Alcohols reduce lateral membrane pressures: predictions from molecular theory

Understanding the Molecular Mechanism of Anesthesia: Effect of General Anesthetics and Structurally Similar Non-Anesthetics on the Properties of Lipid Membranes

General anesthesia can be caused by various, chemically very different molecules, while several other molecules, many of which are structurally rather similar to them, do not exhibit anesthetic effects at all. To understand the origin of this difference and shed some light on the molecular mechanism of general anesthesia, we report here molecular dynamics simulations of the neat dipalmitoylphosphatidylcholine (DPPC) membrane as well as DPPC membranes containing the anesthetics diethyl ether and chloroform and the structurally similar non-anesthetics n-pentane and carbon tetrachloride, respectively. To also account for the pressure reversal of anesthesia, these simulations are performed both at 1 bar and at 600 bar. Our results indicate that all solutes considered prefer to stay both in the middle of the membrane and close to the boundary of the hydrocarbon domain, at the vicinity of the crowded region of the polar headgroups. However, this latter preference is considerably stronger for the (weakly polar) anesthetics than for the (apolar) non-anesthetics. Anesthetics staying in this outer preferred position increase the lateral separation between the lipid molecules, giving rise to a decrease of the lateral density. The lower lateral density leads to an increased mobility of the DPPC molecules, a decreased order of their tails, an increase of the free volume around this outer preferred position, and a decrease of the lateral pressure at the hydrocarbon side of the apolar/polar interface, a change that might well be in a causal relation with the occurrence of the anesthetic effect. All these changes are clearly reverted by the increase of pressure. Furthermore, non-anesthetics occur in this outer preferred position in a considerably smaller concentration and hence either induce such changes in a much weaker form or do not induce them at all.

A Membrane Biophysics Perspective on the Mechanism of Alcohol Toxicity

Motivations for understanding the underlying mechanisms of alcohol toxicity range from economical to toxicological and clinical. On the one hand, acute alcohol toxicity limits biofuel yields, and on the other hand, acute alcohol toxicity provides a vital defense mechanism to prevent the spread of disease. Herein the role that stored curvature elastic energy (SCE) in biological membranes might play in alcohol toxicity is discussed, for both short and long-chain alcohols. Structure-toxicity relationships for alcohols ranging from methanol to hexadecanol are collated, and estimates of alcohol toxicity per alcohol molecule in the cell membrane are made. The latter reveal a minimum toxicity value per molecule around butanol before alcohol toxicity per molecule increases to a maximum around decanol and subsequently decreases again. The impact of alcohol molecules on the lamellar to inverse hexagonal phase transition temperature (T_H) is then presented and used as a metric to assess the impact of alcohol molecules on SCE. This approach suggests the nonmonotonic relationship between alcohol toxicity and chain length is consistent with SCE being a target of alcohol toxicity. Finally, in vivo evidence for SCE-driven adaptations to alcohol toxicity in the literature are discussed.

Alcohol stress on cyanobacterial membranes: New insights revealed by transcriptomics

Cyanobacteria are model photosynthetic prokaryotic organisms often used in biotechnology to produce biofuels including alcohols. The effect of alcohols on cyanobacterial cell physiology and specifically on membrane fluidity is poorly understood. Previous research on various primary aliphatic alcohols found that alcohols with a short hydrocarbon chain (C₁-C₃) do not affect expression of genes related to membrane physical state. In addition, less water-soluble alcohols with a hydrocarbon chain longer than C₈ are found to have a reduced ability to reach cellular membranes hence do not drastically change membrane physical state or induce expression of stress-responsive genes. Therefore, hexan-1-ol (C₆) is suggested to have the most profound effect on cyanobacterial membrane physical state. Here, we studied the effects of hexan-1-ol on the cyanobacterium Synechocystis sp. PCC 6803 transcriptome. The transcriptome data obtained is compared to the previously reported analysis of gene expression induced by benzyl alcohol and butan-1-ol. The set of genes whose expression is induced after exposure to all three studied alcohols is identified. The expression under alcohol stress for several general stress response operons is analyzed, and examples of antisense interactions of RNA are investigated.

Response of microbial membranes to butanol: interdigitation vs. disorder

Biobutanol production by fermentation is potentially a sustainable alternative to butanol production from fossil fuels. However, the toxicity of butanol to fermentative bacteria, resulting largely from cell membrane fluidization, limits production titers and is a major factor limiting the uptake of the technology. Here, studies were undertaken, in vitro and in silico, on the butanol effects on a representative bacterial (i.e. Escherichia coli) inner cell membrane. A critical butanol : lipid ratio for stability of 2 : 1 was observed, computationally, consistent with complete interdigitation. However, at this ratio the bilayer was ∼20% thicker than for full interdigitation. Furthermore, butanol intercalation induced acyl chain bending and increased disorder, measured as a 27% lateral diffusivity increase experimentally in a supported lipid bilayer. There was also a monophasic Tm reduction in butanol-treated large unilamellar vesicles. Both behaviours are inconsistent with an interdigitated gel. Butanol thus causes only partial interdigitation at physiological temperatures, due to butanol accumulating at the phospholipid headgroups. Acyl tail disordering (i.e. splaying and bending) fills the subsequent voids. Finally, butanol short-circuits the bilayer and creates a coupled system where interdigitated and splayed phospholipids coexist. These findings will inform the design of strategies targeting bilayer stability for increasing biobutanol production titers.

