Synthetic antimicrobial oligomers induce a composition-dependent topological transition in membranes

Antimicrobial peptides (AMPs) are cationic amphiphiles that comprise a key component of innate immunity. Synthetic analogues of AMPs, such as the family of phenylene ethynylene antimicrobial oligomers (AMOs), recently demonstrated broad-spectrum antimicrobial activity, but the underlying molecular mechanism is unknown. Homologues in this family can be inactive, specifically active against bacteria, or nonspecifically active against bacteria and eukaryotic cells. Using synchrotron small-angle X-ray scattering (SAXS), we show that observed antibacterial activity correlates with an AMO-induced topological transition of small unilamellar vesicles into an inverted hexagonal phase, in which hexagonal arrays of 3.4-nm water channels defined by lipid tubes are formed. Polarized and fluorescence microscopy show that AMO-treated giant unilamellar vesicles remain intact, instead of reconstructing into a bulk 3D phase, but are selectively permeable to encapsulated macromolecules that are smaller than 3.4 nm. Moreover, AMOs with different activity profiles require different minimum threshold concentrations of phosphoethanolamine (PE) lipids to reconstruct the membrane. Using ternary membrane vesicles composed of DOPG:DOPE:DOPC with a charge density fixed at typical bacterial values, we find that the inactive AMO cannot generate the inverted hexagonal phase even when DOPE completely replaces DOPC. The specifically active AMO requires a threshold ratio of DOPE:DOPC = 4:1, and the nonspecifically active AMO requires a drastically lower threshold ratio of DOPE:DOPC = 1.5:1. Since most gram-negative bacterial membranes have more PE lipids than do eukaryotic membranes, our results imply that there is a relationship between negative-curvature lipids such as PE and antimicrobial hydrophobicity that contributes to selective antimicrobial activity.     

Triggered Liposomal Release through a Synthetic Phosphatidylcholine Analogue Bearing a Photocleavable Moiety Embedded within the sn‐2 Acyl Chain

Liposomes represent promising carriers for drug delivery applications. To maximize this potential, there has been significant interest in developing liposomal systems encapsulating molecular cargo that are highly stable until their contents are released remotely in a controlled manner. Herein, we describe the design, synthesis, and analysis of a photocleavable analogue of the ubiquitous lipid phosphoatidylcholine (PC) for the development of highly stable and controllable photodisruptable membranes. Our strategy was to develop a lipid that closely mimics the structure of PC to optimize favorable properties including biocompatibility and stability of subsequent liposomes when mixed with lipids possessing a broad range of physicochemical properties. Thus, NB‐PC was designed, which contains a photocleavable 2‐nitrobenzyl group embedded within the acyl chain at the sn‐2 position. Following the synthesis of NB‐PC , liposome disruption efficacy was evaluated through photolysis studies involving the detection of nile red release. Studies performed using a range of liposomes with different percentages of NB‐PC , PC, phosphatidylethanolamine (PE), cholesterol, and polyethylene glycol‐PE (PEG‐PE) demonstrated minimal background release in controls, release efficacies that correlate directly with the amount of NB‐PC incorporation, and that release is only minimally impacted by the inclusion of the lipids PE and cholesterol that possess disparate properties. These results demonstrate that the NB‐PC system is a highly stable, flexible, and tunable system for photoinitiated release from liposomal systems.     

Membrane activity of biomimetic facially amphiphilic antibiotics

Membranes are a central feature of all biological systems, and their ability to control many cellular processes is critically important. As a result, a better understanding of how molecules bind to and select between biological membranes is an active area of research. Antimicrobial host defense peptides are known to be membrane-active and, in many cases, exhibit discrimination between prokaryotic and eukaryotic cells. The design of synthetic molecules that capture the biological activity of these natural peptides has been shown. In this report, the interaction between our biomimetic structures and different biological membranes is reported using both model vesicle and in vitro bacterial cell experiments. Compound 1 induces 12% leakage at 20 microg/mL against phosphatidylglycerol (PG)-phosphatidylethanolamine (PE) vesicles vs only 3% leakage at 200 microg/mL against phosphatidyl-L-serine (PS)-phosphatidylcholine (PC) vesicles. Similarly, a 40% reduction in fluorescence is measured in lipid movement experiments for PG-PE compared to 10% for PS-PC at 600 s. A 30 degrees C increase in the phase transition of stearoyl-oleoyl-phosphatidylserine is observed in the presence of 1. These results show that lipid composition is more important for selectivity than overall net charge. Additionally, the overall concentration of a given lipid is another important factor. An effort is made to connect model vesicle studies with in vitro data and naturally occurring lipid compositions.     

