Kinetics and thermodynamics of association of a phospholipid derivative with lipid bilayers in liquid-disordered and liquid-ordered phases

In vitro screening of topical formulation excipients for epithelial toxicity in cancerous and non-cancerous cell lines

Chemical excipients used in topical formulations may be toxic to living skin cells. Here, we compared the in vitro toxicity of some common solubilizing excipients against human melanoma cells, human keratinocytes (HaCaT) and primary skin fibroblasts (FB) as examples of cancerous, immortalized and primary human skin cells, often used as experimental models representative of in vivo conditions. Two distinct endpoint assays (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and crystal violet (CV)) were used. The mechanism of cell death after excipient exposure was assessed through Reactive Oxygen Species (ROS) production, cell membrane integrity and cell cycle progression. Results showed that the surfactants, Labrasol®, Labrafil® and Transcutol®, were less toxic than Triton X-100 (a model irritant) in all cell types whereas the oil, Labrafac®, was non-toxic. The human melanoma WM164 cell line showed the greatest sensitivity toward cytotoxicity after chemical exposure, while the other cell lines were more resistant. The relative excipient cytotoxicity responses observed in the MTT and CV assays were comparable and similar trends were seen in their estimated 50 % inhibitory concentration (IC50) values. DNA fragmentation by flow cytometry after exposing the cells to IC50 concentrations of the excipients showed negligible apoptotic populations. ROS production was increased in all cell types after toxic exposure; however, ROS elevation did not lead to apoptosis. The toxicity profiles of each excipient are not only relevant to their use in formulating safe topical products but also in the potential synergistic efficacy in the topical treatment of melanoma.

Permeation of a Homologous Series of NBD-Labeled Fatty Amines through Lipid Bilayers: A Molecular Dynamics Study

Permeation through biomembranes is ubiquitous for drugs to reach their active sites. Asymmetry of the cell plasma membrane (PM) has been described as having an important role in this process. Here we describe the interaction of a homologous series of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled amphiphiles (NBD-Cn, n = 4 to 16) with lipid bilayers of different compositions (1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC):cholesterol (1:1) and palmitoylated sphingomyelin (SpM):cholesterol (6:4)), including an asymmetric bilayer. Both unrestrained and umbrella sampling (US) simulations (at varying distances to the bilayer center) were carried out. The free energy profile of NBD-Cn at different depths in the membrane was obtained from the US simulations. The behavior of the amphiphiles during the permeation process was described regarding their orientation, chain elongation, and H-bonding to lipid and water molecules. Permeability coefficients were also calculated for the different amphiphiles of the series, using the inhomogeneous solubility-diffusion model (ISDM). Quantitative agreement with values obtained from kinetic modeling of the permeation process could not be obtained. However, for the longer, and more hydrophobic amphiphiles, the variation trend along the homologous series was qualitatively better matched by the ISDM when the equilibrium location of each amphiphile was taken as reference (ΔG = 0), compared to the usual choice of bulk water.

Interaction of a Homologous Series of Amphiphiles with P-glycoprotein in a Membrane Environment-Contributions of Polar and Non-Polar Interactions

The transport of drugs by efflux transporters in biomembranes limits their bioavailability and is a major determinant of drug resistance development by cancer cells and pathogens. A large number of chemically dissimilar drugs are transported, and despite extensive studies, the molecular determinants of substrate specificity are still not well understood. In this work, we explore the role of polar and non-polar interactions on the interaction of a homologous series of fluorescent amphiphiles with the efflux transporter P-glycoprotein. The interaction of the amphiphiles with P-glycoprotein is evaluated through effects on ATPase activity, efficiency in inhibition of [¹²⁵I]-IAAP binding, and partition to the whole native membranes containing the transporter. The results were complemented with partition to model membranes with a representative lipid composition, and details on the interactions established were obtained from MD simulations. We show that when the total concentration of amphiphile is considered, the binding parameters obtained are apparent and do not reflect the affinity for P-gp. A new formalism is proposed that includes sequestration of the amphiphiles in the lipid bilayer and the possible binding of several molecules in P-gp's substrate-binding pocket. The intrinsic binding affinity thus obtained is essentially independent of amphiphile hydrophobicity, highlighting the importance of polar interactions. An increase in the lipophilicity and amphiphilicity led to a more efficient association with the lipid bilayer, which maintains the non-polar groups of the amphiphiles in the bilayer, while the polar groups interact with P-gp's binding pocket. The presence of several amphiphiles in this orientation is proposed as a mechanism for inhibition of P-pg function.

Interaction of amphiphilic coumarin with DPPC/DPPS lipid bilayer: effects of concentration and alkyl tail length

In this work, interactions between amphiphilic amino methyl coumarin and dipalmitoyl-sn-glycero-3-phosphocholine/dipalmitoyl-sn-glycero-3-phosphoserine (DPPC/DPPS) lipid bilayer were investigated. A combination of experimental techniques (zeta potential, fluorescence spectroscopy, and differential scanning calorimetry) along with molecular dynamics simulations was employed to examine the influence of alkyl tail length and concentration of the amphiphilic coumarin on the lipid bilayer. Alkyl tails comprising 5(C₅), 9(C₉), and 12(C₁₂) carbon atoms were conjugated to amino methyl coumarin via a single-step process. The binding and insertion mechanisms of the amphiphilic coumarins were studied in increasing concentrations for short-tailed (C₅) and long-tailed (C₁₂) coumarins. The simulation results show that C₅ coumarin molecules penetrate the lipid bilayer, but owing to the short alkyl tail, they interact primarily with the lipid head groups resulting in lipid bilayer thinning; however, at high concentrations, the C₅ coumarins undergo continuous insertion-ejection from the outer leaflet of the lipid bilayer. In contrast, C₁₂ coumarins interact favorably with the hydrophobic lipid tails and lack the ejection-reinsertion behavior. Instead, the C₁₂ coumarin molecules undergo flip-flops between the outer and inner leaflets of the lipid bilayer. At high concentrations, the high-frequency flip-flops lead to lipid destabilization, causing the lipid bilayer to rupture. The simulation results are in excellent agreement with the toxicity of amphiphilic coumarin activity in cancer cells. The efficacy of amphiphilic coumarins in liposomal lipid bilayers demonstrates the promise of these molecules as a tool in the treatment of cancer.

