Zwitterionic Head Research Articles

Learning Force Field Parameters from Differentiable Particle-Field Molecular Dynamics.

We develop ∂-HylleraasMD (∂-HyMD), a fully end-to-end differentiable molecular dynamics software based on the Hamiltonian hybrid particle-field formalism, and use it to establish a protocol for automated optimization of force field parameters. ∂-HyMD is templated on the recently released HylleraaasMD software, while using the JAX autodiff framework as the main engine for the differentiable dynamics. ∂-HyMD exploits an embarrassingly parallel optimization algorithm by spawning independent simulations, whose trajectories are simultaneously processed by reverse mode automatic differentiation to calculate the gradient of the loss function, which is in turn used for iterative optimization of the force-field parameters. We show that parallel organization facilitates the convergence of the minimization procedure, avoiding the known memory and numerical stability issues of differentiable molecular dynamics approaches. We showcase the effectiveness of our implementation by producing a library of force field parameters for standard phospholipids, with either zwitterionic or anionic heads and with saturated or unsaturated tails. Compared to the all-atom reference, the force field obtained by ∂-HyMD yields better density profiles than the parameters derived from previously utilized gradient-free optimization procedures. Moreover, ∂-HyMD models can predict with good accuracy properties not included in the learning objective, such as lateral pressure profiles, and are transferable to other systems, including triglycerides.

Characterisation of the apparent aqueous solubility enhancement of testosterone analogues in micelles of dodecyl-chained surfactants with different headgroups

The solubilisation capacities of the micelles formed by a range of surfactants possessing a dodecyl (C12) hydrocarbon ‘tail’ and a variety of hydrophilic ‘headgroups’ have been studied for a range of hydrophobic steroids (testosterone (T) and its propionate (TP) and enanthate (TE) esters). The solubilisation studies were performed at ambient temperature, with drug concentration monitored (over 48–72 h) by UV absorbance. The various surfactant systems were characterised in terms of their physicochemical properties, including their surface tension, critical micelle concentration, viscosity, and density. Results show that while the apparent aqueous solubility of the poorly water-soluble steroids increases upon solubilisation within the surfactant micelles, the extent of this increase varies with the nature of the hydrophilic headgroup and the hydrophobicity of the drug. Specifically, changing the surfactant headgroup from non-ionic to zwitterionic, then to ionic, results in a marked increase in solubility of the two least hydrophobic steroids, testosterone and testosterone propionate, suggesting rather counter-intuitively that these steroidal drugs tend to associate to a greater extent with micelles formed by surfactants with more hydrophilic/charged head groups. The more hydrophobic steroid, TE, was solubilised to the greatest extent, being predominantly solubilised within the micelle core. In this case, the influence of surfactant head group on TE solubilisation was less obvious, with surfactants containing ionic and zwitterionic head groups exhibiting high levels of solubilisation. The physicochemical properties that had most influence on solubilisation of the testosterone analogues, as evaluated using multiple linear regression, were micelle shape for T and TP, and critical micelle concentration for TE. Through the findings of this comparative study, the essential molecular descriptors of optimal solubilisation for improved drug delivery and the loci of solubilisation of the testosterone analogues have been identified.

Cationic lipid membranes condense double-strand DNA through ion bridging.

