Hydrophobic/superhydrophobic oxidized metal surfaces showing negligible contact angle hysteresis

The blood coagulation system relies on lipid membrane constituents to act as regulators of the coagulation process upon vascular trauma, and in particular the 2D configuration of the lipid membranes is known to efficiently catalyze enzymatic activity of blood coagulation factors. This work demonstrates a new application of a recently developed methodology to study blood coagulation at lipid membrane interfaces with the use of imaging technology. Lipid membranes with varied net charges were formed on silica supports by systematically using different combinations of lipids where neutral phosphocholine (PC) lipids were mixed with phospholipids having either positively charged ethylphosphocholine (EPC), or negatively charged phosphatidylserine (PS) headgroups. Coagulation imaging demonstrated that negatively charged SiO(2) and membrane surfaces exposing PS (obtained from liposomes containing 30% of PS) had coagulation times which were significantly shorter than those for plain PC membranes and EPC exposing membrane surfaces (obtained from liposomes containing 30% of EPC). Coagulation times decreased non-linearly with increasing negative surface charge for lipid membranes. A threshold value for shorter coagulation times was observed below a PS content of ～6%. We conclude that the lipid membranes on solid support studied with the imaging setup as presented in this study offers a flexible and non-expensive solution for coagulation studies at biological membranes. It will be interesting to extend the present study towards examining coagulation on more complex lipid-based model systems.     

Two components of boundary potentials at the lipid membrane surface: electrokinetic and complementary methods studies

Bilayer lipid membranes (BLM) are commonly used as models for cell membranes to study their interactions with inorganic ions and molecules of biological importance. In this work the principal electrostatic effects at the BLM surface are demonstrated by two methods: by the inner membrane field compensation (IFC) which is applied to planar BLM and sensitive to changes in the total boundary potential; and by electrokinetic measurements in liposome suspensions, sensitive to diffuse (surface) component of this potential. The difference in these two potentials allows us to conclude on changes in the dipole component of the boundary potential caused by structural changes at the membrane–water interface. No difference in the experimental data of both methods was observed for Be²⁺ and other divalent cation adsorption to unchanged phosphatidyl choline (PC) membranes. These data are in a good agreement with the Gouy–Champan–Stern (GCS) theory of diffuse double layer. This theory gives the value of binding constants for Be²⁺ about 400 M⁻¹ and 10⁴ M⁻¹ for DPPC liposomes in the liquid and solid states of the lipids, respectively. Clear isotope effects for normal and heavy water solutions of Be²⁺ were observed both by the electrostatic measurements and by differential scanning calorimetry. In contrast to PC, the electrostatic potentials induced by Be²⁺ and Gd³⁺ adsorption to membranes from phosphatidyiserine (PS) show the difference between the data of mentioned methods — total boundary potential changes are much higher in comparison to the surface potential. Dipole potential changes (about 150 mV) caused by changes in PS head group orientation may be more important in this specific case.     

Effect of tetracaine on model and erythrocyte membranes by DSC and EPR

Summary DSC and EPR experiments were performed on human erythrocyte membranes and DPPC vesicles in order to study the effect of the anaesthetic drug tetracaine on structure and dynamics of the lipid region. Experiments using spin label technique showed that tetracaine induced fluidity changes of the lipid region in the environment of the fatty acid probe molecules incorporated into the membranes in the vicinity of the lipid-water interface. Similarly to EPR observations, DSC measurements reported decrease of the main melting and the pretransition temperature in comparison to control DPPC vesicles, which is the sign of destabilisation of the structure in the head group region of the lipids. Similar effect was observed in the case of erythrocytes where the protein conformation was also controlled in the presence of drug. A separated membrane melting with well distinguished membrane protein phase transition was found that was affected significantly by tetracaine. These results suggest that tetracaine is able to modify not only the internal dynamics of erythrocyte membranes and produce destabilisation of the lipid structure, but the protein system as well. These might lead to further damage of the biological functions.     

