Investigation of temperature-induced phase transitions in DOPC and DPPC phospholipid bilayers using temperature-controlled scanning force microscopy

Structure and Nanomechanics of Model Membranes by Atomic Force Microscopy and Spectroscopy: Insights into the Role of Cholesterol and Sphingolipids

Biological membranes mediate several biological processes that are directly associated with their physical properties but sometimes difficult to evaluate. Supported lipid bilayers (SLBs) are model systems widely used to characterize the structure of biological membranes. Cholesterol (Chol) plays an essential role in the modulation of membrane physical properties. It directly influences the order and mechanical stability of the lipid bilayers, and it is known to laterally segregate in rafts in the outer leaflet of the membrane together with sphingolipids (SLs). Atomic force microscope (AFM) is a powerful tool as it is capable to sense and apply forces with high accuracy, with distance and force resolution at the nanoscale, and in a controlled environment. AFM-based force spectroscopy (AFM-FS) has become a crucial technique to study the nanomechanical stability of SLBs by controlling the liquid media and the temperature variations. In this contribution, we review recent AFM and AFM-FS studies on the effect of Chol on the morphology and mechanical properties of model SLBs, including complex bilayers containing SLs. We also introduce a promising combination of AFM and X-ray (XR) techniques that allows for in situ characterization of dynamic processes, providing structural, morphological, and nanomechanical information.

An Assemblable, Multi-Angle Fluorescence and Ellipsometric Microscope

We introduce a multi-functional microscope for research laboratories that have significant cost and space limitations. The microscope pivots around the sample, operating in upright, inverted, side-on and oblique geometries. At these geometries it is able to perform bright-field, fluorescence and qualitative ellipsometric imaging. It is the first single instrument in the literature to be able to perform all of these functionalities. The system can be assembled by two undergraduate students from a provided manual in less than a day, from off-the-shelf and 3D printed components, which together cost approximately $16k at 2016 market prices. We include a highly specified assembly manual, a summary of design methodologies, and all associated 3D-printing files in hopes that the utility of the design outlives the current component market. This open design approach prepares readers to customize the instrument to specific needs and applications. We also discuss how to select household LEDs as low-cost light sources for fluorescence microscopy. We demonstrate the utility of the microscope in varied geometries and functionalities, with particular emphasis on studying hydrated, solid-supported lipid films and wet biological samples.

Electronic control of H+ current in a bioprotonic device with Gramicidin A and Alamethicin

In biological systems, intercellular communication is mediated by membrane proteins and ion channels that regulate traffic of ions and small molecules across cell membranes. A bioelectronic device with ion channels that control ionic flow across a supported lipid bilayer (SLB) should therefore be ideal for interfacing with biological systems. Here, we demonstrate a biotic-abiotic bioprotonic device with Pd contacts that regulates proton (H+) flow across an SLB incorporating the ion channels Gramicidin A (gA) and Alamethicin (ALM). We model the device characteristics using the Goldman-Hodgkin-Katz (GHK) solution to the Nernst-Planck equation for transport across the membrane. We derive the permeability for an SLB integrating gA and ALM and demonstrate pH control as a function of applied voltage and membrane permeability. This work opens the door to integrating more complex H+ channels at the Pd contact interface to produce responsive biotic-abiotic devices with increased functionality.

Simple Optical Imaging of Nanoscale Features in Free-Standing Films

Measuring thicknesses in thin films with high spatial and temporal resolution is of prime importance for understanding the structure and dynamics in thin films and membranes. In the present work, we introduce fluorescence-interferometry, a method that combines standard reflected light thin film interferometry with simultaneous fluorescence measurements. We apply this method to the thinning dynamics and phase separation in free-standing inverse phospholipid bilayer films. The measurements were carried out using a standard fluorescence microscope using multichannel imaging and yielded subnanometer resolution, which is applied to optically measure the discrete thickness variations across phase-separated membranes.

Nanoscale investigation of the interaction of colistin with model phospholipid membranes by Langmuir technique, and combined infrared and force spectroscopies

Colistin (Polymyxin E), an antimicrobial peptide, is increasingly put forward as salvage for severe multidrug-resistant infections. Unfortunately, colistin is potentially toxic to mammalian cells. A better understanding of the interaction with specific components of the cell membranes may be helpful in controlling the factors that may enhance toxicity. Here, we report a physico-chemical study of model phospholipid (PL) mono- and bilayers exposed to colistin at different concentrations by Langmuir technique, atomic force microscopy (AFM) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The effect of colistin on chosen PL monolayers was examined. Insights into the topographical and elastic changes in the PL bilayers within time after peptide injection are presented via AFM imaging and force spectra. Finally, changes in the PL bilayers' ATR-FTIR spectra as a function of time within three bilayer compositions, and the influence of colistin on their spectral fingerprint are examined together with the time-evolution of the Amide II and νCO band integrated intensity ratios. Our study reveals a great importance in the role of the PL composition as well as the peptide concentration on the action of colistin on PL model membranes.