Effect of dipole moment on amphiphile solubility and partition into liquid ordered and liquid disordered phases in lipid bilayers

Association of amphiphiles with biomembranes is important for their availability at specific locations in organisms and cells, being critical for their biological function. A prominent role is usually attributed to the hydrophobic effect, and to electrostatic interactions between charged amphiphiles and lipids. This work explores a closely related and complementary aspect, namely the contribution made by dipole moments to the strength of the interactions established. Two xanthene amphiphiles with opposite relative orientations of their dipole and amphiphilic moments have been selected (Rhodamine-C₁₄ and Carboxyfluorescein-C₁₄). The membranes studied have distinct lipid compositions, representing typical cell membrane pools, ranging from internal membranes to the outer and inner leaflet of the plasma membrane. A comprehensive study is reported, including the affinity of the amphiphiles for the different membranes, the stability of the amphiphiles as monomers and their tendency to form small clusters, as well as their transverse location in the membrane. The orientation of the amphiphile dipole moment, which determines whether its interaction with the membrane dipole potential is repulsive or attractive, is found to exert a large influence on the association of the amphiphile with ordered lipid membranes. These interactions are also responsible for the formation of small clusters or stabilization of amphiphile monomers in the membrane. The results obtained allow understanding the prevalence of protein lipidation at the N-terminal for efficient targeting to the plasma membrane, as well as the tendency of GPI-anchored proteins (usually lipidated at the C-terminal) to form small clusters in the membrane ordered domains.

The Acinetobacter baumannii Mla system and glycerophospholipid transport to the outer membrane

The outer membrane (OM) of Gram-negative bacteria serves as a selective permeability barrier that allows entry of essential nutrients while excluding toxic compounds, including antibiotics. The OM is asymmetric and contains an outer leaflet of lipopolysaccharides (LPS) or lipooligosaccharides (LOS) and an inner leaflet of glycerophospholipids (GPL). We screened Acinetobacter baumannii transposon mutants and identified a number of mutants with OM defects, including an ABC transporter system homologous to the Mla system in E. coli. We further show that this opportunistic, antibiotic-resistant pathogen uses this multicomponent protein complex and ATP hydrolysis at the inner membrane to promote GPL export to the OM. The broad conservation of the Mla system in Gram-negative bacteria suggests the system may play a conserved role in OM biogenesis. The importance of the Mla system to Acinetobacter baumannii OM integrity and antibiotic sensitivity suggests that its components may serve as new antimicrobial therapeutic targets.

Cholesterol is Inefficiently Converted to Cholesteryl Esters in the Blood of Cardiovascular Disease Patients

Shotgun lipidomic analysis of 203 lipids in 13 lipid classes performed on blood plasma of donors who had just suffered an acute coronary syndrome (ACS, n = 74), or an ischemic stroke (IS, n = 21), or who suffer from stable angina pectoris (SAP, n = 78), and an age-matched control cohort (n = 52), showed some of the highest inter-lipid class correlations between cholesteryl esters (CE) and phosphatidylcholines (PC) sharing a common fatty acid. The concentration of lysophospatidylcholine (LPC) and ratios of concentrations of CE to free cholesterol (Chol) were also lower in the CVD cohorts than in the control cohort, indicating a deficient conversion of Chol to CE in the blood plasma in the CVD subjects. A non-equilibrium reaction quotient, Q′, describing the global homeostasis of cholesterol as manifested in the blood plasma was shown to have a value in the CVD cohorts (Q′_ACS = 0.217 ± 0.084; Q′_IS = 0.201 ± 0.084; Q′_SAP = 0.220 ± 0.071) that was about one third less than in the control cohort (Q′_Control = 0.320 ± 0.095, p < 1 × 10⁻⁴), suggesting its potential use as a rapid predictive/diagnostic measure of CVD-related irregularities in cholesterol homeostasis.

Lipid vesicles chaperone an encapsulated RNA aptamer

The organization of molecules into cells is believed to have been critical for the emergence of living systems. Early protocells likely consisted of RNA functioning inside vesicles made of simple lipids. However, little is known about how encapsulation would affect the activity and folding of RNA. Here we find that confinement of the malachite green RNA aptamer inside fatty acid vesicles increases binding affinity and locally stabilizes the bound conformation of the RNA. The vesicle effectively ‘chaperones’ the aptamer, consistent with an excluded volume mechanism due to confinement. Protocellular organization thereby leads to a direct benefit for the RNA. Coupled with previously described mechanisms by which encapsulated RNA aids membrane growth, this effect illustrates how the membrane and RNA might cooperate for mutual benefit. Encapsulation could thus increase RNA fitness and the likelihood that functional sequences would emerge during the origin of life.

So far little is known about how encapsulation affects the activity and folding of RNA, which is of interest for understanding the origin of cellular life. Here the authors show that encapsulation of functional RNA in vesicles increases RNA activity and improves RNA folding through a biophysical confinement effect.

Incubation of human sperm with micelles made from glycerophospholipid mixtures increases sperm motility and resistance to oxidative stress

Membrane integrity is essential in maintaining sperm viability, signaling, and motility, which are essential for fertilization. Sperm are highly susceptible to oxidative stress, as they are rich in sensitive polyunsaturated fatty acids (PUFA), and are unable to synthesize and repair many essential membrane constituents. Because of this, sperm cellular membranes are important targets of this process. Membrane Lipid Replacement (MLR) with glycerophospholipid mixtures (GPL) has been shown to ameliorate oxidative stress in cells, restore their cellular membranes, and prevent loss of function. Therefore, we tested the effects of MLR on sperm by tracking and monitoring GPL incorporation into their membrane systems and studying their effects on sperm motility and viability under different experimental conditions. Incubation of sperm with mixtures of exogenous, unoxidized GPL results in their incorporation into sperm membranes, as shown by the use of fluorescent dyes attached to GPL. The percent overall (total) sperm motility was increased from 52±2.5% to 68±1.34% after adding GPL to the incubation media, and overall sperm motility was recovered from 7±2% after H2O2 treatment to 58±2.5%)(n = 8, p<0.01) by the incorporation of GPL into sperm membranes. When sperm were exposed to H2O2, the mitochondrial inner membrane potential (MIMP), monitored using the MIMP tracker dye JC-1 in flow cytometry, diminished, whereas the addition of GPL prevented the decrease in MIMP. Confocal microscopy with Rhodamine-123 and JC-1 confirmed the mitochondrial localization of the dyes. We conclude that incubation of human sperm with glycerolphospholipids into the membranes of sperm improves sperm viability, motility, and resistance to oxidizing agents like H2O2. This suggests that human sperm might be useful to test innovative new treatments like MLR, since such treatments could improve fertility when it is adversely affected by increased oxidative stress.