Cationic lipid (CL)-DNA complexes have attracted much attention due to their potential application in somatic gene therapy, such as nonviral transfection vectors. The rational design of efficient synthetic vectors requires a systematic understanding of the underlying mechanisms governing CL-DNA assembly. Experimental studies show that DNA double strands form assemblies in divalent cations when confined between CL lipid systems. The physical mechanism of membrane-induced condensation is not well understood. Herein, we present a computational study of a simplified vector system, quasi-random genomic DNA chains intercalated into DOPC/DOTAP membrane bilayers. Using all-atom molecular dynamics simulations and free energy calculations, we observe attraction between dsDNA pairs when the ionic strength of the solution and mole fraction of DOPC mimic experimental conditions. Strikingly, the same salt conditions favoring condensation in CL lead to repulsion without the lipid. We investigated the role of lipids and confined cations to understand the differences in DNA-DNA interactions. Simulations suggest that the head groups of the lipids in contact with the DNA backbone reduce the DNA charges. The cationic DOTAP and zwitterionic DOPC head groups form direct contact with the DNA backbone. In addition, the DOPC phosphate head groups are glued to the DNA backbone by magnesium cations. The already reduced opposite surface of DNA double strands are brought together with dynamically bridging magnesium cations confined between the two helices. Our study offers physical insight into the mechanism of CL-DNA assembly and may help to facilitate the development of liposome transfer vectors for gene delivery.

Regulation of Gramicidin Channel Function Solely by Changes in Lipid Intrinsic Curvature.

Membrane protein function is regulated by the lipid bilayer composition. In many cases the changes in function correlate with changes in the lipid intrinsic curvature (c0), and c0 is considered a determinant of protein function. Yet, water-soluble amphiphiles that cause either negative or positive changes in curvature have similar effects on membrane protein function, showing that changes in lipid bilayer properties other than c0 are important—and may be dominant. To further investigate the mechanisms underlying the bilayer regulation of protein function, we examined how maneuvers that alter phospholipid head groups effective “size”—and thereby c0—alter gramicidin (gA) channel function. Using dioleoylphospholipids and planar bilayers, we varied the head groups’ physical volume and the electrostatic repulsion among head groups (and thus their effective size). When 1,2-dioleyol-sn-glycero-3-phosphocholine (DOPC), was replaced by 1,2-dioleyol-sn-glycero-3-phosphoethanolamine (DOPE) with a smaller head group (causing a more negative c0), the channel lifetime (τ) is decreased. When the pH of the solution bathing a 1,2-dioleyol-sn-glycero-3-phosphoserine (DOPS) bilayer is decreased from 7 to 3 (causing decreased head group repulsion and a more negative c0), τ is decreased. When some DOPS head groups are replaced by zwitterionic head groups, τ is similarly decreased. These effects do not depend on the sign of the change in surface charge. In DOPE:DOPC (3:1) bilayers, pH changes from 5→9 to 5→0 (both increasing head group electrostatic repulsion, thereby causing a less negative c0) both increase τ. Nor do the effects depend on the use of planar, hydrocarbon-containing bilayers, as similar changes were observed in hydrocarbon-free lipid vesicles. Altering the interactions among phospholipid head groups may alter also other bilayer properties such as thickness or elastic moduli. Such changes could be excluded using capacitance measurements and single channel measurements on gA channels of different lengths. We conclude that changes gA channel function caused by changes in head group effective size can be predicted from the expected changes in c0.

Hydrophobicity and Charge Distribution Effects in the Formation of Bioorganoclays

Interactions of bioorganic moieties with clay minerals have attracted attention not only from the perspective of novel bioclay materials but also because they play a crucial role in our understanding of physical and chemical processes in soils. The aim of the present article is to explore the interactions responsible for the formation of a phosphatidylcholine-kaolinite bioclay by employing a series of classical molecular dynamic simulations. Detailed analysis of the structure and energies of the resulting bioclays reveals that the phosphatidylcholine molecules bind to the kaolinite surface either via their zwitterionic heads or hydrophobic aliphatic tails, depending on the kaolinite surface characteristics and the density of organic coating. The phosphatidylcholine molecules have a tendency to form irregular layers with a preferred parallel orientation of molecules with respect to the kaolinite surface. The tails exhibit varying degrees of flexibility and disorder depending on their distance from the surface and the density of surface coating. Significant differences in the binding can be spotted with respect to the two types of kaolinite basal surfaces, i.e., the hydrophobic siloxane surface, which possesses a considerable dispersion character, and the hydrophilic alumina surface, polarized by the surface hydroxyl groups.

