Lipid Bilayer Interactions Research Articles

Molecular Dynamics Simulation and Analysis of the Antimicrobial Peptide-Lipid Bilayer Interactions.

A great deal of research has been undertaken in order to discover antimicrobial peptides (AMPs) with unexploited mechanisms of action to counteract the health-threatening issues associated with bacterial resistance. The intrinsic effectiveness of AMPs is strongly influenced by their initial interactions with the bacterial cell membrane. Understanding these interactions in the atomistic details is important for the design of the less prone bacteria-resistant peptides. However, these studies always require labor-intensive and difficult steps. With this regard, modeling studies of the AMPs binding to simple lipid membrane systems, e.g., lipid bilayers, is of great advantage. In this chapter, we present an applicable step-by-step protocol to run the molecular dynamics (MD) simulation of the interaction between cyclo-RRWFWR (c-WFW) (a small cyclic AMP) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayer using the Groningen machine for chemical simulations (GROMACS) package. The protocol as described here may simply be optimized for other peptide-lipid systems of interest.

Coarse-Grained Molecular Dynamics Simulations of Membrane-Trehalose Interactions.

It is well established that trehalose (TRH) affects the physical properties of lipid bilayers and stabilizes biological membranes. We present molecular dynamics (MD) computer simulations to investigate the interactions between lipid membranes formed by 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and TRH. Both atomistic and coarse-grained (CG) interaction models were employed, and the coarse graining of DMPC leads to a reduction in the acyl chain length corresponding to a 1,2-dilauroyl-sn-glycero-3-phosphocholine lipid (DLPC). Several modifications of the Martini interaction model, used for CG simulations, were implemented, resulting in different potentials of mean force (PMFs) for DMPC bilayer-TRH interactions. These PMFs were subsequently used in a simple two-site analytical model for the description of sugar binding at the membrane interface. In contrast to that in atomistic MD simulations, the binding in the CG model was not in agreement with the two-site model. Our interpretation is that the interaction balance, involving water, TRH, and lipids, in the CG systems needs further tuning of the force-field parameters. The area per lipid is only weakly affected by TRH concentration, whereas the compressibility modulus related to the fluctuations of the membrane increases with an increase in TRH content. In agreement with experimental findings, the bending modulus is not affected by the inclusion of TRH. The important aspects of lipid bilayer interactions with biomolecules are membrane curvature generation and sensing. In the present investigation, membrane curvature is generated by artificial buckling of the bilayer in one dimension. It turns out that TRH prefers the regions with the highest curvature, which enables the most favorable situation for lipid-sugar interactions.

Selective High-Resolution Detection of Membrane Protein-Ligand Interaction in Native Membranes Using Trityl-Nitroxide PELDOR.

The orchestrated interaction of transmembrane proteins with other molecules mediates several crucial biological processes. Detergent solubilization may significantly alter or even abolish such hetero-oligomeric interactions, which makes observing them at high resolution in their native environment technically challenging. Dipolar electron paramagnetic resonance (EPR) techniques such as pulsed electro-electron double resonance (PELDOR) can provide very precise distances within biomolecules. To concurrently determine the inter-subunit interaction and the intra-subunit conformational changes in hetero-oligomeric complexes, a combination of different spin labels is required. Orthogonal spin labeling using a triarylmethyl (TAM) label in combination with a nitroxide label is used to detect protein-ligand interactions in native lipid bilayers. This approach provides a higher sensitivity and total selectivity and will greatly facilitate the investigation of multimeric transmembrane complexes employing different spin labels in the native lipid environment.

Selective High‐Resolution Detection of Membrane Protein–Ligand Interaction in Native Membranes Using Trityl–Nitroxide PELDOR

AbstractThe orchestrated interaction of transmembrane proteins with other molecules mediates several crucial biological processes. Detergent solubilization may significantly alter or even abolish such hetero‐oligomeric interactions, which makes observing them at high resolution in their native environment technically challenging. Dipolar electron paramagnetic resonance (EPR) techniques such as pulsed electro–electron double resonance (PELDOR) can provide very precise distances within biomolecules. To concurrently determine the inter‐subunit interaction and the intra‐subunit conformational changes in hetero‐oligomeric complexes, a combination of different spin labels is required. Orthogonal spin labeling using a triarylmethyl (TAM) label in combination with a nitroxide label is used to detect protein–ligand interactions in native lipid bilayers. This approach provides a higher sensitivity and total selectivity and will greatly facilitate the investigation of multimeric transmembrane complexes employing different spin labels in the native lipid environment.

