Surface Pressure Of Monolayers Research Articles

Molecular Packing Tunes the Activity of Kluyveromyces lactis β-Galactosidase Incorporated in Langmuir−Blodgett Films

Functional consequences of constraining beta-Gal in bidimensional space were studied at defined molecular packing densities and constant topology. Langmuir-Blodgett films, LB15 and LB35 composed of dipalmitoyl phosphatidylcholine and K. lactis beta-Gal, were obtained by transferring Langmuir films (L) initially packed at 15 and 35 mN/m, respectively, to alkylated glasses. The beta-Gal-monolayer binding equilibrium, mainly the adsorption rate and affinity, depended on the initial monolayer's surface pressure (lower for higher pi i). At pi i = 15 and 35 mN/m, the surface excess (Gamma) followed downward parabolic and power-law tendencies, respectively, as a function of subphase protein concentration. Gamma values in L roughly reflected the protein surface density chemically determined in LBs (0-7.5 ng/mm2 at pi i = 0-35 mN/m and [beta-Gal] subphase = 0-100 microg/mL). The beta-Gal-catalyzed hydrolysis of o-nitrophenyl-galactopyranoside showed a Michaelian kinetics in solution as well as in LB15. KM, KM,LB15, Vmax, and Vmax,LB15 were 5.15 +/- 2.2 and 9.25 +/- 6 mM and 39.63 and 0.0096 +/- 0.0027 micromol/min/mg protein, respectively. The sigmoidal kinetics observed with LB35 was evaluated by Hill's model (K0.5 = 9.55 +/- 0.4 mM, Vmax,35 = 0.0021 micromol/min/mg protein, Hill coefficient n = 9) and Savageau's fractal model (fractal constant K f = 9.84 mM; reaction order for the substrate gs = 9.06 and for the enzyme ge = 0.62). Fractal reaction orders would reflect the fractal organization of the environment, demonstrated by AFM images, more than the molecularity of the reaction. Particular dynamics of the protein-lipid structural coupling in each molecular packing condition would have led to the different kinetic responses.

Chain Conformation of Poly(dimethyl siloxane) at the Air/Water Interface by Sum Frequency Generation

This is to report a study of chain conformation of poly(dimethylsiloxane) (PDMS) in spread monolayers at the air/water interface (A/W) with the aid of vibrational sum frequency spectroscopy (VSFS). We find that methyl groups of PDMS chains at the interface are completely disordered in the dilute regime of the surface density. At higher surface densities, however, the two methyl groups on the repeating unit point into the air asymmetrically; one points more normal to the interface, whereas the other lies more parallel to the interface. In the first collapsed regime, where the surface pressure of the PDMS monolayer reaches a plateau value of 8.7 mN/m, the signal intensity at 2915 cm (-1), assigned to the symmetric vibrational frequency of the methyl groups, is found independent of the surface density. On the basis of this finding, we propose that PDMS chains, in the first collapse regime at the A/W, form asymmetric layers. Thus, our proposal lends support to earlier works by Langevin's group to refute a widely speculated helix model that was based on energy minimization in the crystalline state of PDMS. In short, the energy consideration in the bulk crystalline state does not provide meaningful guidance as to the chain conformation of the monolayer at the A/W.

Electrostatic Contribution to the Surface Pressure of Charged Monolayers Containing Polyphosphoinositides

Structural and functional studies of lateral heterogeneity in biological membranes have underlined the importance of membrane organization in biological function. Most inquiries have focused on steric determinants of membrane organization, such as headgroup size and acyl-chain saturation. This manuscript reports a combination of theory and experiment that shows significant electrostatic contributions to surface pressures in monolayers of phospholipids where the charge spacing is smaller than the Bjerrum length. For molecules with steric cross sections typical of phospholipids in the cell membrane (∼50 Å 2), only polyphosphoinositides achieve this threshold. The most abundant such lipid is phosphatidylinositol bisphosphate, which has between three and four charged groups at physiological conditions. Theory and experiment show that surface pressure increases linearly with phosphatidylinositol bisphosphate net charge and reveal crossing of high and low ionic strength pressure-area isotherms, due to opposing effects of ionic strength in compressed and expanded monolayers. Theory and experiment show that electrostatic effects are negligible for monolayers of univalent lipids, emphasizing the unique importance of electrostatic effects for lateral organization of polyphosphoinositides. Quantitative differences between theory and experiment suggest that attractive interactions between polyphosphoinositides, possibly mediated by hydrogen bonding, can lessen the effect of electrostatic repulsions.

