bluuebunny/crossref_metadata_2025_split huggingface dataset:
Bilayer Lipid Membranes
Authors: Stark G. 
Published: September 2012

AbstractThin lipid membranes formed across an aperture in a teflon foil are frequently used as model systems to simulate the hydrophobic diffusion barrier of biological membranes. These membranes have a thickness of about twice the length of a lipid molecule (bilayer membranes) and are prepared either by a spontaneous transition of a thick lamella made from a solution of lipids in a hydrocarbon [1] or by a direct “addition” of two monomolecular lipid layers on the two water surfaces separated by the teflon foil [2]. The aqueous phases on both sides of these planar membranes are easily accessible by electrodes which may serve to study the electrical properties of the membranes. The whole system allows the application of steady state electrical methods as well as that of fast relaxation methods. The latter include the voltage jump‐current relaxation technique [3,6], the charge pulse method [4,5] as well as the temperature jump method [6]. These kinetic techniques have been used throughout recent years to investigate ion transport phenomena across bilayer lipid membranes.Unmodified lipid membranes have been found to represent high energy barriers for the movement of inorganic cations and anions. A simple equivalent electrical circuit of the membrane consists of a parallel arrangement of the membrane resistance RM and the membrane capacitance CM (typical values: RM = 106‐108μcm2, CM = 0.3‐0.8μF/cm2). A refined version composed of three of such elements (one for the hydrocarbonlike membrane interior, the two others for the polar membrane‐water interfaces) has been suggested on the basis of a.c. measurements [7]. These measurements were, however, confined to the rather small band width of 0.1‐100 Hz. A different possibility of testing such a circuit is to make use of the voltage jump method. The induced current relaxation may be measured with a time resolution of at least 1 μs and very high sensitivity by using signal averaging techniques. For the three element circuit described above the relaxation should consist of two exponential terms. At least five different relaxation times were, however, necessary to obtain a sufficiently good fit to the experimental data [8.9]. This finding indicates that even the refined equivalent circuit is insufficient for a phenomenological description of the high frequency behaviour of bilayer lipid membranes. The complex current relaxation may be interpreted on the basis of cooperative dielectric relaxation phenomena associated with the dipoles of the polar head groups of the lipid molecules.
Ring Micelles in Membranes – the Concept of a Simple Pore Structure
Authors: Boheim G. 
Published: September 2012

AbstractIonic surfactants form spherical micelles in surfactant‐water systems. The kinetics of micelle formation is analyzed by means of conductivity measurements of surfactant monomers. Pressure‐jump experiments exhibit two relaxation processes. The slow process is assigned to a change in micelle concentration and the fast one to a change in micelle size distribution [1].Pore formation of alamethicin‐type antibiotics in two dimensional lipid membranes shows striking similarities to micelle formation. Voltage‐jump experiments demonstrate the existence of two relaxation processes which are accordingly assigned to changes in pore concentration (slow process) and to changes in pore size distribution (fast process). Contrary to the situation with spherical surfactant micelles in the case of alamethicin ring micelles (pores) the conductance of an ensemble of pores is measured. Furthermore, it is possible to analyze single fluctuating pores and the interpretation of the two relaxation processes is confirmed by independent direct measurements [2, 3].Since the molecular models of the two kinds of aggregates are based on the same mechanism: a small nucleus is formed which then grows by uptake of monomers until a steady‐state distribution of aggregates of statistically fluctuating size is reached, the theoretical description of both systems has to be equivalent.
Change of Lecithin Aggregation Due to Valinomycin‐Lipid Interaction
Authors: Walz D. 
Published: September 2012

AbstractValinomycin added to a suspension of lecithin vesicles with chlorophyll a or b incorporated into their membranes induces slight changes in the absorption spectrum of chlorophyll. The analysis of these changes revealed that this effect is not associated with the ionophoric properties of valinomycin but indicates a valinomycinlecithin interaction. As a consequence, the aggregational state of the lecithin molecules is altered in a domain which comprises about 40 lipids around one valinomycin molecule. The ability of the chlorophylls to detect different lipid aggregations arises from their solvatochromic behaviour. Using spectroscopic data for chlorophyll in different solvents, it could be inferred that a change in electrostatic interaction is the main reason for the difference of the spectrum of chlorophyll embedded in lecithin of different aggregational states. This interaction originates from a strong electric field associated with the polar moieties of the lipids which is much larger than the field imposed by transmembrane potentials. Hence the absorbance changes of chlorophyll due to such potentials are rather small compared to those induced by valinomycin.
Fluorescence Spectroscopic and Thermodynamic Studies of Monolayers Containing Pheophytin a and Lecithin
Authors: Heithier H. , Galla H.‐J. , Möhwald H. 
Published: September 2012