Structural perturbation of a dipalmitoylphosphatidylcholine (DPPC) bilayer by warfarin and its bolaamphiphilic analogue: A molecular dynamics study

Compounds with nominally similar biological activity may exhibit differential toxicity due to differences in their interactions with cell membranes. Many pharmaceutical compounds are amphiphilic and can be taken up by phospholipid bilayers, interacting strongly with the lipid-aqueous interface whether or not subsequent permeation through the bilayer is possible. Bolaamphiphilic compounds, which possess two hydrophilic ends and a hydrophobic linker, can likewise undergo spontaneous uptake by bilayers. While membrane-spanning bolaamphiphiles can stabilize membranes, small molecules with this characteristic have the potential to create membrane defects via disruption of bilayer structure and dynamics. When compared to the amphiphilic therapeutic anticoagulant, warfarin, the bolaamphiphilic analogue, brodifacoum, exhibits heightened toxicity that goes beyond superior inhibition of the pharmacological target enzyme. We explore, herein, the consequences of anticoagulant accumulation in a dipalmitoylphosphatidylcholine (DPPC) bilayer. Coarse-grained molecular dynamics simulations reveal that permeation of phospholipid bilayers by brodifacoum causes a disruption of membrane barrier function that is driven by the bolaamphiphilic nature and size of this molecule. We find that brodifacoum partitioning into bilayers causes membrane thinning and permeabilization and promotes lipid flip-flop - phenomena that are suspected to play a role in triggering cell death. These phenomena are either absent or less pronounced in the case of the less toxic, amphiphilic compound, warfarin.

Probing Microscopic Orientation in Membranes by Linear Dichroism

The cell membrane is an ordered environment, which anisotropically affects the structure and interactions of all of its molecules. Monitoring membrane orientation at a local level is rather challenging but could reward crucial information on protein conformation and interactions in the lipid bilayer. We monitored local lipid ordering changes upon varying the cholesterol concentration using polarized light spectroscopy and pyrene as a membrane probe. Pyrene, with a shape intermediate between a disc and a rod, can detect microscopic orientation variations at the level of its size. The global membrane orientation was determined using curcumin, a probe with nonoverlapping absorption relative to that of pyrene. While the macroscopic orientation of a liquid-phase bilayer decreases with increasing cholesterol concentration, the local orientation is improved. Pyrene is found to be sensitive to the local effects induced by cholesterol and temperature on the bilayer. Disentangling local and global orientation effects in membranes could provide new insights into functionally significant interactions of membrane proteins.

Preservation of Supported Lipid Membrane Integrity from Thermal Disruption: Osmotic Effect

Preservation of structural integrity under various environmental conditions is one major concern in the development of the supported lipid membrane (SLM)-based devices. It is common for SLMs to experience temperature shifts from manufacture, processing, storage, and transport to operation. In this work, we studied the thermal adaption of the supported membranes on silica substrates. Homogenous SLMs with little defects were formed through the vesicle fusion method. The mass and fluidity of the bilayers were found to deteriorate from a heating process but not a cooling process. Fluorescence characterizations showed that the membranes initially budded as a result of heating-induced lipid lateral area expansion, followed by the possible fates including maintenance, retraction, and fission, among which the last contributes to the irreversible compromise of the SLM integrity and spontaneous release of the interlipid stress accumulated. Based on the mechanism, we developed a strategy to protect SLMs from thermal disruption by increasing the solute concentration in medium. An improved preservation of the membrane mass and fluidity against the heating process was observed, accompanied by a decrease in the retraction and fission of the buds. Theoretical analysis revealed a high osmotic energy penalty for the fission, which accounts for the depressed disruption. This osmotic-based protection strategy is facile, solute nonspecific, and long-term efficient and has little impact on the original SLM properties. The results may help broaden SLM applications and sustain the robustness of SLM-based devices under multiple thermal conditions.

Spontaneous vesicle formation in a deep eutectic solvent

Solvent penetration experiments and small-angle X-ray scattering reveal that phospholipids dissolved in a deep eutectic solvent (DES) spontaneously self-assemble into vesicles above the lipid chain melting temperature. This means DESs are one of the few nonaqueous solvents that mediate amphiphile self-assembly, joining a select set of H-bonding molecular solvents and ionic liquids.

