Lipid-Chaperone Hypothesis: A Common Molecular Mechanism of Membrane Disruption by Intrinsically Disordered Proteins

An increasing number of human diseases has been shown to be linked to aggregation and amyloid formation by intrinsically disordered proteins (IDPs). Amylin, amyloid-β, and α-synuclein are, indeed, involved in type-II diabetes, Alzheimer's, and Parkinson's, respectively. Despite the correlation of the toxicity of these proteins at early aggregation stages with membrane damage, the molecular events underlying the process is quite complex to understand. In this study, we demonstrate the crucial role of free lipids in the formation of lipid-protein complex, which enables an easy membrane insertion for amylin, amyloid-β, and α-synuclein. Experimental results from a variety of biophysical methods and molecular dynamics results reveal that this common molecular pathway in membrane poration is shared by amyloidogenic (amylin, amyloid-β, and α-synuclein) and nonamyloidogenic (rat IAPP, β-synuclein) proteins. Based on these results, we propose a "lipid-chaperone" hypothesis as a unifying framework for protein-membrane poration.

Protein Adsorption at the Air-Water Interface by a Charge Sensing Interferometric Technique

Protein uptake at the interface of a millimeter-sized air bubble in water is investigated by a recently developed differential interferometric technique. The technique allows the study of capillary waves with amplitudes around 10^-9 m, excited at the surface of the bubble by an electric field of intensity on the order of 10 V/cm. When one studies the resonant modes of the bubble (radial and shape modes), it is possible to assess variations of interfacial properties and, in particular, of the net surface charge as a function of bulk protein concentration. Sensing the interfacial charge, the technique enables us to follow the absorption process in conditions of low concentrations, not easily assessable by other methods. We focus on bovine serum albumin (BSA) and lysozyme as representatives of typical globular proteins. To provide comprehensive insight into the novelty of the technique, we also investigated the equilibrium adsorption of sodium dodecyl sulfate (SDS) ionic surfactant for bulk concentrations at hundreds of times lower than the Critical Micelle Concentration (CMC). Results unveil how the absorption of charged molecules affects the amplitudes of the bubble resonant modes even before affecting the frequencies in a transition-like fashion. Different adsorption models are proposed and developed. They are validated against the experimental findings by comparing frequency and amplitude data. By measuring the charging rate of the bubble interface, we have followed the absorption kinetics of BSA and lysozyme recognizing a slow, energy barrier limited phenomena with characteristic times in agreement with data in the literature. The evaluation of the surface excess concentration (Γ) of BSA and SDS at equilibrium is obtained by monitoring charge uptake. At the investigated low bulk concentrations, reliable comparisons with literature data from equilibrium surface tension isotherm models are reported.

Helical Inclusions in Phospholipid Membranes: Lipid Adaptation and Chiral Order

The lipid bilayer is a flexible matrix that is able to adapt in response to the perturbation induced by inclusions, such as peptides and proteins. Here we use molecular dynamics simulations with a coarse-grained model to investigate the effect of a helical inclusion on a lipid bilayer in the liquid disordered phase. We show that the helical inclusion induces a collective tilt of acyl chains, with a small, yet unambiguous difference between a right- and a left-handed inclusion. This behavior is rationalized using the elastic continuum theory: The magnitude of the chiral (twist) deformation of the bilayer is determined by the interaction at the lipid/inclusion interface, and the decay length is controlled by the elastic properties of the bilayer. The lipid reorganization can thus be identified as a generic mechanism that, together with specific interactions, contributes to chiral recognition in phospholipid bilayers. An enhanced response is expected in highly ordered environments, such as rafts in biomembranes, with a potential impact on membrane-mediated interactions between inclusions.

Phospholipids Critical Micellar Concentrations Trigger Different Mechanisms of Intrinsically Disordered Proteins Interaction with Model Membranes

Amyloidogenic proteins are involved in many diseases, including Alzheimer's, Parkinson's, and type II diabetes. These proteins are thought to be toxic for cells because of their abnormal interaction with the cell membrane. Simpler model membranes (LUVs) have been used to study the early steps of membrane-protein interactions and their subsequent evolution. Phospholipid LUVs formed in water solution establish a chemical equilibrium between self-assembled LUVs and a small amount of phospholipids in water solution (CMC). Here, using both experimental and molecular dynamics simulations approach we demonstrate that the insertion of IAPP, an amyloidogenic peptide involved in diabetes, in membranes is driven by free lipids in solution in dynamic equilibrium with the self-assembled lipids of the bilayer. It is suggested that this could be a general mechanism lying at the root of membrane insertion processes of self-assembling peptides.

