Lateral order in mixed lipid bilayers and its influence on ion translocation by gramicidin: a model for the structure-function relationship in membranes

Using small-angle neutron scattering to detect nanoscopic lipid domains

The cell plasma membrane is a complex system, which is thought to be capable of exhibiting non-random lateral organization. Studies of live cells and model membranes have yielded mechanisms responsible for the formation, growth, and maintenance of nanoscopic heterogeneities, although the existence and mechanisms that give rise to these heterogeneities remain controversial. Small-angle neutron scattering (SANS) is a tool ideally suited to interrogate lateral heterogeneity in model membranes, primarily due to its unique spatial resolution (i.e., ~5-100nm) and its ability to resolve structure with minimal perturbation to the membrane. In this review we examine several methods used to analyze the SANS signal arising from freely suspended unilamellar vesicles containing lateral heterogeneity. Specifically, we discuss an analytical model for a single, round domain on a spherical vesicle. We then discuss a numerical method that uses Monte Carlo simulation to describe systems with multiple domains and/or more complicated morphologies. Also discussed are several model-independent approaches that are sensitive to membrane heterogeneity. The review concludes with several recent applications of SANS to the study of membrane raft mixtures.

Interactions of ibuprofen with hybrid lipid bilayers probed by complementary surface-enhanced vibrational spectroscopies

The incorporation of small molecules into lipid bilayers is a process of biological importance and clinical relevance that can change the material properties of cell membranes and cause deleterious side effects for certain drugs. Here we report the direct observation, using surface-enhanced Raman and IR spectroscopies (SERS, SEIRA), of the insertion of ibuprofen molecules into hybrid lipid bilayers. The alkanethiol-phospholipid hybrid bilayers were formed onto gold nanoshells by self-assembly, where the underlying nanoshell substrates provided the necessary enhancements for SERS and SEIRA. The spectroscopic data reveal specific interactions between ibuprofen and phospholipid moieties and indicate that the overall hydrophobicity of ibuprofen plays an important role in its intercalation in these membrane mimics.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Comparative in vitro study of contraceptive agents with anti-HIV activity: gramicidin, nonoxynol-9, and gossypol

Gramicidin, a polypeptide antibiotic derived from Bacillus brevis, was compared in vitro with the established contraceptive virucidal agents nonoxynol-9 and gossypol for activity against human immunodeficiency virus (HIV) infection. The effective antiviral 10 ng/ml concentration of gramicidin required for complete HIV inactivation was a thousand-fold lower than the dose observed for nonoxynol-9 or gossypol. Gramicidin, routinely used as a contraceptive agent in the former Soviet Union, should be considered for in vivo trials as a spermicide with potent antiviral activity.

Ca(2+)-induced lateral phase separation in black lipid membranes and its coupling to the ion translocation by gramicidin

We analyze the single-channel current fluctuations of gramicidin incorporated into biomolecular lipid membranes (BLM) of binary mixtures of phosphatidylcholine (PC) and phosphatidylglycerol (PG) as a function of the Ca2+ concentration in the electrolyte (0.5 M CsCl, pH 6) solution. At low Ca2+ levels (cCa2+ < 10(-6) M) a monomodal conductance histogram and a single average lifetime suggests a homogeneous mixture over the full range of composition (PG(1-x)PCx, 0 < or = x < or = 1). At higher Ca2+ concentrations phase separation processes are inferred from the appearance of bimodal conductance histograms. The two channel populations (in the two coexisting phases) can also be distinguished through their different average lifetimes. By a systematic variation of the mole fractions of the two lipid components we derive the respective phase boundaries and thus the full Ca2+ concentration-composition phase diagram.

Lateral order in binary lipid alloys and its coupling to membrane functions

Densitometry, Raman spectroscopy and small angle neutron scattering are employed to elucidate the miscibility behavior of lipid mixtures organized as liposomal dispersions. First, temperature-composition-phase diagrams for several binary alloys of dialkyl-lecithins differing in chain lengths by an increasing number of CH2-groups are derived. A mixture of dimyristoyllecithin and distearoyllecithin (delta CH2 = 4) shows a peritectic phase behavior with a miscibility gap in the gel state. In the fluid phase, at high enough temperatures, homogeneous mixtures of the two components are formed at all molar ratios. However, upon approaching a critical point by either lowering the temperature or increasing the hydrostatic pressure, critical concentration fluctuations are observed. If one component of a binary mixture is charged, electrostatic interactions can be used to induce phase separation at constant temperature. This is demonstrated for Ca2(+)-driven demixing in alloys of lecithin and negatively charged phosphatidylglycerol. The influence of the various concepts for the induction of lateral structure formation in lipid membranes on integral functional units like ionophores is demonstrated by analysing the single channel current fluctuations of gramicidin in bimolecular lipid membranes. Ca2+, as well as polyelectrolyte-induced phase separations are shown and discussed as examples for the important (lateral) order-function relationship in biomembranes.

pH-control of the miscibility properties of a binary lipid alloy and its influence on the ion transport by gramicidin

We studied the coupling of a membrane function (the transport of ions by the pore forming polypeptide gramicidin) to chemically driven phase changes in black membranes of binary lipid mixtures. In particular, we investigated the influence of the aqueous pH value on the fluid-fluid demixing effect of Ca2+ to phosphatidylcholine/phosphatidylglycerol bilayers. It is found that one can switch, under certain conditions, between a homogeneously mixed and a phase separated membrane by changing the pH. We interpret this as being caused by the change in the degree of dissociation of one of the lipid components.

Phase separation in bimolecular mixed lipid membranes induced by polylysine

We demonstrate, for the first time, polylysine-induced phase separation in a bimolecular lipid membrane of a lecithin/phosphatidylglycerol-mixture by analysing the single channel current fluctuations of gramicidin. The bimodal conductance histograms are direct evidence for the incorporation of the transport system into the two coexisting phases of different composition.