alamethicin and 1-2-oleoylphosphatidylcholine

An all-atom molecular dynamics simulation of the archetype barrel-stave alamethicin (alm) pore in a 1,2-dioleoyl-sn-glycero-3-phosphocholine bilayer at 313 K indicates that ∼7 μs is required for equilibration of a preformed 6-peptide pore; the pore remains stable for the duration of the remaining 7 μs of the trajectory, and the structure factors agree well with experiment. A 5 μs simulation of 10 surface-bound alm peptides shows significant peptide unfolding and some unbinding, but no insertion. Simulations at 363 and 413 K with a -0.2 V electric field yield peptide insertion in 1 μs. Insertion is initiated by the folding of residues 3-11 into an α-helix, and mediated by membrane water or by previously inserted peptides. The stability of five alm pore peptides at 413 K with a -0.2 V electric field demonstrates a significant preference for a transmembrane orientation. Hence, and in contrast to the cationic antimicrobial peptide described in the following article, alm shows a strong preference for the inserted over the surface-bound state.

Topics: Alamethicin; Animals; Anti-Bacterial Agents; Antimicrobial Cationic Peptides; Electromagnetic Fields; Fish Proteins; Fishes; Fungal Proteins; Glycerylphosphorylcholine; Hydrophobic and Hydrophilic Interactions; Lipid Bilayers; Molecular Dynamics Simulation; Phosphatidylcholines; Protein Binding; Protein Conformation, alpha-Helical; Protein Folding; Trichoderma; Viscosity

Planar lipid bilayers suspended in apertures provide a controlled environment for ion channel studies. However, short lifetimes and poor mechanical stability of suspended bilayers limit the experimental throughput of bilayer electrophysiology experiments. Although bilayers are more stable in smaller apertures, ion channel incorporation through vesicle fusion with the suspended bilayer becomes increasingly difficult. In an alternative bilayer stabilization approach, we have developed shaped apertures in SU8 photoresist that have tapered sidewalls and a minimum diameter between 60 and 100 μm. Bilayers formed at the thin tip of these shaped apertures, either with the painting or the folding method, display drastically increased lifetimes, typically >20 h, and mechanical stability, being able to withstand extensive perturbation of the buffer solution. Single-channel electrical recordings of the peptide alamethicin and of the proteoliposome-delivered potassium channel KcsA demonstrate channel conductance with low noise, made possible by the small capacitance of the 50 μm thick SU8 septum, which is only thinned around the aperture, and unimpeded proteoliposome fusion, enabled by the large aperture diameter. We anticipate that these shaped apertures with micrometer edge thickness can substantially enhance the throughput of channel characterization by bilayer lipid membrane electrophysiology, especially in combination with automated parallel bilayer platforms.

Topics: Alamethicin; Bacterial Proteins; Electric Capacitance; Epoxy Compounds; Fluorescence; Light; Lipid Bilayers; Liposomes; Membrane Fusion; Microscopy, Electron, Scanning; Phosphatidylcholines; Phosphatidylglycerols; Potassium Channels

A detailed knowledge of the interaction between bacterial membranes and antibiotics provides important information to prevent high levels of antibiotic resistance exhibited by pathogenic strains. We investigated by energy dispersive X-ray diffraction (EDXD) the structure ordering of dioleoyl-phosphatidylcholine (DOPC) lipid interacting with antimicrobial peptide alamethicin, varying the lipid/peptide (L/P) molar ratio under two different hydration levels. In conditions of full hydration (100%) we found that the bilayer thickness is constant between L/P=20 and L/P=80 indicating that in this range, the system has reached the threshold value for the channel formation, while at the relative hydration of 45% a linear decrease of the bilayer thickness as function of L/P was revealed. The kinetic study of the complex alamethicin-DOPC at different L/P values, shows that the Bragg peak energy variation versus the hydration time has a biexponential behavior characterized by two different time constants.