Nanometric Surface Oscillation Spectroscopy of Water-Poor Microemulsions

Selectively exchanging metal complexes between emulsified water-poor microemulsions and concentrated solutions of mixed electrolytes is the core technology for strategic metal recycling. Nanostructuration triggered by solutes present in the organic phase is understood, but little is known about fluctuations of the microemulsion-water interface. We use here a modified version of an optoelectric device initially designed for air bubbles, in order to evidence resonant electrically induced surface waves of an oily droplet suspended in an aqueous phase. Resonant waves of nanometer amplitude of a millimeter-sized microemulsion droplet containing a common ion-specific extractant diluted by dodecane and suspended in a solution of rare earth nitrate are evidenced for the first time with low excitation fields (5 V/cm). From variation of the surface wave spectrum with rare earth concentration, we evidence uptake of rare-earth ions at the interface and at higher concentration the formation of a thin "crust" of liquid crystal forming at unusually low concentration, indicative of a surface induced phase transition. The effect of the liquid crystal structure on the resonance spectrum is backed up by a model, which is used to estimate crust thickness.

Chirality Enhancement of Porphyrin Supramolecular Assembly Driven by a Template Preorganization Effect

Cationic polylysine promotes, under neutral conditions, the spontaneous aggregation of opposite charged ZnTPPS in water. Spectroscopic investigations evidence a different preorganization of ZnTPPS onto the polypeptide matrix depending on the chain length. Spinodal decomposition theory in confined geometry is used to model this mechanism by considering the time evolution of a homogeneous distribution of randomly adsorbed particles (porphyrins) onto a rodlike polyelectrolyte (polymer) of variable length L.

An interferometric technique to study capillary waves

We describe a new interferometric technique to study gas-liquid and liquid-liquid interfaces. Bubbles and drops are subjected to an alternating electric field which excites capillary oscillations at the interface, if charged. Bubble or drop deformation is detected by the change of the internal optical path of a laser beam crossing perpendicular to the oscillation axis. Due to the closed geometry, a discrete spectrum of stationary oscillation frequencies (normal modes) is excited. The interferometric nature of the measurement and the resonant nature of the oscillation modes concur in allowing for high sensitivity, in the sub-nanometric region. We present a detailed description of the experimental setup and examples of applications of the technique to the study of both gas-liquid and liquid-liquid interfaces, either naked or with adsorbed surfactant monolayers, for bubbles and drops with diameter~1mm. In particular, the resonance frequencies and the width of the resonance peaks depend on the surface tension and the viscous dampening, respectively. We show that, by this new technique, properties of the interface can be accessed with confidence at the sub-nanometer scale, and surface phenomena, like the monolayer phase transition or the peculiarities of adsorption/desorption processes, can be unraveled in concentration regimes which are too low for existing methods.

Anomalous Behavior of Ultra-Low-Amplitude Capillary Waves. A Glimpse of the Viscoelastic Properties of Interfacial Water?

We investigate, both theoretically and by a differential interferometric technique, the behavior of large-wavelength capillary waves (of the order of 10^-4 m) selectively excited at the surface of drops and bubbles with typical eigenfrequencies of the order of 10² Hz. The resonance peaks of gas bubbles or hydrocarbon drops in water (radius less than 1 mm) highlight anomalously small dissipation in the region of ultralow (sub-nanometric) oscillation amplitudes, reaching a plateau at higher amplitudes. This is in sharp contrast to the usual oscillating systems, where an anomalous behavior holds at large amplitudes alone. Dissipation is strongly dependent on the excited vibrational modes and, in spite of remarkable numerical differences, water-vapor and water-hydrocarbon interfaces exhibit the same overall trend. A phenomenological model was developed, based on the assumption that water possesses a threshold viscoelasticity, above which it behaves like a regular viscous fluid. The well-known Deborah number was then estimated within the anomalous region and found to lie in the range of viscoelastic fluids. In agreement with previous studies of nanohydrodynamics (e.g., atomic force microscopy measurements with sub-nanometric tip motions), the present one lends support to the idea that every self-aggregating fluid exhibits yield stress behavior, including classical Newtonian fluids like water. The essential requirement is that the applied perturbation lie below a critical threshold, above which viscous behavior is recovered. Our differential interferometric technique seems particularly suitable for this type of studies, as it allows measurement of long-wavelength capillary waves with sub-nanometric resolution on the oscillation amplitudes.