Phototriggered Local Anesthesia

We report a phototriggerable formulation enabling in vivo repeated and on-demand anesthesia with minimal toxicity. Gold nanorods (GNRs) that can convert near-infrared (NIR) light into heat were attached to liposomes (Lip-GNRs), enabling light-triggered phase transition of their lipid bilayers with a consequent release of payload. Lip-GNRs containing the site 1 sodium channel blocker tetrodotoxin and the α2-adrenergic agonist dexmedetomidine (Lip-GNR-TD) were injected subcutaneously in the rat footpad. Irradiation with an 808 nm continuous wave NIR laser produced on-demand and repeated infiltration anesthesia in the rat footpad in proportion to the irradiance, with minimal toxicity. The ability to achieve on-demand and repeated local anesthesia could be very beneficial in the management of pain.

Sensing membrane thickness: Lessons learned from cold stress

The lipid bilayer component of biological membranes is important for the distribution, organization, and function of bilayer spanning proteins. These physical barriers are subjected to bilayer perturbations. As a consequence, nature has evolved proteins that are able to sense changes in the bilayer properties and transform these lipid-mediated stimuli into intracellular signals. A structural feature that most signal-transducing membrane-embedded proteins have in common is one or more α-helices that traverse the lipid bilayer. Because of the interaction with the surrounding lipids, the organization of these transmembrane helices will be sensitive to membrane properties, like hydrophobic thickness. The helices may adapt to the lipids in different ways, which in turn can influence the structure and function of the intact membrane proteins. We review recent insights into the molecular basis of thermosensing via changes in membrane thickness and consider examples in which the hydrophobic matching can be demonstrated using reconstituted membrane systems. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

A study of molecular adsorption of a cationic surfactant on complex surfaces with atomic force microscopy

The study of molecular adsorption on solid surfaces is of broad interest. However, so far the study has been restricted to idealized flat smooth rigid surfaces which are rarely the case in real world applications. Here we describe a study of molecular adsorption on a complex surface of the submicron fibers of a fibrous membrane of regenerated cellulose in aqueous media. We use a cationic surfactant, cetyltrimethylammonium chloride (CTAC), as the adsorbing molecule. We study the equilibrium adsorption of CTAC molecules on the same area of the fibers by sequentially immersing the membrane in pure water, 1 mM and then a 20 mM solution of CTAC. Atomic force microscopy (AFM) is applied to study the adsorption. The force-volume mode is used to record the force-deformation curves of the adsorbed molecules on the fiber surface. We suggest a model to separate the forces due to the adsorbed molecules from the elastic deformation of the fiber. Interestingly, knowledge of the surface geometry is not required in this model provided the surface is made of elastically homogeneous material. Different models are investigated to estimate the amount of the adsorbed molecules based on the obtained force curves. The exponential steric repulsion model fits the force data the best. The amount of the adsorbed surfactant molecules and its dependence on the concentration are found to be reasonable compared to the data previously measured by means of Raman scattering done on a flat surface of silica.

Biological potential of nanomaterials strongly depends on the suspension media: experimental data on the effects of fullerene C₆₀ on membranes

Fullerenes (C60) are some of the most promising carbon nanomaterials to be used for medical applications as drug delivery agents. Computational and experimental studies have proposed their ability to enter cells by penetrating lipid bilayers. The aim of our study was to provide experimental evidence on whether pristine C60 in physiological media could penetrate cell membranes. The effect was tested on phospholipid vesicles (liposomes) composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, and validated on isolated human red blood cells (RBCs). We incubated the liposomes in an aqueous suspension of C60 and dissolved the lipids and C60 together in chloroform and subsequently formatted the liposomes. By differential scanning calorimetry measurements, we assessed the effect of C60 on the phospholipid thermal profile. The latter was not affected after the incubation of liposomes in the C60 suspension; also, a shape transformation of RBCs did not occur. Differently, by dispersing both C60 and the phospholipids in chloroform, we confirmed the possible interaction of C60 with the bilayer. We provide experimental data suggesting that the suspension medium is an important factor in determining the C60-membrane interaction, which is not always included in computational studies. Since the primary particle size is not the only crucial parameter in C60-membrane interactions, it is important to determine the most relevant characteristics of their effects on membranes.

Phase transition behaviors of the supported DPPC bilayer investigated by sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM)

The phase transition behaviors of a supported bilayer of dipalmitoylphosphatidyl-choline (DPPC) have been systematically evaluated by in situ sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM).