OPTICAL SPECTROSCOPY OF BILAYER MEMBRANES

Abstract— The absorption and fluorescence spectroscopy of natural and model bilayer lipid membranes is reviewed. Basic structural features of biological membranes and the relative advantages of black lipid membranes (BLM) and of liposomes are discussed. Theoretical considerations show that the wavelengths of absorption maxima are affected by the refractive index and dielectric constant of the medium surrounding the chromophore. Techniques of obtaining photoelectric action spectra, direct absorption spectra, and reflection spectra of BLM are described. Polarized spectra can give information about the orientation of membrane constituents and show, for example, that the porphyrin ring of chlorophyll in BLM is tilted at 45 ± 5° to the membrane surface. Absorption maxima of chlorophyll in BLM are compared with solution spectra of various chlorophyll adducts and aggregates. It is concluded that chlorophyll in BLM exists largely as solvated monomer and dimer (or oligomer), depending on concentration, and is not coordinated with water. From the theory of fluorescence spectroscopy it follows that aggregation and the polarity of the environment affect the fluorescence yield and lifetime of a membrane component, and also the wavelength of its emission maximum. The microviscosity of the membrane matrix affects the anisotropy of fluorescence. Techniques of steady-state fluorescence spectroscopy and of fluorescence lifetime measurements are reviewed. Examples of the use of fluorescent probes in membrane studies are given. Certain probes such as anilinonaphthalene sulfonate (ANS) preferentially bind to membrane proteins. The location of a probe in a particular membrane region can be pinpointed from its fluorescence yield and emission maximum. The orientation of the hydrocarbon chains of membrane lipids has been found, from fluorescence polarization of certain probes, to be normal to the membrane surface as postulated a priori on the basis of the lipid bilayer model. Anisotropy of fluorescence shows that elongated probe molecules rotate rapidly about their long axes when surrounded by phospholipids but become immobilized when bound to proteins. Changes in intensity and anisotropy of fluorescence as function of temperature have demonstrated the existence of phase transitions and phase equilibria of membrane lipids. Excimer fluorescence has been used as a measure of the available lipid core volume of membranes. Mechanisms of energy transfer between membrane components are reviewed. The theoretical dependence of energy transfer on distance and orientation for several rigid and fluid membrane models is discussed in terms of the structural information it can provide. Fluorescence sensitization resulting from energy transfer within and across bilayer membranes has been demonstrated in various systems. Quantitative measurement of energy transfer efficiency in BLM has shown that such transfer is about five times more efficient than in solutions at comparable donor-acceptor distances. Lipid membranes can be viewed as structures which maintain their components at high concentrations, in a reactive state, and at favourable orientations.     

Membrane potential and electrostatics of phospholipid bilayers with asymmetric transmembrane distribution of anionic lipids

It is well-established that native plasma membranes are characterized by an asymmetric distribution of charged (anionic) lipids across the membrane. To clarify how the asymmetry can affect membrane electrostatics, we have performed extensive atomic-scale molecular dynamics simulations of asymmetric lipid membranes composed of zwitterionic (phosphatidylcholine (PC) or phosphatidylethanolamine (PE)) and anionic (phosphatidylserine (PS)) leaflets. It turns out that the asymmetry in transmembrane distribution of anionic lipids gives rise to a nonzero potential difference between the two sides of the membrane. This potential arises from the difference in surface charges of the two leaflets. The magnitude of the intrinsic membrane potential was found to be 238 mV and 198 mV for PS/PC and PS/PE membranes, respectively. Remarkably, this potential is of the same sign as the membrane potential in cells. Our findings, being in reasonable agreement with available experimental data, lend support to the idea that the transmembrane lipid asymmetry typical of most living cells contributes to the membrane potential.     

