Interfacial properties of gramicidin and gramicidin-lipid mixtures measured with static and dynamic monolayer techniques

Activity of the Gramicidin A Ion Channel in a Lipid Membrane with Switchable Physical Properties

Lipid bilayer membranes formed from the artificial 1,3-diamidophospholipid Pad-PC-Pad have the remarkable property that their hydrophobic thickness can be modified in situ: the particular arrangement of the fatty acid chains in Pad-PC-Pad allows them to fully interdigitate below 37 °C, substantially thinning the membrane with respect to the noninterdigitated state. Two stimuli, traversing the main phase transition temperature of the lipid or addition of cholesterol, have previously been shown to disable the interdigitated state. Both manipulations cause an increase in hydrophobic thickness of about 25 Å due to enhanced conformational entropy of the lipids. Here, we characterize the interdigitated state using electrophysiological recordings from free-standing lipid-membranes formed on micro structured electrode cavity arrays. Compared to standard membranes made from 1,2-diphytanoyl-sn-glycero-3-phosphocholin (DPhPC), pure Pad-PC-Pad membranes at room temperature had lowered electroporation threshold and higher capacitance. Ion channel formation by the peptide Gramicidin A was clearly facilitated in pure Pad-PC-Pad membranes at room temperature, with activity occurring at significantly lower peptide concentrations and channel dwell times increased by 2 orders of magnitude with respect to DPhPC-membranes. Both elevation of temperature beyond the phase transition and addition of cholesterol reduced channel dwell times, as expected if the reduced membrane thickness stabilized channel formation due to decreased hydrophobic mismatch.

Comparison between the behavior of different hydrophobic peptides allowing membrane anchoring of proteins

Membrane binding of proteins such as short chain dehydrogenase reductases or tail-anchored proteins relies on their N- and/or C-terminal hydrophobic transmembrane segment. In this review, we propose guidelines to characterize such hydrophobic peptide segments using spectroscopic and biophysical measurements. The secondary structure content of the C-terminal peptides of retinol dehydrogenase 8, RGS9-1 anchor protein, lecithin retinol acyl transferase, and of the N-terminal peptide of retinol dehydrogenase 11 has been deduced by prediction tools from their primary sequence as well as by using infrared or circular dichroism analyses. Depending on the solvent and the solubilization method, significant structural differences were observed, often involving α-helices. The helical structure of these peptides was found to be consistent with their presumed membrane binding. Langmuir monolayers have been used as membrane models to study lipid-peptide interactions. The values of maximum insertion pressure obtained for all peptides using a monolayer of 1,2-dioleoyl-sn-glycero-3-phospho-ethanolamine (DOPE) are larger than the estimated lateral pressure of membranes, thus suggesting that they bind membranes. Polarization modulation infrared reflection absorption spectroscopy has been used to determine the structure and orientation of these peptides in the absence and in the presence of a DOPE monolayer. This lipid induced an increase or a decrease in the organization of the peptide secondary structure. Further measurements are necessary using other lipids to better understand the membrane interactions of these peptides.

Surface rheology and phase transitions of monolayers of phospholipid/cholesterol mixtures

The dynamic surface elasticity and the surface dilational viscosity of three binary phospholipid/cholesterol mixtures were determined with axisymmetric drop shape analysis on a harmonically oscillating pendent drop. Dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and dioleoylphosphatidylcholine were used to explore the rheological properties and phase transitions of mixtures of saturated and unsaturated phospholipids with cholesterol. The growth rates for surface dilational viscosity and dynamic elasticity are parallel for all film pressures studied. Characteristic breaks and plateaus could be found for these growth rates, indicating phase transitions. For dipalmitoylphosphatidylcholine/cholesterol and dimyristoylphosphatidylcholine/cholesterol mixtures, phase diagrams with six regions separated by phase boundaries were found, which are in good agreement with phase transitions reported in the literature for static measurements of isotherms and isobars on a Langmuir film balance and from fluorescence microscopy. Some phase boundaries were only found by dynamic, but not by static, elasticity measurements. Imaging methods revealed phase separations produced by the formation of condensed stoichiometric complexes leading to micron-sized and mostly circular domains. The effects of these complexes on monolayer rheology in liquid/liquid phases is described. Furthermore, liquid/solid and solid phase transitions are discussed.