Effect of Acyl Chain Length on the Rate of Phospholipid Flip-Flop and Intermembrane Transfer

The rate at which phospholipids equilibrate between different membranes and between the non-polar environments in biological fluids is of high importance in the understanding of biomembrane diversity, as well as in the development of liposomes for drug delivery. In this work, we characterize the rate of insertion into and desorption from POPC bilayers for a homologous series of amphiphiles with the fluorescent NBD group attached to phosphoethanolamines of different acyl chain lengths, NBD-diC _n -PE with n = 6, 8, 10, and 12. The rate of translocation between bilayer leaflets was also characterized, providing all the relevant parameters for their interaction with lipid bilayers. The results are complemented with data for NBD-diC₁₄-PE obtained from literature (Abreu et al. Biophys J 87:353-365, 2004; Moreno et al. Biophys J 91:873-881, 2006). The rate of translocation between the POPC leaflets is not dependent on the length of the acyl chains, while this affects strongly the rate of desorption from the bilayer. Insertion in the POPC bilayer is not diffusion controlled showing a significant dependence on the acyl chain length and on temperature. The results obtained are compared with those previously reported for NBD-LysoC₁₄-PE (Sampaio et al. Biophys J 88:4064-4071, 2005), and with the homologous series of single chain amphiphiles NBD-C _n (Cardoso et al. J Phys Chem B 114:16337-16346, 2010; J Phys Chem B 115:10098-10108, 2011). This allows the establishment of important relations between the rate constants for interaction with the lipid bilayers and the structural properties of the amphiphiles, namely the total surface and the cross-section of their non-polar region.

Partition of Amphiphilic Molecules to Lipid Bilayers by ITC: Low-Affinity Solutes

A protocol is developed to allow the accurate characterization of partition to lipid bilayers for solutes with low affinity, using isothermal titration calorimetry. The methodology proposed is suitable for studies using complex membranes, such as intact biomembranes or whole cells. In the method developed, the association is characterized at increasing solute concentrations. This allows the characterization of solute partition into unperturbed membranes, as well as effects induced by high solute concentrations. Most druglike molecules are expected to interact with low-to-moderate affinity with relevant cell membranes. This is due to both the need for a relatively high aqueous solubility of the drug and the poor binding properties of the cell membranes. The methodology is applied to characterize the interaction of antibiotic Rifampicin with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and with lipid bilayers representative of bacterial membranes.

Interaction of NBD-labelled fatty amines with liquid-ordered membranes: a combined molecular dynamics simulation and fluorescence spectroscopy study

A complete homologous series of fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl-(NBD) labelled fatty amines of varying alkyl chain lengths, NBD-Cn, inserted in 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC) or N-palmitoyl sphingomyelin (SpM) bilayers, with 50 mol% and 40 mol% cholesterol (Chol), respectively, was studied using atomistic molecular dynamics simulations. For all amphiphiles in both bilayers, the NBD fluorophore locates at the interface, in a more external position than that previously observed for pure POPC bilayers. This shallower location of the NBD group agrees with the lower fluorescent quantum yield, shorter fluorescence lifetime, and higher ionisation constants (smaller pKa) determined experimentally. The more external location is also consistent with the changes measured in steady-state fluorescence anisotropy from POPC to POPC/Chol (1 : 1) vesicles. Accordingly, the equilibrium location of the NBD group within the various bilayers is mainly dictated by bilayer compositions, and is mostly unaffected by the length of the attached alkyl chain. Similarly to the behaviour observed in POPC bilayers, the longer-chained NBD-Cn amphiphiles show significant mass density near the mixed bilayers' midplanes, and the alkyl chains of the longer derivatives, mainly NBD-C16, penetrate the opposite bilayer leaflet to some extent. However, this effect is quantitatively less pronounced in these ordered bilayers than in POPC. Similarly to POPC bilayers, the effects of these amphiphiles on the structure and dynamics of the host lipid were found to be relatively mild, in comparison with acyl-chain phospholipid analogues.

Toxicity of nanoparticle surface coating agents: Structure-cytotoxicity relationship

Surface coating agents for metal nanoparticles, cationic alkyl ammonium bromides, and anionic alkyl sulfates were tested against human skin keratinocytes (HaCaT) and blood T lymphocytes (TIB-152). The surfactants of short chain (C8) are not cytotoxic, but as chain length increases, their cytotoxicity increases and levels off at C12 for cationic surfactants against both cell lines and for anionic surfactants against the TIB-152, but C14 for anionic surfactants against HaCaT. The cationic surfactants are more toxic than the anionic surfactants for HaCaT; while with similar cytotoxicity for TIB-152 cells. di- and tetra-Alkyl ammonium salts are more cytotoxic than the mono-substituted.