Molecular Determinants of Sphingomyelin Specificity of a Eukaryotic Pore-forming Toxin

Sphingomyelin (SM) is abundant in the outer leaflet of the cell plasma membrane, with the ability to concentrate in so-called lipid rafts. These specialized cholesterol-rich microdomains not only are associated with many physiological processes but also are exploited as cell entry points by pathogens and protein toxins. SM binding is thus a widespread and important biochemical function, and here we reveal the molecular basis of SM recognition by the membrane-binding eukaryotic cytolysin equinatoxin II (EqtII). The presence of SM in membranes drastically improves the binding and permeabilizing activity of EqtII. Direct binding assays showed that EqtII specifically binds SM, but not other lipids and, curiously, not even phosphatidylcholine, which presents the same phosphorylcholine headgroup. Analysis of the EqtII interfacial binding site predicts that electrostatic interactions do not play an important role in the membrane interaction and that the two most important residues for sphingomyelin recognition are Trp(112) and Tyr(113) exposed on a large loop. Experiments using site-directed mutagenesis, surface plasmon resonance, lipid monolayer, and liposome permeabilization assays clearly showed that the discrimination between sphingomyelin and phosphatidylcholine occurs in the region directly below the phosphorylcholine headgroup. Because the characteristic features of SM chemistry lie in this subinterfacial region, the recognition mechanism may be generic for all SM-specific proteins.

Molecular Organization, Structural Orientation, and Surface Topography of Monoacylated β-Cyclodextrins in Monolayers at the Air−Aqueous Interface

The surface behavior of monoacylated beta-cyclodextrins, with hydrocarbon chains of 16, 14, and 10 carbons, has been assessed by the measurement of the surface pressure, surface (dipole) potential, optical reflectivity, and surface topography in monolayers at the air-water interface. For all the derivatives studied, the intermolecular organization adopted along compression-decompression isotherms reveals a rich variety of packing states which imply profound reorganization of the hydrophobic and hydrophilic moieties of the beta-cyclodextrin derivatives in the film, depending on the lateral surface pressure. The intermolecular arrangements are consistent with the adoption of a different and defined orientation of the cyclic oligosaccharide unit, relative to the interfacial plane and the aqueous subphase. This is different from the behavior of the per-substituted derivatives, and none of the changes exhibited by the monosubstituted forms are consistent with the oligosaccharide ring remaining in a fixed orientation along the interface when the surface pressure is varied.

Fatty acid monolayer dissociation and collapse: Effect of pH and cations

Surface pressure versus specific molecular area ( π– A) isotherms of stearic acid monolayers have been studied by varying the pH of the aqueous subphase containing monovalent (Na) or different divalent (Mg, Ca, Ba, Zn and Cd) cations. The lowering of the tilted to untilted phase transition pressure ( π t) with increasing pH of the subphase is correlated with an increase of the headgroup dissociation of the acid monolayer. On pure water, the surface pressure drops after the collapse point ( π c) to a lower pressure value ( π l). The pressure-gap Δ π = π c − π l changes with increasing dissociation, depending upon the particular cation. After a certain pH, no pressure gap is observed and π remains constant or increases slowly after π c. This transition from constant area collapse to constant pressure collapse of the monolayer is observed for all monovalent and divalent cations near total monolayer dissociation.