Drunken Membranes: Short-Chain Alcohols Alter Fusion of Liposomes to Planar Lipid Bilayers

Although the effects of ethanol on protein receptors and lipid membranes have been studied extensively, ethanol's effect on vesicles fusing to lipid bilayers is not known. To determine the effect of alcohols on fusion rates, we utilized the nystatin/ergosterol fusion assay to measure fusion of liposomes to a planar lipid bilayer (BLM). The addition of ethanol excited fusion when applied on the cis (vesicle) side, and inhibited fusion on the trans side. Other short-chain alcohols followed a similar pattern. In general, the inhibitory effect of alcohols (trans) occurs at lower doses than the excitatory (cis) effect, with a decrease of 29% in fusion rates at the legal driving limit of 0.08% (w/v) ethanol (IC50 = 0.2% v/v, 34 mM). Similar inhibitory effects were observed with methanol, propanol, and butanol, with ethanol being the most potent. Significant variability was observed with different alcohols when applied to the cis side. Ethanol and propanol enhanced fusion, butanol also enhanced fusion but was less potent, and low doses of methanol mildly inhibited fusion. The inhibition by trans addition of alcohols implies that they alter the planar membrane structure and thereby increase the activation energy required for fusion, likely through an increase in membrane fluidity. The cis data are likely a combination of the above effect and a proportionally greater lowering of the vesicle lysis tension and hydration repulsive pressure that combine to enhance fusion. Alternate hypotheses are also discussed. The inhibitory effect of ethanol on liposome-membrane fusion is large enough to provide a possible biophysical explanation of compromised neuronal behavior.

Molecular mechanisms of ethanol-associated oro-esophageal squamous cell carcinoma

Alcohol drinking is a major etiological factor of oro-esophageal squamous cell carcinoma (OESCC). Both local and systemic effects of ethanol may promote carcinogenesis, especially among chronic alcoholics. However, molecular mechanisms of ethanol-associated OESCC are still not well understood. In this review, we summarize current understandings and propose three mechanisms of ethanol-associated OESCC: (1) Disturbance of systemic metabolism of nutrients: during ethanol metabolism in the liver, systemic metabolism of retinoids, zinc, iron and methyl groups is altered. These nutrients are known to be associated with the development of OESCC. (2) Disturbance of redox metabolism in squamous epithelial cells: when ethanol is metabolized in oro-esophageal squamous epithelial cells, reactive oxygen species are generated and produce oxidative damage. Meanwhile, ethanol may also disturb fatty-acid metabolism in these cells. (3) Disturbance of signaling pathways in squamous epithelial cells: due to its physico-chemical properties, ethanol changes cell membrane fluidity and shape, and may thus impact multiple signaling pathways. Advanced molecular techniques in genomics, epigenomics, metabolomics and microbiomics will help us elucidate how ethanol promotes OESCC.

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.

n-Butanol partitioning into phase-separated heterogeneous lipid monolayers

Cellular adaptation to elevated alcohol concentration involves altering membrane lipid composition to counteract fluidization. However, few studies have examined the biophysical response of biologically relevant heterogeneous membranes. Lipid phase behavior, molecular packing, and elasticity have been examined by surface pressure-area (π-A) analysis in mixed monolayers composed of saturated dipalmitoylphosphatidylcholine (DPPC) and unsaturated dioleoylphosphatidylcholine (DOPC) as a function of DOPC and n-butanol concentration. n-Butanol partitioning into DPPC monolayers led to lipid expansion and increased elasticity. Greater lipid expansion occurred with increasing DOPC concentration, and a maximum was observed at equimolar DPPC:DOPC consistent with n-butanol partitioning between coexisting liquid expanded (LE, DOPC) phases and liquid condensed (LC, DPPC) domains. This led to distinct changes in the size and morphology of LC domains. In DOPC-rich monolayers the effect of n-butanol adsorption on π-A behavior was less pronounced due to DOPC tail kinking. These results point to the importance of lipid composition and phase coexistence on n-butanol partitioning and monolayer restructuring.