Influence of lipid composition on membrane activity of antimicrobial phenylene ethynylene oligomers

Host defense peptides (HDPs), part of the innate immune system, selectively target the membranes of bacterial cells over that of host cells. As a result, their antimicrobial properties have been under intense study. Their selectivity strongly depends on the chemical and mostly structural properties of the lipids that make up different cell membranes. The ability to synthesize HDP mimics has recently been demonstrated. To better understand how these HDP mimics interact with bilayer membranes, three homologous antimicrobial oligomers (AMOs) 1-3 with an m-phenylene ethynylene backbone and alkyl amine side chains were studied. Among them, AMO 1 is nonactive, AMO 2 is specifically active, and AMO 3 is nonspecifically active against bacteria over human red blood cells, a standard model for mammalian cells. The interactions of these three AMOs with liposomes having different lipid compositions are characterized in detail using a fluorescent dye leakage assay. AMO 2 and AMO 3 caused more leakage than AMO 1 from bacteria membrane mimic liposomes composed of PE/PG lipids. The use of E. coli lipid vesicles gave the same results. Further changes of the lipid compositions revealed that AMO 2 has selectively higher affinity toward PE/PG and E. coli lipids than PC, PC/PG or PC/PS lipids, the major components of mammalian cell membranes. In contrast, AMO 3 is devoid of this lipid selectivity and interacts with all liposomes with equal ease; AMO 1 remains inactive. These observations suggest that lipid type and structure are more important in determining membrane selectivity than lipid headgroup charges for this series of HDP mimics.     

Changes of conductance and compressibility of bilayer lipid membranes induced by oligonucleotide-cationic polyene antibiotic complexes

The positively charged polyene molecule amphotericin B 3-dimethylaminopropylamide (AMA) is an efficient agent for the delivery of antisense oligodeoxyribonucleotides (ODN) into target cells. In the present study, bilayer lipid membrane (BLM) conductance, elasticity modulus perpendicular to the membrane plane, surface potential and electrical capacitance were measured by conductance and electrostriction methods in the presence of AMA, pure or complexed to 20-mer single stranded ODN at different ratios. Pure AMA did not induce changes in conductance of cholesterol-containing BLM, but did induce an increase in elasticity modulus and surface potential. ODN/AMA complexes changed BLM properties depending on the charge ratio. The most pronounced effect on membrane conductance was observed for positively charged ODN/AMA complexes (charge ratio rho-/+=0.1), while for negatively charged complexes these changes were less marked/apparent, correlating to substantially lower binding constants. The effect of ODN/AMA complexes on elasticity modulus and charge potential was biphasic. After an increase in both values, a decrease was observed for higher incubation times and ODN/AMA concentrations. These results are interpreted as indicating that the membrane property changes result from the large AMA aggregates induced by the presence of the negatively charged ODN, which condensate on these aggregates. It is suggested that the decrease of elasticity modulus and surface potential in the presence of increasing incubation time and AMA concentration result from desorption of the complexes in the complex-free compartment of the BLM cell, or appearance of a non-linear conductance of the lipid bilayer. The first alternative would explain the AMA-induced transmembrane transfer of ODN.     

Local anesthetics facilitate ion transport across lipid planar bilayer membranes under an electric field: dependence on type of lipid bilayer

In order to elucidate the role of structural change of lipid membrane bilayer in the mode of action of local anesthetic, we studied the effects of local anesthetics, charged tetracaine and uncharged benzocaine, on ion permeability across various lipid planar bilayers (PC, mixed PC/PS (4/1, mol/mol); mixed PC/PE (1/1, mol/mol); mixed PC/SM (4/1, mol/mol)) under a constant applied voltage. The membrane conductances increased in the order of PC  PC/PS ≤ PC/SM  PC/PE. When the constant voltage of −100 or −70 mV was applied through the lipid bilayer membranes in the presence of positively charged tetracaine, the fluctuating current pulses with the large amplitude generated, but not appeared in the absence of tetracaine. The addition of uncharged benzocaine generated the fluctuating currents with the small amplitude. Both charged tetracaine and uncharged benzocaine facilitated electrophoretically the transport of small ions such as KCl in the buffer solution through the fluctuating pores in the lipid bilayer membranes formed by interaction with the local anesthetic under the negative applied membrane potential. The current pulses also contained actual transport of charged tetracaine together with the transport of the small ions. The amplitude and the duration time of the electrical current generated by adding the local anesthetics were dependent on the type of the lipid, the applied voltage and its voltage polarity.     