Topics: Alamethicin; Antimicrobial Cationic Peptides; Kinetics; Phosphatidylcholines; X-Ray Diffraction

We study fully hydrated bilayers of two di-monounsaturated phospholipids diC18:1PC (DOPC) and diC22:1PC with varying amounts of alamethicin (Alm). We combine the use of X-ray diffuse scattering and molecular dynamics simulations to determine the orientation of alamethicin in model lipids. Comparison of the experimental and simulated form factors shows that Alm helices are inserted transmembrane at high humidity and high concentrations, in agreement with earlier results. The X-ray scattering data and the MD simulations agree that membrane thickness changes very little up to 1/10 Alm/DOPC. In contrast, the X-ray data indicate that the thicker diC22:1PC membrane thins with added Alm, a total decrease in thickness of 4 A at 1/10 Alm/diC22:1PC. The different effect of Alm on the thickness changes of the two bilayers is consistent with Alm having a hydrophobic thickness close to the hydrophobic thickness of 27 A for DOPC; Alm is then mismatched with the 7 A thicker diC22:1PC bilayer. The X-ray data indicate that Alm decreases the bending modulus (K(C)) by a factor of approximately 2 in DOPC and a factor of approximately 10 in diC22:1PC membranes (P/L approximately 1/10). The van der Waals and fluctuational interactions between bilayers are also evaluated through determination of the anisotropic B compressibility modulus.

Topics: Alamethicin; Lipid Bilayers; Lipids; Models, Molecular; Phosphatidylcholines; Protein Conformation; Scattering, Radiation; Solvents; X-Rays

The lipid matrix, or the lipid bilayer, of cell membranes is a natural binding site for amphipathic molecules, including antimicrobial peptides, pore-forming proteins, and many drugs. The unique property of pore-forming antimicrobial peptides is that they exhibit a threshold concentration (called the lethal concentration or the minimum inhibitory concentration) for activity, below which no effect is seen. Without this property, antimicrobial peptides would not be effective self-defense weapons, because they would have harmed all cells at any concentration. The question is what gives rise to this unique property? This study provides a free energy description for the origin of a threshold concentration. The same free energy applied differently also explains the binding of drugs that shows no threshold concentrations. The idea is compared with theories of micellar solutions that require a large oligomer size (n 15) to achieve a threshold concentration. The elasticity of lipid bilayers makes the phenomena in membranes different. The majority of antimicrobial peptides have a large negative binding energy to the bilayer interface, but the binding causes an expansion in the membrane area, or equivalently a thinning in the membrane thickness. This elastic energy of membrane thinning elevates the energy level of interfacial binding with the peptide concentration, hence gives rise to a threshold concentration for forming pores containing as few as four peptides.

Topics: Alamethicin; Algorithms; Animals; Antimicrobial Cationic Peptides; Bees; Curcumin; Elasticity; Lipid Bilayers; Melitten; Models, Molecular; Phosphatidylcholines; Thermodynamics

X-ray scattering features induced by aggregates of alamethicin (Alm) were obtained in oriented stacks of model membranes of DOPC(diC18:1PC) and diC22:1PC. The first feature obtained near full hydration was Bragg rod in-plane scattering near 0.11 A(-1) in DOPC and near 0.08 A(-1) in diC22:1PC at a 1:10 Alm:lipid ratio. This feature is interpreted as bundles consisting of n Alm monomers in a barrel-stave configuration surrounding a water pore. Fitting the scattering data to previously published molecular dynamics simulations indicates that the number of peptides per bundle is n = 6 in DOPC and n >or= 9 in diC22:1PC. The larger bundle size in diC22:1PC is explained by hydrophobic mismatch of Alm with the thicker bilayer. A second diffuse scattering peak located at q(r) approximately 0.7 A(-1) is obtained for both DOPC and diC22:1PC at several peptide concentrations. Theoretical calculations indicate that this peak cannot be caused by the Alm bundle structure. Instead, we interpret it as being due to two-dimensional hexagonally packed clusters in equilibrium with Alm bundles. As the relative humidity was reduced, interactions between Alm in neighboring bilayers produced more peaks with three-dimensional crystallographic character that do not index with the conventional hexagonal space groups.