Interaction of Bile Salts with Model Membranes Mimicking the Gastrointestinal Epithelium: A Study by Isothermal Titration Calorimetry

Bile salts (BS) are biosurfactants synthesized in the liver and secreted into the intestinal lumen where they solubilize cholesterol and other hydrophobic compounds facilitating their gastrointestinal absorption. Partition of BS toward biomembranes is an important step in both processes. Depending on the loading of the secreted BS micelles with endogeneous cholesterol and on the amount of cholesterol from diet, this may lead to the excretion or absorption of cholesterol, from cholesterol-saturated membranes in the liver or to gastrointestinal membranes, respectively. The partition of BS toward the gastrointestinal membranes may also affect the barrier properties of those membranes affecting the permeability for hydrophobic and amphiphilic compounds. Two important parameters in the interaction of the distinct BS with biomembranes are their partition coefficient and the rate of diffusion through the membrane. Altogether, they allow the calculation of BS local concentrations in the membrane as well as their asymmetry in both membrane leaflets. The local concentration and, most importantly, its asymmetric distribution in the bilayer are a measure of induced membrane perturbation, which is expected to significantly affect its properties as a cholesterol donor and hydrophobic barrier. In this work we have characterized the partition of several BS, nonconjugated and conjugated with glycine, to large unilamellar vesicles (LUVs) in the liquid-disordered phase and with liquid-ordered/liquid-disordered phase coexistence, using isothermal titration calorimetry (ITC). The partition into the liquid-disordered bilayer was characterized by large partition coefficients and favored by enthalpy, while association with the more ordered membrane was weak and driven only by the hydrophobic effect. The trihydroxy BS partitions less efficiently toward the membranes but shows faster translocation rates, in agreement with a membrane protective effect of those BS. The rate of translocation through the more ordered membrane was faster, indicating accumulation of BS at specific locations in this membrane.

Fusion of raft-like lipid bilayers operated by a membranotropic domain of the HSV-type I glycoprotein gH occurs through a cholesterol-dependent mechanism

A wealth of evidence indicates that lipid rafts are involved in the fusion of the viral lipid envelope with the target cell membrane. However, the interplay between these sterol- and sphingolipid-enriched ordered domains and viral fusion glycoproteins has not yet been clarified. In this work we investigate the molecular mechanism by which a membranotropic fragment of the glycoprotein gH of the Herpes Simplex Virus (HSV) type I (gH625) drives fusion of lipid bilayers formed by palmitoyl oleoyl phosphatidylcholine (POPC)-sphingomyelin (SM)-cholesterol (CHOL) (1 : 1 : 1 wt/wt/wt), focusing on the role played by each component. The comparative analysis of the liposome fusion assays, Dynamic Light Scattering (DLS), spectrofluorimetry, Neutron Reflectivity (NR) and Electron Spin Resonance (ESR) experiments, and Molecular Dynamics (MD) simulations shows that CHOL is fundamental for liposome fusion to occur. In detail, CHOL stabilizes the gH625-bilayer association by specific interactions with the peptide Trp residue. The interaction with gH625 causes an increased order of the lipid acyl chains, whose local rotational motion is significantly hampered. SM plays only a minor role in the process, favoring the propagation of lipid perturbation to the bilayer inner core. The stiffening of the peptide-interacting bilayer leaflet results in an asymmetric perturbation of the membrane, which is locally destabilized thus favoring fusion events. Our results show that viral fusion glycoproteins are optimally suited to exert a high fusogenic activity on lipid rafts and support the relevance of cholesterol as a key player of membrane-related processes.

Behaviour of NBD-head group labelled phosphatidylethanolamines in POPC bilayers: a molecular dynamics study

An NBD-diC₁₆PE/POPC bilayer with typical fluorophore inverted-snorkelling configurations, and mass density profiles across the membrane. The wide distribution of the NBD fluorophore lies at the origin of its complex emission kinetics.

High-throughput formation of lipid bilayer membrane arrays with an asymmetric lipid composition

We present a micro-device in which more than 10,000 asymmetric lipid bilayer membranes are formed at a time on micro-chamber arrays. The arrayed asymmetric lipid bilayers, where lipid compositions are different between the inner and outer leaflets, are formed with high efficiency of over 97% by injecting several types of liquids into a micro-device that has hydrophilic-in-hydrophobic surfaces. The lipid compositional asymmetry is an intrinsic property of bio-membranes, and therefore, this micro-device extends the versatility of artificial lipid-bilayer systems, which were previously limited to symmetric bilayer formation, and could contribute to the understanding of the role of lipid compositional asymmetry in cell physiology and also to further analytical and pharmacological applications.

Homeostasis of free cholesterol in the blood: a preliminary evaluation and modeling of its passive transport

The rate of noncatalyzed transfer of cholesterol (Chol) among lipoproteins and cells in the blood is of fundamental importance as a baseline to assess the role of active transport mechanisms, but remains unknown. Here we address this gap by characterizing the associa-tion of the Chol analog, ergosta-5,7,9(11),22-tetraen-3β-ol (DHE), with the lipoproteins VLDL, LDL, HDL2, and HDL3 Combining these results with data for the association of DHE with liposomes, we elaborated a kinetic model for the noncatalyzed exchange of free Chol among blood compartments. The computational results are in good agreement with experimental values. The small deviations are explained by the nonequilibrium distribution of unesterified Chol in vivo, due to esterification and entry of new unesterified Chol, and eventual effects introduced by incubations at low temperatures. The kinetic profile of the homeostasis of unesterified Chol in the blood predicted by the model developed in this work is in good agreement with the observations in vivo, highlighting the importance of passive processes.

Replication of simulated prebiotic amphiphile vesicles controlled by experimental lipid physicochemical properties

We present a new embodiment of the graded autocatalysis replication domain (GARD) for the growth, replication and evolution of lipid vesicles based on a semi-empirical foundation using experimentally measured kinetic values of selected extant lipid species. Extensive simulations using this formalism elucidated the details of the dependence of the replication and properties of the vesicles on the physicochemical properties and concentrations of the lipids, both in the environment and in the vesicle. As expected, the overall concentration and number of amphiphilic components strongly affect average replication time. Furthermore, variations in acyl chain length and unsaturation of vesicles also influence replication rate, as do the relative concentrations of individual lipid types. Understanding of the dependence of replication rates on physicochemical parameters opens a new direction in the study of prebiotic vesicles and lays the groundwork for future studies involving the competition between lipid vesicles for available amphiphilic monomers.