AbstractMixed monolayers of pheophytin a and L‐α‐dimyristoyllecithin (DML) are investigated on the water surface. The studies gain their special value from the simultaneous measurement of surface pressure and fluorescence intensity as a function of the covered area per molecule. A phase separation in the liquid state of the mono‐layer is established. Phase (1) exists almost exclusively of pheophytin molecules. Phase (2) exists essentially of DML domains solubilizing pheophytin in a concentration of 15 mol‐%. During the DML main transition the pheophytin solubility decreases to about 2 mol‐%, the excess pheophytin being precipitated within the surface layer. During the pheophytin main transition an ordered structure below the surface layer is formed. A stabilizing interaction between the pheophytin domains and their environment is observed and discussed. A sharp fluorescence change at a pressure below 0.5 dyn/cm indicates another phase transition. It very probably involves an unwrapping of pheophytin from water molecules.
Energy Transfer and Fluorescence Quenching in Chlorophyll Containing Vesicles
Authors: Luisetti J. , Galla H.‐J. , Möhwald H. 
Published: September 2012

AbstractFluorescence spectroscopic experiments are reported on dimyristoyllecithin (DML) vesicles containing pheophytin a, chlorophyll a or chlorophyll b. From the temperature as well as from the concentration dependence of the fluorescence intensity a phase separation at low temperatures is observed: The chlorophylls are solubilized by the DML phase only up to concentrations of the order of 1 mole‐%. the excess molecules being precipitated in domains containing almost exclusively the chlorophylls. – Whereas at high temperatures (30°C) for chlorophyll a containing vesicles a phase separation could not be established, this was possible in the pheophytin case. The pheophytin solubility in the liquid DML phase amounts to about 10 mole‐%, which is consistent with monolayer studies of the system. – The efficiency of energy transfer from chlorophyll b to chlorophyll a increases with decreasing temperature for small chlorophyll concentrations (< 1 mol‐%). This is due to the decrease in intermolecular distance on solidification of the lipid. The temperature dependence of the transfer efficiency is reversed at high chlorophyll concentrations (≤ 20 mole‐%). This is interpreted as a consequence of the decrease of the chlorophyll solubility in the DML phase that causes a decrease in the chlorophyll‐chlorophyll distance.
Structure and Dynamics of Phospholipid Multihilayers : An E.S.R. Study Using Biradicals as Spin Probes
Authors: Kothe G. , Ohmes E. , Meler P. , Blume A. 
Published: September 2012

AbstractThe Alexander‐Kaplan method is applied to develop a com‐ prehensive line shape model, which accounts for the E.S.R. spectra of biradicals in the fast‐rotational and slow‐motional region [1]. The spin hamiltonian employed considers Zeeman, exchange, dipole‐dipole and hyperfine interactions of the two unpaired electrons. Orientational order is described by an orientational distribution function with adjustable parameters.Temperature and angular dependent E.S.R. spectra of biradicals in phospholipid multibilayers are studied [2]. Computer simulations provide the orientational distribution functions and rotational correlation times of the spin probes. They occupy two different sites in the bilayer membrane.The orientation and motion of the spin probes is related to the structure and dynamics of the phospholipid phases. In the gel phase of dimiristoyllecith in the tilt angle is 6 = 30°. At the pre‐transition temperature 6 decreases. A small spread in 6 indicates a long periodic ripple on the bilayer surface. In the liquid crystalline phase 6 = 0°. The viscosity of the phospholipid bilayers changes abruptly at the phase transitions and varies between q = 0.2 P at 310 K and q = 3.0 P at 275 K. The fusion activation energy E, for all three phases is nearly constant and amounts to 28.9 kJ/mol I E, s 33.5 kJ/mol.Many important motions' in membranes occur with rotational times τr> 10−7 s, but the E.S.R. Of the monly employed nitroxide spin probes have been found useful, Only measuring faster motions C3i. With biradical 'pin probes, however, slower motions can be detected. This particular sensitivity of biradicals results from the anisotropy of the electron dipolar interactions, not available in monoradicals.
Lipid‐Protein‐Interaction in Model Membranes: Binding of Melittin to Lecithin Bilayer Vesicles
Authors: Galla H.‐J. , Hartmann W. , Sackmann E. 
Published: September 2012

