Bilayer Partition Coefficients of Alkanols: Predicted Effects of Varying Lipid Composition

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.

Characterization of lipid composition and diffusivity in OLA generated vesicles

Giant Unilamellar Vesicles (GUVs) are a versatile tool in many branches of science, including biophysics and synthetic biology. Octanol-Assisted Liposome Assembly (OLA), a recently developed microfluidic technique enables the production and testing of GUVs within a single device under highly controlled experimental conditions. It is therefore gaining significant interest as a platform for use in drug discovery, the production of artificial cells and more generally for controlled studies of the properties of lipid membranes. In this work, we expand the capabilities of the OLA technique by forming GUVs of tunable binary lipid mixtures of DOPC, DOPG and DOPE. Using fluorescence recovery after photobleaching we investigated the lateral diffusion coefficients of lipids in OLA liposomes and found the expected values in the range of 1 μm²/s for the lipid systems tested. We studied the OLA derived GUVs under a range of conditions and compared the results with electroformed vesicles. Overall, we found the lateral diffusion coefficients of lipids in vesicles obtained with OLA to be quantitatively similar to those in vesicles obtained via traditional electroformation. Our results provide a quantitative biophysical validation of the quality of OLA derived GUVs, which will facilitate the wider use of this versatile platform.

Fluorinated Alcohols' Effects on Lipid Bilayer Properties

Fluorinated alcohols (fluoroalcohols) have physicochemical properties that make them excellent solvents of peptides, proteins, and other compounds. Like other alcohols, fluoroalcohols also alter membrane protein function and lipid bilayer properties and stability. Thus, the questions arise: how potent are fluoroalcohols as lipid-bilayer-perturbing compounds, could small residual amounts that remain after adding compounds dissolved in fluoroalcohols alter lipid bilayer properties sufficiently to affect membranes and membrane protein function, and do they behave like other alcohols? To address these questions, we used a gramicidin-based fluorescence assay to determine the bilayer-modifying potency of selected fluoroalcohols: trifluoroethanol (TFE), HFIP, and perfluoro-tert-butanol (PFTB). These fluoroalcohols alter bilayer properties in the low (PFTB) to high (TFE) mM range. Using the same assay, we determined the bilayer partitioning of the alcohols. When referenced to the aqueous concentrations, the fluoroalcohols are more bilayer perturbing than their nonfluorinated counterparts, with the largest fluoroalcohol, PFTB, being the most potent and the smallest, TFE, the least. When referenced to the mole fractions in the membrane, however, the fluoroalcohols have equal or lesser bilayer-perturbing potency than their nonfluorinated counterparts, with TFE being more bilayer perturbing than PFTB. We compared the fluoroalcohols' molecular level bilayer interactions using atomistic molecular dynamics simulations and showed how, at higher concentrations, they can cause bilayer breakdown using absorbance measurements and ³¹P nuclear magnetic resonance.

n-Alcohol Length Governs Shift in Lo-Ld Mixing Temperatures in Synthetic and Cell-Derived Membranes

A persistent challenge in membrane biophysics has been to quantitatively predict how membrane physical properties change upon addition of new amphiphiles (e.g., lipids, alcohols, peptides, or proteins) in order to assess whether the changes are large enough to plausibly result in biological ramifications. Because of their roles as general anesthetics, n-alcohols are perhaps the best-studied amphiphiles of this class. When n-alcohols are added to model and cell membranes, changes in membrane parameters tend to be modest. One striking exception is found in the large decrease in liquid-liquid miscibility transition temperatures (Tmix) observed when short-chain n-alcohols are incorporated into giant plasma membrane vesicles (GPMVs). Coexisting liquid-ordered and liquid-disordered phases are observed at temperatures below Tmix in GPMVs as well as in giant unilamellar vesicles (GUVs) composed of ternary mixtures of a lipid with a low melting temperature, a lipid with a high melting temperature, and cholesterol. Here, we find that when GUVs of canonical ternary mixtures are formed in aqueous solutions of short-chain n-alcohols (n ≤ 10), Tmix increases relative to GUVs in water. This shift is in the opposite direction from that reported for cell-derived GPMVs. The increase in Tmix is robust across GUVs of several types of lipids, ratios of lipids, types of short-chain n-alcohols, and concentrations of n-alcohols. However, as chain lengths of n-alcohols increase, nonmonotonic shifts in Tmix are observed. Alcohols with chain lengths of 10-14 carbons decrease Tmix in ternary GUVs of dioleoyl-PC/dipalmitoyl-PC/cholesterol, whereas 16 carbons increase Tmix again. Gray et al. observed a similar influence of the length of n-alcohols on the direction of the shift in Tmix. These results are consistent with a scenario in which the relative partitioning of n-alcohols between liquid-ordered and liquid-disordered phases evolves as the chain length of the n-alcohol increases.

