Techniques that can probe nanometer length scales, such as small-angle neutron scattering (SANS), have become increasingly popular to detect phase separation in membranes. But to extract the phase composition and domain structure from the SANS traces, complementary information is needed. Here, we present a SANS, calorimetry and densitometry study of a mixture of two saturated lipids that exhibits solidus-liquidus phase coexistence: 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (dDPPC, tail-deuterated DPPC) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC). With calorimetry, we investigated the phase diagram for this system and found that the boundary traces for both multilamellar vesicles (MLVs) as well as 50 nm unilamellar vesicles overlap. Because the solidus boundary was mostly inaccessible by calorimetry, we investigated it by both SANS and molecular volume measurements for a 1:1 dDPPC:DLPC lipid mixture. From the temperature behavior of the molecular volume for the 1:1 dDPPC:DLPC mixture, as well as the individual molecular volume of each lipid species, we inferred that the liquidus phase consists of only fluid-state lipids while the solidus phase consists of lipids that are in gel-like states. Using this solidus-liquidus phase model, the SANS data were analyzed with an unrestricted shape model analysis software: MONSA. The resulting fits show irregular domains with dendrite-like features as those previously observed on giant unilamellar vesicles (GUVs). The surface pair correlation function describes a characteristic domain size for the minority phase that decreases with temperature, a behavior found to be consistent with a concomitant decrease in membrane mismatch between the liquidus and solidus phases.

The antineoplastic drug Docetaxel is a second generation taxane which is used against a great variety of cancers. The drug is highly lipophilic and produces a great array of severe toxic effects that limit its therapeutic effectiveness. The study of the interaction between Docetaxel and membranes is very scarce, however, it is required in order to get clues in relation with its function, mechanism of toxicity and possibilities of new formulations. Using phosphatidylcholine biomimetic membranes, we examine the interaction of Docetaxel with the phospholipid bilayer combining an experimental study, employing a series of biophysical techniques like Differential Scanning Calorimetry, X-Ray Diffraction and Infrared Spectroscopy, and a Molecular Dynamics simulation. Our experimental results indicated that Docetaxel incorporated into DPPC bilayer perturbing the gel to liquid crystalline phase transition and giving rise to immiscibility when the amount of the drug is increased. The drug promotes the gel ripple phase, increasing the bilayer thickness in the fluid phase, and is also able to alter the hydrogen-bonding interactions in the interfacial region of the bilayer producing a dehydration effect. The results from computational simulation agree with the experimental ones and located the Docetaxel molecule forming small clusters in the region of the carbon 8 of the acyl chain palisade overlapping with the carbonyl region of the phospholipid. Our results support the idea that the anticancer drug is embedded into the phospholipid bilayer to a limited amount and produces structural perturbations which might affect the function of the membrane.

This study aims to investigate the interactions appearing when the beta-2-glycoprotein-1 binds to a lipid bilayer. The inter- and intra-molecular forces acting between the two macromolecular systems have been investigated using a molecular dynamics simulation method. The importance of water bridges has also been addressed. Additionally, the viscoelastic response of the bilayer has been studied. In detail, the (saturated-chain) 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and (unsaturated-chain) 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) bilayers have been chosen to test their behavior near the protein. Both of the lipids have a polar head but different chemical structures and are similar to the main phospholipids present in the synovial fluid. This study is meaningful for further explaining the worsening friction properties in articular cartilage, as the inactivation of phospholipid bilayers by beta-2-glycoprotein-1 is believed to be a cause of the destruction of cartilage in most rheumatic diseases and osteoarthritis. It was found that the protein binds stronger to the DPPC bilayer than to the POPE, but in both cases, it has the potential to change the local bilayer stability. Nevertheless, the binding forces are placed within a small area (only a few lipids contribute to the binding, creating many interactions). However, together, they are not stronger than the covalent bonds between C-O, thus, potentially, it is possible to push the lipids into the bilayer but detaching the lipids\' heads from the tail is not possible. Additionally, the protein causes water displacement from the vicinity of the bilayer, and this may be a contributor to the instability of the bilayer (disrupting the water bridges needed for the stabilization of the bilayer, especially in the case of DPPC where the heads are not so well stabilized by H-bonds as they are in POPE). Moreover, it was found that the diffusivity of lipids in the DPPC bilayer bound to the protein is significantly different from the diffusivity of the ones which are not in contact with the protein. The POPE bilayer is stiffer due to intramolecular interactions, which are stronger than in the DPPC; thus, the viscous to elastic effects in the POPE case are more significant than in the case of the DPPC. It is, therefore, harder to destabilize the POPE bilayer than the DPPC one.