Topics: Alamethicin; Hydrophobic and Hydrophilic Interactions; Ion Channels; Lipid Bilayers; Membrane Lipids; Models, Chemical; Models, Molecular; Molecular Dynamics Simulation; Phosphatidylcholines; Scattering, Radiation; X-Ray Diffraction

The aggregation properties of an antibiotic membrane-active peptide alamethicin at the air-water interface have been studied using interfacial rheology and fluorescence microscopy techniques. Fluorescence microscopy of alamethicin monolayers revealed a coexistence of liquid expanded (LE) and solid phases at the surface concentrations studied. Interfacial oscillatory shear measurements on alamethicin monolayers indicate that its viscoelastic properties are determined by the area fraction of the solid domains. The role of zwitterionic phospholipids dioleoylphosphatidyl choline (DOPC) and dioleoylphosphatidyl ethanolamine (DOPE) on the peptide aggregation behavior was also investigated. Fluorescence microscopy of alamethicin/phospholipid monolayers revealed an intermediate phase (I) in addition to the solid and LE phase. In mixed monolayers of phospholipid (L)/alamethicin (P), with increase in L/P, the monolayer transforms from a viscoelastic to a viscous fluid with the increase in area fraction of the intermediate phase. Further, a homogeneous mixing of alamethicin/lipid molecules is observed at L/P > 4. Our studies also confirm that the viscoelasticity of alamethicin/phospholipid monolayers is closely related to the alamethicin/phospholipid interactions at the air-water interface.

Topics: Air; Alamethicin; Anti-Bacterial Agents; Chemistry, Pharmaceutical; Elasticity; Microscopy, Fluorescence; Oscillometry; Peptides; Phosphatidylcholines; Phosphatidylethanolamines; Phospholipids; Rheology; Surface Properties; Viscosity; Water

Using dilatometry and small-angle X-ray diffraction, we have studied under bulk conditions the structural changes and elastic response of dioleoyl phosphatidylcholine bilayers to alamethicin. With increasing peptide concentration, we found a progressive thinning of the membrane. However, in contrast to previously published reports, this thinning exhibits exponential behavior. Furthermore, an increase in alamethicin content resulted in an increased lateral area per lipid and a swelling of the multibilayers which can be attributed to a decrease in the bilayer's bending rigidity by approximately 50%. At the same time, hydration and van der Waals forces remained unaffected by the presence of the peptide. Interestingly, all elastic and structural parameters followed the same exponential form found for the membrane thickness, implying a common underlying mechanism for all of these structural parameters. Our results can be understood by introducing an additional entropy term into the free-energy description of peptide incorporation, a term previously not considered. As a result, we have been able to reconcile recent controversies regarding the effect of peptides on membrane thinning.

Topics: Alamethicin; Anti-Bacterial Agents; Calorimetry; Entropy; Lipid Bilayers; Membrane Fluidity; Phosphatidylcholines; Scattering, Small Angle; X-Ray Diffraction