Does a methionine-to-norleucine substitution in PGLa influence peptide-membrane interactions?

Yes. To understand the molecular mechanisms of amphiphilic membrane-active peptides, it is essential to study their interactions with lipid bilayers under near-native conditions. Amino acid composition largely determines the non-specific properties of peptides, on the basis of the physicochemical properties of the side chains. The resultant effects on peptides' functional properties include influences on the conformation, structural dynamics and binding affinities within the peptide interactome. Here, we studied the effect of substituting oxidation-prone methionine (Met) with non-oxidizable norleucine (Nle) in the model α-helical antimicrobial peptide PGLa, through systematic comparison of PGLa with the 2Met/2Nle mutant. Both peptides were evaluated for their bacteriostatic and hemolytic activities (using in situ assays), for their conformational preferences in isotropic solutions (using circular dichroism spectropolarimetry) and for their abilities to modulate membrane curvature (using a solid-state 31P NMR assay). We determined the membrane-bound states in detail and characterized the orientational dynamics of both peptides in oriented phospholipid membranes by solid-state 19F NMR spectroscopy. On the one hand, the bioactivity results, the structure in the diluted membrane-mimicking environments and the strong inhibition of the negative membrane curvature were comparable between PGLa and the mutant. On the other hand, the alignments in DMPC bilayer were qualitatively the same but differed in absolute values – the more hydrophobic Nle residue inserted deeper in the membrane core. Furthermore, the mutant peptide displayed a significantly reduced ability to re-orient from the monomeric, surficial to the putative dimeric, tilted state. Overall, these results confirm the functional isosterism of Nle and Met in the helical membrane-active peptides but highlight differences in the ways in which the two residues affect non-specific binding to the lipid bilayer and homomeric peptide-peptide interactions.

Insights into the Nature of Anesthetic-Protein Interactions: An ONIOM Study.

Anesthetics have been employed widely to relieve surgical suffering, but their mechanism of action is not yet clear. For over a century, the mechanism of anesthesia was previously thought to be via lipid bilayer interactions. In the present work, a rigorous three-layer ONIOM(M06-2X/6-31+G*:PM6:AMBER) method was utilized to investigate the nature of interactions between several anesthetics and actual protein binding sites. According to the calculated structural features, interaction energies, atomic charges, and electrostatic potential surfaces, the amphiphilic nature of anesthetic-protein interactions was demonstrated for both inhalational and injectable anesthetics. The existence of hydrogen and halogen bonding interactions between anesthetics and proteins was clearly identified, and these interactions served to assist ligand recognition and binding by the protein. Within all complexes of inhalational or injectable anesthetics, the polarization effects play a dominant role over the steric effects and induce a significant asymmetry in the otherwise symmetric atomic charge distributions of the free ligands in vacuo. This study provides new insight into the mechanism of action of general anesthetics in a more rigorous way than previously described. Future rational design of safer anesthetics for an aging and more physiologically vulnerable population will be predicated on this greater understanding of such specific interactions.

Biopolymer-Lipid Bilayer Interaction Modulates the Physical Properties of Liposomes: Mechanism and Structure.

This study was conducted to elucidate the conformational dependence of the modulating ability of chitosan, a positively charged biopolymer, on a new type of liposome composed of mixed lipids including egg yolk phosphatidylcholine (EYPC) and nonionic surfactant (Tween 80). Analysis of the dynamic and structure of bilayer membrane upon interaction with chitosan by fluorescence and electron paramagnetic resonance techniques demonstrated that, in addition to providing a physical barrier for the membrane surface, the adsorption of chitosan extended and crimped chains rigidified the lipid membrane. However, the decrease in relative microviscosity and order parameter suggested that the presence of chitosan coils disturbed the membrane organization. It was also noted that the increase of fluidity in the lipid bilayer center was not pronounced, indicating the shallow penetration of coils into the hydrophobic interior of bilayer. Microscopic observations revealed that chitosan adsorption not only affected the morphology of liposomes but also modulated the particle aggregation and fusion. Especially, a number of very heterogeneous particles were visualized, which tended to confirm the role of chitosan coils as a "polymeric surfactant". In addition to particle deformation, the membrane permeability was also tuned. These findings may provide a new perspective to understand the physiological functionality of biopolymer and design biopolymer-liposome composite structures as delivery systems for bioactive components.