Acute effects of ethanol on the transfer of nicotine and two dietary carcinogens in human placental perfusion

Many mothers use, against instructions, alcohol during pregnancy. Simultaneously mothers are exposed to a wide range of other environmental chemicals. These chemicals may also harm the developing fetus, because almost all toxic compounds can go through human placenta. Toxicokinetic effects of ethanol on the transfer of other environmental compounds through human placenta have not been studied before. It is known that ethanol has lytic properties and increases the permeability and fluidity of cell membranes. We studied the effects of ethanol on the transfer of three different environmental toxins: nicotine, PhIP (2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine) and NDMA (N-nitrosodimethylamine) in placental perfusion. We tested in human breast cancer adenocarcinoma cell line MCF-7 whether ethanol affects ABCG2/BCRP, which is also the major transporter in human placenta. We found that the transfer of ethanol is comparable to that of antipyrine, which points to passive diffusion as the transfer mechanism. Unexpectedly, ethanol had no statistically significant effect on the transfer of the other studied compounds. Neither did ethanol inhibit the function of ABCG2/BCRP. These experiments represent only the effects of acute exposure to ethanol and chronic exposure remains to be studied.

Modulation of KvAP unitary conductance and gating by 1-alkanols and other surface active agents

The actions of alcohols and anesthetics on ion channels are poorly understood. Controversy continues about whether bilayer restructuring is relevant to the modulatory effects of these surface active agents (SAAs). Some voltage-gated K channels (Kv), but not KvAP, have putative low affinity alcohol-binding sites, and because KvAP structures have been determined in bilayers, KvAP could offer insights into the contribution of bilayer mechanics to SAA actions. We monitored KvAP unitary conductance and macroscopic activation and inactivation kinetics in PE:PG/decane bilayers with and without exposure to classic SAAs (short-chain 1-alkanols, cholesterol, and selected anesthetics: halothane, isoflurane, chloroform). At levels that did not measurably alter membrane specific capacitance, alkanols caused functional changes in KvAP behavior including lowered unitary conductance, modified kinetics, and shifted voltage dependence for activation. A simple explanation is that the site of SAA action on KvAP is its entire lateral interface with the PE:PG/decane bilayer, with SAA-induced changes in surface tension and bilayer packing order combining to modulate the shape and stability of various conformations. The KvAP structural adjustment to diverse bilayer pressure profiles has implications for understanding desirable and undesirable actions of SAA-like drugs and, broadly, predicts that channel gating, conductance and pharmacology may differ when membrane packing order differs, as in raft versus nonraft domains.

Using ergosterol to mitigate the deleterious effects of ethanol on bilayer structure

In wine fermentations, yeast is exposed to concentrated ethanol solutions. Ergosterol, a sterol that is found in lower eukaryotic membranes, helps preserve the structural integrity of yeast membranes in stressful environmental conditions. A premature arrest in ethanol production due to unknown metabolic changes in yeasts results in undesirably large concentrations of residual sugar and may be caused by the formation of an ethanol-induced interdigitated phase. We use atomistic molecular dynamics simulations to examine the induction of the interdigitated phase in model yeast membranes that contain either 0, 10, 20, 25 mol % ergosterol in ethanol concentrations of 0, 10, 15 vol %. The 25 mol % ergosterol system shows a similar level of interdigitation for the 0 and 10 vol % ethanol solutions, indicating that ergosterol molecules in this system are able to effectively counteract the disruptive behavior of ethanol molecules. However, at a 15 vol % ethanol solution, the amount of interdigitation triples and this ethanol concentration is similar to the concentrations found in stuck fermentations. The other three ergosterol concentrations studied (0, 10, 20 mol %) show larger quantities of interdigitation in the 10 vol % ethanol solution than the 0 vol % solution. Thus, the 25 mol % ergosterol bilayer, which is representative of the ergosterol concentrations seen in yeast membranes, is unique in the systems examined in its ability to delay the onset of ethanol-induced interdigitation. The concentration of ergosterol affects the permeability of a fluid-phase bilayer, where the 10 mol % ergosterol bilayer is more permeable to ethanol than either a bilayer containing no ergosterol molecules or a bilayer containing 20/25 mol % ergosterol. This lipid permeability appears to be correlated with the existence of a lipid region whose lipids neither have direct contact with ergosterol molecules nor exhibit bulk lipid/lipid interactions.