Potential Controls the Interaction of Liposomes with Octadecanol-Modified Au Electrodes: An in Situ AFM Study

The formation of supported lipid bilayers using liposomes requires interaction with the solid surface, rupture of the liposome, and spreading to cover the surface with a lipid bilayer. This can result in a less-than-uniform coating of the solid surface. Presented is a method that uses the electrochemical poration of an adsorbed lipid-like layer on a Au electrode to control the interaction of 100 nm DOPC liposomes. An octadecanol-coated Au-on-mica surface was imaged using tapping-mode AFM during the application of potential in the presence or absence of liposomes. When the substrate potential was made negative enough, defects formed in the adsorbed layer and new taller features were observed. More features were observed and existing features increased in size with time spent at this negative poration potential. The new features were 1.8-2.0 nm higher than the octadecanol-coated gold surface, half the thickness of a DOPC bilayer. These features were not observed in the absence of liposomes when undergoing the same potential perturbation. In the presence of liposomes, the application of a poration potential was needed to initiate the formation of these taller features. Once the applied potential was removed, the features stopped growing and no new regions were observed. The size of these new regions was consistent with the footprint of a flattened 100 nm liposome. It is speculated that the DOPC liposomes were able to interact with the defects and became soluble in the octadecanol, creating a taller region that was limited in size to the liposome that adsorbed and became incorporated. This AFM study confirms previous in situ fluorescence measurements of the same system and illustrates the use of a potential perturbation to control the formation of these regions of increased DOPC content.

Assemblies of pore-forming toxins visualized by atomic force microscopy

A number of pore-forming toxins (PFTs) can assemble on lipid membranes through their specific interactions with lipids. The oligomeric assemblies of some PFTs have been successfully revealed either by electron microscopy (EM) and/or atomic force microscopy (AFM). Unlike EM, AFM imaging can be performed under physiological conditions, enabling the real-time visualization of PFT assembly and the transition from the prepore state, in which the toxin does not span the membrane, to the pore state. In addition to characterizing PFT oligomers, AFM has also been used to examine toxin-induced alterations in membrane organization. In this review, we summarize the contributions of AFM to the understanding of both PFT assembly and PFT-induced membrane reorganization. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Effect of superparamagnetic iron oxide nanoparticles on fluidity and phase transition of phosphatidylcholine liposomal membranes

Superparamagnetic iron oxide nanoparticles (SPIONs) with multifunctional properties have shown great promise in theranostics. The aim of our work was to compare the effects of SPIONs on the fluidity and phase transition of the liposomal membranes prepared with zwitterionic phosphatidylcholine lipids. In order to study if the surface modification of SPIONs has any influence on these membrane properties, we have used four types of differently functionalized SPIONs, such as: plain SPIONs (primary size was shown to bê11 nm), silica-coated SPIONs, SPIONs coated with silica and functionalized with positively charged amino groups or negatively charged carboxyl groups (the primary size of all the surface-modified SPIONs was ~20 nm). Small unilamellar vesicles prepared with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine lipids and multilamellar vesicles prepared with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine lipids were encapsulated or incubated with the plain and surface-modified SPIONs to determine the fluidity and phase transition temperature of the bilayer lipids, respectively. Fluorescent anisotropy and differential scanning calorimetric measurements of the liposomes that were either encapsulated or incubated with the suspension of SPIONs did not show a significant difference in the lipid ordering and fluidity; though the encapsulated SPIONs showed a slightly increased effect on the fluidity of the model membranes in comparison with the incubated SPIONs. This indicates the low potential of the SPIONs to interact with the nontargeted cell membranes, which is a desirable factor for in vivo applications.

Structural Significance of Lipid Diversity as Studied by Small Angle Neutron and X-ray Scattering

We review recent developments in the rapidly growing field of membrane biophysics, with a focus on the structural properties of single lipid bilayers determined by different scattering techniques, namely neutron and X-ray scattering. The need for accurate lipid structural properties is emphasized by the sometimes conflicting results found in the literature, even in the case of the most studied lipid bilayers. Increasingly, accurate and detailed structural models require more experimental data, such as those from contrast varied neutron scattering and X-ray scattering experiments that are jointly refined with molecular dynamics simulations. This experimental and computational approach produces robust bilayer structural parameters that enable insights, for example, into the interplay between collective membrane properties and its components (e.g., hydrocarbon chain length and unsaturation, and lipid headgroup composition). From model studies such as these, one is better able to appreciate how a real biological membrane can be tuned by balancing the contributions from the lipid's different moieties (e.g., acyl chains, headgroups, backbones, etc.).

A Parameterization of Cholesterol for Mixed Lipid Bilayer Simulation within the Amber Lipid14 Force Field

The Amber Lipid14 force field is expanded to include cholesterol parameters for all-atom cholesterol and lipid bilayer molecular dynamics simulations. The General Amber and Lipid14 force fields are used as a basis for assigning atom types and basic parameters. A new RESP charge derivation for cholesterol is presented, and tail parameters are adapted from Lipid14 alkane tails. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers are simulated at a range of cholesterol contents. Experimental bilayer structural properties are compared with bilayer simulations and are found to be in good agreement. With this parameterization, another component of complex membranes is available for molecular dynamics with the Amber Lipid14 force field.