Interaction of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-labeled fatty amines with 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine bilayers: a molecular dynamics study

A complete homologous series of fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled fatty amines of varying alkyl chain length, NBD-C(n), inserted in 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, was studied using atomistic molecular dynamics (MD) simulations. For all amphiphiles, the NBD fluorophore locates near the glycerol backbone/carbonyl region of POPC and establishes stable hydrogen bonding with POPC ester oxygen atoms. Small differences observed in the transverse location of the fluorophore correlate with other calculated parameters and with small discrepancies recently measured in the photophysical properties of the molecules. The longer-chained NBD-C(n) amphiphiles show significant mass density near the bilayer midplane, and the chains of these derivatives interdigitate to some extent the opposite bilayer leaflet. This phenomenon leads to a slower lateral diffusion for the longer-chained derivatives (n > 12). Effects of these amphiphiles on the structure and dynamics of the host lipid were found to be relatively mild, in comparison with acyl-chain-labeled NBD probes. The molecular details obtained by this work allow the rationalization of the nonmonotonic behavior, recently obtained experimentally, for the photophysical parameters of the amphiphiles and the kinetic and thermodynamic parameters for their interaction with the POPC membranes.

Chain-length dependence of insertion, desorption, and translocation of a homologous series of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-labeled aliphatic amines in membranes

We present a complete characterization of the kinetics of interaction between the homologous series of fluorescent fatty amines with the fluorescent moiety 7-nitrobenz-2-oxa-1,3-diazol-4-yl covalently bound to the amine group, NBD-C(n) (n = 8-16), and a lipid bilayer in the liquid disordered phase. The insertion into and the desorption from the lipid bilayer, as well as the rate of translocation across the two bilayer leaflets, has been measured at different temperatures, allowing an estimation of the thermodynamic parameters in the formation of the transition state. This is the first report on the complete characterization of the kinetics of the interaction of a large series of structurally homologous amphiphiles. In a recent paper from this research group, the equilibrium interaction of NBD-C(n) (n = 4-10) with POPC bilayers and serum albumin was reported. This information allows the calculation of the equilibrium distribution of the amphiphiles among the aqueous phase, serum proteins, and biomembranes. The data presented in this manuscript complement its characterization with information on the kinetics of the interactions, making possible the quantitative evaluation of their pharmacokinetics. The rate of translocation is shown to decrease with increasing alkyl chain length up to n = 12, becoming relatively insensitive to further increases in n. The Gibbs free energy variation associated with the rate of desorption from the lipid bilayer increased linearly with n, with ΔΔG(‡o) = 3.4 ± 0.5 kJ mol(-1) per methylene group. It was also found that the process of insertion in the lipid bilayer is not diffusion-limited, although it is close to this limit for the smaller amphiphiles in the homologous series at high temperatures.

In vitro surfactant structure-toxicity relationships: implications for surfactant use in sexually transmitted infection prophylaxis and contraception

BACKGROUND

The need for woman-controlled, cheap, safe, effective, easy-to-use and easy-to-store topical applications for prophylaxis against sexually transmitted infections (STIs) makes surfactant-containing formulations an interesting option that requires a more fundamental knowledge concerning surfactant toxicology and structure-activity relationships.

METHODOLOGY/PRINCIPAL FINDINGS

We report in vitro effects of surfactant concentration, exposure time and structure on the viability of mammalian cell types typically encountered in the vagina, namely, fully polarized and confluent epithelial cells, confluent but non-polarized epithelial-like cells, dendritic cells, and human sperm. Representatives of the different families of commercially available surfactants--nonionic (Triton X-100 and monolaurin), zwitterionic (DDPS), anionic (SDS), and cationic (C(n)TAB (n = 10 to 16), C(12)PB, and C(12)BZK)--were examined. Triton X-100, monolaurin, DDPS and SDS were toxic to all cell types at concentrations around their critical micelle concentration (CMC) suggesting a non-selective mode of action involving cell membrane destabilization and/or destruction. All cationic surfactants were toxic at concentrations far below their CMC and showed significant differences in their toxicity toward polarized as compared with non-polarized cells. Their toxicity was also dependent on the chemical nature of the polar head group. Our results suggest an intracellular locus of action for cationic surfactants and show that their structure-activity relationships could be profitably exploited for STI prophylaxis in vaginal gel formulations. The therapeutic indices comparing polarized epithelial cell toxicity to sperm toxicity for all surfactants examined, except C(12)PB and C(12)BZK, does not justify their use as contraceptive agents. C(12)PB and C(12)BZK are shown to have a narrow therapeutic index recommending caution in their use in contraceptive formulations.

CONCLUSIONS/SIGNIFICANCE

Our results contribute to understanding the mechanisms involved in surfactant toxicity, have a predictive value with regard to their safety, and may be used to design more effective and less harmful surfactants for use in topical applications for STI prophylaxis.

Physical effects underlying the transition from primitive to modern cell membranes

To understand the emergence of Darwinian evolution, it is necessary to identify physical mechanisms that enabled primitive cells to compete with one another. Whereas all modern cell membranes are composed primarily of diacyl or dialkyl glycerol phospholipids, the first cell membranes are thought to have self-assembled from simple, single-chain lipids synthesized in the environment. We asked what selective advantage could have driven the transition from primitive to modern membranes, especially during early stages characterized by low levels of membrane phospholipid. Here we demonstrate that surprisingly low levels of phospholipids can drive protocell membrane growth during competition for single-chain lipids. Growth results from the decreasing fatty acid efflux from membranes with increasing phospholipid content. The ability to synthesize phospholipids from single-chain substrates would have therefore been highly advantageous for early cells competing for a limited supply of lipids. We show that the resulting increase in membrane phospholipid content would have led to a cascade of new selective pressures for the evolution of metabolic and transport machinery to overcome the reduced membrane permeability of diacyl lipid membranes. The evolution of phospholipid membranes could thus have been a deterministic outcome of intrinsic physical processes and a key driving force for early cellular evolution.

Cholesterol slows down the lateral mobility of an oxidized phospholipid in a supported lipid bilayer

We investigated the mobility and phase-partitioning of the fluorescent oxidized phospholipid analogue 1-palmitoyl-2-glutaroyl-sn-glycero-3-phospho-N-Alexa647-ethanolamine (PGPE-Alexa647) in supported lipid bilayers. Compared to the conventional phospholipid dihexadecanoylphosphoethanolamine (DHPE)-Bodipy we found consistently higher diffusion constants. The effect became dramatic when immobile obstacles were inserted into the bilayer, which essentially blocked the diffusion of DHPE-Bodipy but hardly influenced the movements of PGPE-Alexa647. In a supported lipid bilayer made of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), the differences in probe mobility leveled off with increasing cholesterol content. Using coarse-grained molecular dynamics simulations, we could ascribe this effect to increased interactions between the oxidized phospholipid and the membrane matrix, concomitant with a translation in the headgroup position of the oxidized phospholipid: at zero cholesterol content, its headgroup is shifted to the outside of the DOPC headgroup region, whereas increasing cholesterol concentrations pulls the headgroup into the bilayer plane.