Hydrophobic resin acids (RAs) are synthesized by conifer trees as part of their defense mechanisms. One of the functions of RAs in plant defense is suggested to be the perturbation of the cellular membrane. However, there is a vast diversity of chemical structures within this class of molecules, and there are no clear correlations to the molecular mechanisms behind the RA\'s toxicity. In this study we unravel the molecular interactions of the three closely related RAs dehydroabietic acid, neoabietic acid, and the synthetic analogue dichlorodehydroabietic acid with dipalmitoylphosphatidylcholine (DPPC) model membranes and the polar lipid extract of soybeans. The complementarity of the biophysical techniques used (NMR, DLS, NR, DSC, Cryo-TEM) allowed correlating changes at the vesicle level with changes at the molecular level and the co-localization of RAs within DPPC monolayer. Effects on DPPC membranes are correlated with the physical chemical properties of the RA and their toxicity.

The interaction of coenzyme Q10 (CoQ10) in a monolayer of 1,2-dipalmitoyl-sn-glysero-3-phospho-L-choline (DPPC), in a monolayer of 1,2-dierucoyl-sn-glysero-3-phospho-L-choline (DEPC), in a monolayer of 1-palmitoyl-2-oleoyl-sn-glysero-3-phospho-L-serine (POPS) and in a monolayer of total lipid extract from pig brain (PB) has been investigated by using the Langmuir monolayer technique. Surface pressure (π)-mean molecular area (mma) isotherms have been measured for pure lipid monolayers and lipid monolayers with 0.5, 1.0, 2.0, 5.0 and 10.0 mol% CoQ10 concentrations. At the biological concentration (1.0-3.0 mol%) of CoQ10, intercalation of CoQ10 occurs in the lipid acyl chains of DPPC, POPS and PB monolayers. Above the biological concentration of CoQ10, the CoQ10 molecule induces domain formation in the monolayers of DPPC, POPS and PB lipids. The DEPC monolayer behavior deviates from the other lipids in this study. At 2.0 mol% the CoQ10 promotes very dense lipid packing, and the CoQ10 molecule is located parallel to the DEPC acyl chains at all concentrations.

Cytochrome c (Cyt c) is an essential component of the inner mitochondrial respiratory chain because of its function of transferring electrons. The feature is closely related to the interaction between Cyt c and membrane lipids. We used Langmuir-Blodgett monolayer technique combined with AFM to study the interaction of Cyt c with lipid monolayers at air-buffer interface. In our work, by comparing the mixed Cyt c-anionic (DPPS) and Cyt c-zwitterionic (DPPC/DPPE) monolayers, the adsorption capacity of Cyt c on lipid monolayers is DPPS>DPPE>DPPC, which is attributed to their different headgroup structures. π-A isothermal data show that Cyt c (v=2.5 μL) molecules are at maximum adsorption quantity on lipid monolayer. Moreover, Cyt c molecules would form aggregations and drag some lipids with them into subphase if the protein exceeds the maximum adsorption quantity. π-T curve indicates that it takes more time for Cyt c molecular conformation to rearrange on DPPE monolayer than on DPPC. The compressibility study reveals that the adsorption or intermolecular aggregation of Cyt c molecules on lipid monolayer will change the membrane fluidization. In order to quantitatively estimate Cyt c molecular adsorption properties on lipid monolayers, we fit the experimental isotherm with a simple surface state equation. A theoretical model is also introduced to analyze the liquid expanded (LE) to liquid condensed (LC) phase transition of DPPC monolayer. The results of theoretical analysis are in good agreement with the experiment.

The localization and interaction of six naturally occurring flavones (FLV, 5HF, 6HF, 7HF, CHY and BLN) in DPPC bilayers were studied using DSC and multi-nuclear NMR. DSC results indicate that FLV and 6HF interact with alkyl chains. The (1)H NMR shows interaction of flavones with the sn-glycero region. Ring current induced chemical shifts indicate that 6HF and BLN acquire parallel orientation in bilayers. 2D NOESY spectra indicate partitioning of the B-ring into the alkyl chain region. The DSC, NMR and binding studies indicate that 5HF and 7HF are located near head group region, while 6HF, CHY and BLN are located in the vicinity of sn-glycero region, and FLV is inserted deepest in the membrane.