Effect of Gramicidin on Phospholipid-Modified Monolayers and on Ion Transfer at a Liquid–Liquid Interface

The interaction of hybrid lipid/gramicidin A (gA) monolayers with dextran sulfate (DS) and the effect of this interaction on ion transfer at a liquid-liquid interface is reported. The interfacial and physicochemical properties are studied with Langmuir-Blodgett (LB) and electrochemical techniques. The results obtained from compression isotherms demonstrate that the interactions between the different species in the hybrid monolayer vary according to the chemical nature of the lipid (hydrocarbon region and charge of the head group). Interfacial capacitance measured with AC voltammetry indicates that the DS chains form a rather flat and compact layer when adsorbed to either zwitterionic or negatively charged phospholipid monolayers, and that calcium, even at low concentrations, interacts with the monolayers. These results are successfully described by a model based on the solution of the Poisson-Boltzmann equation in the interfacial region. Ion transfer and interactions with the lipid/gA/DS-modified monolayers were also studied with electrochemical techniques. Admittance data show that although the studied ions are not using gA channels for the transfer through the lipid membranes, the incorporation of gA in the lipid domain and the adsorption of DS at the interface have a significant effect on ion transfer across the monolayers. This effect can be explained as a consequence of the modified surface charge and of the compactness of the lipid domain due to its interaction with gA and to calcium and DS adsorption at the interface. The ion-transfer rate, therefore, depends on the composition of the monolayer and the chemical nature of the ion.

Investigations of antimicrobial peptides in planar film systems

Planar systems--monolayers and films--constitute a useful platform for studying membrane-active peptides. Here, we summarize varied approaches for studying peptide organization and peptide-lipid interactions at the air/water interface, and focus on three representative antimicrobial membrane--associated peptides-alamethicin, gramicidin, and valinomycin. Experimental data, specifically surface pressure/area isotherms and Brewster angle microscopy images, provided information on peptide association and the effects of the lipid monolayers on peptide surface organization. In general, film analysis emphasized the effects of lipid layers in promoting peptide association and aggregation at the air/water interface. Importantly, the data demonstrated that in many cases peptide domains are phase-separated within the phospholipid monolayers, suggesting that this behavior contributes to the biological actions of membrane-active antimicrobial peptides.

Maxwell displacement current allows to study structural changes of gramicidin A in monolayers at the air-water interface

We applied methods of measurement Maxwell displacement current (MDC) pressure-area isotherms and dipole potential for analysis of the properties of gramicidin A (gA) and mixed gA/DMPC monolayers at an air-water interface. The MDC method allowed us to observe the kinetics of formation of secondary structure of gA in monolayers at an air-water interface. We showed, that secondary structure starts to form at rather low area per molecule at which gA monolayers are in gaseous state. Changes of the MDC during compression can be attributed to the reorientation of dipole moments in a gA double helix at area 7 nm(2)/molecule, followed by the formation of intertwined double helix of gA. The properties of gA in mixed monolayers depend on the molar fraction of gA/DMPC. At higher molar fractions of gA (around 0.5) the shape of the changes of dipole moment of mixed monolayer was similar to that for pure gA. The analysis of excess free energy in a gel (18( ) degrees C) and in a liquid-crystalline phase (28( ) degrees C) allowed us to show influence of the monolayer structural state on the interaction between gA and the phospholipids. In a gel state and at the gA/DMPC molar ratio below 0.17 the aggregates of gA were formed, while above this molar ratio gA interacts favorably with DMPC. In contrast, for DMPC in a liquid-crystalline state aggregation of gA was observed for all molar fractions studied. The effect of formation ordered structures between gA and DMPC is more pronounced at low temperatures.