Oscillations of Bubble Shape Cause Anomalous Surfactant Diffusion: Experiments, Theory, and Simulations

We investigate, both theoretically and experimentally, the role played by the oscillations of the cell membrane on the capture rate of substances freely diffusing around the cell. To obtain quantitative results, we propose and build up a reproducible and tunable biomimetic experimental model system to simulate the phenomenon of an oscillation-enhanced (or depressed) capture rate (chemoreception) of a diffusant. The main advantage compared to real biological systems is that the different oscillation parameters (type of deformation, frequencies, and amplitudes) can be finely tuned. The model system that we use is an anchored gas drop submitted to a diffusive flow of charged surfactants. When the surfactant meets the surface of the bubble, it is reversibly adsorbed. Bubble oscillations of the order of a few nanometers are selectively excited, and surfactant transport is accurately measured. The surfactant concentration past the oscillating bubbles was detected by conductivity measurements. The results highlight the role of surface oscillations on the diffusant capture rate. Particularly unexpected is the onset of intense overshoots during the adsorption process. The phenomenon is particularly relevant when the bubbles are exposed to intense forced oscillations near resonance.

Trapping of Sodium Dodecyl Sulfate at the Air-Water Interface of Oscillating Bubbles

We report that at very low initial bulk concentrations, a couple of hundred times below the critical micellar concentration (CMC), anionic surfactant sodium dodecyl sulfate (SDS) adsorbed at the air-water interface of a gas bubble cannot be removed, on the time scale of the experiment (hours), when the surrounding solution is gently replaced by pure water. Extremely sensitive interferometric measurements of the resonance frequency of the bubble-forced oscillations give precise access to the concentration of the surfactant monolayer. The bulk-interface dynamic exchange of SDS molecules is shown to be inhibited below a concentration which we believe refers to a kind of gas-liquid phase transition of the surface monolayer. Above this threshold we recover the expected concentration-dependent desorption. The experimental observations are interpreted within simple energetic considerations supported by molecular dynamics (MD) calculations.

Optimizing the crowding strategy: sugar-based ionic micelles in the dilute-to-condensed regime

In the present study, we explore the effect of concentration on micelles made by different gangliosides, which are ionic biological glycolipids bearing multisugar headgroups with huge steric hindrance. Moreover, strong preferential interactions exist among like-conformer headgroups that can keep the ganglioside micelles in a trapped configuration. We extend the well-known ionic-amphiphiles paradigm, where local condensation and micelle crowding are matched by forming larger aggregates at increasing concentration. In fact, we force the balance between interparticle and intraparticle interactions while allowing for like conformers to modulate rebalancing. In the vast experimental framework, obtained by Small Angle X-ray scattering (SAXS) experiments, a theoretical model, accounting for a collective conformational transition of the bulky headgroups, is developed and successfully tested. It allows us to shed some light on the nature and coupling of the intermolecular forces involved in the interactions among glycolipid micelles. Energy minimization leads to complex behavior of the aggregation number on increasing concentration, fully consistent with the experimental landscape. From a biological perspective, this result could be reflected in the properties of ganglioside-enriched rafts on cell membranes, with a nonlinear structural response to approaching bodies such as charged proteins.

Hydrodynamic enhancement of the diffusion rate in the region between two fluctuating membranes in close opposition: a theoretical and computational study

Periodic variation of the distance between two weakly adhering bodies gives rise to a huge tangential motions of the sandwiched solvent layer (squeezing flow). Oscillations either can be induced by an external applied field or can spontaneously arise from the coupling with the solvent heat bath. First we calculated by the Navier-Stokes equation the components of the fluid velocity near two oscillating juxtaposed plates. Then we evaluated the influence of plate oscillations on the transport properties of a trace diffusant dissolved at t = 0 in the outer medium for both deterministic and stochastic excitations. By employing both analytical (Fokker-Planck) and coarse-grained molecular dynamics (MD) simulations, we proved that the entry and migration rates of the diffusant sharply increases with the oscillation amplitudes. Enhancement was related to relevant parameters like oscillation frequency, fluid layer thickness, fluid viscosity, and temperature. An extension to the case of oscillating multistacked lamellae has been also made. Theoretical and MD results suggest a significant enhancement of the diffusant flux even in the worse situation of thermally excited small amplitude fluctuations. Excitation arising from other sources (e.g., microwave or ultrasound irradiation of solid-fluid layered systems) could have a dramatic effect on the transport phenomena. Possible implications to relevant biological problems have been discussed.