Incorporation of charged residues in the CYP2J2 F-G loop disrupts CYP2J2–lipid bilayer interactions

CYP2J2 epoxygenase is an extrahepatic, membrane bound cytochrome P450 (CYP) that is primarily found in the heart and mediates endogenous fatty acid metabolism. CYP2J2 interacts with membranes through an N-terminal anchor and various non-contiguous hydrophobic residues. The molecular details of the motifs that mediate membrane interactions are complex and not fully understood. To gain better insights of these complex protein–lipid interactions, we employed molecular dynamics (MD) simulations using a highly mobile membrane mimetic (HMMM) model that enabled multiple independent spontaneous membrane binding events to be captured. Simulations revealed that CYP2J2 engages with the membrane at the F-G loop through hydrophobic residues Trp-235, Ille-236, and Phe-239. To explore the role of these residues, three F-G loop mutants were modeled from the truncated CYP2J2 construct (Δ34) which included Δ34-I236D, Δ34-F239H and Δ34-I236D/F239H. Using the HMMM coordinates of CYP2J2, the simulations were extended to a full POPC membrane which showed a significant decrease in the depth of insertion for each of the F-G loop mutants. The CYP2J2 F-G loop mutants were expressed in E. coli and were shown to be localized to the cytosolic fraction at a greater percentage relative to construct Δ34. Notably, the functional data demonstrated that the double mutant, Δ34-I236D/F239H, maintained native-like enzymatic activity. The membrane insertion characteristics were examined by monitoring CYP2J2 Trp-quenching fluorescence spectroscopy upon binding nanodiscs containing pyrene phospholipids. Relative to the Δ34 construct, the F-G loop mutants exhibited lower Trp quenching and membrane insertion. Taken together, the results suggest that the mutants exhibit a different membrane topology in agreement with the MD simulations and provide important evidence towards the involvement of key residues in the F-G loop of CYP2J2.

Multi-scale Simulation of Carbon Nanotubes Interactions with Cell Membrane: DFT Calculations and Molecular Dynamic Simulation

Due to unique physiochemical properties of carbon nanotubes (CNTs), they are utilized in biomedicine applications including biomedical engineering, tissue engineering, drug delivery, gene therapy and biosensor engineering. Studies have been shown that carbon nanotube have a large propensity to interact with biomembranes; therefore, the toxicity of these nanomaterials and understanding the way CNTs interact with cell become more important. Recently, enormous efforts have been made to understand the molecular mechanism of CNTs and lipid bilayer interaction. Because of the relative large scale of lipid bilayer, quantum ab-initio methods are not common for such systems; consequently, both all atom and coarse grained molecular dynamic simulation (MD) are widely performed. For simulation of such system it is important to apply the correct physical and structural properties of carbon nanotube in the simulation, for instance with density functional theory (DFT) modeling it has been clear that, for open-ended CNTs not only is there unsaturated bonds at the end of tube, but also has a significant dipole moment which should not neglected and their quantity correspond to nanotube chirality and diameter. The aim of this work is to discuss the partial charges distribution along the carbon nanotubes and describe how they affect in CNTs -lipid bilayer interaction by free energy calculation. Our Simulation consist of two stage, first partial charges for pristine and functional carbon nanotube were determined by DFT calculation then results were applied to molecular dynamic force field. The DFT simulation computes partial charges quantity at the ends of nanotube and its results indicate that partial charges for functional nanotubes are higher than pristine nanotubes. The MD simulation determines that nanotubes interaction energy with biomembrane is highly strong, about 120kcal/mol.

Water and Lipid Bilayers.

Water is crucial to the structure and function of biological membranes. In fact, the membrane's basic structural unit, i.e. the lipid bilayer, is self-assembled and stabilized by the so-called hydrophobic effect, whereby lipid molecules unable to hydrogen bond with water aggregate in order to prevent their hydrophobic portions from being exposed to water. However, this is just the beginning of the lipid-bilayer-water relationship. This mutual interaction defines vesicle stability in solution, controls small molecule permeation, and defines the spacing between lamella in multi-lamellar systems, to name a few examples. This chapter will describe the structural and dynamical properties central to these, and other water- lipid bilayer interactions.