Main phase transition in lipid bilayers: Phase coexistence and line tension in a soft, solvent-free, coarse-grained model

We devise a soft, solvent-free, coarse-grained model for lipid bilayer membranes. The nonbonded interactions take the form of a weighted-density functional, which allows us to describe the thermodynamics of self-assembly and packing effects of the coarse-grained beads in terms of a density expansion of the equation of state and weighting functions that regularize the microscopic bead densities, respectively. Identifying the length and energy scales via the bilayer thickness and the thermal energy scale, k(B)T, the model qualitatively reproduces key characteristics (e.g., bending rigidity, area per molecule, and compressibility) of lipid membranes. We employ this model to study the main phase transition between the fluid and the gel phase of the bilayer membrane. We accurately locate the phase coexistence using free energy calculations and also obtain estimates for the bare and the thermodynamic line tension.

Real-time partitioning of octadecyl rhodamine B into bead-supported lipid bilayer membranes revealing quantitative differences in saturable binding sites in DOPC and 1:1:1 DOPC/SM/cholesterol membranes

Quantitative analysis of the staining of cell membranes with the cationic amphiphile, octadecyl rhodamine B (R18), is confounded by probe aggregation and changes to the probes' absorption cross section and emission quantum yield. In this paper, flow cytometry, quantum-dot-based fluorescence calibration beads, and FRET were used to examine real-time transfer of R18 from water to two limiting models of the cellular plasma membrane, namely, a single-component disordered membrane, dioleoyl-L-alpha-phosphatidylcholine (DOPC), and a ternary mixture of DOPC, cholesterol, and sphingomyelin (DSC) membranes, reconstituted on spherical and monodisperse glass beads (lipobeads). The quenching of R18 was analyzed as the probe concentration was raised from 0 to 10 mol % in membranes. The data show a > 2-fold enhancement in the quenching level of the probes that were reconstituted in DSC relative to DOPC membranes at the highest concentration of R18. We have parametrized the propagation of concentration-dependent quenching as a function of real-time binding of R18 to lipobeads. In this way, phenomenological kinetics of serum-albumin-mediated transfer of R18 from the aqueous phase to DOPC and DSC membranes could be evaluated under optimal conditions where the critical aggregation concentration (CAC) of the probe is defined as 14 nM. The mass action kinetics of association of R18 with DOPC and DSC lipobeads are shown to be similar. However, the saturable capacity for accepting exogenous probes is found to be 37% higher in DOPC relative to that for DSC membranes. The difference is comparable to the disparity in the average molecular areas of DOPC and DSC membranes. Finally, this analysis shows little difference in the spectral overlap integrals of the emission spectrum of a fluorescein derivative donor and the absorption spectrum of either monomeric or simulated spectrum of dimeric R18. This approach represents a first step toward a nanoscale probing of membrane heterogeneity in living cells by analyzing differential local FRET among sites of unique receptor expression in living cells.

Sandwiched graphene--membrane superstructures

We demonstrate by molecular dynamics simulations that graphene sheets could be hosted in the hydrophobic interior of biological membranes formed by amphiphilic phospholipid molecules. Our simulation shows that these hybrid graphene--membrane superstructures might be prepared by forming hydrated micelles of individual graphene flakes covered by phospholipids, which can be then fused with the membrane. Since the phospholipid layers of the membrane electrically isolate the embedded graphene from the external solution, the composite system might be used in the development of biosensors and bioelectronic materials.

Partition of amphiphilic molecules to lipid bilayers by isothermal titration calorimetry

The partition of the amphiphile sodium dodecyl sulfate (SDS) between an aqueous solution and a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer was followed by isothermal titration calorimetry (ITC) as a function of the total concentration of SDS. It was found that the obtained partition coefficient is strongly affected by the ligand concentration, even after correction for the charge imposed in the bilayer by the bound SDS. The partition coefficient decreased as the total concentration of SDS increased, with this effect being significant for local concentrations of SDS in the lipid bilayer above 5 molar%. At those high local concentrations, the properties of the lipid bilayer are strongly affected, leading to nonideal behavior and concentration-dependent apparent partition coefficients. It is shown that with the modern ITC instruments available, the concentrations of SDS can be drastically reduced while maintaining a good signal-to-noise ratio. The intrinsic parameters of the interaction with unperturbed membranes can be obtained from the asymptotic behavior of the apparent parameters as a function of the ligand concentration for both nonionic and ionic solutes. A detailed analysis is performed, and a spreadsheet is provided to obtain the interaction parameters with and without correction for electrostatics.

Lipid flip-flop driven mechanical and morphological changes in model membranes

We study, using dissipative particle dynamics simulations, the effect of active lipid flip-flop on model fluid bilayer membranes. We consider both cases of symmetric as well as asymmetric flip-flops. Symmetric flip-flop leads to a steady state of the membrane with an effective temperature higher than that of the equilibrium membrane and an effective surface tension lower than that of the equilibrium membrane. Asymmetric flip-flop leads to transient conformational changes in the membrane in the form of bud or blister formation, depending on the flip rate.