Anomalous viscosity effect in the early stages of the ion-assisted adhesion/fusion event between lipid bilayers: a theoretical and computational study

The effect of viscosity on the encounter rate of two interacting membranes was investigated by combining a non-equilibrium Fokker-Planck model together with extensive Molecular Dynamics (MD) calculations. The encounter probability and stabilization of transient contact points represent the preliminary steps toward short-range adhesion and fusion of lipid leaflets. To strengthen our analytical model, we used a Coarse Grained MD method to follow the behavior of two charged palmitoyl oleoyl phosphatidylglycerol membranes embedded in a electrolyte-containing box at different viscosity regimes. Solvent friction was modulated by varying the concentration of a neutral, water-soluble polymer, polyethylene glycol, while contact points were stabilized by divalent ions that form bridges among juxtaposed membranes. While a naïve picture foresees a monotonous decrease of the membranes encounter rate with solvent viscosity, both the analytical model and MD simulations show a complex behavior. Under particular conditions, the encounter rate could exhibit a maximum at a critical viscosity value or for a critical concentration of bridging ions. These results seem to be confirmed by experimental observations taken from the literature.

Out of equilibrium divergence of dissipation in an oscillating bubble coated by surfactants

We report measurements of the relaxation and resonance frequency of forced oscillating bubbles covered by a layer of surface-active molecules, the anionic surfactant sodium dodecyl sulfate (SDS). Less systematic investigations have been also carried out on neutral and cationic surfactants. A divergence of the viscous damping is observed at a very low bulk concentration. Subtle variations in the resonance peak are also measured. Bubble oscillations are driven by an electric field and measured with a sensitive interferometric technique. Results are interpreted with a model which takes care of the coupling between the dynamics of fluid surface oscillations and the properties of a surfactant monolayer in the vicinity of the phase transition from a gas-like distribution to a liquid-like assembly (the so-called gas-LE transition). Important charge effects are also considered. The basic assumptions of the model (cooperative adsorption of the surfactant at the air-water interface and coupling between the shape of the deformed surface and the local surfactant concentration) have been fully confirmed by extensive coarse-grained molecular dynamics simulations on model systems.

New interferometric technique to evaluate the electric charge of gas bubbles in liquids

We report a new interferometric technique to measure the electric charge at the gas-liquid interface of a bubble in a liquid. The bubble rests by buoyancy against an electrode, and an alternating electric field excites its capillary oscillations. The oscillation amplitude of the quadrupolar mode frequency is measured by the interferometer, and it is used to evaluate the electric charge. The mode frequency scales with the square root of the interfacial tension and with a -(3)/(2) power law as a function of the bubble radius. For bubbles in the millimeter diameter range in pure water, the measured negative charge scales with the square of the radius, hence, giving a constant surface charge density on the order of 1.8 × 10(-5) C m(-2), which is rather consistent with the electrophoretic values reported in the literature.

The thermodynamics of simple biomembrane mimetic systems

Insight into the forces governing a system is essential for understanding its behavior and function. Thermodynamic investigations provide a wealth of information that is not, or is hardly, available from other methods. This article reviews thermodynamic approaches and assays to measure collective properties such as heat adsorption / emission and volume variations. These methods can be successfully applied to the study of lipid vesicles (liposomes) and biological membranes. With respect to instrumentation, differential scanning calorimetry, pressure perturbation calorimetry, isothermal titration calorimetry, dilatometry, and acoustic techniques aimed at measuring the isothermal and adiabatic processes, two- and three-dimensional compressibilities are considered. Applications of these techniques to lipid systems include the measurement of different thermodynamic parameters and a detailed characterization of thermotropic, barotropic, and lyotropic phase behavior. The membrane binding and / or partitioning of solutes (proteins, peptides, drugs, surfactants, ions, etc.) can also be quantified and modeled. Many thermodynamic assays are available for studying the effect of proteins and other additives on membranes, characterizing non-ideal mixing, domain formation, bilayer stability, curvature strain, permeability, solubilization, and fusion. Studies of membrane proteins in lipid environments elucidate lipid-protein interactions in membranes. Finally, a plethora of relaxation phenomena toward equilibrium thermodynamic structures can be also investigated. The systems are described in terms of enthalpic and entropic forces, equilibrium constants, heat capacities, partial volume changes, volume and area compressibility, and so on, also shedding light on the stability of the structures and the molecular origin and mechanism of the structural changes.