We present the results of a comparative differential calorimetric and Fourier transform infrared spectroscopic study of the effect of cholesterol and five of its analogues on the thermotropic phase behaviour and organization of dipalmitoylphosphatidylcholine bilayer membranes. These sterols/steroids differ in both the nature and stereochemistry of the polar head group at C3 (βOH, αOH or C=O) and in the position of the double bond (C4-C5 in ring A or C5-C6 in ring B). In the three Δ(5) sterols/steroid series, the concentration of these compounds required to abolish the DPPC pretransition, inversely related to their relative ability to disorder gel state DPPC bilayers, decreases in the order βOH>αOH>C=O and these differences in concentration are significant. However, in the Δ(4) series, these concentrations are more similar, regardless of polar head group nature or stereochemistry. Similarly, the residual enthalpy of the main phase transition of DPPC at 50 mol.% sterol/steroid, which is inversely related to the miscibility of these compounds in the DPPC bilayer, also increases in the order βOH>αOH>C=O, but this effect is attenuated in the Δ(4) as opposed to the Δ(5) series. Both of these results indicate that the presence of a double bond at C4-C5 in ring A, as compared to a C5-C6 double bond in ring B, reduces the effect of variations in the structure of the polar group at C3 on the properties of the host DPPC bilayer. The movement of the double bond from C5 to C4 in the two sterol pairs results in a greater decrease in the temperature and enthalpy of both the pretransition and the main phase transition, whereas the opposite result is observed in the ketosteroid pair. Similarly, the ability of these compounds to order the DPPC hydrocarbon chains decreases in the order βOH>αOH>C=O in both series of compounds, but in the two sterol pairs, hydrocarbon chain ordering is greater for the Δ(5) than the Δ(4) sterols, whereas the opposite is the case for the steroid pair. All of these results indicate that the typical effects of sterols/steroids in increasing the packing density and thermal stability of fluid lipid bilayers are optimal when an OH group rather than C=O group is present at C3, and that this OH group is more effective in the equatorial rather than the axial orientation. We can explain all of our sterol results by noting that the shift of the double bond from Δ(5) to Δ(4) introduces of a bend in ring A, which in turn destroys the coplanarity of the steroid fused ring system and reduces the goodness of sterol packing in the host DPPC bilayer. However, this conformational change should also occur in the ketosteroid pair, yet our experimental results indicate that the presence of the Δ(4) double bond is less disruptive than a double bond at Δ(5). We suggest that the presence of keto-enol tautomerism in the conjugated Δ(4) ketosteroid, but not in the nonconjugated Δ(5) compound, may provide additional H-bonding opportunities to adjacent DPPC molecules in the bilayer, which can overcome the unfavourable conformational change in ring A induced by the Δ(4) double bond.

The dynamical translocation of lipids from one leaflet to another due to membrane permeabilization driven by nanosecond, high-intensity (>100kV/cm) electrical pulses has been probed. Our simulations show that lipid molecules can translocate by diffusion through water-filled nanopores which form following high voltage application. Our focus is on multiple pulsing, and such simulations are relevant to gauge the time duration over which nanopores might remain open, and facilitate continued lipid translocations and membrane transport. Our results are indicative of a N(½) scaling with pulse number for the pore radius. These results bode well for the use of pulse trains in biomedical applications, not only due to cumulative behaviors and in reducing electric intensities and pulsing hardware, but also due to the possibility of long-lived thermo-electric physics near the membrane, and the possibility for pore coalescence.

The lipopeptide surfactin exhibits promising antimicrobial activities which are hampered by haemolytic toxicity. Rational design of new surfactin molecules, based on a better understanding of membrane:surfactin interaction, is thus crucial. We here performed bioimaging of lateral membrane lipid heterogeneity in adherent living human red blood cells (RBCs), as a new relevant bioassay, and explored its potential to better understand membrane:surfactin interactions. RBCs show (sub)micrometric membrane domains upon insertion of BODIPY analogs of glucosylceramide (GlcCer), sphingomyelin (SM) and phosphatidylcholine (PC). These domains exhibit increasing sensitivity to cholesterol depletion by methyl-β-cyclodextrin. At concentrations well below critical micellar concentration, natural cyclic surfactin increased the formation of PC and SM, but not GlcCer, domains, suggesting preferential interaction with lipid assemblies with the highest vulnerability to methyl-β-cyclodextrin. Surfactin not only reversed disappearance of SM domains upon cholesterol depletion but further increased PC domain abundance over control RBCs, indicating that surfactin can substitute cholesterol to promote micrometric domains. Surfactin sensitized excimer formation from PC and SM domains, suggesting increased lipid recruitment and/or diffusion within domains. Comparison of surfactin congeners differing by geometry, charge and acyl chain length indicated a strong dependence on acyl chain length. Thus, bioimaging of micrometric lipid domains is a visual powerful tool, revealing that intrinsic lipid domain organization, cholesterol abundance and drug acyl chain length are key parameters for membrane:surfactin interaction. Implications for surfactin preferential location in domains or at their boundaries are discussed and may be useful for rational design of better surfactin molecules.