A selective Nile Red based solvatochromic probe: A study of fluorescence in LUVs and GUVs model membranes

In this work, a membrane probe (NR18) with a Nile Red-based chromophore, a long tail of 18 carbon atoms and a zwitterionic head was synthesized. The probe is optimized to remain anchored to the phospholipid bilayer without internalizing. NR18 was fully characterized chemically and optically and showed marked solvatochromism. Incorporation into model membranes of LUVs and GUVs having different compositions, demonstrated that the probe could be used to discriminate the lipid order in membranes, based on the fluorescent radiation emitted. Spectrophotometric and fluorimetric measurements on LUVs showed that the emission wavelength increases with temperature, confirming the increase in fluidity and hydration in membranes at higher temperatures. DFT simulations helped us to clarify the impact of NR18 on the membrane thickness and the order parameter showing that NR18 does not cause substantial perturbation in the bilayer. Moreover, we demonstrated that the presence of an enaminic terminal group is mainly responsible for the ability of NR18 to have a different fluorescence in Ld and Lo membranes.

Mechanical and Electrical Interaction of Biological Membranes with Nanoparticles and Nanostructured Surfaces.

In this review paper, we theoretically explain the origin of electrostatic interactions between lipid bilayers and charged solid surfaces using a statistical mechanics approach, where the orientational degree of freedom of lipid head groups and the orientational ordering of the water dipoles are considered. Within the modified Langevin Poisson–Boltzmann model of an electric double layer, we derived an analytical expression for the osmotic pressure between the planar zwitterionic lipid bilayer and charged solid planar surface. We also show that the electrostatic interaction between the zwitterionic lipid head groups of the proximal leaflet and the negatively charged solid surface is accompanied with a more perpendicular average orientation of the lipid head-groups. We further highlight the important role of the surfaces’ nanostructured topography in their interactions with biological material. As an example of nanostructured surfaces, we describe the synthesis of TiO2 nanotubular and octahedral surfaces by using the electrochemical anodization method and hydrothermal method, respectively. The physical and chemical properties of these nanostructured surfaces are described in order to elucidate the influence of the surface topography and other physical properties on the behavior of human cells adhered to TiO2 nanostructured surfaces. In the last part of the paper, we theoretically explain the interplay of elastic and adhesive contributions to the adsorption of lipid vesicles on the solid surfaces. We show the numerically predicted shapes of adhered lipid vesicles corresponding to the minimum of the membrane free energy to describe the influence of the vesicle size, bending modulus, and adhesion strength on the adhesion of lipid vesicles on solid charged surfaces.