Single-Molecule FRET Detection of GXXXG-Mediated Transmembrane Helix-Helix Interactions

Helix-helix interactions in lipid bilayers are principal processes that determine the folding, oligomerization, and conformational change of helical transmembrane proteins. Not only the amino acid sequence of the protein but also the composition of surrounding lipids significantly affect the stability of the interaction. The GXXXG motif is frequently found at interaction interface of the transmembrane region, and proposed to mediate helix associations via hydrogen bonding between Cα-H donor and the backbone C=O acceptor. However, energetic/kinetic contributions of the motif have not been well characterized.In this study, we investigated the effect of a GXXXG-motif introduced into the center of the host transmembrane helix (AALALAA)3, examined by a single molecule FRET technique. The host helices are known to weakly self-associate in antiparallel orientations in POPC vesicles. In contrast, the GXXXG motif significantly stabilized a parallel association of the helices with lifetimes of subseconds. We also found that cholesterol suppressed the GXXXG-mediated parallel associations, demonstrating the importance of lipid environment on the helix-helix interaction

Nanoparticle–lipid bilayer interactions studied with lipid bilayer arrays

The widespread environmental presence and commercial use of nanoparticles have raised significant health concerns as a result of many in vitro and in vivo assays indicating toxicity of a wide range of nanoparticle species. Many of these assays have identified the ability of nanoparticles to damage cell membranes. These interactions can be studied in detail using artificial lipid bilayers, which can provide insight into the nature of the particle-membrane interaction through variation of membrane and solution properties not possible with cell-based assays. However, the scope of these studies can be limited because of the low throughput characteristic of lipid bilayer platforms. We have recently described an easy to use, parallel lipid bilayer platform which we have used to electrically investigate the activity of 60 nm diameter amine and carboxyl modified polystyrene nanoparticles (NH2-NP and COOH-NP) with over 1000 lipid bilayers while varying lipid composition, bilayer charge, ionic strength, pH, voltage, serum, particle concentration, and particle charge. Our results confirm recent studies finding activity of NH2-NP but not COOH-NP. Detailed analysis shows that NH2-NP formed pores 0.3-2.3 nm in radius, dependent on bilayer and solution composition. These interactions appear to be electrostatic, as they are regulated by NH2-NP surface charge, solution ionic strength, and bilayer charge. The ability to rapidly measure a large number of nanoparticle and membrane parameters indicates strong potential of this bilayer array platform for additional nanoparticle bilayer studies.

Interactions of Lipid Membranes with Fibrillar Protein Aggregates.

Amyloid fibrils are an intriguing class of protein aggregates with distinct physicochemical, structural and morphological properties. They display peculiar membrane-binding behavior, thus adding complexity to the problem of protein-lipid interactions. The consensus that emerged during the past decade is that amyloid cytotoxicity arises from a continuum of cross-β-sheet assemblies including mature fibrils. Based on literature survey and our own data, in this chapter we address several aspects of fibril-lipid interactions, including (i) the effects of amyloid assemblies on molecular organization of lipid bilayer; (ii) competition between fibrillar and monomeric membrane-associating proteins for binding to the lipid surface; and (iii) the effects of lipids on the structural morphology of fibrillar aggregates. To illustrate some of the processes occurring in fibril-lipid systems, we present and analyze fluorescence data reporting on lipid bilayer interactions with fibrillar lysozyme and with the N-terminal 83-residue fragment of amyloidogenic mutant apolipoprotein A-I, 1-83/G26R/W@8. The results help understand possible mechanisms of interaction and mutual remodeling of amyloid fibers and lipid membranes, which may contribute to amyloid cytotoxicity.