Miscibility of Gramicidin A−Ethyl Nonadecanoate in Langmuir Monolayers in the Presence of Salts Dissolved in the Subphase

The behavior of binary mixed Langmuir monolayers from gramicidin A (GA) and ethyl nonadecanoate (EN), spread on aqueous subphases containing NaCl and CaCl2, was investigated on the basis of the analysis of surface pressure-average area per molecule (pi-A) isotherms complemented with Brewster angle microscopy (BAM) images. Compression modulus versus surface pressure (C(S-1)-pi) curves indicate the existence of interactions in the GA-EN mixed monolayers at low surface pressures (below 5 mN m(-1)). However, for mixtures in which the ester is the predominant component, both GA and EN are miscible within regions from fully expanded to collapse. To examine the interactions between both components in the studied system, values of the mean molecular area per molecule (A12) were plotted as a function of molar fraction of gramicidin A (X(GA)). A12-X(GA) plots exhibit negative deviations from ideality at high surface pressures, wherein beta-helices of GA are vertically oriented in respect to the interface. However, at surface pressures below the plateau transition, which is due to reorientation of GA, the binary system obeys the additive rule. Brewster angle microscopy (BAM) was applied for a direct visualization of the monolayers morphologies. The obtained images prove that for molar ratios of GA > or = 0.3 and at surface pressures above 5 mN m(-1), both components are immiscible at the interface. The observed negative deviations from the additively rule were attributed to the formation of a three-dimensional phase in the mixed film, which provokes its contraction at the interface.

Surface pressure measurements of human tears and individual tear film components indicate that proteins are major contributors to the surface pressure

PURPOSE

Tear film stability has been associated with a low surface tension (high surface pressure), which has been attributed to a variety of tear film components. In this study, we examined the contribution of various tear proteins, mucin, and meibomian lipids to the surface pressure of human tears.

METHODS

A Langmuir trough was used to measure and compare the surface activities of albumin, lipocalin, beta-lactoglobulin, lactoferrin, lysozyme, secretory IgA, mucin, meibomian lipid, and tears.

RESULTS

All proteins exhibited surface activity. The surface pressure-area (Pi-A) profiles of most protein films at equilibrium surface pressure (Pieq) were sigmoidal and showed hysteresis between the expansion and compression phases of the cycle. Pieq of most proteins took 4-9 hours to occur. By contrast, the Pi-A profiles for meibomian lipid films were hyperbolic rather than sigmoidal and had little hysteresis, and Pieq was attained within 1 hour. The Pi-A profiles of mucin films showed mostly hyperbolic characteristics with small hysteresis. The Pi-A profiles of films of tears were sigmoidal, showed strong hysteresis, and reached Pieq at about 5 hours. Partitioning of the proteins and whole tears into the subphase also occurred.

CONCLUSION

Comparison between the dynamic Pi-A profiles of tears and those of individual tear film components shows that tear film proteins not only are capable of surface activity but also are major contributors to the surface activity of the tear film.

The effect of interaction between K+ ions and gramicidin D on the lecithin membrane interfacial tension

The effect of the presence of gramicidin D in a lecithin membrane on its interfacial tension has been studied. The studies have been carried out at various forming solution compositions and at various potassium ion concentrations in the electrolyte solution. Potassium chloride was used as the electrolyte. The complex was formed between the gramicidin molecule and K(+) ion. The following parameters describing the complex were determined: the surface area occupied by GK(+) complex (A(GK(+))), the interfacial tension of the GK(+) membrane complex (gamma(GK+)), and the stability constant of the gramicidin-K(+) complex (K). These values are 156 A(2), 1.89 mN m(-1) and 0.033 m(3) mol(-1), respectively.

Conformation of gramicidin a in water: Inference from analysis of hydrogen/deuterium exchange behavior by matrix assisted laser desorption ionization mass spectrometry

Gramicidin A (the major component of gramicidin D) is a highly hydrophobic peptide with very little solubility in water. Hence, the conformation of this peptide has been extensively investigated in organic solvents and model membranes, but not in water. The peptide adopts a beta6.3-helical conformation in the monomeric and dimeric forms. We have investigated the conformation of gramicidin A in water by monitoring hydrogen-deuterium exchange by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. Our results indicate that gramicidin A is monomeric and exists in a highly folded conformation. The metal ion bound forms are clearly discernible in the monomers. The presence of the dimeric form is not observed. It is unlikely this is due to the operating conditions or the method used, as both hetero- and homodimers in gramicidin D are detected when methanol is used as a solvent. The present study also establishes that the linear gramicidins retain a history of solvent environment when ions are generated by matrix-assisted laser desorption ionization and analyzed by time-of-flight.