Abstract P-38: Tunable Soft Networks of Wormlike Micelles and Clay Particles

Background: Over the past few decades, there has been a great deal of interest in the aqueous self-assembly of surfactant molecules into giant wormlike micelles (WLMs). These cylindrical aggregates undergo reversible breakdown processes and in favorable cases can grow up to few tens of micrometers that is comparable with the length of high molecular weight polymer. The viscoelastic properties of WLMs can be easily modified by different additives like salts or polymers. A new emerging research area consists of tuning the WLM solution properties by inorganic nanoparticles. It suggests, in particular, the use of networks of entangled WLMs as a matrix for producing soft nanocomposites with different kinds of embedded nanoparticles that are promising for controlled release, template synthesis, and oilfield applications. These materials can combine adaptive rheological properties of the WLM matrix and the functionality of nanofiller. Methods: Rheometry and cryo-transmission electron microscopy were combined to investigate the structure and properties of mixed WLMs of zwitterionic oleylamidopropyl dimethyl betaine and anionic sodium dodecyl sulfate surfactants and platelike particles of bentonite clay. Results: This system demonstrates the formation of giant linear long-lived WLMs, which even at extremely low surfactant concentrations reach a sufficient length to entangle with each other and form a temporally persistent network. The stability of these micelles can be due to electrostatic attraction between the headgroups of the anionic and zwitterionic surfactants and favorable volume/length hydrophobic ratio in the surfactant mixture. At increasing surfactant concentration, the long-lived linear micelles transform into fast-breaking branched micelles. Stable viscoelastic suspensions of clay particles in semi-dilute solutions of WLM were elaborated. They represent a novel type of soft nanocomposite with the tunable matrix. Structural studies revealed that the clay is dispersed in a dense network of entangled WLM in the form of 100-nm tactoids. Rheological investigations demonstrated that clay particles can induce an increase of viscosity and relaxation time by up to one order of magnitude. The effect of the clay becomes more pronounced with increasing content of anionic surfactant, when the micelles become branched. This behavior was explained by the stabilization of micelle-nanoclay junction points due to the screening of the repulsion between positively charged fragments of zwitterionic head groups by added anionic surfactant. Conclusion: The pronounced effect of nanoparticles on the viscoelasticity of the network formed by branched WLMs was observed for the first time. The nanoparticles-WLM junctions were confirmed by cryo-TEM data. The elaborated systems are of interest for many industrial applications.

Neutron reflectometry study of the interface between two immiscible electrolyte solutions: Effects of electrolyte concentration, applied electric field, and lipid adsorption.

The properties of lipid monolayers at the interface between two immiscible electrolyte solutions (ITIES) have attracted much attention over the last 30 years. This is mainly because of the biological relevance of lipids and the possibility of controlling ion and electron transfer across one leaflet of a cellular membrane. In the last decade, the electrochemical characterization of phosphatidylcholine (PC) adsorbed monolayers at ITIES suggested that the transfer of aqueous cations across the interface is facilitated by the complexation of aqueous cations with the PC zwitterionic head groups, followed by the depletion of lipids from the interface. In this work, we present a study on the effects of applied electric fields and electrolyte concentration on the interfacial structure of the ITIES by combining neutron reflectometry (NR) and electrochemical characterization techniques including the effects of an adsorbed lipid layer. Our results confirm that lipid depletion occurs as cations are transferred. However, we found that the presence of lipids favors the intermixing of the two-liquid phases on a length scale of a few tens of nanometers. To our knowledge, this has been the first NR-electrochemistry study of the ITIES. We believe that our findings could open new possibilities for coupling bioelectrochemical characterization and scattering based techniques at the liquid-liquid interface.

The biophysical interaction of ferulic acid with liposomes as biological membrane model: The effect of the lipid bilayer composition

Molecules able to prevent lipid peroxidation have attracted attention due to their potential to act against lipid peroxidation-induced disorders including cancer and neurological diseases, such as Alzheimer's disease. Ferulic acid (FA) is a phenolic compound with antioxidant properties but in vivo evidence remains limited. Since the interaction of drugs with biological membranes plays a key role on their therapeutic activity, the biophysical interaction between FA and mimetic cell membrane models was addressed in this study. Liposomes with increasing levels of complexity composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), DMPC:cholesterol (CHOL) [85:15] and DMPC:CHOL:sphingomyelin (SM) [75:15:10] were used as in vitro biomembrane models. Electrophoretic light scattering technique was used to assess the effect of FA on the membrane surface charge. Derivative UV–Vis spectrophotometry, dynamic light scattering, and fluorescence quenching were the techniques chosen to determine the FA's partition into the bilayer, the effect of FA on membrane fluidity and the location of FA within the membrane, respectively. The results suggest that FA is located near to the membranes' surface interacting electrostatically with the zwitterionic polar heads of phospholipids. Also, the FA's affinity to the models is highly dependent on the lipid composition, with a significant reduction of FA's partition with the increase of the membranes' complexity. As consequence, FA only affected the membrane fluidity of the simplest model (DMPC) by increasing its rigidity, with no effect observed in rigid membranes (DMPC:CHOL and DMPC:CHOL:SM). Thus, the results indicate that the FA-membrane interactions depend on the lipid composition, complexity, and physical state of membranes.