The influence of 1-alkanols and external pressure on the lateral pressure profiles of lipid bilayers

The suggestion by Robert Cantor, that drug-induced pressure changes in lipid bilayers can change the conformational equilibrium between open and closed states of membrane proteins and thereby cause anesthesia, attracted much attention lately. Here, we studied the effect of both large external pressure and of 1-alkanols of different chain lengths--some of them anesthetics, others not--on the lateral pressure profiles across dimyristoylphosphatidylcholine (DMPC) bilayers by molecular dynamics simulations. For a pure DMPC bilayer, high pressure both reduced and broadened the tension at the interface hydrophobic/hydrophilic and diminished the repulsion between the phospholipid headgroups. Whereas the effect of ethanol on the lateral pressure profile was similar to the effect of a large external pressure on a DMPC bilayer, long-chain 1-alkanols significantly amplified local maxima and minima in the lateral pressure profile. For most 1-alkanols, external pressure had moderate effects and did not reverse the changes 1-alkanols exerted on the pressure profile. Nevertheless, assuming the bent helix model as a simple geometric model for the transmembrane region of a membrane protein, protein conformational equilibria were shifted in opposite directions by addition of 1-alkanols and additional application of external pressure.

Influence of ethanol on lipid membranes: from lateral pressure profiles to dynamics and partitioning

We have combined experiments with atomic-scale molecular dynamics simulations to consider the influence of ethanol on a variety of lipid membrane properties. We first employed isothermal titration calorimetry together with the solvent-null method to study the partitioning of ethanol molecules into saturated and unsaturated membrane systems. The results show that ethanol partitioning is considerably more favorable in unsaturated bilayers, which are characterized by their more disordered nature compared to their saturated counterparts. Simulation studies at varying ethanol concentrations propose that the partitioning of ethanol depends on its concentration, implying that the partitioning is a nonideal process. To gain further insight into the permeation of alcohols and their influence on lipid dynamics, we also employed molecular dynamics simulations to quantify kinetic events associated with the permeation of alcohols across a membrane, and to characterize the rotational and lateral diffusion of lipids and alcohols in these systems. The simulation results are in agreement with available experimental data and further show that alcohols have a small but non-vanishing effect on the dynamics of lipids in a membrane. The influence of ethanol on the lateral pressure profile of a lipid bilayer is found to be prominent: ethanol reduces the tension at the membrane-water interface and reduces the peaks in the lateral pressure profile close to the membrane-water interface. The changes in the lateral pressure profile are several hundred atmospheres. This supports the hypothesis that anesthetics may act by changing the lateral pressure profile exerted on proteins embedded in membranes.

A quantitative coarse-grain model for lipid bilayers

A simplified particle-based computer model for hydrated phospholipid bilayers has been developed and applied to quantitatively predict the major physical features of fluid-phase biomembranes. Compared with available coarse-grain methods, three novel aspects are introduced. First, the main electrostatic features of the system are incorporated explicitly via charges and dipoles. Second, water is accurately (yet efficiently) described, on an individual level, by the soft sticky dipole model. Third, hydrocarbon tails are modeled using the anisotropic Gay-Berne potential. Simulations are conducted by rigid-body molecular dynamics. Our technique proves 2 orders of magnitude less demanding of computational resources than traditional atomic-level methodology. Self-assembled bilayers quantitatively reproduce experimental observables such as electron density, compressibility moduli, dipole potential, lipid diffusion, and water permeability. The lateral pressure profile has been calculated, along with the elastic curvature constants of the Helfrich expression for the membrane bending energy; results are consistent with experimental estimates and atomic-level simulation data. Several of the results presented have been obtained for the first time using a coarse-grain method. Our model is also directly compatible with atomic-level force fields, allowing mixed systems to be simulated in a multiscale fashion.

How alcohol chain-length and concentration modulate hydrogen bond formation in a lipid bilayer

Molecular dynamics simulations are used to measure the change in properties of a hydrated dipalmitoylphosphatidylcholine bilayer when solvated with ethanol, propanol, and butanol solutions. There are eight oxygen atoms in dipalmitoylphosphatidylcholine that serve as hydrogen bond acceptors, and two of the oxygen atoms participate in hydrogen bonds that exist for significantly longer time spans than the hydrogen bonds at the other six oxygen atoms for the ethanol and propanol simulations. We conclude that this is caused by the lipid head group conformation, where the two favored hydrogen-bonding sites are partially protected between the head group choline and the sn-2 carbonyl oxygen. We find that the concentration of the alcohol in the ethanol and propanol simulations does not have a significant influence on the locations of the alcohol/lipid hydrogen bonds, whereas the concentration does impact the locations of the butanol/lipid hydrogen bonds. The concentration is important for all three alcohol types when the lipid chain order is examined, where, with the exception of the high-concentration butanol simulation, the alcohol molecules having the longest hydrogen-bonding relaxation times at the favored carbonyl oxygen acceptor sites also have the largest order in the upper chain region. The lipid behavior in the high-concentration butanol simulation differs significantly from that of the other alcohol concentrations in the order parameter, head group rotational relaxation time, and alcohol/lipid hydrogen-bonding location and relaxation time. This appears to be the result of the system being very near to a phase transition, and one occurrence of lipid flip-flop is seen at this concentration.