely long time but spontaneously disappears after the achiral diamines are removed by a stronger acid, indicating the dynamic nature of the helicity memory. Here we report that such a dynamic memory could be "stored" after the pendant was converted to its methyl ester with diazomethane, resulting in the generation of a phosphorus stereogenic center with optical activity. The esterification enantioselectively proceeded through chirality transfer from the induced helical conformation or the helicity memory of the polyacetylene backbone. Although the enantioselectivity was low, the pendant chirality was significantly amplified in the polymer backbone at low temperatures, resulting in higher optical activity as an excess single-handed helix than that expected from the enantiomeric excess of the pendants.     

Aqueous sodium dehydrocholate-sodium deoxycholate mixtures at low concentration

The behavior of the sodium dehydrocholate (NaDHC)-sodium deoxycholate (NaDC) mixed system was studied by a battery of methods that examine effects caused by the different components of the system: monomers, micelles, and both components. The behavior of the mixed micellar system was studied by the application of Rubingh's model. The obtained results show that micellar interaction was repulsive when the aggregates were rich in NaDHC. The gradual inclusion of NaDC in micelles led to a structural transformation in the aggregates and the interaction became attractive. The bile salts' behavior in mixed monolayers at the air-solution interface was also investigated. Mixed monolayers are monotonically rich in NaDC, giving a stable and compact adsorbed layer. Results have shown that the interaction in both micelles and monolayer is not ideal and such behavior is assumed to be due to a structural factor in their hydrocarbon backbone.