Antimicrobial peptides are known to form pores in cell membranes. We study this process in model bilayers of various lipid compositions. We use two of the best-studied peptides, alamethicin and melittin, to represent peptides making two types of pores, that is, barrel-stave pores and toroidal pores. In both cases, the key control variable is the concentration of the bound peptides in the lipid bilayers (expressed in the peptide-lipid molar ratio, P/L). The method of oriented circular dichroism (OCD) was used to monitor the peptide orientation in bilayers as a function of P/L. The same samples were scanned by X-ray diffraction to measure the bilayer thickness. In all cases, the bilayer thickness decreases linearly with P/L and then levels off after P/L exceeds a lipid-dependent critical value, (P/L)*. OCD spectra showed that the helical peptides are oriented parallel to the bilayers as long as P/L < (P/L)*, but as P/L increases over (P/L)*, an increasing fraction of peptides changed orientation to become perpendicular to the bilayer. We analyzed the data by assuming an internal membrane tension associated with the membrane thinning. The free energy containing this tension term leads to a relation explaining the P/L-dependence observed in the OCD and X-ray diffraction measurements. We extracted the experimental parameters from this thermodynamic relation. We believe that they are the quantities that characterize the peptide-lipid interactions related to the mechanism of pore formation. We discuss the meaning of these parameters and compare their values for different lipids and for the two different types of pores. These experimental parameters are useful for further molecular analysis and are excellent targets for molecular dynamic simulation studies.

Topics: Alamethicin; Animals; Anti-Bacterial Agents; Circular Dichroism; Ion Channels; Lipid Bilayers; Melitten; Membranes, Artificial; Models, Chemical; Phosphatidylcholines; Protein Binding; Spectroscopy, Fourier Transform Infrared; Thermodynamics; X-Ray Diffraction

Ceratotoxin A (CtxA), a 36-residue alpha-helical cationic peptide isolated from the medfly Ceratitis capitata, exhibits strong antibacterial activity. To determine its mode of action against bacteria, we investigated the behavior of ceratotoxin A by incorporating it into planar lipid bilayers. Macroscopic and single channel conductance experiments showed that ceratotoxin A forms voltage-dependent ion channels in bilayers according to the barrel-stave model. The characteristics of the channel suggest that the C-terminal regions form bundles of five or six helices embedded in the membrane, such that the N-terminal moieties lie on the polar side of the lipid bilayer.

Topics: Alamethicin; Animals; Anti-Bacterial Agents; Ceratitis capitata; Electric Conductivity; Insect Proteins; Lipid Bilayers; Phosphatidylcholines

Incorporation of the helical antimicrobial peptide alamethicin from aqueous phase into hydrated phases of dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) was investigated within a range of peptide concentrations and temperatures by time-resolved synchrotron X-ray diffraction. It was found that alamethicin influences the organizations of the non-bilayer-forming (DOPE) and the bilayer-forming (DOPC) lipids in different ways. In DOPC, only the bilayer thickness was affected, while in DOPE new phases were induced. At low peptide concentrations (<1.10(-4) M), an inverted hexagonal (H(II)) phase was observed as with DOPE dispersions in pure buffer solution. A coexistence of two cubic structures was found at the critical peptide concentration for induction of new lipid/peptide phases. The first one Q224 (space group Pn3m) was identified within the entire temperature region studied (from 1 to 45 degrees C) and was found in coexistence with H(II)-phase domains. The second lipid/peptide cubic structure was present only at temperatures below 16 degrees C and its X-ray reflections were better fitted by a Q212 (P4(3)32) space group, rather than by the expected Q229 (Im3m) space group. At alamethicin concentrations of 1 mM and higher, a nonlamellar phase transition from a Q224 cubic phase into an H(II) phase was observed. Within the investigated range of peptide concentrations, lamellar structures of two different bilayer periods were established with the bilayer-forming lipid DOPC. They correspond to lipid domains of associated and nonassociated helical peptide. The obtained X-ray results suggest that the amphiphilic alamethicin molecules adsorb from the aqueous phase at the lipid head group/water interface of the DOPE and DOPC membranes. At sufficiently high (>1.10(-4) M) solution concentrations, the peptide is probably accommodated in the head group region of the lipids thus inducing structural features of mixed lipid/peptide phases.