Lipid headgroups alter huntingtin aggregation on membranes

Huntington's Disease is a fatal neurodegenerative disorder caused by expansion of a glutamine repeat region (polyQ) beyond a critical threshold within exon1 of the huntingtin protein (htt). As a consequence of polyQ expansion, htt associates into a variety of aggregate species that are thought to underlie cellular toxicity. Within cells, htt associates with numerous membranous organelles and surfaces that exert influence on the aggregation process. In particular, the first 17 amino acids at the N-terminus of htt (Nt17) serve as a lipid-binding domain that is intrinsically disordered in bulk solution but adopts an amphipathic α-helical structure upon binding membranes. Beyond this, Nt17 is implicated in initiating htt fibrillization. As the interaction between Nt17 and lipid membranes is likely influenced by lipid properties, the impact of lipid headgroups on htt-exon1 aggregation, membrane activity, and the ability to form protein:lipid complexes was determined. Htt-exon1 with a disease-length polyQ domain (46Q) was exposed to lipid vesicles comprised of lipids with either zwitterionic (POPC and POPE) or anionic (POPG and POPS) headgroups. With zwitterionic head groups, large lipid to peptide ratios were required to have a statistically significant impact on htt aggregation. Anionic lipids enhanced htt fibrillization, even at low lipid:protein ratios, and this was accompanied by changes in aggregate morphology. Despite the larger impact of anionic lipids, htt-exon1(46Q) was more membrane active with zwitterionic lipid systems. The ability of Nt17 to form complexes with lipids was also mediated by lipid headgroups as zwitterionic ionic lipids more readily associated with multimeric forms of Nt17 in comparison with anionic lipids. Collectively, these results highlight the complexity of htt/membrane interactions and the resulting impact on the aggregation process.

Geometry and molecular arrangement of phosphatidylcholine-montmorillonite bioclays via classical molecular dynamics simulation

Inspired by nature, a class of new perspective materials (here referred to as bioclays) can be prepared by combining clay minerals with charged bioorganic moieties. Here we present a comprehensive computational study of a phosphatidylcholine-montmorillonite bioclay composite by employing a series of classical molecular dynamics simulations. Our detailed analysis of the structure and energies of the resulting bioclays reveals that the phosphatidylcholine molecules bind to the montmorillonite surface through their zwitterionic heads, forming layers with the aliphatic tails stretched preferably parallel to the montmorillonite surface. The tails exhibit varying degrees of flexibility and disorder depending on their distance from the surface and density of the surface coverage. The observed detachment of naturally occurring Na+ cations caused by the presence of the phosphatidylcholine zwitterionic heads suggests that cation-exchange is very likely the driving mechanism for the phosphatidylcholine adsorption on montmorillonite. Once the first layer of phosphatidylcholine forms (corresponding to a complete surface saturation, estimated as 0.1 μg cm−2), more molecules can still be attached and stabilized by the mutual phosphatidylcholine intermolecular interactions.

A Flavone-Based Solvatochromic Probe with A Low Expected Perturbation Impact on the Membrane Physical State.

The study of the cell membrane is an ambitious and arduous objective since its physical state is regulated by a series of processes that guarantee its regular functionality. Among the different methods of analysis, fluorescence spectroscopy is a technique of election, non-invasive, and easy to use. Besides, molecular dynamics analysis (MD) on model membranes provides useful information on the possibility of using a new probe, following its positioning in the membrane, and evaluating the possible perturbation of the double layer. In this work, we report the rational design and the synthesis of a new fluorescent solvatochromic probe and its characterization in model membranes. The probe consists of a fluorescent aromatic nucleus of a 3-hydroxyflavone moiety, provided with a saturated chain of 18 carbon atoms and a zwitterionic head so to facilitate the anchoring to the polar heads of the lipid bilayer and avoid the complete internalization. It was possible to study the behavior of the probe in GUV model membranes by MD analysis and fluorescence microscopy, demonstrating that the new probe can efficiently be incorporated in the lipid bilayer, and give a color response, thanks to is solvatochromic properties. Moreover, MD simulation of the probe in the membrane supports the hypothesis of a reduced perturbation of the membrane physical state.