A coiled coil switch mediates cold sensing by the thermosensory protein DesK

The thermosensor histidine kinase DesK from Bacillus subtilis senses changes in membrane fluidity initiating an adaptive response. Structural changes in DesK have been implicated in transmembrane signaling, but direct evidence is still lacking. On the basis of structure-guided mutagenesis, we now propose a mechanism of DesK-mediated signal sensing and transduction. The data indicate that stabilization/destabilization of a 2-helix coiled coil, which connects the transmembrane sensory domain of DesK to its cytosolic catalytic region, is crucial to control its signaling state. Computational modeling and simulations reveal couplings between protein, water and membrane mechanics. We propose that membrane thickening is the main driving force for signal sensing and that it acts by inducing helix stretching and rotation prompting an asymmetric kinase-competent state. Overall, the known structural changes of the sensor kinase, as well as further dynamic rearrangements that we now predict, consistently link structure determinants to activity modulation.

Properties of Langmuir and solid supported lipid films with sphingomyelin

Biological cell membranes play a crucial role in various biological processes and their functionality to some extent is determined by the hydrophilic/hydrophobic balance. A significant progress in understanding the membrane structure was the discovery of laterally segregated lipid domains, called the lipid rafts. These raft domains are of ordered lamellar liquid-crystalline phase, while rest of the membrane exists in a relatively disordered lamellar liquid-crystalline phase. Moreover, the chemical constitution of the lipid rafts consists of a higher content (up to 50%) of cholesterol (Chol) and sphingomyelin (SM). Sphingomyelin also plays a significant role in the red cells of blood and nerves, in some diseases, as a precursor to ceramides, and other sphingolipid metabolites. In this paper properties of Langmuir and solid supported mixed lipid films of DPPC/SM, DOPC/SM, and Chol/SM are described. Special attention has been paid to wetting properties (hydrophobic/hydrophilic balance) of these films transferred onto a hydrophilic glass surface. To our knowledge such results have not yet been published in the literature. The properties were determined via contact angle measurements and then calculation of the films' apparent surface free energy. The films' wettability and their apparent surface free energy strongly depend on their composition. The energy is affected by both the structure of hydrocarbon chains of glycerophospholipids (DPPC and DOPC) and their interactions with SM. Properties of mixed Chol/SM monolayer depend also on the film stoichiometry. At a low Chol content (XChol=0.25) the interactions between SM and Chol are strong and hence the formation of binary complex is possible. This is accompanied by a decrease in the film surface free energy in comparison to that of pure SM monolayer, contrary to a higher Chol content where the monolayer energy increases. This suggests that cholesterol is excluded from the membrane thus increasing the film hydrophilicity. These results are consistent with the literature data and somehow confirm the hypothesis of lipid raft formation. The roughness of the investigated monolayer surfaces was also determined using optical profilometry. The roughness parameters of the DPPC, SM, and mixed DPPC/SM generally correlate with the changes of their apparent surface free energy, i.e. with the decreasing roughness the apparent surface free energy also decreases. However, this is not the case for mixed DOPC/SM monolayers. Although the roughness increases with SM content the apparent surface free energy decreases. Therefore some other factors, like the presence of unsaturated bonds in the DOPC molecule, influence the film phase state and the energy too. More experiments are needed to explain this hypothesis.

Electrospinning deposition of hydrogel fibers used as scaffold for biomembranes. Thermal stability of DPPC corroborated by ellipsometry

DPPC bilayers were deposited over thin hydrogel scaffolds using the Langmuir-Blodgett technique (with DPPC thickness ∼ 6.2 nm). Wrinkled hydrogels films were used to maintain a moist environment in order to enhance DPPC bilayer stability. Polymer mixtures were prepared using HEMA (as a base monomer) and DEGDMA, PEGDA575, PEGDA700 or AAm (as crosslinking agents); a thermal initiator was added to obtain a final pre-hydrogel (oligomer) with an adequate viscosity for thin film formation. This mixture was deposited as wrinkled film/fibers over hydrophilic silicon wafers using an electrospinning technique. Later, these samples were exposed to UV light to trigger photopolymerization, generating crosslinking bonds between hydrogel chains; this process also generated remnant surface stresses in the films that favored wrinkle formation. In the cases where DEGDMA and AAm were used as crosslinking agents, HEMA was added in higher amounts. The resultant polymer film surface showed homogenous layering with some small isolated clusters. If PEGDA575/700 was used as the crosslinking agent, we observed the formation of polymer wrinkled thin films, composed by main and secondary chains (with different dimensions). Moreover, water absorption and release was found to be mediated through surface morphology, ordering and film thickness. The thermal behavior of biomembranes was examined using ellipsometry techniques under controlled heating cycles, allowing phases and phase transitions to be detected through slight thickness variations with respect to temperature. Atomic force microscopy was used to determinate surface roughness changes according to temperature variation, temperature was varied sufficiently for the detection and recording of DPPC phase limits. Contact angle measurements corroborated and quantified system wettability, supporting the theory that wrinkled hydrogel films act to enhance DPPC bilayer stability during thermal cycles.