The psychotropic drug olanzapine (Zyprexa®) increases the area of acid glycerophospholipid monolayers

The typical antipsychotics chlorpromazine (CPZ) and trifluoperazine (TFP) increase the mean molecular area (mma) of acidic, but not neutral, glycerophospholipids in monolayers at pH 7.36 measured by the Langmuir technique. The atypical antipsychotic olanzapine (OLP 1 1 The uncharged and positively charged forms of OLP are designated OLP 0 and OLP +, respectively. ) is structurally similar to TFP. We have therefore studied the effects of OLP on glycerophospholipid monolayers and in comparison with CPZ. Olanzapine (10 µM, in subphase, pH 7.36) influenced the isotherms (surface pressure versus mma) in monolayers of the neutral dipalmitoyl phosphatidylcholine (DPPC) and the acidic dipalmitoyl phosphatidylserine (DPPS) or 1-palmitoyl-2-oleoylphosphatidylserine (POPS) in the increasing order of mma: DPPS < DPPC < POPS at both lower and higher temperature. Thus, presence of an unsaturated acyl in PS increased the drug-induced effect on mma. The mma in the absence of drugs was lower at lower temperatures than at higher temperatures. OLP affected mma to a greater extent than CPZ, and caused the greatest interaction at surface pressure of 30 mN/m at higher temperatures. In contrast, CPZ gave the largest effect in the monolayers at surface pressure 30 mN/m at lower temperatures. CPZ did not alter the isotherms of DPPC, at lower or higher temperature, and only affected the packing of the DPPS and POPS monolayers. In contrast, OLP altered the isotherms of DPPC. It is suggested that the drugs affect the monolayer packing by intercalating between the glycerophospholipid molecules. Since CPZ has major side effects, while OLP has few, this may indicate that there is poor correlation between side effects and effects of the drugs on phospholipid monolayers.

Hexagonal Nanoarchitecture of Composite Monolayer of Magnetite Nanoparticles and Geminus Surfactant 1,3‐Propylenebis(dodecyldimethylammonium) Dibromide

AbstractNegatively charged magnetite nanoparticles with an average size of about 10 nm have been synthesized by a chemical coprecipitation method using sodium dodecyl benzene sulphonate as a surface modifying reagent. Composite Langmuir monolayer of Fe3O4 nanoparticles and geminus surfactant 1,3‐propylenebis(dodecyldimethylammonium) dibromide (C12‐C3‐C12) was prepared on the subphase of Fe3O4 nanoparticle hydrosols. In the presence of the magnetite nanoparticles, the collapse pressure of the composite monolayer and the limited mean molecular area of C12‐C3‐C12 are higher than those on pure water subphase. Transmission electron microscopy observation of a C12‐C3‐C12/Fe3O4 nanoparticle complex shows that Fe3O4 nanoparticles and geminus surfactant had an unexpected hexagonal nanoarchitecture at the air‐liquid interface when the surface pressure of the composite monolayer increased to about 12 mN·m−1. A mechanism for constructing the particular nanopatterned configuration of the C12‐C3‐C12/Fe3O4 nanoparticle complex in the Langmuir layer directly from the unique molecular structure of the geminus surfactant and the interfacial interactions between C12‐C3‐C12 and the components in the subphase was proposed.

Biphasic behavior of bupivacaine and cardiolipin-containing biomimetic membrane interaction

Intravenous administration of bupivacaine, a lipophilic long-acting local anesthetic can cause serious cardiotoxicity. However, the mechanisms underlying this response are uncertain. Cardiolipin (CL) is the predominant polyglycerophospholipid that exists in the mitochondrial membrane. In a recent study we reported that bupivacaine, but not lidocaine, disrupts cardiolipin-containing liposomes by increasing membrane permeability. This study further investigated the change in surface pressure of phospholipid monolayers after incorporation of local anesthetics. From this study, we found that relatively high concentrations of bupivacaine evoked a biphasic behavior when model membrane contained CL. At low bupivacaine concentration, such biphasic behavior was not observed even though surface pressure increased initially. In contrast, lidocaine had no significant effects on membrane surface pressure and integrity even in the presence of CL. These data are consistent with our previous findings using liposomes. We suggest that in vivo cardiotoxic effects of bupivacaine on cardiolipin-containing mitochondrial membranes can be related and predicted from in vitro biphasic behavior of bupivacaine interaction with biomimetic phospholipid monolayers.