Topics: Alamethicin; Amino Acid Sequence; Anti-Bacterial Agents; Lipid Bilayers; Macromolecular Substances; Models, Molecular; Molecular Sequence Data; Phosphatidylcholines; Phosphatidylethanolamines; Protein Conformation; Thermodynamics; Water; X-Ray Diffraction

The aqueous-membrane partitioning of alamethicin, a voltage-gated channel-forming peptide, was measured as a function of the membrane spontaneous curvature. EPR spectroscopy was used to measure the partitioning of the peptide in lipid compositions formed from dioleoylphosphatidylcholine (DOPC) and varied percentages of dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylethanolamine-N-methyl (DOPE-Me), or dioleoylphosphatidylethanolamine-N,N-dimethyl (DOPE-Me2). When the mole fraction of DOPE in mixtures of DOPC/DOPE is increased the binding of alamethicin decreases, and the increase in binding free energy is found to be linearly dependent upon the mole fraction of DOPE in the mixture. Addition of DOPE-Me or DOPE-Me2 also increases the binding free energy, except that the effect is reduced relative to that of DOPE. The free-energy increase per mole fraction of DOPE was found to be 1400 cal/mol, whereas for DOPE-Me and DOPE-Me2 the free-energy changes were 980 and 630 cal/mol, respectively. When the free-energy changes for alamethicin binding are compared with the previously determined spontaneous curvatures for mixtures of DOPC/DOPE and DOPC/DOPE-Me, the free energy of binding is found to be linearly dependent upon the spontaneous curvature of the bilayer lipids. The effects of membrane lipid unsaturation on the partitioning of alamethicin were also measured and are qualitatively consistent with this conclusion. The sensitivity to spontaneous curvature and the cooperativity that is seen in the binding curves for alamethicin are postulated to be a result of a localized thinning of the bilayer promoted by this peptide.

Topics: Alamethicin; Amino Acid Sequence; Binding Sites; Electron Spin Resonance Spectroscopy; Fatty Acids, Unsaturated; Ion Channel Gating; Ion Channels; Lipid Bilayers; Models, Molecular; Molecular Sequence Data; Phosphatidylcholines; Phosphatidylethanolamines; Thermodynamics

Amide-resolved hydrogen-deuterium exchange-rate constants were measured for backbone amides of alamethicin reconstituted in dioleoylphosphatidylcholine vesicles by an exchange-trapping method combined with high-resolution nuclear magnetic resonance spectroscopy. In vesicles containing alamethicin at molar ratios between 1:20 and 1:100 relative to lipid, the exchange-rate constants increased with increasing volume of the D20 buffer in which the vesicles were suspended, indicating that exchange under these conditions is dominated by partitioning of the peptide into the aqueous phase. This was supported by observation of a linear relationship between the exchange-rate constants for amides in membrane-reconstituted alamethicin and those for amides in alamethicin dissolved directly into D2O buffer. Significant protection of amides from exchange with D2O buffer in membrane-reconstituted alamethicin is interpreted in terms of stabilization by helical hydrogen bonding. Under conditions in which amide exchange occurred by partitioning of the peptide into solution, only lower limits for hydrogen-bond stabilities in the membrane were determined; all the potentially hydrogen-bonded amides of alamethicin are at least 1000-fold exchange protected in the membrane-bound state. When partitioning of alamethicin into the aqueous phase was suppressed by hydration of reconstituted vesicles in a limiting volume of water [D2O:dioleoylphosphatidylcholine:alamethicin; 220:1:0.05; (M:M:M)], the exchange-protection factors exhibited helical periodicity with highly exchange-protected, and less well-protected, amides on the nonpolar and polar helix faces, respectively. The exchange data indicate that, under the conditions studied, alamethicin adopts a stable helical structure in DOPC bilayers in which all the potentially hydrogen-bonded amides are stabilized by helical hydrogen bonds. The protection factors define the orientation of the peptide helix with respect to an aqueous phase, which is either the bulk solution or water within parallel or antiparallel transmembrane arrays of reconstituted alamethicin.