Calcium modulates the mechanical properties of anionic phospholipid membranes

Using micropipette aspiration and fluorescence techniques, we have studied the material properties of charged lipid vesicles in calcium solutions. Vesicles were composed of phosphatidylglycerol (PG)/phosphatidylcholine (PC) or phosphatidic acid (PA)/PC mixtures. For the case of PG/PC membranes, we measure no effect of anionic lipid fraction on elasticity but a monotonic decrease up to 20% for tension required to induce membrane failure. Both of these observations are rationalized by a model we have developed to describe membrane electrostatic interactions in a two-component salt solution and the resulting changes in membrane properties. Critical tensions measured for PA/PC membranes, on the other hand, did not depend on anionic lipid fraction and were uniformly approximately 35% lower than PG/PC vesicles. This is likely due to a lateral phase separation in the membrane. By combining mechanical properties with fluorescence observations we propose that the PA-rich phase separates into small unconnected domains.     

The Effect of Lipid Composition on the Permeability of Fluorescent Markers from Photosensitized Membranes

There is evidence indicating that the cellular locus of PDT action by amphiphilic sensitizers are the cellular membranes. The photosensitization process causes oxidative damage to membrane components that can result in the cell's death. However, it was not yet established whether lipid oxidation can cause free passage of molecules through the membrane and, as a result, be the primary cause of the cell's death. In this work, we studied the effect of liposomes' lipid composition on the kinetics of the leakage of three fluorescent dyes, calcein, carboxyfluorescein and DTAF, which were trapped in the intraliposomal aqueous phase, after photosensitization with the photosensitizer deuteroporphyrin. We found that as the degree of fatty acid unsaturation increased, the photosensitized passage of these molecules through the lipid bilayer increased. We also found that the rate of leakage of these molecules was affected by their size and bulkiness as well as by their net electric charge. In liposomes that are composed of a lipid mixture similar to that of natural membranes, the observed passage of molecules through the membrane is slow. Thus, the photodynamic damage to lipids does not appear to be severe enough to be an immediate, primary cause of cell death in biological photosensitization.     

ELECTRONIC PROCESSES AND PHOTOELECTRIC ASPECTS OF BILAYER LIPID MEMBRANES

Abstract— Owing to the complexity of biological membranes, many model systems have been studied in order to gain insight into the molecular mechanism of specific functions. One such model membrane extensively investigated in the past decade is the so-called bilayer lipid membrane (BLM). With suitable modifications, a BLM less than 100 A thick separating two aqueous solutions has been used as a model for a variety of biological membranes. This paper is devoted to a review of the properties and electronic processes of modified BLM.
Recent experiments using these membranes which contain photosynthetic pigments or dyes have demonstrated that, upon illumination, an EMF and a current can be generated. The connection between the photoelectric BLM and light-sensitive biological membranes and the rationale for this work are described.
Additionally, the effects of physical chemical parameters such as electric field, temperature, light intensity, duration of illumination and chemical agents (electron acceptors, donors, uncouplers, etc.) on the photoresponses of BLM are discussed. Other results indicate that BLM containing photoactive compounds behave similar to that of a silicon solar cell with one side of the membrane reducing and the other side oxidizing. The transverse pathway for the electron across the BLM could be provided by carotenoids such as β-carotene. Photoelectric BLM of this type represents a unique kind of energy transducing system and may well be useful in the conversion of solar energy into electricity and/or other forms of energy.     