Impedance analysis of phosphatidylcholine membranes modified with gramicidin D

Electrochemical impedance spectroscopy (EIS) was used to the study of gramicidin D (GD) dimerization and to transport of monovalent cations across lipid bilayers by the dimers. Phosphatidylcholine (PC) membranes were studied, unmodified and modified with very low GD concentrations in the presence of various potassium ion concentrations. A new method was proposed to determine the parameters used to describe the gramicidin dimer: gramicidin surface concentration (c(GG)), area occupied by individual channel (A(G)) and gramicidin dimerization equilibrium constant (K(GG)). It was shown that electrochemical impedance spectroscopy measurements of lipid bilayer membranes yielded the K(GG) and A(G) values of the same order of magnitude as other measurement techniques.

Structure-function relationship of reduced cytochrome c probed by complete solution structure determination in 30% acetonitrile/water solution

The complete solution structure of ferrocytochrome c in 30% acetonitrile/70% water has been determined using high-field 1D and 2D (1)H NMR methods and deposited in the Protein Data Bank with codes 1LC1 and 1LC2. This is the first time a complete solution protein structure has been determined for a protein in nonaqueous media. Ferrocyt c retains a native protein secondary structure (five alpha-helices and two omega loops) in 30% acetonitrile. H18 and M80 residues are the axial heme ligands, as in aqueous solution. Residues believed to be axial heme ligands in the alkaline-like conformers of ferricyt c, specifically H33 and K72, are positioned close to the heme iron. The orientations of both heme propionates are markedly different in 30% acetonitrile/70% water. Comparative structural analysis of reduced cyt c in 30% acetonitrile/70% water solution with cyt c in different environments has given new insight into the cyt c folding mechanism, the electron transfer pathway, and cell apoptosis.

Proton NMR study of chemically modified horse heart ferricytochrome c confirms the presence of histidine and lysine-ligated conformers in 30% acetonitrile solution

Comparison of the 1H NMR spectra for guanidinated ferricyt c and chloro(terpyridine)platinum(II)-modified ferricyt c in 30% acetonitrile (ACN) solution with that for ferricyt c in 30% ACN is reported. The absence of the heme methyl proton resonances characteristic of the IV*-form (Lys-ligated) in the NMR spectrum of guanidinated ferricyt c in 30% ACN solution confirms that a lysine-ligated form of ferricyt c is produced in 30% ACN solution. The absence of the 8-methyl heme proton resonance of the V*-form in the NMR spectrum of chloro(terpyridine)platinum(II)-modified ferricyt c in 30% ACN solution demonstrates that a bis-His-ligated form of ferricyt c is produced in 30% ACN, not a hydroxide ligated form, as previously proposed. The revised assignment for the V* form of ferricyt c in mixed media explains differences between the exchange network we previously reported for ferricyt c in 30% ACN [Protein Sci. 10 (2001) 2291] as versus that reported by Dopner et al. at high pH [J. Am. Chem. Soc. 120 (1998) 11246]. Lys- and His-ligated forms are known to be produced in the presence of denaturants in protein folding studies of ferricyt c. Consequently, the exchange network between these non-native forms of ferricyt c in 30% ACN may have biological relevance for ferricyt c folding.

Spectroscopic [correction of eSpectroscopic] and structural properties of valine gramicidin A in monolayers at the air-water interface

Monomolecular films of valine gramicidin A (VGA) were investigated in situ at the air-water interface by x-ray reflectivity and x-ray grazing incidence diffraction as well as polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). These techniques were combined to obtain information on the secondary structure and the orientation of VGA and to characterize the shoulder observed in its pi-A isotherm. The thickness of the film was obtained by x-ray reflectivity, and the secondary structure of VGA was monitored using the frequency position of the amide I band. The PM-IRRAS spectra were compared with the simulated ones to identify the conformation adopted by VGA in monolayer. At large molecular area, VGA shows a disordered secondary structure, whereas at smaller molecular areas, VGA adopts an anti-parallel double-strand intertwined beta(5.6) helical conformation with 30 degrees orientation with respect to the normal with a thickness of 25 A. The interface between bulk water and the VGA monolayer was investigated by x-ray reflectivity as well as by comparing the experimental and the simulated PM-IRRAS spectra on D(2)O and H(2)O, which suggested the presence of oriented water molecules between the bulk and the monolayer.