Influence of External NaCl Salt on Membrane Rigidity of Neutral DOPC Vesicles.

Sodium chloride (NaCl) is a very common molecule in biotic and abiotic aqueous environments. In both cases, variation of ionic strength is inevitable. In addition to the osmotic variation posed by such perturbations, the question of whether the interactions of monovalent ions Na+ and Cl-, especially with the neutral head groups of phospholipid membranes are impactful enough to change the membrane rigidity, is still not entirely understood. We investigated the dynamics of 1,2-di-(octadecenoyl)-sn-glycero-3-phosphocholine (DOPC) vesicles with zwitterionic neutral head groups in the fluid phase with increasing external salt concentration. At higher salt concentrations, we observe an increase in bending rigidity from neutron spin echo (NSE) spectroscopy and an increase in bilayer thickness from small-angle X-ray scattering (SAXS). We compared different models to distinguish membrane undulations, lipid tail motions, and the translational diffusion of the vesicles. All of the models indicate an increase in bending rigidity by a factor of 1.3-3.6. We demonstrate that even down to t > 10 ns and for Q > 0.07 Å-1, the observed NSE relaxation spectra are influenced by translational diffusion of the vesicles. For t < 5 ns, the lipid tail motion dominates the intermediate dynamic structure factor. As the salt concentration increases, this contribution diminishes. We introduced a time-dependent analysis for the bending rigidity that highlights only a limited Zilman-Granek time window in which the rigidity is physically meaningful.

A comprehensive computational study and simulation of innovative zwitterionic materials for enhanced poly (vinylidene fluoride) membrane hydrophilicity

The goal of this study is to design a novel zwitterionic (ZW)-poly (vinylidene fluoride) (PVDF) membrane with high hydrophilicity potential using the pair interaction energy decomposition analysis (PIEDA) integrated with fragment molecular orbital (FMO) method. In addition, the differential hydration and efficiency of salt rejection of the novel zwitterion and original PVDF were investigated using molecular dynamics simulation (MDS). Within this study computational methods were applied to investigate the performance of zwitterionic moieties derived from three different anionic groups in the ZW head, specifically, carboxylate, sulfonate, and phosphate. This approach was used in addition to the inclusion of a linker between the ZW head and the PVDF backbone, such as trimethyl ammonium groups and hydroxyl group for an increase in PVDF membrane hydrophilicity. The quantum chemical calculations were employed to examine the hydration structure of moieties, the number of hydrogen bonding instances, and hydration free energy. The interactions between the ZW moieties on PVDF membranes with water molecules confirmed that they depended on the charged groups and the chemical groups between charged groups. The results pointed to differences in hydrophilicity, membrane water uptake due to their structural properties depending on the types of anionic groups involved, polar groups between charged groups, and the hydrophilic groups as a linker between charged groups of the zwitterions to the PVDF polymer backbone. The double zwitterionic PMAL®-C8-CB–OH–SB-PVDF was formed through protonated carboxyl group on backbone of copolymer PMAL®-C8, and protonated nitrogen atom of amide group. This double zwitterion showed strong electrostatic interactions between individual water and secondary ammonium and Oxygen of carboxybetaine, compared to PMAL®-C8-OH-SB-PVDF model. The simulated results using MDS confirmed the hydrophilicity of PMAL®-C8-CB–OH–SB-PVDF and showed that the positive and negative centers of zwitterionic polymer chains on PVDF membrane surface can interact with the ions, contributing into the increase of charge density. Our designed hydrophilic zwitterion PVDF membrane, and especially the double zwitterion membrane, is an exciting development that can be used in a broad range of water applications.