Simple interpretative model for the anomalous behavior of the excess surface area in mixed systems with large composition fluctuations: a theoretical analysis and an experimental investigation of mixed phospholipid/omega-3 fatty acid Langmuir-Blodgett films

We derive an elementary phenomenological lattice hole model to describe the complex behavior of the excess surface area as a function of the composition in two-component monolayers. The model accounts for the coupling between the composition fluctuations and the density (holes) concentration in mixed films. When the composition fluctuations are large and the hole-composition coupling parameters are strong, our model predicts the occurrence of a double maximum (or minimum) in the plot of the excess surface area A(E) against the film composition. The theoretical results are compared with excess surface area measurements of Langmuir-Blodgett films of mixed phospholipid/omega-3 fatty acids performed at different temperature, pH, and pressure.

Lamellar stacking split by in-membrane clustering of bulky glycolipids

We developed a simple model to investigate the effect of lipid clustering on the local interlayer distance in a cluster of interacting lamellae. The model, based on nonequilibrium thermodynamics and linear stability theories, explores the early stages of the lamella-lamella phase separation process where the lateral diffusion is much faster than the interlamellar lipid exchange. Results indicate, in the early stages, the presence of locally distorted regions with a higher concentration of one lipid component and an anomalous repeat distance. Experimental cases are presented, consisting of multilamellar-oriented depositions of phospholipids containing minority amounts of ganglioside or sphingomyelin under a low-hydration condition. The minority components are known to form domains within the phospholipid bilayer matrix. The low water content inhibits the lipid exchange among nearby lamellae and strengthens lamella-lamella interaction, allowing for a straightforward comparison with the model. Small-angle and wide-angle neutron diffraction experiments were performed in order to detect interlayer distances and local chain order, respectively. Lamellar stacking splitting has been observed for the ganglioside-containing lamellae, induced by in-phase lipid clustering. In excess water and after long equilibration times, these local structures may further evolve, leading to coexisting lamellar phases with different lipid compositions and interlayer distances.

Switching Direction of Laterally Ordered Monolayers Induced by Transfer Instability

Langmuir-Blodgett monolayers may show nanoscopic periodic patterns parallel and/or perpendicular to the transfer direction. The experimental findings are interpreted by a nonequilibrium model based on the stability of surfactant concentration and film thickness coupled fluctuations near the meniscus of a surfactant-covered receding thin film. In the high and low transfer speed limits, periodic fluctuations of the fluid subphase thickness, respectively perpendicular and parallel to the transfer, are selected. A qualitative phase diagram shows how transfer speed and film density manage the pattern shape.

Dielectric behavior of lipid vesicles: the case of L-alpha-dipalmitoylphosphatidylcholine vesicles as a function of size and temperature

We present an extensive set of radio wave dielectric relaxation spectroscopy measurements of aqueous suspensions of different size unilamellar L-alpha-dipalmitoylphosphatidylcholine (DPPC) vesicles, in a temperature range between 15 and 55 C, where the lipidic bilayer experiences structural transitions from the gel to the rippled phase (at the pretransition temperature) and from the rippled to the liquid phase (at the main transition temperature). The dielectric spectra have been analyzed in the light of the Cole-Cole relaxation function, and the main dielectric parameters-the dielectric increment Deltaepsilon and the mean relaxation frequency omega(0)--have been evaluated as a function of temperature. These parameters display a very complex phenomenology, depending on the structural arrangement of the lipid-water interface. The structural parameters that govern the dielectric behavior of these systems associated with the lipid bilayer have been recognized within a recent dynamic mean-field model we have proposed, aimed to predict the dipolar relaxation of an array of strongly interacting dipoles anchored to a flat or corrugated surface. They are the prefactor A(T) of the distance-dependent part of the effective dipolar interaction energy, the term Gamma(vis), that takes into account the damping of the dipolar motion, the average dipolar distance related to the area a(0) per polar head, and the bilayer thickness. The present analysis furnishes, from a phenomenological point of view, the dependence of these parameters on the temperature and on the vesicle size.