Superficially active water in lipid membranes and its influence on the interaction of an aqueous soluble protease

The purpose of this paper is to demonstrate that the interaction of an aqueous soluble enzyme with lipid membranes is influenced by the lipid composition of the interphase. The results show that the interaction of an aqueous soluble protease, Rennet from Mucor miehei, depends on the exposure of the carbonyl and phosphate groups at the membrane interphase. The changes produced by the protease on the surface pressure of monolayers of dimyristoylphosphatidylcholine (DMPC); dioleoylphosphatidylcholine (DOPC); diphytanoylphosphatidylcholine (DPhPC); dipalmitoylphosphatidylcholine (DPPC); di- O-tetradecylphosphatidyl-choline [D(ether)PC]; dimyristoylphosphatidylethanolamine (DMPE); di- O-tetradecyl-phosphatidylethanolamine [D(ether)PE] were measured at different initial surface pressures. The meaning of the Δ Π vs. Π curves was interpreted in the light of the concept of interphase given by Defay and Prigogine [R. Defay, I. Prigogine, Surface Tension and Adsorption, John Wiley & Sons, New York, 1966, pp. 273–277] considering the interphase as a bidimensional solution of polar head groups. With this approach, and based on reported evidences that carbonyls and phosphates are the main hydration sites of the lipid membranes, it is suggested that the mechanism of interaction of aqueous soluble protein involves water beyond the hydration shell. At high surface pressure, only water strongly bound to carbonyl and phosphate groups is present and the interaction is not occurring. In contrast, at low surface pressures, the protease-membrane interaction is a function of acyl chain for different polar groups. This is interpreted, as a consequence of the changes in the interfacial tension produced by the displacement of water confined between the hydrated head groups.

Monoglycerides and β-Lactoglobulin Adsorbed Films at the Air−Water Interface. Structure, Microscopic Imaging, and Shear Characteristics

In this work we have analyzed the structural, topographical, and shear characteristics of mixed monolayers formed by adsorbed beta-lactoglobulin (beta-lg) and spread monoglyceride (monopalmitin or monoolein) on a previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm, Brewster angle microscopy (BAM), and surface shear characteristics were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The pi-A isotherm and BAM images deduced for adsorbed beta-lactoglobulin-monoglyceride mixed films at pi lower than the equilibrium surface pressure of beta-lactoglobulin (pi(e)(beta-lg)) indicate that beta-lactoglobulin and monoglyceride coexist at the interface. However, the interactions between protein and monoglyceride are somewhat weak. At higher surface pressures (at pi > or = pi(e)(beta-lg)) a protein displacement by the monoglyceride from the interface takes place. The surface shear viscosity (eta(s)) of mixed films is very sensitive to protein-monoglyceride interactions and displacement as a function of monolayer composition (protein/monoglyceride fraction) and surface pressure. Shear can induce change in the morphology of monoglyceride and beta-lactoglobulin domains, on the one hand, and segregation between domains of the film-forming components on the other hand. In addition, the displacement of beta-lactoglobulin by the monoglycerides is facilitated under shear conditions.

Structural and shear characteristics of adsorbed sodium caseinate and monoglyceride mixed monolayers at the air–water interface

The structural and shear characteristics of mixed monolayers formed by an adsorbed Na-caseinate film and a spread monoglyceride (monopalmitin or monoolein) on the previously adsorbed protein film have been analyzed. Measurements of the surface pressure ( π)–area ( A) isotherm and surface shear viscosity ( η s ) were obtained at 20 °C and at pH 7 in a modified Wilhelmy-type film balance. The structural and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. At surface pressures lower than the equilibrium surface pressure of Na-caseinate (at π < π e CS ), both Na-caseinate and monoglyceride coexist at the interface, with a structural polymorphism or a liquid expanded structure due to the presence of monopalmitin or monoolein in the mixture, respectively. At higher surface pressures, collapsed Na-caseinate residues may be displaced from the interface by monoglyceride molecules. For a Na-caseinate–monopalmitin mixed film the η s value varies greatly with the surface pressure (or surface density) of the mixed monolayer at the interface. In general, the greater the surface pressure, the greater are the values of η s . However, the values of η s for a Na-caseinate–monoolein mixed monolayer are very low and practically do not depend on the surface pressure. The collapsed Na-caseinate residues displaced from the interface by monoglyceride molecules at π > π e CS have important repercussions on the shear characteristics of the mixed films.