Binding of Aβ Monomer to DMPC Bilayer using Isobaric-Isothermal Replica Exchange Molecular Dynamics

Interactions between Aβ peptides and cellular membranes have been implicated as a possible cause of the cytotoxicity associated with Alzheimer's disease; however, little is known about the molecular mechanisms predominating Aβ and lipid bilayer interactions. To understand Aβ-lipid interactions at molecular level, we have performed all-atom explicit solvent replica exchange molecular dynamics simulations in the isobaric-isothermal ensemble of the Aβ monomer binding to the zwitterionic DMPC bilayer. Upon binding, Aβ undergoes a dramatic structural transition from random coil state in water to a state predominated by stable helical structure in the peptide's hydrophobic C-terminal and, to a lesser extent, the central hydrophobic cluster. In addition, binding to the lipid bilayer induces the formation of the intrapeptide Asp23-Lys28 salt bridge. The peptide's C-terminal and central hydrophobic cluster do not only govern binding to the bilayer but they also penetrate into the bilayer core. In contrast, the polar N-terminal and turn region remain solvated and mainly form interactions with the bilayer surface. This partial insertion of Aβ reduces the local density of lipids and creates an indentation in the bilayer. Surprisingly, peptide insertion does not significantly distort lipid structural properties or enhance water permeation. Taken together, these results suggest that Aβ aggregation in the presence of a zwitterionic lipid bilayer is likely to be different from that in bulk water. Small structural perturbations in lipids induced by the Aβ monomer may constitute the molecular basis of its low cytotoxicity.

Role of Headgroup Dipole Interactions in Phosphatidylcholine and Phosphatidylserine Bilayers

The presence of a dielectric gradient in lipid bilayers is critical for membrane peptide folding in vivo. In addition, the hydrogen bonding ability of lipid head groups is known to affect bilayer properties. To reveal the role of headgroup dielectric properties in phospholipid bilayers, we have developed a polarizable Coarse-Grained (pCG) model, where the dipole moment of polar groups, such as serine and ester groups, can be adjusted through the addition of two extra “dummy” particles inside a CG bead. These dummy particles can mimic the hydrogen bonding network that can exist among head groups. The addition of polarization effects into CG beads has been proven effective in prior work regarding de novo peptide secondary structure folding with no external bias (Ganesan, 2014).In this work, we have explored the role of dipole interactions in monocomponent lipid bilayers using POPC (Palmitoyl Oleoyl Phosphatidyl-Choline) and POPS (Palmitoyl Oleoyl Phosphatidyl-Serine) lipids. An excellent agreement is observed on several structural and dynamical bilayer properties when compared to all-atom simulation data.We will present results on the role of dipole-dipole and dipole-charge interactions in shaping the clustering of PS lipids. Since dipole interactions are influenced by charge screening, the model is sensitive to changes in salt concentration. Our results suggest that headgroup dipole interactions may participate in PS/PC lipid phase separation as observed in experiments.Reference“The role of backbone dipole interactions in the formation of secondary and super-secondary structures of proteins”, Ganesan S. J., Matysiak S., Journal of Chemical Theory and Computation (2014).

Chelating Agent Induction of Multiphase Coexistence in Lipid Multilayers

Previous work on lipid interactions has shown that zwitterionic buffers affect the attractive and repulsive forces between bilayers. The equilibrium spacing between lipid multilayers as measured by small angle x-ray scattering is determined by the balance of van der Waals (vdW), electrostatic and fluctuation forces. We will present a series of small angle x-ray scattering experiments that explores how chelating agents such as ethylenediaminetetraacetic acid (EDTA) modify the interactions between membrane. EDTA was shown to induce a phase coexistence in multilamellar systems of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) [1]. In this study we show that the effect of EDTA depends on other mobile ions in solution and lipid type. These data require a reevaluation of theoretical models of lipid bilayer interactions. [1] Johnson et al, Langmuir 2014.

Sorting of integral membrane proteins mediated by curvature-dependent protein-lipid bilayer interaction.

Cell membrane proteins, both bound and integral, are known to preferentially accumulate at membrane locations with curvatures favorable to their shape. This is mainly due to the curvature dependent interaction between membrane proteins and their lipid environment. Here, we analyze the effects of the protein-lipid bilayer interaction energy due to mismatch between the protein shape and the principal curvatures of the surrounding bilayer. The role of different macroscopic parameters that define the interaction energy term is elucidated in relation to recent experiment in which the lateral distribution of a membrane embedded protein potassium channel KvAP is measured on a giant unilamellar lipid vesicle (reservoir) and a narrow tubular extension - a tether - kept at constant length. The dependence of the sorting ratio, defined as the ratio between the areal density of the protein on the tether and on the vesicle, on the inverse tether radius is influenced by the strength of the interaction, the intrinsic shape of the membrane embedded protein, and its abundance in the reservoir. It is described how the values of these constants can be extracted from experiments. The intrinsic principal curvatures of a protein are related to the tether radius at which the sorting ratio attains its maximum value. The estimate of the principal intrinsic curvature of the protein KvAP, obtained by comparing the experimental and theoretical sorting behavior, is consistent with the available information on its structure.