Soft nanocomposites based on nanoclay particles and mixed wormlike micelles of surfactants

Stable viscoelastic suspensions of bentonite clay particles in semi-dilute solutions of wormlike micelles of mixed zwitterionic (oleyl amidopropyl dimethyl carboxybetaine) and anionic (sodium dodecyl sulfate) surfactants were elaborated. They represent a novel type of soft nanocomposite with tunable viscoelastic matrix. Structural studies revealed that the clay is dispersed in a dense network of entangled wormlike micelles in the form of 100-nm tactoids. Rheological investigations demonstrated that clay particles can induce an increase of viscosity by up to one order of magnitude. The effect of the clay becomes more pronounced with increasing content of anionic surfactant in the mixture with zwitterionic surfactant, when the micelles become branched. This behavior was explained by the stabilization of micelle-nanoclay junction points due to the screening of the repulsion between positively charged fragments of zwitterionic head groups by added anionic surfactant. As a result of stabilization, the lifetime of surfactant-nanoclay junctions becomes much larger than the breaking time of micelles. The elaborated systems are of interest for many industrial applications.

Characterizing the Locus of a Peripheral Membrane Protein-Lipid Bilayer Interaction Underlying Protein Export Activity in E. coli.

Quantitative characterization of the strength of peripheral membrane protein-lipid bilayer interactions is fundamental in the understanding of many protein targeting pathways. SecA is a peripheral membrane protein that plays a central role in translocating precursor proteins across the inner membrane of E. coli. The membrane binding activity of the extreme N-terminus of SecA is critical for translocase function. Yet, the mechanical strength of the interaction and the kinetic pathways that this segment of SecA experiences when in proximity of an E. coli polar lipid bilayer has not been characterized. We directly measured the N-terminal SecA-lipid bilayer interaction using precision single molecule atomic force microscope (AFM)-based dynamic force spectroscopy. To provide conformational data inaccessible to AFM, we also performed all-atom molecular dynamics simulations and circular dichroism measurements. The N-terminal 10 amino acids of SecA have little secondary structure when bound to zwitterionic lipid head groups, but secondary structure, which rigidifies the lipid-bound protein segment, emerges when negatively charged lipids are present. Analysis of the single molecule protein-lipid dissociation data converged to a well-defined lipid-bound-state lifetime in the absence of force, τ0lipid = 0.9 s, which is well separated from and longer than the fundamental time scale of the secretion process, defined as the time required to translocate a single amino acid residue (∼50 ms). This value of τ0lipid is likely to represent a lower limit of the in vivo membrane-bound lifetime due to factors including the minimal system employed here.

An Atomistic Molecular Dynamics Study of Titanium Dioxide Adhesion to Lipid Bilayers

Titanium dioxide (TiO2) nanoparticles are found in an array of consumer and industrial products, and human exposure to these nanoparticles involves interaction with biological membranes. To understand the effect of the membrane lipid composition on bilayer perturbation by TiO2 we performed all-atom molecular dynamics simulations of nanosized TiO2 interacting with three model single component bilayers differing only in their head group composition: the zwitterionic DOPC, which is overall neutral containing negatively charged phosphate and positively charged choline in its head, DOPG, which is overall anionic containing negatively charged phosphate and neutral glycerol and the anionic DOPS, containing negatively charged phosphate attached to the hydroxyl side-chain of the amino acid, serine containing negatively charged carboxyl and positively charged ammonium. The nanoparticle adheres to all three bilayers causing a negative curvature on their top leaflet. However, the local deformation of DOPG was more pronounced than DOPC and DOPS. The anionic DOPG, which is the thinnest of the three bilayers, interacted most strongly with the TiO2. DOPS has the next strongest interaction, however this bilayer's high bending modulus enables it to resist deformation by the nanoparticle. DOPC has the weakest interaction with the nanoparticle of the three as it has the highest bending modulus and its zwitterionic head groups have strong cohesive interactions amongst themselves. We also observed a non-uniform response of the bilayers: the orientational order of the lipids near the nanoparticle decreases, while that of the lipids away from the nanoparticle increases. The overall thickness and bending modulus of DOPG increased upon contact with the nanoparticle owing to overall stiffening of the bilayer despite local softening, while the average structural and mechanical properties of DOPC and DOPS remain unchanged, which can be explained in part by the greater bilayer bending elasticicty of DOPC and DOPS. The above findings suggest regions of biological membranes populated by anionic lipids with weaker bending elasticity will be more susceptible to perturbation by TiO2 nanoparticles than zwitterionic-rich regions.