Static and Dynamic Microscopy of the Chemical Stability and Aggregation State of Silver Nanowires in Components of Murine Pulmonary Surfactant

The increase of production volumes of silver nanowires (AgNWs) and of consumer products incorporating them may lead to increased health risks from occupational and public exposures. There is currently limited information about the putative toxicity of AgNWs upon inhalation and incomplete understanding of the properties that control their bioreactivity. The lung lining fluid (LLF), which contains phospholipids and surfactant proteins, represents a first contact site with the respiratory system. In this work, the impact of dipalmitoylphosphatidylcholine (DPPC), Curosurf, and murine LLF on the stability of AgNWs was examined. Both the phospholipid and protein components of the LLF modified the dissolution kinetics of AgNWs, due to the formation of a lipid corona or aggregation of the AgNWs. Moreover, the hydrophilic proteins, but neither the hydrophobic surfactant proteins nor the phospholipids, induced agglomeration of the AgNWs. Finally, the generation of a secondary population of nanosilver was observed and attributed to the reduction of Ag(+) ions by the surface capping of the AgNWs. Our findings highlight that combinations of spatially resolved dynamic and static techniques are required to develop a holistic understanding of which parameters govern AgNW behavior at the point of exposure and to accurately predict their risks on human health and the environment.

Thin and ordered hydrogel films deposited through electrospinning technique; a simple and efficient support for organic bilayers

Thermal behavior of Dipalmitoylphosphatidylcholine (DPPC) bilayers deposited over hydrogel fibers was examined. Thus, membrane stability, water absorption-release, phase transitions and phase transition temperatures were studied through different methods during heating cycles. Hydrogel films were realized using an oligomer mixture (HEMA-PEGDA575/photo-initiator) with adequate viscosity. Then, the fibers were deposited over silicon wafers (hydrophilic substrate) through electrospinning technique using four different voltages: 15, 20, 25 and 30 kV. The films were then exposed to UV light, favoring polymer chain crosslinking and interactions between hydrogel and substrate. For samples deposited at 20 and 25 kV, hierarchical wrinkle folds were observed at surface level, their arrangement distribution depends directly on thickness and associated point defects. DPPC bilayers were then placed over hydrogel scaffold using Langmuir-Blodgett technique. Field emission scanning electron microscopy (FE-SEM) analysis were used to investigate sample surface, micrographies show homogeneous layer formation with chain polymer order/disorder related to applied voltage during hydrogel deposition process, among other parameters. According to the results obtained, it is possible to conclude that the oligomer deposited at 20 kV produce thin homogenous films (~40 nm) with enhanced ability to absorb water and release it in a controlled way during heating cycles. These scaffold properties confer to DPPC membrane thermal stability, which allow an easy detection of phase(s) and phase transitions. Thermal behavior was also studied via Atomic Force Microscopy (roughness analysis). Contact angle measurements corroborate system wettability, supporting the theory that hydrogel thin films act as DPPC membrane enhancers for thermal stability against external stimuli.

Influence of polar and nonpolar carotenoids on structural and adhesive properties of model membranes

Carotenoids, which are known primarily for their photoprotective and antioxidant properties, may also strongly influence the physical properties of membranes. The localization and orientation of these pigments in the lipid bilayer depends on their structure and is determined by their interactions with lipid molecules. This affects both phase behavior and the mechanical properties of membranes. Differential scanning calorimetry (DSC) and atomic force microscopy (AFM) allowed us to gain a direct insight into the differences between the interaction of the non-polar β-carotene and polar zeaxanthin embedded into DPPC liposomes. DSC results showed that zeaxanthin, having polar ionone rings, interacts more strongly with the membrane lipids than β-carotene. The decrease in molar heat capacity by a factor of 2 with a simultaneous broadening of the main phase transition (gel-to-liquid crystalline phase transition) as compared to the two other systems studied suggests some increased length of the coupled interactions between the polar xanthophyll and lipids. Long-distance interactions lead to the formation of larger clusters which may exhibit higher flexibility than small clusters when only short-distance interactions occur. AFM experiments show that adhesive forces are 2 and 10 times higher for DPPC membranes enriched in β-carotene and zeaxanthin, respectively, than those observed for an untreated system. Temperature dependent measurements of adhesion revealed that subphases can be formed in the gel lamellar state of DPPC bilayers. The presence of the non-polar carotenoid enhanced the effect and even a bifurcation of the substates was detected within a temperature range of 30.0-32.5°C prior to pretransition. It is the first time when the presence of subphases has been demonstrated. This knowledge can be helpful in better understanding the functioning of carotenoids in biological membranes. AFM seem to be a very unique and sensitive method for detecting such fine changes in the lipid bilayers.

Mechanics of lipid bilayers: What do we learn from pore-spanning membranes?