AbstractWe have studied the interaction of melittin with lecithin bilayer membranes. Fluorescence polarisation measurements were performed beside the use of excimer forming pyrene derivatives.1. Diphenyl hexatriene, a fluorescence polarisation probe, was used to investigate the lipid phase transition. Addition of melittin leads to an increase of the transition temperature. Characteristic breaks were obtained plotting Tu as a function of the melittin concentration. The change in the width of the phase transition gives an indication of phase separation phenomenon.2. Pure pyrene lecithin membranes were agitated by melittin. A dramatic decrease in the excimer to monomer ratio was observed showing the same characteristic breaks.3. No change in the lipid phase transition temperature was obtained using pyrene decanoic acid and pyrene lecithin as probe molecules. Pyrene lecithin is involved in the lipid‐protein‐complex and therefore prevented from undergoing the lipid phase transition. Rigid patches surrounding the peptide molecule were formed. The pyrene decanoic acid is squeezed out from these regions leading to an increase of the excimer yield in the bulk lipid.4. High melittin concentration (c > 10 mole‐%) leads to a membrane disruption followed by the formation of a stable melittin micell furnished with lipids.
Infrared Spectroscopic Study of the Interaction of Alamethicin with Oriented Bilayers of Lecithin
Authors: Fringeli U. P. , Fringeli M. , Günthard H. 
Published: September 2012

AbstractAlamethicin is a linear peptide of 19 residues [1,2]. When added to lipid bilayer membranes a correctly applied electric field induces pores with fluctuating diameter [3,4]. Our investigation deals with peptide‐lipid interaction in the absence of an electric field.Alamethicin and L‐α‐dipalmitoyl‐lecithin/alamethicin mixtures (molar ratios 25:1 to 250:1) were dissolved in chloroform. Oriented layers (thickness 100–500 Å) are obtained by evaporation of the solvent. Incorporation of alamethicin into the lecithin bilayers required exposure of the sample to liquid water for several hours.The same samples are obtained, when alamethicin is dissolved in the aqueous surrounding of oriented lecithin bilayers. The most prominent feature of this system is a significant change of peptide structure in going from dry or hydrated (90% rel. humidity, 25°C) to aqueous environment. In both states alamethicin is incorporated into the lecithin bilayer. In the former one about 90% of the peptide assumes a helical structure with the helix axis parallel to the hydrocarbon chains. By H–D‐exchange measurements it turned out that the helix flexibility is greater near the C‐terminus, which is found to interact with the polar headgroups of lecithin. However, in aqueous environment most probably the helices are unfolded, spanning now the lipid bilayer. The shape of the amide I band is typical for associated, unfolded alamethicin molecules (parallel β‐structure). The number of associated molecules varies from dimers to multimers. Considerable reduction of helical content in going from unpolar to polar solvent has also been observed by CD and NMR [5]. Concerning the influence of alamethicin on lecithin structure it should be mentioned that significant interaction between PO2− and ‐N(CH3)3+ groups of lecithin and Glu18, Gln19 of the peptide C‐terminus is observed. Hydrocarbon chain ordering is not influenced in the dry and hydrated state, however, it is significantly reduced in aqueous environment.
Temperature Dependent Aggregation of Bacteriorhodopsin in Dipalmitoyl‐ and Dimyristoylphosphatidylcholine Vesicles
Authors: Heyn M. P. , Cherry R. J. 
Published: September 2012