Comparative interaction of alpha-helical and beta-sheet amphiphilic isopeptides with phospholipid monolayers

The two sequential amphiphilic peptide isomers, (Leu-Lys-Lys-Leu)4 and (Leu-Lys)8, were chosen as models for alpha-helical and beta-sheet peptides, respectively. In order to evaluate the contribution of the secondary structure of a peptide to its penetration into cellular membranes, interactions of these isopeptides with L-alpha-dimyristoyl phosphatidylcholine (DMPC) monolayers were studied. Both isopeptides penetrate into DMPC monolayers up to an exclusion pressure of approximately 27 mN/m, but a discontinuity is observed in the penetration profile of the alpha-helical (LKKL)4. The main parameters extracted from the compression isotherms of the mixed peptide/DMPC monolayers-namely, transition pressure, mean area, elasticity modulus, and energy of mixing-were analyzed. These analyses indicate that the alpha-helical isomer interacts strongly with DMPC by forming a 1:32 (LKKL)4-DMPC complex. When engaged in this complex, (LKKL)(4) behaves as an hydrophobic helix and has a tendency to become vertically oriented in the course of the compression process. The beta-sheet (LK)8 interacts more weakly with DMPC and no complex can be detected.

Molecular organization of the human serotonin transporter at the air/water interface

The serotonin transporter (SERT) is the target of several important antidepressant and psychostimulant drugs. It has been shown that under defined conditions, the transporter spread at the air/water interface was able to bind its specific ligands. In this paper, the interfacial organization of the protein has been assessed from dynamic surface pressure and ellipsometric measurements. For areas comprising between 10,400 and 7,100 A(2)/molecule, ellipsometric measurements reveal an important change in the thickness of the SERT film. This change was attributed to the reorientation of the transporter molecules from a horizontal to their natural predictive transmembrane orientation. The thickness of the SERT film at 7,100 A(2)/molecule was found to be approximately equal to 84 A and coincided well with the theoretical value estimated from the calculations based on the dimensions of alpha-helices containing membrane proteins. These data suggest that the three-dimensional arrangement of the SERT may be represented as a box with lengths d(z)=83--85 A and d(y) or d(x)=41--47 A.

The monolayer technique: a potent tool for studying the interfacial properties of antimicrobial and membrane-lytic peptides and their interactions with lipid membranes

Erudites of the antiquity already knew the calming effect of oil films on the sea waves. But one had to wait until 1774 to read the first scientific report on oil films from B. Franklin and again 1878 to learn the thermodynamic analysis on adsorption developed by J. Gibbs. Then, in 1891, Agnes Pockels described a technique to manipulate oil films by using barriers. Finally, in 1917, I. Langmuir introduced the experimental and theoretical modern concepts on insoluble monolayers. Since that time, and because it has been found to provide invaluable information at the molecular scale, the monolayer technique has been more and more extensively used, and, during the past decade, an explosive increase in the number of publications has occurred. Over the same period, considerable and ever-increasing interest in the antimicrobial peptides of various plants, bacteria, insects, amphibians and mammals has grown. Because many of these antimicrobial peptides act at the cell membrane level, the monolayer technique is entirely suitable for studying their physicochemical and biological properties. This review describes monolayer experiments performed with some of these antimicrobial peptides, especially gramicidin A, melittin, cardiotoxins and defensin A. After giving a few basic notions of surface chemistry, the surface-active properties of these peptides and their behavior when they are arranged in monomolecular films are reported and discussed in relation to their tridimensional structure and their amphipathic character. The penetration of these antimicrobial peptides into phospholipid monolayer model membranes, as well as their interactions with lipids in mixed films, are also emphasized.