The mechanical properties of biological membranes have become increasingly important not only from a biophysical viewpoint but also as they play a substantial role in the information transfer in cells and tissues. This minireview summarizes some of our recent understanding of the mechanical properties of artificial model membranes with particular emphasis on membranes suspending an array of pores, so called pore-spanning membranes. A theoretical description of the mechanical properties of these membranes might pave the way to biophysically describe and understand the complex behavior of native biological membranes. This article is part of a Special Issue entitled: Mechanobiology.

Asymmetric lipid membranes: towards more realistic model systems

Despite the ubiquity of transbilayer asymmetry in natural cell membranes, the vast majority of existing research has utilized chemically well-defined symmetric liposomes, where the inner and outer bilayer leaflets have the same composition. Here, we review various aspects of asymmetry in nature and in model systems in anticipation for the next phase of model membrane studies.

Thermosensing via transmembrane protein-lipid interactions

Cell membranes are composed of a lipid bilayer containing proteins that cross and/or interact with lipids on either side of the two leaflets. The basic structure of cell membranes is this bilayer, composed of two opposing lipid monolayers with fascinating properties designed to perform all the functions the cell requires. To coordinate these functions, lipid composition of cellular membranes is tailored to suit their specialized tasks. In this review, we describe the general mechanisms of membrane-protein interactions and relate them to some of the molecular strategies organisms use to adjust the membrane lipid composition in response to a decrease in environmental temperature. While the activities of all biomolecules are altered as a function of temperature, the thermosensors we focus on here are molecules whose temperature sensitivity appears to be linked to changes in the biophysical properties of membrane lipids. This article is part of a Special Issue entitled: Lipid-protein interactions.

Mechanisms of membrane binding of small GTPase K-Ras4B farnesylated hypervariable region

K-Ras4B belongs to a family of small GTPases that regulates cell growth, differentiation and survival. K-ras is frequently mutated in cancer. K-Ras4B association with the plasma membrane through its farnesylated and positively charged C-terminal hypervariable region (HVR) is critical to its oncogenic function. However, the structural mechanisms of membrane association are not fully understood. Here, using confocal microscopy, surface plasmon resonance, and molecular dynamics simulations, we observed that K-Ras4B can be distributed in rigid and loosely packed membrane domains. Its membrane binding domain interaction with phospholipids is driven by membrane fluidity. The farnesyl group spontaneously inserts into the disordered lipid microdomains, whereas the rigid microdomains restrict the farnesyl group penetration. We speculate that the resulting farnesyl protrusion toward the cell interior allows oligomerization of the K-Ras4B membrane binding domain in rigid microdomains. Unlike other Ras isoforms, K-Ras4B HVR contains a single farnesyl modification and positively charged polylysine sequence. The high positive charge not only modulates specific HVR binding to anionic phospholipids but farnesyl membrane orientation. Phosphorylation of Ser-181 prohibits spontaneous farnesyl membrane insertion. The mechanism illuminates the roles of HVR modifications in K-Ras4B targeting microdomains of the plasma membrane and suggests an additional function for HVR in regulation of Ras signaling.

An investigation of the carbon nanotube--Lipid interface and its impact upon pulmonary surfactant lipid function

Multiwalled carbon nanotubes (MWCNTs) are now synthesized on a large scale, increasing the risk of occupational inhalation. However, little is known of the MWCNT-pulmonary surfactant (PS) interface and its effect on PS functionality. The Langmuir-Blodgett trough was used to evaluate the impact of MWCNTs on fundamental properties of PS lipids which influence PS function, i.e. compression resistance and maximum obtainable pressure. Changes were found to be MWCNT length-dependent. 'Short' MWCNTs (1.1 μm, SD = 0.61) penetrated the lipid film, reducing the maximum interfacial film pressure by 10 mN/m (14%) in dipalmitoylphosphatidylcholine (DPPC) and PS, at an interfacial MWCNT-PS lipid mass ratio range of 50:1 to 1:1. 'Long' commercial MWCNTs (2.1 μm, SD = 1.2) caused compression resistance at the same mass loadings. 'Very long' MWCNTs (35 μm, SD = 19) sequestered DPPC and were squeezed out of the DPPC film. High resolution transmission electron microscopy revealed that all MWCNT morphologies formed DPPC coronas with ordered arrangements. These results provide insight into how nanoparticle aspect ratio affects the interaction mechanisms with PS, in its near-native state at the air-water interface.