Kinetics and thermodynamics of the association of dehydroergosterol with lipid bilayer membranes

We have examined the detailed kinetics and thermodynamics of the association of Ergosta-5,7,9(11),22-tetraen-3beta-ol (dehydroergosterol, DHE) with lipid bilayers prepared from 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), a 1:1 binary mixture of POPC and cholesterol (Chol), and a 6:4 binary mixture of egg sphingomyelin (SpM) and Chol. Association of DHE with all three membranes was shown to be entropically driven, most so in the case of SpM-Chol bilayers. Equilibrium partition coefficients for partitioning of DHE between the lipid phase and the aqueous phase were shown to be similar for POPC and POPC-Chol bilayers between 15 and 35 degrees C. Partitioning into the SpM-Chol bilayer is favored at higher temperatures and there is a crossover in solubility preference at approximately 25 degrees C. Insertion (k(+)) and desorption (k(-)) rate constants were shown to be very similar for POPC and POPC-Chol bilayer membranes, but were lower for SpM-Chol bilayers. Similar results were previously reported by us for the association of other amphiphiles with these membranes. We propose a model for the microscopic structure of a POPC-Chol (1:1) bilayer membrane that is consistent with these observations.

Ligand binding to cytochrome P450 3A4 in phospholipid bilayer nanodiscs: the effect of model membranes

The membrane-bound protein cytochrome P450 3A4 (CYP3A4) is a major drug-metabolizing enzyme. Most studies of ligand binding by CYP3A4 are currently carried out in solution, in the absence of a model membrane. Therefore, there is little information concerning the membrane effects on CYP3A4 ligand binding behavior. Phospholipid bilayer Nanodiscs are a novel model membrane system derived from high density lipoprotein particles, whose stability, monodispersity, and consistency are ensured by their self-assembly. We explore the energetics of four ligands (6-(p-toluidino)-2-naphthalenesulfonic acid (TNS), alpha-naphthoflavone (ANF), miconazole, and bromocriptine) binding to CYP3A4 incorporated into Nanodiscs. Ligand binding to Nanodiscs was monitored by a combination of environment-sensitive ligand fluorescence and ligand-induced shifts in the fluorescence of tryptophan residues present in the scaffold proteins of Nanodiscs; binding to the CYP3A4 active site was monitored by ligand-induced shifts in the heme Soret band absorbance. The dissociation constants for binding to the active site in CYP3A4-Nanodiscs were 4.0 microm for TNS, 5.8 microm for ANF, 0.45 microm for miconazole, and 0.45 microm for bromocriptine. These values are for CYP3A4 incorporated into a lipid bilayer and are therefore presumably more biologically relevant that those measured using CYP3A4 in solution. In some cases, affinity measurements using CYP3A4 in Nanodiscs differ significantly from solution values. We also studied the equilibrium between ligand binding to CYP3A4 and to the membrane. TNS showed no marked preference for either environment; ANF preferentially bound to the membrane, and miconazole and bromocriptine preferentially bound to the CYP3A4 active site.

Investigation of domain formation in sphingomyelin/cholesterol/POPC mixtures by fluorescence resonance energy transfer and Monte Carlo simulations

We have recently proposed a phase diagram for mixtures of porcine brain sphingomyelin (BSM), cholesterol (Chol), and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) on the basis of kinetics of carboxyfluorescein efflux induced by the amphipathic peptide δ-lysin. Although that study indicated the existence of domains, phase separations in the micrometer scale have not been observed by fluorescence microscopy in BSM/Chol/POPC mixtures, though they have for some other sphingomyelins (SM). Here we examine the same BSM/Chol/POPC system by a combination of fluorescence resonance energy transfer (FRET) and Monte Carlo simulations. The results clearly demonstrate that domains are formed in this system. Comparison of the FRET experimental data with the computer simulations allows the estimate of lipid-lipid interaction Gibbs energies between SM/Chol, SM/POPC, and Chol/POPC. The latter two interactions are weakly repulsive, but the interaction between SM and Chol is favorable. Furthermore, those three unlike lipid interaction parameters between the three possible lipid pairs are sufficient for the existence of a closed loop in the ternary phase diagram, without the need to involve multibody interactions. The calculations also indicate that the largest POPC domains contain several thousand lipids, corresponding to linear sizes of the order of a few hundred nanometers.

Spontaneous, intervesicular transfer rates of fluorescent, acyl chain-labeled phosphatidylcholine analogs

It was recently shown that the structure of the fluorophore attached to the acyl chain of phosphatidylcholine analogs determines their mechanism of transport across the plasma membrane of yeast cells (Elvington et al., J. Biol Chem. 280:40957, 2005). In order to gain further insight into the physical properties of these fluorescent phosphatidylcholine (PC) analogs, the rate and mechanism of their intervesicular transport was determined. The rate of spontaneous exchange was measured for PC analogs containing either NBD (7-nitrobenz-2-oxa-1,3-diazol-4-yl), Bodipy FL (4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene), Bodipy 530 (4,4-difluoro-5,7-diphenyl-4-bora-3a,4a-diaza-s-indacene), or Bodipy 581 (4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene) attached to a five or six carbon acyl chain in the sn-2 position. The rate of transfer between phospholipid vesicles was measured by monitoring the increase in fluorescence as the analogs transferred from donor vesicles containing self-quenching concentrations to unlabeled acceptor vesicles. Kinetic analysis indicated that the transfer of each analog occurred by diffusion through the water phase as opposed to transfer during vesicle collisions. The vesicle-to-monomer dissociation rate constants differed by over four orders of magnitude: NBD-PC (k(dis)=0.115 s(-1); t(1/2)=6.03 s); Bodipy FL-PC (k(dis)=5.2x10(-4); t(1/2)=22.2 min); Bodipy 530-PC (k(dis)=1.52x10(-5); t(1/2)=12.6 h); and Bodipy 581-PC (k(dis)=5.9x10(-6); t(1/2)=32.6 h). The large differences in spontaneous rates of transfer through the water measured for these four fluorescent PC analogs reflect their hydrophobicity and may account for their recognition by different mechanisms of transport across the plasma membrane of yeast.