Preferential partitioning of melittin into the air/water interface: structural and thermodynamic implications

The membrane active agent melittin has been investigated with regard to the formation of a Langmuir monolayer and the accordingly induced surface activities. We show that in spite of its considerable solubility in an aqueous medium, this peptide nevertheless largely accumulates in the air/water interface unless the lateral pressure is raised beyond a certain threshold value depending on the pH in the subphase. The true surface concentrations have been determined by means of a recently developed novel method based on thermodynamic principles. It affords an access to the partitioning equilibrium between the surface and subphase domains, provided the latter surrounding is not excessively preferred. In the present case this approach was used to derive quantitative information on the pertinent interfacial structure and thermodynamics. In particular, the apparent molecular area and the Gibbs energy of mutual interaction in the monolayer could be evaluated as a function of the applied surface pressure. The data suggest the existence of two structural conversions in the course of an increasing lateral compression. The surface-associated peptide accordingly assumes three different states of successively reduced area requirements, supposedly owing to an orientational transition involving a straightening up of a helical conformation. This conclusion is corroborated by surface potential measurements reflecting corresponding changes of the effective dipole moment perpendicular to the surface.

Structure, energetics, and dynamics of lipid-protein interactions: A molecular dynamics study of the gramicidin A channel in a DMPC bilayer

The microscopic details of lipid-protein interactions are examined using molecular dynamics simulations of the gramicidin A channel embedded in a fully hydrated dimyristoyl phosphatidylcholine (DMPC) bilayer. A novel construction protocol was used to assemble the initial configurations of the membrane protein complex for the simulations. Three hundred systems were constructed with different initial lipid placement and conformations. Seven systems were simulated with molecular dynamics. One system was simulated for a total of 600 psec, four were simulated for 300 psec, and two for 100 psec. Analysis of the resulting trajectories shows that the bulk solvent-membrane interface region is much broader than traditionally pictured in simplified continuum theories: its width is almost 15 angstroms. In addition, lipid-protein interactions are far more varied, both structurally and energetically, than is usually assumed: the total interaction energy between the gramicidin A and the individual lipids varies from 0 to -50 kcal/mol. The deuterium quadrupolar splittings of the lipid acyl chains calculated from the trajectories are in good agreement with experimental data. The lipid chains in direct contact with the GA are ordered but the effect is not uniform due to the irregular surface of the protein. Energy decompositions shows that the most energetically favorable interactions between lipid and protein involve nearly equal contributions from van der Waals and electrostatic interactions. The tryptophans, located near the bulk-membrane interface, appear to be particularly important in mediating both hydrogen bonding interactions with the lipid glycerol backbone and water and also in forming favorable van der Waals contacts with the hydrocarbon chains. In contrast, the interactions of the leucine residues with the lipids, also located near the interface, are dominated by van der Waals interactions with the hydrocarbon lipid chains.

Synthesis of acylated gramicidins and the influence of acylation on the interfacial properties and conformational behavior of gramicidin A

Five gramicidin A analogs were synthesized in which various acyl chains, differing in length and unsaturation, were covalently coupled to the C-terminal ethanolamine group. The analogs were characterized by various spectroscopic techniques and their molecular properties were investigated using monolayer techniques and circular dichroism. It is demonstrated that neither the interfacial properties nor the conformational behavior of gramicidin A at the air/water interface are seriously affected upon acylation. It is proposed that at the limiting area the gramicidin molecule is oriented with its C-terminus towards the subphase with the covalently coupled acylchain located parallel to the helical axis in between the protruding tryptophans. Circular dichroism experiments, in which gramicidin-containing vesicles were prepared from different organic solvents, indicate that the presence of a covalently coupled fatty acylchain tends to stabilize the beta 6.3 helical conformation. It is demonstrated that, like for gramicidin A, also for the acylgramicidins the single-stranded beta 6.3 helical conformation, or channel conformation, is the preferred conformation upon incorporation in bilayers.

The intrinsic molecular potential of glyceryl monooleate layers and its effect on the conformation and orientation of an inserted molecule: example of gramicidin A

It is shown by explicit calculation that the distribution of the atomic charges in the constituent molecules of a lipid monolayer or bilayer of glyceryl monooleate creates an intrinsic potential difference between the head region and the hydrocarbon region which tends to repel positive charges towards the exterior and attract negative charges to the interior. The analogies and differences between a bilayer and a monolayer are analyzed. The possible consequences of the intrinsic potential gradient in a lipid layer on the preferred orientation and conformation of a polar neutral molecule are illustrated on the case of a gramicidin A monomer.