Intermicellar interactions may induce anomalous size behavior in micelles carrying out bulky heads with multiple spatial arrangements

We report experimental and theoretical results on the concentration dependence of the micellar size of GM1 and GM1acetyl gangliosides, five-sugar-headed anionic glycolipids. Contrary to one of the mainstays of colloid science, that the aggregation number of amphiphile aggregates grows with concentration, an anomalous region is found at intermediate concentrations, where a sharp decrease of the aggregation number occurs. Experiments were performed by small-angle X-ray and neutron scattering (SAXS and SANS). Two models are discussed, reproducing the observed behavior of either GM1acetyl or GM1. The first one is a conventional picture of interacting micelles where a reduction in the molecular surface area, leading to an increase of the aggregate dimension, is paid to reduce intermicellar interactions: it foresees a monotonous increase of the aggregation number with concentration. The second one accounts for a conformational bistability of the bulky headgroups of GM1, modifying the amphiphilic molecular surface area and protrusion from the aggregate surface, and contributing to the inter- and intramicellar interaction balance. Energy minimization leads to a complex behavior of the aggregation number, which is consistent with the anomalous behavior of GM1.

Concentration fluctuations and collective properties in mixed liquid systems: Rayleigh-Brillouin spectra oftert-butyl alcohol/ 2,2′-dimethylbutane liquid mixture

Rayleigh-Brillouin spectra have been measured in a range of temperatures and compositions of t-butyl alcohol/2,2(')-dimethylbutane liquid mixture. The mixture mole fraction has been varied from pure alkane (x(TBA)=0) to pure alcohol (x(TBA)=1) at temperatures between 283 and 323 K. In the same composition and temperature ranges the authors also executed measurements of mass density, shear viscosity, and refractive index. From light scattering spectra the authors have extracted the hypersound velocities and adiabatic compressibilities and evaluated their excess values. Moreover, the authors attempted to evaluate the isothermal (40 degrees C) Landau-Placzek ratios at various mole fractions, but these values proved to be subject to significant errors due to great uncertainty in the central component intensity measurements. Thus, in discussing the results, this latter quantity was considered only from a qualitative point of view. These results highlight a nonideal behavior of the studied liquid mixture with a probable azeotropic composition around x(TBA)=0.7 due to formation of small clusters of hydrogen-bonded alcohol tetramers that are completely surrounded by solvent molecules and analogous or smaller clusters. These clusters, shaped as inverse micelles, offer their hydrophobic moiety towards the molecules that constitute the solvation shell, resulting in a low polarity solution structure that minimizes the solute-solvent interactions. Differences in thermal and compositional behavior of excess molar volumes and adiabatic compressibilities have been interpreted by attributing different weights to the solute-solvent interaction forces and to the hydrogen bond connectivity effects.

Supra-aggregates of fiber-forming anisotropic molecules

In this paper, the self-organization of fiber-forming anisotropic molecules is inspected both theoretically and experimentally. In the first part, a theoretical model which extends the de Gennes theory of thin films to assemblies of strongly anisotropic molecules is reported. The model predicts that solid supported thin films made up of fiber-forming discotic molecules can grow with both tangential and radial arrangement of the fibers, respectively leading to the formation of compact and holed supra-aggregates. These last systems form according to the following picture. The tangential growth minimizes the number of unfavorable free ends but introduces elastic strain especially in the central region of the aggregate. To reduce the elastic strain, some molecules are displaced from the central region toward the periphery of the growing aggregate, producing a localized well. In the second part of the paper, we experimentally face the above issue by depositing a strongly anisotropic disk-shaped molecule (rhodamine 123) onto different solid substrates through a spin coating procedure. By employing scanning force microscopy (SFM), the formation of thermodynamically favored fiberlike supramolecules as well as of compact and holed submicron-sized supra-aggregates has been demonstrated. The observed phenomena have been found to depend on the interplay of different parameters such as molecular concentration, evaporation time, and substrate composition. As main features, both theory and experiments show that holed supra-aggregates are more stable beyond a critical aggregation size and that the formation of holes is favored at high supersaturation. The theory seems valuable in extending previous dewetting models developed for fluid films with isotropic interaction forces.