AbstractBacteriorhodopsin has been incorporated into large unilamellar vesicles of dipalmitoyl‐ and dimyristoylphosphatidylcholine. The state of aggregation of bacteriorhodopsin in these vesicles was investigated using X‐ray diffraction, freeze fracture electron microscopy, circular dichroism and rotational diffusion measurements.At temperatures below the lipid phase transition, the X‐ray diffraction results show that bacteriorhodopsin crystallizes into patches with the same hexagonal lattice as is observed in the purple membrane. This hexagonal lattice is clearly visible in the electron micrographs. Measurements of the rotational diffusion of the protein, using the decay of flash‐induced dichroism, show that the protein is completely immobilized. In agreement with these results the circular dichroism spectra exhibit the typical exciton coupling bands of the native lattice.Above the phase transition, the lattice disaggregates and the protein molecules are monomeric provided the lipid to protein ratio is sufficiently high. In the X‐ray diffraction pictures both the protein and the lipid long‐range order are absent. The freeze fracture electron micrographs show that the bacteriorhodopsin particles are randomly distributed in the plane of the lipid bilayer. In harmony with these observations rotational diffusion measurements demonstrate that the bacteriorhodopsin molecules are free to rotate about an axis normal to the plane of the membrane. The rotational correlation time shows a marked dependence on the lipid to protein ratio. The absence of exciton coupling bands in the circular dichroism spectra indicates that specific bacteriorhodopsin aggregates are absent.It seems probable that the mechanism of bacteriorhodopsin crystallisation involves the growth of crystalline lipid areas from which the protein is excluded.
Permeability of Cyclic Nucleotides Across Black Lipid Membranes
Authors: Micelli S. , Gallucci E. , Lippe C. 
Published: September 2012

AbstractThere are many evidences that cyclic AMP is the intracellular mediator or ADH effect in the urinary bladder, and that theophylline, an active inhibitor of cyclic AMP phosphodiesterase, increases cellular pool of cyclic AMP.Recently, some Authors [1‐3] reported that the effect of cyclic AMP on urea permeability in toad and frog urinary bladders is evident after treatment with theophylline.Mendoza and Nakamoto [4], studying the action of theophylline when the serosal medium contained high potassium concentration concluded that theophylline “stimulates osmotic water flow and short‐circuit current across the toad bladder by a mechanism other than by inhibition of cyclic nucleotide phosphodiesterase or alteration in permeability to cyclic AMP”. Furthermore, Urakabe et al. [5] have suggested that theophylline reduces the permeability of the toad bladder to cyclic nucleotide by decreasing its efflux from the epithelial cells.In the present work we studied the permeability of the cyclic AMP (AMPc), monobutyryl cyclic AMP (mb‐AMPc), dibutyryl cyclic AMP (db‐AmPc), 5'‐AMP, adenine, adenosine, urea and acetamide across the black lipid membranes, and the effect of theophylline on their permeability.
Photochemically Induced Reactions in Vesicles
Authors: Morris D. , Thomas J. K. 
Published: September 2012

AbstractExcited states of pyrene P, were produced in lecithin vesicles by excitation with a‐very short < 1 nsec pulse of Cerenkov light. Several different reactions of excited pyrene in the vesicles were subsequently studied:
The reaction of excited pyrene with ground state pyrene to produce excimers
The quenching of P with hydrophillic quenchers located mainly in the aqueous phase and
With hydrophobic quenchers located in the lipid phase.
The kinetics of the reaction depend uniquely on the spacial configuration of the reactants and, two dimensional as well as conventional three dimensional kinetics were observed. In particular data in type a) shows a tendency for pyrene to aggregate' in the lipid phase, as cited earlier by Sackmann et al.
Pulse Radiolysis Studies in Model Lipid Systems. The Influence of Aggregation on Kinetic Behavior of OH Induced Radicals in Aqueous Sodium Linoleate
Authors: Patterson L. K. , Hasegawa K. 
Published: September 2012