Hybrid copolymer-phospholipid vesicles: phase separation resembling mixed phospholipid lamellae, but with mechanical stability and control

Vesicles whose bilayer membranes contain phospholipids mixed with co-polymers or surfactants comprise new hybrid materials having potential applications in drug delivery, sensors, and biomaterials. Here we describe a model polymer-phospholipid hybrid membrane system exhibiting strong similarities to binary phospholipid mixtures, but with more robust membrane mechanics. A lamella-forming graft copolymer, PDMS-co-PEO (polydimethylsiloxane-co-polyethylene oxide) was blended with a high melting temperature phospholipid, DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), over a broad compositional range. The resulting giant hybrid unilamellar vesicles were compared qualitatively and quantitatively to analogous mixed phospholipid membranes in which a low melting temperature phospholipid, DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine), was blended with DPPC. The mechanical properties of the hybrid vesicles, even when phase separated, were robust with high lysis stresses and strains approaching those of the pure copolymer vesicles. The temperature-composition phase diagram of the hybrid vesicles closely resembled that of the mixed phospholipids; with only slightly greater nonidealities in the hybrid compared with DOPC/DPPC mixed membranes. In both systems, it was demonstrated that tension could be used to manipulate DPPC solidification into domains of patchy or striped morphologies that exhibited different tracer incorporation. The patch and stripe-shaped domains are thought to be different solid DPPC polymorphys: ripple and tilt (or gel). This work demonstrates that in mixed-phospholipid bilayers where a high-melting phospholipid solidifies on cooling, the lower-melting phospholipid may be substituted by an appropriate copolymer to improve mechanical properties while retaining the underlying membrane physics.

Contribution of temperature to deformation of adsorbed vesicles studied by nanoplasmonic biosensing

With increasing temperature, biological macromolecules and nanometer-sized aggregates typically undergo complex and poorly understood reconfigurations, especially in the adsorbed state. Herein, we demonstrate the strong potential of using localized surface plasmon resonance (LSPR) sensors to address challenging questions related to this topic. By employing an LSPR-based gold nanodisk array platform, we have studied the adsorption of sub-100-nm diameter 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid vesicles on titanium oxide at two temperatures, 23 and 50 °C. Inside this temperature range, DPPC lipid vesicles undergo the gel-to-fluid phase transition accompanied by membrane area expansion, while DOPC lipid vesicles remain in the fluid-phase state. To interpret the corresponding measurement results, we have derived general equations describing the effect of deformation of adsorbed vesicles on the LSPR signal. At the two temperatures, the shape of adsorbed DPPC lipid vesicles on titanium oxide remains nearly equivalent, while DOPC lipid vesicles become less deformed at higher temperature. Adsorption and rupture of DPPC lipid vesicles on silicon oxide were also studied for comparison. In contrast to the results obtained on titanium oxide, adsorbed vesicles on silicon oxide become more deformed at higher temperature. Collectively, the findings demonstrate that increasing temperature may ultimately promote, hinder, or have negligible effect on the deformation of adsorbed vesicles. The physics behind these observations is discussed, and helps to clarify the interplay of various, often hidden, factors involved in adsorption of biological macromolecules at interfaces.

Lipid exchange and transfer on nanoparticle supported lipid bilayers: effect of defects, ionic strength, and size

Lipid exchange/transfer has been compared for zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (DMPC) small unilamellar vesicles (SUVs) and for the same lipids on silica (SiO2) nanoparticle supported lipid bilayers (NP-SLBs) as a function of ionic strength, temperature, temperature cycling, and NP size, above the main gel-to-liquid crystal phase transition temperature, Tm, using d- and h-DMPC and DPPC. Increasing ionic strength decreases the exchange kinetics for the SUVs, but more so for the NP-SLBs, due to better packing of the lipids and increased attraction between the lipid and support. When the NP-SLBs (or SUVs) are cycled above and below Tm, the exchange rate increases compared with exchange at the same temperature without cycling, for similar total times, suggesting that defects provide sites for more facile removal and thus exchange of lipids. Defects can occur: (i) at the phase boundaries between coexisting gel and fluid phases at Tm; (ii) in bare regions of exposed SiO2 that form during NP-SLB formation due to mismatched surface areas of lipid and NPs; and (iii) during cycling as the result of changes in area of the lipids at Tm and mismatched thermal expansion coefficient between the lipids and SiO2 support. Exchange rates are faster for NP-SLBs prepared with the nominal amount of lipid required to form a NP-SLB compared with NP-SLBs that have been prepared with excess lipids to minimize SiO2 patches. Nanosystems prepared with equimolar mixtures of NP-SLBs composed of d-DMPC (d(DMPC)-NP-SLB) and SUVs composed of h-DMPC (h(DMPC)-SUV) show that the calorimetric transition of the "donor" h(DMPC)-SUV decreases in intensity without an initial shift in Tm, indicating that the "acceptor" d(DMPC)-NP-SLB can accommodate more lipids, through either further fusion or insertion of lipids into the distal monolayer. Exchange for d/h(DMPC)-NP-SLB is in the order 100 nm SiO2 > 45 nm SiO2 > 5 nm SiO2.