Electrostatic and Hydrophobic Interactions Governing the Interaction and Binding of β-Lactoglobulin to Membranes

The role of electrostatic and hydrophobic interactions in the binding and penetration of beta-lactoglobulin (betaLG) to preformed lipid membranes was studied using various phospholipid micelles and vesicles. Zwitterionic lysophospholipid micelles are able to induce the beta-sheet to alpha-helix transition, as judged by circular dichroism (CD), but the degree of transition is dramatically below and the amount of lipid required above that for anionic phospholipids with equivalent hydrocarbon chains. Anionic phospholipids with short hydrocarbon chains induce only low alpha-helical content in betaLG as compared to phospholipids with the same head group but longer hydrocarbon chains. These results suggest that both electrostatic and hydrophobic interactions are indispensable in betaLG-lipid interaction. Furthermore, air-water interface monolayer surface pressure and fluorescence anisotropy studies reveal that the membrane insertion of betaLG strongly depends on the nature of phospholipids, given the identical headgroup, particularly lipid packing. These results are supported by urea denaturation and acrylamide fluorescence quenching tests and by the FTIR-ATR polarization results for betaLG in multilayers on a surface. Under the same experimental conditions, the membrane binding and insertion of betaLG as well as the stability of the betaLG-lipid complexes can be enhanced by lowering the pH. Collectively, electrostatic interactions play a crucial role in all the processes involved in the betaLG-lipid interaction, while the presence of hydrophobic interaction remains necessary. Finally, betaLG biological function in the transport of fatty acids was tested by demonstrating the release of 2-AS from a 2-AS-betaLG complex on binding to lipids.

Transition from Nanodomains to Microdomains Induced by Exposure of Lipid Monolayers to Air

The morphology of monolayers prepared from ternary lipid mixtures that have coexisting fluid phases has been examined by atomic force microscopy for samples transferred to mica before and after exposure to air. Mixtures of 1,2-dioleoyl- sn-glycero-3-phosphocholine and cholesterol with either egg sphingomyelin or 1,2-dipalmitoyl- sn-glycero-3-phosphocholine were studied at several surface pressures. Both lipid mixtures have a combination of small islands and large microdomains at low surface pressure (5–10 mN/m) for monolayers deposited in either air or nitrogen. By contrast, monolayers have small interconnected nanodomains when deposited under nitrogen at 30 mN/m but mixtures of large microdomains and small nanodomains when transferred after exposure to air. These results are consistent with an earlier report that concluded that the formation of large domains at high surface pressures (>30 mN/m) for monolayers exposed to air is caused by lipid oxidation. However, the higher spatial resolution available with atomic force microscopy indicates that exposure of the monolayers to air leads to an increase in the size of preexisting nanodomains, rather than a change in the miscibility pressure. Examination of changes in surface morphology as a function of surface pressure demonstrate a gradual evolution in size and surface coverage for both nano- and microdomains, before formation of a network of interconnected nanodomains. Similar studies for binary mixtures in the absence of cholesterol indicate that lipid oxidation results in analogous changes in domain size for monolayers with coexisting gel and fluid phases. These results illustrate the importance of using techniques capable of probing the nanoscale organization of membranes.

Two-dimensional order in mammalian pre-ocular tear film

We report a grazing incidence x-ray diffraction (GIXD) investigation of the surface lipid layer of the pre-ocular tear film. For the first time we demonstrate the existence of 2D order over a wide range of surface pressures in this system, with typical spicing of 3.75 Å and 4.16 Å independent of the monolayer surface pressure. Analogous lipid ordering is also found in an artificial lipid mixture of the major lipid components of the tear film, suggesting that the 2D ordering is set by generic lipid–lipid interactions. Fluorescence microscopy of the natural and artificial tear film mixture reveals the co-existence of a dilute and a much more condensed phase in the amphiphilic lipid matrix over the pressure range of 15–45 mN/m investigated by GIXD, plus an additional structure due to the much more hydrophobic part of the mixture. This evidence supports the previous hypothesis that tear film has a layered structure.