Steered molecular dynamics simulations of cobra cytotoxin interaction with zwitterionic lipid bilayer: no penetration of loop tips into membranes

Cobra cytotoxins, small proteins of three-fingered toxin family, unspecifically damage membranes in different cells and artificial vesicles. However, the molecular mechanism of this damage is not yet completely understood. We used steered molecular dynamics simulations to study the interaction of cardiotoxin A3 from Naja atra cobra venom with hydrated 1-palmitoyl-2-oleoyl-1-sn-3-phosphatidylcholine (POPC) bilayer. The studied system included one cytotoxin molecule, 64 lipid molecules (32 molecules in each monolayer) and 2500 water molecules. It was found that the toxin interacted with zwitterionic bilayer formed by POPC. During first nanosecond of simulation the toxin molecule was oriented toward membrane surface by loops' basement including cytotoxin regions Cys14-Asn19 and Cys38-Ser46. This orientation was stable enough and was not changed during next 6 ns of simulation. The obtained data suggest that cytotoxin molecule cannot penetrate into membrane composed of zwitterionic lipids without some auxiliary interaction.     

Agar-supported lipid bilayers — basic structures for biosensor design. Electrical and mechanical properties

The lipid bilayer is widely accepted as the basic structure of all biological membranes. Known as BLM (bilayer lipid membrane), it can be prepared artificially. Suitably modified, the BLM serves as a very appropriate model for biological membranes. Recent investigations have verified the high analytical potential of artificial lipid membranes. With a structure and composition almost identical to the lipid moiety of biomembranes, the BLM may serve as an ideal host for receptor molecules of biological origin, thus becoming a transducer which could “see” the environment the way the living cell does. For the construction of lipid bilayer based biosensors; however, stable, easy to prepare and long-lasting lipid membranes are required. With this aim in mind, we have prepared lipid bilayer membranes which use an agar gel as support. This as-BLM (agar-supported BLM) has been shown to possess the same electrical, mechanical and dynamic properties the conventional BLM is famous for, along with the benefits of long-term stability and considerably elevated breakdown voltages. Its preparation on the tip of an agar-filled Teflon tube of 0.5 mm diameter is easy and can be performed even by less-skilled personnel.

In an attempt of further miniaturization the concept of the as-BLM was applied to thin-film micro-systems manufactured by standard micro-electronic techniques. The result is a lipid bilayer system, which, while preserving all the essential properties of the bilayer lipid membrane, can serve as a basic building block for cheap, disposable biosensoric systems.     

Fluorescent lipids: functional parts of fusogenic liposomes and tools for cell membrane labeling and visualization

In this paper a rapid and highly efficient method for controlled incorporation of fluorescent lipids into living mammalian cells is introduced. Here, the fluorescent molecules have two consecutive functions: First, they trigger rapid membrane fusion between cellular plasma membranes and the lipid bilayers of their carrier particles, so called fusogenic liposomes, and second, after insertion into cellular membranes these molecules enable fluorescence imaging of cell membranes and membrane traffic processes. We tested the fluorescent derivatives of the following essential membrane lipids for membrane fusion: Ceramide, sphingomyelin, phosphocholine, phosphatidylinositol-bisphosphate, ganglioside, cholesterol, and cholesteryl ester. Our results show that all probed lipids could more efficiently be incorporated into the plasma membrane of living cells than by using other methods. Moreover, labeling occurred in a gentle manner under classical cell culture conditions reducing cellular stress responses. Staining procedures were monitored by fluorescence microscopy and it was observed that sphingolipids and cholesterol containing free hydroxyl groups exhibit a decreased distribution velocity as well as a longer persistence in the plasma membrane compared to lipids without hydroxyl groups like phospholipids or other artificial lipid analogs. After membrane staining, the fluorescent molecules were sorted into membranes of cell organelles according to their chemical properties and biological functions without any influence of the delivery system.     