A mechanistic study of the permeation kinetics through biomembrane models: gemcitabine-phospholipid bilayer interaction

The kinetics of the interaction between Gemcitabine (a new anticancer drug) and phospholipid membrane models was investigated. This kind of study is of particular importance both in hypothesizing the interaction of Gemcitabine with mammalian cell membranes and in evaluating the potentiality of liposomes as a Gemcitabine delivery system. Unilamellar (LUV) and multilamellar (MLV) membrane models were made up of dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidic acid sodium salt (DMPA), or a DMPC-DMPA mixture (1:1 molar ratio). Gemcitabine-phospholipid vesicle interaction was studied by differential scanning calorimetry (DSC) measurements performed at different time intervals. The findings showed slower permeation kinetics of Gemcitabine through MLV than LUV which, at the same lipid/water ratio, are characterized by a larger lipid surface in contact with the drug aqueous solution. Another interesting difference between LUV and MLV is the onset of a transient two-peak structure during the DSC scans of MLVs. The effect is due to the unequal distribution of the drug between the outer and inner bilayers of the multilamellar vesicles during the permeation kinetics. At equilibrium the two-peak structure merges into a unique peak. This finding may provide useful information about the lipid bilayer permeability in model membranes.

Pressure-induced formation of diblock copolymer "micelles" in supercritical fluids. A combined study by small angle scattering experiments and mean-field theory. II. Kinetics of the unimer-aggregate transition

We developed a simple time-dependent mean-field theory to describe the phase separation kinetics of either homopolymers or AB-diblock copolymers in supercritical (SC) fluids. The model, previously used to describe the phase behavior of AB-block copolymers under the assumption of strong solvent selectivity for just one copolymer chain, has been extended to study the kinetics of the phase separation process. Time resolved small angle x-ray scattering (TR-SAXS) measurements have been performed on different AB-diblock copolymers containing a perfluorinated chain and dissolved in SC-CO2. The data obtained over a wide range of pressure and temperature confirm our theoretical predictions. Particularly interesting is the presence of two relaxation frequencies for the homogeneous solution --> spherical aggregate transition, where the two relaxation processes depend on the depth of the pressure jump and on temperature. The whole phenomenon could be explained as an initial SC solvent/polymer phase separation followed by a slow reorientation process to form spherical aggregates driven by the copolymer solvophilic moiety.

Pressure-induced formation of diblock copolymer “micelles” in supercritical fluids. A combined study by small angle scattering experiments and mean-field theory. I. The critical micellization density concept

We developed a simple mean-field theory to describe polymer and AB diblock copolymer phase separation in supercritical (SC) fluids. The highly compressible SC fluid has been described by using a phenomenological hole theory, properly extended to consider the solvent/polymer/vacancy pseudoternary mixture. The model has been applied to describe the phase behavior of AB-diblock copolymers under the assumption of a strong solvent selectivity for just one copolymer chain. In our model the solvent selectivity is a strong function of the external pressure because in compressible fluids vacancies reduce the number of favorable solvent-polymer contacts. The combined effect of the pressure on the average solvent quality and selectivity for a single polymer chain makes the phase behavior of a diblock copolymer in SC fluids quite complex. Small angle neutron and x-ray scattering (SANS and SAXS) measurements have been performed on SC-CO2 solutions of different AB-diblock copolymers containing a perfluorinated chain. The data obtained over a wide range of pressure and temperature confirm our theoretical predictions.

Cooperative behavior of ganglioside molecules in model systems

A concise discussion of the role of different geometrical conformational states in the process of self-assembling of gangliosides is given. The report focuses on the effects of the geometrical variations occurring in the head group region of gangliosides as reflected on the geometrical properties of the whole assembly. Collective phenomena happening at the water interfacial region are found to be coupled to the phase transition of the lipid moiety, that is, to the well-known order-disorder conformational transition involving the hydrophobic tails. The possible biological relevance of the head group bistability is envisaged.