Study of Orientation and Penetration of LAH4 into Lipid Bilayer Membranes: pH and Composition Dependence

LAH4 is an antimicrobial peptide that is believed to possess both antibiotic and DNA delivery capabilities. It is one of a number of membrane-active peptides that show increased affinity toward anionic lipids. Herein, we have performed molecular dynamics simulations to compare LAH4 effects on anionic palmitoyl-oleoyl-phosphatidylglycerol bilayer, which approximate a prokaryotic membrane environment and zwitterionic palmitoyl-oleoyl-phosphatidylcholine bilayer, which approximate a eukaryotic membrane environment. One particular interest in this work is to study how different kinds of lipid bilayers respond to the attraction of LAH4. Remarkably, our data have shown that the depth of peptide penetration strongly depends on membrane composition and pH. At acidic pH, LAH4 has exhibited a high tendency to interact strongly with and be adsorbed on anionic membrane. We have also shown that electrostatic interactions between His11 and the phosphor atoms of bilayers should have a significant impact on the penetration of LAH4. These results provide insights into the interactions of LAH4 and lipid bilayers at the atomic level, which is useful to understand cell selectivity and mechanism of the peptide action.

Ultrafast molecular dynamics of biofuel extraction for microalgae and bacteria milking: blocking membrane folding pathways to damaged lipid-bilayer conformations with nanomicelles

Cell milking is a 100% renewable green energy for CO2 by extraction of biofuels inside the cytosol of photosynthetic micro-organisms as microalgae and bacteria. The cells are exposed to a hydrophobic solvent forming holes and cracks through their membranes from which the biofuels can leak out. In protein folding, the goal would be to find pathways to the unique functional protein conformer. However, in the lipid-bilayer interaction with the solvent for milking, the objective is to block the pathways for damaged membrane conformations of low free energy with undesired nanostructures, using the solvent properties, as shown with an ab initio structural bioinformatic model. Statistical thermodynamics is used to compute the free energy (including entropy) from the molecular dynamics trajectory of the biomolecular system with many conformational changes. This model can be extended to the general problem of biomolecules folding as for proteins and nucleic acids. Using an adaptation of the Einstein diffusion law, the conformational change dynamics of the lipid bilayer depends on the two diffusion coefficients of the solvent: D1 before the irreversible folding transition time and the much smaller D2 thereafter. In contrast to the n-hexane and n-heptane hydrocarbons of smaller size, the residual D2 = 4.7 × 10−7 cm2/s of the n-decane solvent, with the highest partition coefficient among the three extractors, is the only to present a D2 value that is significantly below the critical threshold of 10−6 cm2/s. Therefore, the membrane would resist to long hydrocarbons and the exposed cells would remain viable for milking.

Structural Disruption of Phospholipid Bilayers over a Range of Length Scales by n-Butanol

We report on the exposure of planar multicomponent lipid bilayers supported on mica to n-butanol. The bilayer contains 49 mol % 1,2-dioleoyl-sn-phosphatidylcholine (DOPC), 10 mol % cholesterol, 40 mol % sphingomyelin, and 1 mol % sulforhodamine-tagged 1,2-dioleoyl-sn-phosphatidylethanolamine (SR-DOPE). Phase separation of the cholesterol domains is seen within the bilayer structure, and exposure of this supported bilayer to controlled amounts of n-butanol in the aqueous overlayer produces morphological changes over a range of length scales. We report steady state fluorescence imaging, fluorescence lifetime imaging, and fluorescence anisotropy decay imaging for these bilayers. These data are consistent with literature reports on the interactions of lipid bilayers with n-butanol and provide molecular-scale insight relative to bilayer organization that has not been available to date. The exposure of these bilayers to n-butanol leads to more extensive disruption of the bilayer than is seen for their exposure to ethanol.