Local partition coefficients govern solute permeability of cholesterol-containing membranes

The permeability of lipid membranes for metabolic molecules or drugs is routinely estimated from the solute's oil/water partition coefficient. However, the molecular determinants that modulate the permeability in different lipid compositions have remained unclear. Here, we combine scanning electrochemical microscopy and molecular-dynamics simulations to study the effect of cholesterol on membrane permeability, because cholesterol is abundant in all animal membranes. The permeability of membranes from natural lipid mixtures to both hydrophilic and hydrophobic solutes monotonously decreases with cholesterol concentration [Chol]. The same is true for hydrophilic solutes and planar bilayers composed of dioleoyl-phosphatidylcholine or dioleoyl-phosphatidyl-ethanolamine. However, these synthetic lipids give rise to a bell-shaped dependence of membrane permeability on [Chol] for very hydrophobic solutes. The simulations indicate that cholesterol does not affect the diffusion constant inside the membrane. Instead, local partition coefficients at the lipid headgroups and at the lipid tails are modulated oppositely by cholesterol, explaining the experimental findings. Structurally, these modulations are induced by looser packing at the lipid headgroups and tighter packing at the tails upon the addition of cholesterol.

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.

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.

Alcohols reduce lateral membrane pressures: predictions from molecular theory

We explore the effects of alcohols on fluid lipid bilayers using a molecular theory with a coarse-grained model. We show that the trends predicted from the theory in the changes in area per lipid, alcohol concentration in the bilayer, and area compressibility modulus, as a function of alcohol chain length and of the alcohol concentration in the solvent far from the bilayer, follow those found experimentally. We then use the theory to study the effect of added alcohol on the lateral pressure profile across the membrane, and find that added alcohol reduces the surface tensions at both the headgroup/solvent and headgroup/tailgroup interfaces, as well as the lateral pressures in the headgroup and tailgroup regions. These changes in lateral pressures could affect the conformations of membrane proteins, providing a nonspecific mechanism for the biological effects of alcohols on cells.

Lack of enantiomeric specificity in the effects of anesthetic steroids on lipid bilayers

The most important target protein for many anesthetics, including volatile and steroid anesthetics, appears to be the type A gamma-amino butyric acid receptor (GABA(A)R), yet direct binding remains to be demonstrated. Hypotheses of lipid-mediated anesthesia suggest that lipid bilayer properties are changed by anesthetics and that this in turn affects the functions of proteins. While other data could equally well support direct or lipid-mediated action, enantiomeric specificity displayed by some anesthetics is not reflected in their interactions with lipids. In the present study, we studied the effects of two pairs of anesthetic steroid enantiomers on bilayers of several compositions, measuring potentially relevant physical properties. For one of the pairs, allopregnanolone and ent-allopregnanolone, the natural enantiomer is 300% more efficacious as an anesthetic, while for the other, pregnanolone and ent-pregnanolone, there is little difference in anesthetic potency. For each enantiomer pair, we could find no differences. This strongly favors the view that the effects of these anesthetics on lipid bilayers are not relevant for the main features of anesthesia. These steroids also provide tools to distinguish in general the direct binding of steroids to proteins from lipid-mediated effects.

Packing properties of 1-alkanols and alkanes in a phospholipid membrane

We have used vibrating tube densitometry to investigate the packing properties of four alkanes and a homologous series of ten alcohols in fluid-phase membranes of dimyristoyl phosphatidylcholine (DMPC). It was found that the volume change of transferring these compounds from their pure states into the membrane, DeltaV(m)(pure-->mem), was positive for small (C4-C6) 1-alkanols while it was negative for larger alcohols and all alkanes. The magnitude of DeltaV(m)(pure-->mem) ranged from about +4 cm3/mol for alcohols with an alkyl chain about half the length of the fatty acids of DMPC, to -10 to -15 cm3/mol for the alkanes and long chain alcohols. On the basis of these observations, previously published information on the structure of the membrane-solute complexes and the free volume properties of (pure) phospholipid membranes, we suggest that two effects dominate the packing properties of hydrophobic solutes in DMPC. First, perturbation of the tightly packed interfacial zone around the ester bonds and first few methylene groups of DMPC brings about a positive contribution to DeltaV(m)(pure-->mem). This effect dominates the volume behavior for alcohols like 1-butanol, 1-pentanol and 1-hexanol. More hydrophobic solutes penetrate into the membrane core, which is loosely packed. In this region, they partially occupy interstitial (or free-) volume, which bring about a denser molecular packing and generate a negative contribution to DeltaV(m)(pure-->mem).