Topics: Alamethicin; Amides; Anti-Bacterial Agents; Biophysical Phenomena; Biophysics; Circular Dichroism; Deuterium; Drug Stability; Hydrogen; Hydrogen Bonding; Liposomes; Magnetic Resonance Spectroscopy; Models, Molecular; Molecular Structure; Phosphatidylcholines; Water

Topics: Alamethicin; Amides; Ion Channels; Lipid Bilayers; Models, Molecular; Molecular Structure; Phosphatidylcholines; Protein Conformation; Thermodynamics

Alamethicin is a 20-amino-acid peptide that produces a voltage-dependent conductance in membranes. We investigated the state of aggregation of alamethicin in egg phosphatidylcholine and dioleoylphosphatidylcholine membranes by examining the EPR spectra obtained from an active analog of this peptide that is spin-labeled at its C-terminus. The dependence of both the linewidth and signal intensity as a function of peptide concentration exhibit exchange broadening as the peptide concentration is increased; however, the exchange rates are linear with peptide concentration as is expected for the simple diffusion of monomers. In addition, the spin-exchange rates obtained from the linebroadening are consistent with collisional rates that are predicted from free Brownian diffusion. The results provide strong evidence that in the absence of a membrane potential, alamethicin is largely monomeric in these membranes.

Topics: Alamethicin; Electron Spin Resonance Spectroscopy; Indicators and Reagents; Kinetics; Liposomes; Membrane Potentials; Models, Structural; Peptides; Phosphatidylcholines; Protein Structure, Secondary

The interaction of the voltage-dependent channel-forming peptide alamethicin with dioleoylphosphatidylcholine (DOPC) small unilamellar vesicles (SUV) has been studied using circular dichroism spectroscopy over a range of wavelengths and temperatures. Evidence is presented for the existence of two distinct membrane-bound states of the peptide which reflect different extents of peptide-peptide interaction. An elevated temperature is found to diminish the apparent peptide-peptide interaction. These results provide insight into the general problem of helix-helix interaction in membranes and provide experimental support for the proposal [Popot, J. L., & Engelman, D. M. (1990) Biochemistry 29, 4031-4037] that these interactions can be enthalpically favorable.

Topics: Alamethicin; Circular Dichroism; Ion Channels; Membranes, Artificial; Phosphatidylcholines; Protein Binding; Temperature

With few exceptions, membrane lipids are usually regarded as a kind of filler or passive solvent for membrane proteins. Yet, cells exquisitely control membrane composition. Many phospholipids found in plasma membrane bilayers favor packing into inverted hexagonal bulk phases. It was suggested that the strain of forcing such lipids into a bilayer may affect membrane protein function, such as the operation of transmembrane channels. To investigate this, we have inserted the peptide alamethicin into bilayer membranes composed of lipids of empirically determined inverted hexagonal phase "spontaneous radii" Ro, which will have expectably different degrees of strain when forced into bilayer form. We observe a correlation between measured Ro and the relative probabilities of different conductance states. States of higher conductance are more probable in dioleoylphosphatidylethanolamine, the lipid of highest curvature, 1/Ro, than in dioleoylphosphatidylcholine, the lipid of lowest curvature.

Topics: Alamethicin; Biophysical Phenomena; Biophysics; Electric Conductivity; Lipid Bilayers; Membrane Lipids; Membrane Proteins; Phosphatidylcholines; Phosphatidylethanolamines

Strong aggregation of incorporated alamethicin in the bilayer of lipid vesicles has been observed spectroscopically at aqueous peptide concentrations above a critical value c*. On the other hand, in conventional gating studies with planar lipid films, the onset of conducting pore formation can be characterized by a threshold voltage V.. We present experimental evidence of a direct correspondence between the effects on c* and V. when these parameters are modulated by adding NaCl (to the aqueous medium) or cholesterol (to the lipid moiety). A quantitative analysis supports the idea that the measured aggregation actually results in pore formation, the voltage-dependence being due to an electric field effect on the partition equilibrium of the peptide between the aqueous and the lipid phases.