AbstractThe kinetic and spectral behavior of radicals formed by hydroxyl radical attack on linoleate anions has been studied by pulse radiolysis. Reactivity of ·OH toward this surfactant is an order of magnitude greater in monomeric form [k·OH+linoleate= 8.0 ± 109M−1sec−1] than in micellar form [k·OH+lin(micelle)= 1.0 ± 109M−1sec−1]. Abstraction of a hydrogen atom from the doubly allylic position gives rise to an intense absorption in the UV region (δmax= 282–286 nm, ε 3·104M−1cm−1) which may be used as a probe of radical activity at that site. This abstraction may occur, to a small extent, directly via ·OH attack. However, greater than 90% of initial attack occurs at other sites. Subsequent secondary abstraction of doubly allylic H atoms appears to occur predominantly by: (1) intramolecular processes in monomers, (2) intermolecular processes in micelles. Disappearance of radicals by secondary processes is slower in the micellar pseudo phase than in monomeric solution.
Behaviour of Mixtures of Different Surfactant Types in Solution and on Interfaces
Authors: KurzendC.‐W C.‐P. , Schwuger M.‐J. , Lange H. 
Published: September 2012

AbstractThe formation of mixed micelles and mixed adsorption layers in aqueous solutions of homologous and non‐homologous surfactants was investigated according to various complementary methods. – Mixed solutions of homologous surfactants show ideal mixing behaviour in the mixed micelles and mixed monolayers, in which the homologue with the longer chain prevails. – Mixed micelles and mixed monolayers of anionic and nonionic surfactants, however, show a strong deviation of the ideal behaviour within the meaning of an enhanced association and surface activity. The cause is the decrease of the electric repulsion of the ionic groups of the anionic surfactants by the ethylene oxide chains of the nonionic surfactants which are incorporated in the micelles and the monolayers. In the range of small concentrations mainly micelles of the nonionic component are formed at first. With increasing concentration there occurs an enhanced incorporation of the anionic surfactant in the micelle even below the critical micelle concentration of the pure anionic surfactant. Investigations on the equilibrium adsorption on hydrophobic interfaces clearly contribute to the problematic nature of non‐homologous surfactants.
Liquid Crystalline Structures in the Lecithin‐Cholate‐Water System Studied by NMR and X‐Ray Diffraction Methods
Authors: Arvidson G. , Fontell K. , Johansson L. B.‐Å , Lindblom G. , Ulmius J. , Wennerström H. 
Published: September 2012

AbstractAlthough bile salts have several properties typical for a surfactant substance they show features that differ from those of ordinary amphiphiles. For instance, they do not aggregate in the cooperative manner and they never form lyotropic liquid crystalline phases. However, they have a large capacity to break lecithin bilayers by forming mixed micellar aggregates.The phase diagram for egg yolk lecithin‐sodium cholate‐water has been determined by Small and coworkers [1]. The extension and appearance of the different phases are similar to those in the phase diagrams for ordinary soap‐alcohol‐water systems. Concerning the structures on the molecular level of the different phases, suggestions have been given by Small, but no unambiguous experimental evidence for most of these structures have been reported.Here we present some different NMR studies in order to inquire into the molecular packing and order in the aggregates. Complementary x‐ray studies are also performed. 1H‐NMR shows that, in contrast to what is found for ordinary soap systems, spectra for lecithin‐cholate systems are essentially the same for the so called hexagonal (E) and lamellar phases at a given lecithin/cholate ratio. The order parameter calculated from 2H quadrupole splittings of the‐methyl groups of dilauroyl lecithin vary continuously between the E and lamellar phases, contrary to data obtained for soap systems. All these findings show that the molecular ordering is very similar in the two phases.It is also found that the molecular order decreases strongly with increasing cholate concentration.The x‐ray diffraction picture for the E‐phase shows reflexions that do not unambiguously fit the hexagonal structure.2H water quadrupole splittings of a between glass plates macroscopically oriented E‐phase sample showed that it orients uniaxially with at least a three‐fold symmetry around the symmetry axis.Lipid diffusion coefficients can be conveniently obtained with pulsed NMR for both cubic [2] and lamellar [3] phases. The diffusion coefficients obtained are of the same order of magnitude, leading to the conclusion that the molecular diffusion occur over macroscopical distances in the cubic phase [2,3]. Thus the cubic phase consists of continuous lipid aggregates. Preliminary diffusion data obtained for the E‐phase indicate that also this phase consists of continuous amphiphile aggregates. This is not compatible with the structure previously proposed, where micelles are stacked to form long rods.