Translocation of phospholipids and dithionite permeability in liquid-ordered and liquid-disordered membranes

We present a detailed study of the translocation rate of two headgroup-labeled phospholipid derivatives, one with two acyl chains, NBD-DMPE, and the other with a single acyl chain, NBD-lysoMPE, in lipid bilayer membranes in the liquid-disordered state (POPC) and in the liquid-ordered states (POPC/cholesterol (Chol), molar ratio 1:1, and sphingomyelin (SpM)/Chol, molar ratio 6:4). The study was performed as a function of temperature and the thermodynamic parameters of the translocation process have been obtained. The most important findings are 1), the translocation of NBD-DMPE is significantly faster than the translocation of NBD-lysoMPE for all bilayer compositions and temperatures tested; and 2), for both phospholipid derivatives, the translocation in POPC bilayers is approximately 1 order of magnitude faster than in POPC/Chol (1:1) bilayers and approximately 2-3 orders of magnitude faster than in SpM/Chol (6:4) bilayers. The permeability of the lipid bilayers to dithionite has also been measured. In liquid disordered membranes, the permeability rate constant obtained is comparable to the translocation rate constant of NBD-DMPE. However, in liquid-ordered bilayers, the permeability of dithionite is significantly faster then the translocation of NBD-DMPE. The change in enthalpy and entropy associated with the formation of the activated state in the translocation and permeation processes has also been obtained.

ABC lipid transporters: extruders, flippases, or flopless activators?

Many mammalian ABC transporters move membrane lipids to acceptor lipid assemblies in the extracellular aqueous milieu. Because the desorption from the membrane costs more energy than provided by two ATPs, the transporter probably only translocates the lipid to a partially hydrophilic site on its extracellular face. From this high-energy site, the lipid may efficiently move to the acceptor, which ideally is bound to the transporter, or, in the absence of an acceptor, fall back into the membrane. If the lipid originated from the cytosolic membrane surface, this represents lipid flop and is probably a side activity of the transporters.

Partitioning of membrane molecules between raft and non-raft domains: Insights from model-membrane studies

The special physical and functional properties ascribed to lipid rafts in biological membranes reflect their distinctive organization and composition, properties that are hypothesized to rest in part on the differential partitioning of various membrane components between liquid-ordered and liquid-disordered lipid environments. This review describes the principles and findings of recently developed methods to monitor the partitioning of membrane proteins and lipids between liquid-ordered and liquid-disordered domains in model membranes, and how these approaches can aid in elucidating the properties of rafts in biological membranes.

1,2-diacyl-phosphatidylcholine flip-flop measured directly by sum-frequency vibrational spectroscopy

Sum-frequency vibrational spectroscopy (SFVS) is used to measure the intrinsic rate of lipid flip-flop for 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in planar-supported lipid bilayers (PSs). Asymmetric PSLBs were prepared using the Langmuir-Blodgett/Langmuir-Schaefer method by placing a perdeuterated lipid analog in one leaflet of the PSLB. SFVS was used to directly measure the asymmetric distribution of the native lipid within the membrane by measuring the decay in the CH3 v(s) intensity at 2875 cm(-1) with time and as a function of temperature. An average activation energy of 220 kJ/mol for the translocation of DMPC, DPPC, and DSPC was determined. A decrease in alkyl chain length resulted in a substantial increase in the rate of flip-flop manifested as an increase in the Arrhenius preexponential factor. The effect of lipid labeling was investigated by measuring the exchange of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-n,n-Dimethyl-n-(2',2',6',6'-tetramethyl-4'-piperidyl) (TEMPO-DPPC). The rate of TEMPO-DPPC flip-flop was an order-of-magnitude slower compared to DPPC. An activation energy of 79 kJ/mol was measured which is comparable to that previously measured by electron spin resonance. The results of this study illustrate how SFVS can be used to directly measure lipid flip-flop without the need for a fluorescent or spin-labeled lipid probe, which can significantly alter the rate of lipid translocation.

Kinetics and thermodynamics of association of a fluorescent lysophospholipid derivative with lipid bilayers in liquid-ordered and liquid-disordered phases

We have measured the rates of insertion into, desorption from, and spontaneous interlayer translocation (flip-flop) of the fluorescent lysophospholipid derivative NBD-lyso-1-myristoylphosphatidylethanolamine in l(d) and l(o) phase lipid bilayer membranes. The lipid bilayers, studied as LUV, were prepared from pure 1-palmitoyl-2-oleoylphosphatidylcholine, in the l(d) phase; and from two Chol-containing binary lipid mixtures, 1-palmitoyl-2-oleoylphosphatidylcholine and Chol (molar ratio of 1:1) and SpM and Chol (molar ratio of 6:4), both in the l(o) phase. Insertion, desorption, and translocation rate constants and equilibrium constants for association of the amphiphile monomer with the lipid bilayers were measured between 15 degrees C and 35 degrees C, and the standard free energies, enthalpies, and entropies, as well as the activation energies for these processes were derived from these data. The equilibrium partition coefficients for partitioning of the amphiphile between the aqueous phase and the different membrane phases were also derived, and an estimation was made of hypothetical partition coefficients and the respective energetic parameters for partitioning between the different lipid phases if these were to coexist in the same membrane. We show that, contrary to general belief, the association of NBD-lysoMPE with lipid bilayers is not a diffusion-controlled process, the rate-limiting step in insertion being the formation of a free area in the membrane surface of an adequate size for insertion to occur.

Lipid Rafts have Different Sizes Depending on Membrane Composition: A Time-resolved Fluorescence Resonance Energy Transfer Study

The ternary lipid system palmitoylsphingomyelin (PSM)/palmitoyloleoylphosphatidylcholine (POPC)/cholesterol is a model for lipid rafts. Previously the phase diagram for that mixture was obtained, establishing the composition and boundaries for lipid rafts. In the present work, this system is further studied in order to characterize the size of the rafts. For this purpose, a time-resolved fluorescence resonance energy transfer (FRET) methodology, previously applied with success to a well-characterized phosphatidylcholine/cholesterol binary system, is used. It is concluded that: (1) the rafts on the low raft fraction of the raft region are small (below 20 nm), whereas on the other side the domains are larger; (2) on the large domain region, the domains reach larger sizes in the ternary system (> approximately 75-100 nm) than in binary systems phosphatidylcholine/cholesterol (between approximately 20 and approximately 75-100 nm); (3) the raft marker ganglioside G(M1) in small amounts (and excess cholera toxin subunit B) does not affect the general phase behaviour of the lipid system, but can increase the size of the rafts on the small to intermediate domain region. In summary, lipid-lipid interactions alone can originate lipid rafts on very different length scales. The conclusions presented here are consistent with the literature concerning both model systems and cell membrane studies.