Structure of a Tc holotoxin pore provides insights into the translocation mechanism

Tc toxins are modular toxin systems of insect and human pathogenic bacteria. They are composed of a 1.4-MDa pentameric membrane translocator (TcA) and a 250-kDa cocoon (TcB and TcC) encapsulating the 30-kDa toxic enzyme (C terminus of TcC). Binding of Tc toxins to target cells and a pH shift trigger the conformational transition from the soluble prepore state to the membrane-embedded pore. Subsequently, the toxic enzyme is translocated and released into the cytoplasm. A high-resolution structure of a holotoxin embedded in membranes is missing, leaving open the question of whether TcB-TcC has an influence on the conformational transition of TcA. Here we show in atomic detail a fully assembled 1.7-MDa Tc holotoxin complex from Photorhabdus luminescens in the membrane. We find that the 5 TcA protomers conformationally adapt to fit around the cocoon during the prepore-to-pore transition. The architecture of the Tc toxin complex allows TcB-TcC to bind to an already membrane-embedded TcA pore to form a holotoxin. Importantly, assembly of the holotoxin at the membrane results in spontaneous translocation of the toxic enzyme, indicating that this process is not driven by a proton gradient or other energy source. Mammalian lipids with zwitterionic head groups are preferred over other lipids for the integration of Tc toxins. In a nontoxic Tc toxin variant, we can visualize part of the translocating toxic enzyme, which transiently interacts with alternating negative charges and hydrophobic stretches of the translocation channel, providing insights into the mechanism of action of Tc toxins.

Polar Interactions Play an Important Role in the Energetics of the Main Phase Transition of Phosphatidylcholine Membranes.

Conformational changes of membrane proteins are accompanied by deformation in the surrounding lipid bilayer. To gain insight into the energetics of membrane deformation, the phase behavior of dimyristoylphosphatidylcholine (DMPC) membranes in the presence of the dipole potential, ψd, modifiers was investigated by differential scanning calorimetry. 7-Ketocholesterol, which weakens ψd and reduces membrane-perpendicular dipole–dipole repulsion, causes a discrete second peak on the high-temperature side of the main transition, whereas 6-ketocholestanol, which strengthens ψd and increases membrane-perpendicular dipole–dipole repulsion, merely produces a shoulder. Measurements on pure DMPC vesicles showed that the observed temperature profile could not be explained by a single endothermic process, that is, breaking of van der Waals forces between hydrocarbon chains alone. Removal of NaCl from the buffer caused an increase in the main transition temperature and the appearance of an obvious shoulder, implicating polar interactions. Consideration of the phosphatidylcholine (PC) head group dipole moment indicates direct interactions between PC dipoles that are unlikely to account for the additional process. It seems more likely that the breaking of an in-plane hydrogen-bonded network consisting of hydrating water dipoles together with zwitterionic lipid head groups is responsible. The evidence presented supports the idea that the breaking of van der Waals forces between lipid tails required for the main phase transition of PC membranes is coupled to partial breaking of a hydrogen-bonded network at the membrane surface.