Adsorption of Carboxylate-Modified Gold Nanoparticles on an Octadecylamine Monolayer at the Air/Liquid Interface

Gold nanoparticles (Au NPs) were prepared and surface-modified by mercaptosuccinic acid (MSA) to render a surface with carboxylic acid groups (MSA-Au). Octadecylamine (ODA) was used as a template monolayer to adsorb the Au NPs dispersed in the subphase. The effect of MSA concentration on the incorporation of Au NPs on the ODA monolayer and the relevant behavior of the mixed monolayer were studied using the pressure-area (pi-A) isotherm and transmission electron microscopy (TEM) observations. The experimental results showed that the adsorbed density of Au NPs is low without the surface modification by MSA. When MSA was added into the Au NP-containing subphase, the incorporation amount of Au NPs increased with increasing MSA concentration up to approximately 1 x 10-5 M for the particle density of 1.3 x 1011 particles/mL. With a further increase in the MSA concentration, the adsorbed particle density decreases due to competitive adsorption between the free MSA molecules and the MSA-Au NPs. It is inferred that free MSA molecules adsorb more easily than the MSA-Au NPs on the ODA monolayer. Therefore, an excess amount of MSA present in the subphase is detrimental to the incorporation of gold particles. The study on the monolayer behavior also shows that the pi-A isotherm of the ODA monolayer shifts right when small amounts of Au NPs or free MSA molecules are incorporated. However, when larger amounts of particles are adsorbed at the air/liquid interface, a left shift of the pi-A isotherm appears, probably due to the adsorption of ODA molecules onto the particle surface and the transferring of the particles from beneath the ODA monolayer to the air/water interface. According to the present method, it is possible to prepare uniform particulate films of controlled densities by controlling the particle concentration in the subphase, the MSA concentration, and the surface pressure of a mixed monolayer.

Three-component microemulsions formed using pH-degradable 1,3-dioxolane alkyl ethoxylate surfactants

A series of three nonionic surfactants which undergo acid-catalyzed hydrolysis, O-[(2,2-dialkyl-1,3-dioxolan-4-yl) methoxy] – O′-methoxy poly(ethylene glycol), or cyclic ketal (CK), were synthesized. The surfactants shared similar HLB values (9.2–9.8) but differed in the relative length of their two alkyl tails. Water/CK/isooctane microemulsion systems formed by each of the three surfactants also shared similar phase behavior, physical properties, and surfactant hydrolysis kinetics. The apparent critical microemulsion concentration for three CK surfactants, 0.060–0.075 g g −1 (in both water and isooctane), and the efficiency, the minimum amount of surfactant required to totally mix water and oil into single microemulsion phase, 0.35–0.40 g g −1, are significantly higher than values possessed by regular, linear, alkyl ethoxylate surfactants. The underlying causes for the high values were investigated. Due to the broad distribution of its ethoxylate head group size, CK surfactants were distributed among all phases within two- and three-phases systems, with CKs possessing shorter ethoxylate groups partitioning to the oil phase and CKs with larger ethoxylate groups to the aqueous phase. Small-angle neutron scattering experiments suggest that CK exists as monomers or very small pre-micellar aggregates in isooctane at 12% and as ellipsoidal aggregates in water at 2.5–25%. Surface pressure versus surface area measurements of CK surfactant monolayers at the water-air interface demonstrate two differences from linear alkyl ethoxylate monolayers: the occurrence of a collapse at moderately low surface pressure (30 mN m −1) and high surface area per molecule (0.59 nm 2) and the inability to undergo dissolution when subjected to a constant, moderately high surface pressure (25 mN m −1). The results suggest that CK molecules are not able to pack as efficiently as linear alkyl ethoxylates at interfaces, and are strongly anchored to monolayers in aqueous media.