Comparative study of the dark and light-induced toxicity of lipofuscin granules from human retinal pigment epithelium and their chromophore A2E on the cardiolipin liposome model

The damaging effect of lipofuscin granules from the human retinal pigment epithelium and fluorophore A2E was studied on models of calcein- and ascorbate-loaded cardiolipin liposomes and outer segments of the bovine eye photoreceptor cells in dark and under visible light irradiation. In dark fluorophore A2E induces the release of calcein from calcein-loaded liposomes and reduces the lifetime of the artificial bilayer lipid membrane prepared from dioleyl phosphatidilcholine. A similar detergent-like action A2E exhibits towards ascorbate-loaded liposomes, significantly accelerating the release of ascorbate in dark. In the presence of A2E, irradiation with the full visible light (390?C700 nm) stimulates both the release of ascorbate from liposomes and accelerates the destruction of the bilayer lipid membrane. Retinal pigment epithelium lipofuscin granules also accelerate the release of ascorbate from ascorbate-loaded liposomes under visible light irradiation; the blue light (457.9 nm) was twice as more efficient as the green light (514.5 nm). The preliminary irradiation of A2E with the visible light decreases its detergent-like action on the cardiolipin liposomal membranes under the dark conditions and the photosensitizing effect on the lipid peroxidation of the outer segments of photoreceptor cells. Unlike A2E, the visible light irradiation of a suspension of lipofuscin granules under similar conditions does not noticeably decrease their sensitizing activity towards lipid peroxidation. It is assumed that the phototoxicity of retinal pigment epithelium lipofuscin granules is related not only to A2E in their composition, but depends mainly on the content of other photosensitizers (chromophores) in the granules.     

ENERGY TRANSFER FROM CAROTENOIDS TO CHLOROPHYLL a IN BLACK LIPID MEMBRANES

Abstract— Black lipid membranes (BLM) were prepared from extracts of Chlorella and spinach chloroplasts. Excitation spectra of the 730 nm fluorescence of chlorophyll a in the BLM contained peaks identified as due to carotenoids and which therefore indicate sensitization of the chlorophyll fluorescence by them. The efficiency of this energy transfer was evaluated by comparison of the actual excitation spectra with those corresponding to 0 and 100 per cent transfer efficiency. Efficiencies were of the order of 40–50 per cent in BLM, but only 10 per cent in pigment solutions, when the mean distance between pigment molecules was 23 Å in both systems. The fluorescence quantum yield of chlorophyll a in such solutions was only 2 per cent of that found in BLM. Enhancement of energy transfer in BLM is considered to be mainly due to suppression of competing deactivation processes of excited carotenoid states, such as diffusional quenching by ground-state molecules and internal conversion. Favorable orientation of pigment molecules in the BLM constitutes a further enhancement factor.     

The structure of ions and zwitterionic lipids regulates the charge of dipolar membranes

In pure water, zwitterionic lipids form lamellar phases with an equilibrium water gap on the order of 2 to 3 nm as a result of the dominating van der Waals attraction between dipolar bilayers. Monovalent ions can swell those neutral lamellae by a small amount. Divalent ions can adsorb onto dipolar membranes and charge them. Using solution X-ray scattering, we studied how the structure of ions and zwitterionic lipids regulates the charge of dipolar membranes. We found that unlike monovalent ions that weakly interact with all of the examined dipolar membranes, divalent and trivalent ions adsorb onto membranes containing lipids with saturated tails, with an association constant on the order of ～10 M(-1). One double bond in the lipid tail is sufficient to prevent divalent ion adsorption. We suggest that this behavior is due to the relatively loose packing of lipids with unsaturated tails that increases the area per lipid headgroup, enabling their free rotation. Divalent ion adsorption links two lipids and limits their free rotation. The ion-dipole interaction gained by the adsorption of the ions onto unsaturated membranes is insufficient to compensate for the loss of headgroup free-rotational entropy. The ion-dipole interaction is stronger for cations with a higher valence. Nevertheless, polyamines behave as monovalent ions near dipolar interfaces in the sense that they interact weakly with the membrane surface, whereas in the bulk their behavior is similar to that of multivalent cations. Advanced data analysis and comparison with theory provide insight into the structure and interactions between ion-induced regulated charged interfaces. This study models biologically relevant interactions between cell membranes and various ions and the manner in which the lipid structure governs those interactions. The ability to monitor these interactions creates a tool for probing systems that are more complex and forms the basis for controlling the interactions between dipolar membranes and charged proteins or biopolymers for encapsulation and delivery applications.     