From achiral porphyrins to template-imprinted chiral aggregates and further. Self-replication of chiral memory from scratch

The presence of 10-13 M of chiral clusters of aromatic amino acids addresses aggregation of opposite-charged achiral porphyrin towards the formation of smart chiral assemblies. The latter supramolecular complexes are able to self-propagate and transfer their chiral information with a 100% yield. The chiral bias occurs through a correlated sequence of induction, memory, and amplification of chirality that strongly recalls possible prebiotic scenarios.

Spectroscopic and molecular dynamics simulation studies of the interaction of insulin with glucose

The interaction between monomeric insulin and monosaccharides has been investigated through circular dichroism, fluorescence spectroscopy and two dimensional nuclear magnetic resonance. CD spectra indicate that D-glucose interacts with monomeric insulin whereas D-galactose, D-mannose and 2-deoxy-D-glucose have a lower effect. Fluorescence emission was quenched at sugar concentrations of 5-10 mM. Titration with the different sugars produces a quenching of the tyrosine spectrum from which a binding free energy value for the insulin-sugar complexes has been evaluated. Transfer nuclear Overhauser enhancement NMR experiments indicate the existence of dipolar interactions at short interatomic distances between C-1 proton of D-glucose in the beta form and the monomeric insulin. Further, NMR total correlation spectra experiments revealed that the hormone is in the monomeric form and that upon addition of glucose no aggregation occurs. The interaction does not involve relevant changes in the secondary structure of insulin suggesting that the interaction occur at the side chain level. Molecular dynamics simulations and modeling studies, based on the dynamic fluctuations of potential binding moiety sidechains, argued from results of NMR spectroscopy, provide additional informations to locate the putative binding sites of D-glucose to insulin.

A model for the origin of photosynthesis--III. The ultraviolet photochemistry of uroporphyrinogen

The photochemical ramifications of the high ultraviolet flux on the primordial earth prior to the formation of the ozone layer have been considered in a study of the ultraviolet photochemistry of uroporphyrinogen (urohexahydroporphyrin), a colorless compound which absorbs strongly at wavelengths less than 220 nanometers. Urohexahydroporphyrin was investigated since it is the first macrocycle formed on the biosynthetic pathway of chlorophyll and can be used to test the hypothesis that the biosynthetic pathway to chlorophyll recapitulates the evolutionary history of photosynthesis. When urohexahydroporphyrin is illuminated in aqueous anaerobic solution, hydrogen gas is produced. More hydrogen gas is produced in the presence of a colloidal platinum catalyst. The products of the photooxidation of urohexahydroporphyrin are urotetrahydroporphyrin (uroporphomethene) and uroporphyrin. This research shows how the oxidation of uroporphyrinogen to uroporphyrin, the first biogenetic porphyrin, could have occurred anaerobically and abiotically on the primordial earth.

Binding of Lipid Vesicles to Protein-Coated Solid Polymer Surfaces: A Model for Cell Adhesion to Artificial Biocompatible Materials

The adhesion of lipid vesicles (liposomes) having controlled chemical and physical structure to polymer supported human serum albumin (HSA) thin layers was investigated by a spectrofluorimetric technique. The vesicle lipid bilayer was labeled with a small amount of an apolar fluorescent probe (diphenylexathriene) and the vesicle suspension was set in contact with the protein film. After washing and drying, the adhering vesicles containing sample was dissolved in chloroform and the homogeneous solution was analyzed by standard spectrofluorimetric techniques. Different parameters of the lipid bilayer, suspending solution, and protein film were varied and their influence on the liposome binding was investigated. Concerning the lipid bilayer, we studied the effect of liposome surface charge by using different mixtures of neutral (dipalmitoyl-phosphatidylcholine) and charged (dipalmitoyl-phosphatidic acid) phospholipids and the fluid or gel nature of the lipid bilayer (switched on and off by temperature variation). Variations of the local environment involve Ca(2+) and H(+) changes in the millimolar range as well as different hydrodynamical flows (in the range 0.1-10 cm/s). Preliminary measurements using different protein layers were also performed. Results show: (a) negligible adhesion without the protein layer, (b) the presence of a maximum for the liposome adhesion vs ion concentration (depending on the liposome composition and kind of the adsorbed ions), (c) a much stronger adhesion for vesicles in the fluid phase (overcoming the entropy-driven desorption increase with temperature), and (d) a dramatic lowering of the adhesion capability under hydrodynamic flow. Points a-c have been interpreted on the basis of a simple mechanoelectrical model. Copyright 2000 Academic Press.