Adsorption of water-soluble polymers with surfactant character.: Adsorption kinetics and equilibrium properties

A comparative study between Langmuir and Gibbs monolayers of a hyperbranched polyol, poly(propylene glycol) homopolymers, and poly(propylene glycol)–poly(ethylene glycol) copolymers with different structure and molecular weight, is reported. Dynamic surface tension (DST) and surface pressure measurements have been carried out to characterize these amphiphilic water-soluble polymers. The adsorption kinetics results are consistent with a rapid diffusion stage followed by a slow reorganization at the air–water interface. The characteristic times of these steps, calculated by the Joos model, point out differences among the polymers in the diffusion rate and rearrangement mechanisms for diluted solutions. Short time analysis of DST data leads to diffusion coefficients in qualitative agreement with the diffusion times calculated with Joos' model. Spread monolayers remain stable for long periods of time. The desorption process seems quite inoperative. As a consequence, the surface pressure of the spread monolayers can be studied over a broad surface concentration range. 2D first-order phase transitions have been evidenced from plateaux observed in Langmuir and Gibbs isotherms. It has been found that Gibbs monolayers lead to lower surface tension states than the Langmuir ones.

Two-Dimensional Polymeric Nanomaterials through Cross-linking of Polybutadiene-b-Poly(ethylene oxide) Monolayers at the Air/Water Interface

Two-dimensional polymeric nanomaterials consisting of a continuously cross-linked polybutadiene (PB) two-dimensional network with poly(ethylene oxide) (PEO) domains of controlled sizes trapped within the PB network were synthesized. To reach that goal, novel (PB(Si(OEt)3)-b-PEO)3 star block copolymers were designed by hydrosilylation of the pendant double bonds of (PB-b-PEO)3 star block copolymer precursors with triethoxysilane. The (PB(Si(OEt)3)-b-PEO)3 star block copolymers were characterized by 1H NMR and IR spectroscopy. Self-condensation of the triethoxysilane pendant groups under acidic conditions led to a successful cross-linking of the polybutadiene blocks directly at the air/water interface without any additives or reagents. This strategy was found more efficient than radical cross-linking of (PB-b-PEO)3 with AIBN to get a homogeneously cross-linked monolayer of controlled and fixed morphology as demonstrated by the easy mechanical removal of the cross-linked Langmuir film from the water surface. As shown by AFM imaging, this strategy allows the accurate control of the PEO "pore" size depending on the monolayer surface pressure applied during the cross-linking reaction. The subphase pH and surface pressure influence on the cross-linking kinetics and monolayer morphologies were investigated by Langmuir trough studies (isotherm and isobar experiments) and AFM imaging.

The N-Terminal Domain of Bcl-xL Reversibly Binds Membranes in a pH-Dependent Manner

Bcl-xL regulates apoptosis by maintaining the integrity of the mitochondrial outer membrane by adopting both soluble and membrane-associated forms. The membrane-associated conformation does not require a conserved, C-terminal transmembrane domain and appears to be inserted into the bilayer of synthetic membranes as assessed by membrane permeabilization and critical surface pressure measurements. Membrane association is reversible and is regulated by the cooperative binding of approximately two protons to the protein. Two acidic residues, Glu153 and Asp156, that lie in a conserved hairpin of Bcl-xLDeltaTM appear to be important in this process on the basis of a 16% increase in the level of membrane association of the double mutant E153Q/D156N. Contrary to that for the wild type, membrane permeabilization for the mutant is not correlated with membrane association. Monolayer surface pressure measurements suggest that this effect is primarily due to less membrane penetration. These results suggest that E153 and D156 are important for the Bcl-xLDeltaTM conformational change and that membrane binding can be distinct from membrane permeabilization. Taken together, these studies support a model in which Bcl-xL activity is controlled by reversible insertion of its N-terminal domain into the mitochondrial outer membrane. Future studies with Bcl-xL mutants such as E153Q/D156N should allow determination of the relative contributions of membrane binding, insertion, and permeabilization to the regulation of apoptosis.