Effect of NaCl and KCl on phosphatidylcholine and phosphatidylethanolamine lipid membranes: insight from atomic-scale simulations for understanding salt-induced effects in the plasma membrane

To gain a better understanding of how monovalent salt under physiological conditions affects plasma membranes, we have performed 200 ns atomic-scale molecular dynamics simulations of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) lipid bilayers. These two systems provide representative models for the outer and inner leaflets of the plasma membrane, respectively. The implications of cation-lipid interactions in these lipid systems have been considered in two different aqueous salt solutions, namely NaCl and KCl, and the sensitivity of the results on the details of interactions used for ions is determined by repeating the simulations with two distinctly different force fields. We demonstrate that the main effect of monovalent salt on a phospholipid membrane is determined by cations binding to the carbonyl region of a membrane, while chloride anions mostly stay in the water phase. It turns out that the strength and character of the cation-lipid interactions are quite different for different types of lipids and cations. PC membranes and Na+ ions demonstrate strongest interactions, leading to notable membrane compression. This finding was confirmed by both force fields (Gromacs and Charmm) employed for the ions. The binding of potassium ions to PC membranes (and the overall effect of KCl), in turn, was found to be much weaker mainly due to the larger size of a K+ ion compared to Na+. Furthermore, the effect of KCl on PC membranes was found to be force-field sensitive: The binding of a potassium ion was not observed at all in simulations performed with the Gromacs force-field, which seems to exaggerate the size of a K+ ion. As far as PE lipid bilayers are concerned, they are found to be influenced by monovalent salt to a significantly lesser extent compared to PC bilayers, which is a direct consequence of the ability of PE lipids to form both intra- and intermolecular hydrogen bonds and hence to adopt a more densely packed bilayer structure. Whereas for NaCl we observed weak binding of Na+ cations to the PE lipid-water interface, in the case of KCl we witnessed almost complete lack of cation binding. Overall, our findings indicate that monovalent salt ions affect lipids in the inner and outer leaflets of plasma cell membranes in substantially different ways.     

Water order profiles on phospholipid/cholesterol membrane bilayer surfaces

Water is pivotal in the stabilization of macromolecular biological structures, although the dynamic ensemble structure of water near to molecular surfaces has yet to be fully understood. We show, through molecular simulation and fluorescence measurements, that water at the membrane surface is substantially more ordered than bulk water, due to a loss of hydrogen bonding between water molecules, coupled with an alignment of lipid and water dipole moments. Ordering of the water leads to a gradient in the effective dielectric permittivity, which is evident in both the molecular simulations and the fluorescence measurements. A lower effective dielectric permittivity was correlated with a decreasing degree of hydrogen bonding over the same spatial range. The water molecules closest to the lipid headgroup oxygen atoms form hydrogen bonds which exhibit a mean lifetime of 6.3 ps, compared with a mean lifetime of water-water hydrogen bonds of less than 2 ps. Membranes made up purely of phosphatidylcholine (PC) were compared with those made with a PC/cholesterol ratio relevant to cell membranes. Clear differences were found between these membrane configurations. These observations point to molecular structural differences in the surface environments of membranes and may underlie regional differences in the surface biophysical properties of membrane microdomains.