Topics: Alamethicin; Anti-Bacterial Agents; Cholesterol; Circular Dichroism; Electric Conductivity; Ion Channels; Kinetics; Lipid Bilayers; Macromolecular Substances; Membrane Lipids; Membrane Potentials; Phosphatidylcholines; Sodium Chloride

The reaction of fluorescence-labeled alamethicin with unilamellar phospholipid vesicles (DOPC and DMPC) has been investigated in a stopped-flow apparatus. Clearly single exponential time functions have been observed at temperatures above the phase transition of the bilayer. This can be interpreted in terms of an essentially one-step incorporation process. The pseudo first-order forward rate is found to be quite fast, falling in a range somewhat below the diffusion controlled upper bound. The data are quantitatively very well described on the basis of a simple mechanism. This comprises diffusion of peptide into the bilayer accompanied by a more or less slower change of the secondary structure. Aggregation of the incorporated molecules at higher concentrations is indicated to be comparatively rapid.

Topics: Alamethicin; Anti-Bacterial Agents; Dimyristoylphosphatidylcholine; Kinetics; Lipid Bilayers; Models, Biological; Phosphatidylcholines

Interaction of the pore-forming antibiotic alamethicin with small unilamellar vesicles of dioleoylphosphatidylcholine has been studied by means of circular dichroism. The data strongly suggest that alamethicin does not bind to the surface of the vesicles but incorporates into the lipid phase to a fairly large extent. Furthermore, aggregation of the peptide in the membrane is apparent from the existence of a 'critical concentration'. Quantitative evaluation and interpretation of the data rest on a quite generally applicable thermodynamic analysis. The underlying phenomenon is treated in terms of a partition equilibrium between the aqueous and lipid media. In the bilayer phase non-ideal interactions (described by appropriate activity coefficients) as well as aggregate formation are considered. Using this approach the relevant parameters of the alamethicin-lipid system have been determined (yielding, in particular, a partition coefficient of 1.3 X 10(3) for the monomeric peptide and a critical aqueous concentration of 2.5 microM). Finally, the possible relevance of these results for the voltage-dependent gating of alamethicin is briefly pointed out.

Topics: Alamethicin; Anti-Bacterial Agents; Cell Aggregation; Chemical Phenomena; Chemistry, Physical; Circular Dichroism; Lipid Bilayers; Mathematics; Membrane Lipids; Membranes; Phosphatidylcholines; Thermodynamics

The properties of alamethicin channels in dioleyl phosphatidylcholine bilayers were studied in 1 M LaCl3 and were compared with those in 1 M NaCl. Single-channel recordings demonstrated that the mean single-channel life-time is about 0.25 s in NaCl but only about 17 ms in LaCl3. Whereas in NaCl the conductance levels 2 and 3 are mostly populated, in LaCl3 the levels 0 and 1 are preferentially adopted. The single-level conductance are slightly smaller in LaCl3 if the higher bulk solution conductivity of LaCl3 is taken into account. Multipore experiments confirmed earlier results (Boheim, G., Irmscher, G. and Jung, G. (1978) Biochim. Biophys. Acta 507, 485--506) that the bilayer conductance is less strongly dependent on voltage in LaCl3 than in NaCl solution. Current-fluctuation analysis showed that this effect can be explained by a less strong dependence on voltage of the pore-formation rate as well as of the mean channel life-time in LaCl3. The data can be interpreted as an increased lateral diffusion mobility of the alamethicin monomers in the bilayer. This can be the result of the binding of La3+ to the polar headgroups which can induce cluster formation of the phospholipids.

Topics: Alamethicin; Anti-Bacterial Agents; Electric Conductivity; Ion Channels; Lanthanum; Lipid Bilayers; Mathematics; Models, Biological; Phosphatidylcholines; Sodium; Thermodynamics