Profiling and quantification of Drosophila melanogaster lipids using liquid chromatography/mass spectrometry

We present here the findings of global profiling of Drosophila lipids using liquid chromatography/tandem mass spectrometry (LC/MS/MS) on an LTQ-Orbitrap instrument. In addition, we present a multiple reaction monitoring (LC-MRM) method for the absolute quantification of the major phosphatidylethanolamine (PE) and phosphatidylcholine (PC) lipids of Drosophila. Using both normal- and reversed-phase LC followed by accurate mass analysis and MS/MS on an LTQ-Orbitrap instrument, we evaluated the lipid composition of the fruit fly Drosophila melanogaster. A total of 74 lipid species were identified consisting of glycerphospholipids belonging to the PE, PC, phosphatidylglycerol (PG), phosphatidylinositol (PI) and phosphatidylserine (PS) classes including several plasmanyl PE species, as well as triacylglycerides, cardiolipins, ceramides, and PE ceramides. Individual PE and PC phospholipids were then quantified using an LC-MRM approach. Reversed-phase chromatography followed by monitoring on a QTrap 4000 instrument of 21 MRM transitions combined with calibration curves constructed using internal standards enabled the absolute quantification of 28 PE and PC lipid species with limits of quantification of 3 and 5 pg/μL, respectively. Internal standards accounted for the differences in ionization efficiencies of PE and PC phospholipids, facilitating more accurate lipid abundance measurements. The method presented here builds on previous Drosophila work by making the quantification of absolute lipid abundance possible and will be of interest to scientists who study variation and changes in the degree of unsaturation, fatty acid carbon length, and head-group concentration among individuals of different genotypes in response to environmental, genetic, or physiological perturbation in small insects. It will also be particularly useful to biologists interested in adaptation and acclimation of cellular membranes in response to thermal heterogeneity.     

Responses of Transmembrane Peptide and Lipid Chains to Hydrophobic Mismatch

Hydrophobic mismatch between the hydrophobic length of membrane proteins and hydrophobic thickness of membranes is a crucial factor in controlling protein function and assembly. We combined fluorescence with circular dichroism(CD) and attenuated total reflection infrared(ATR-IR) spectroscopic methods to investigate the behaviors of the peptide and lipids under hydrophobic mismatch using a model peptide from the fourth transmembrane domain of natural resistance-associated macrophage protein 1(Nramp1), the phosphatidylcholines(PCs) and phosphatidylglycerols(PGs) with different lengths of acyl chains(14:0, 16:0 and 18:0). In all PG lipid membranes, the peptide forms stable a-helix structure, and the helix axis is parallel to lipid chains. The helical span and orientation hardly change in varying thickness of PG membranes, while the lipid chains can deform to accommodate to the hydrophobic surface of embedded peptide. By comparison, the helical structures of the model peptide in PC lipid membranes are less stable. Upon incorporation with PC lipid membranes, the peptide can deform itself to accommodate to the hydrophobic thickness of lipid membranes in response to hydrophobic mismatch. In addition, hydrophobic mismatch can increase the aggregation propensity of the peptide in both PC and PG lipid membranes and the peptide in PC membranes has more aggregation tendency than that in PG membranes.     

Membrane orientation of MSI-78 measured by sum frequency generation vibrational spectroscopy

Antimicrobial peptides (AMPs) selectively disrupt bacterial cell membranes to kill bacteria whereas they either do not or weakly interact with mammalian cells. The orientations of AMPs in lipid bilayers mimicking bacterial and mammalian cell membranes are related to their antimicrobial activity and selectivity. To understand the role of AMP-lipid interactions in the functional properties of AMPs better, we determined the membrane orientation of an AMP (MSI-78 or pexiganan) in various model membranes using sum frequency generation (SFG) vibrational spectroscopy. A solid-supported single 1,2-dipalmitoyl-an-glycero-3-[phospho-rac-(1-glycerol)] (DPPG) bilayer or 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) bilayer was used as a model bacterial cell membrane. A supported 1,2-dipalmitoyl-an-glycero-3-phosphocholine (DPPC) bilayer or a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer was used as a model mammalian cell membrane. Our SFG results indicate that the helical MSI-78 molecules are associated with the bilayer surface with ～70° deviation from the bilayer normal in the negatively charged gel-phase DPPG bilayer at 400 nM peptide concentration. However, when the concentration was increased to 600 nM, MSI-78 molecules changed their orientation to make a 25° tilt from the lipid bilayer normal whereas multiple orientations were observed for an even higher peptide concentration in agreement with toroidal-type pore formation as reported in a previous solid-state NMR study. In contrary, no interaction between MSI-78 and a zwitterionic DPPC bilayer was observed even at a much higher peptide concentration (～12,000 nM). These results demonstrate that SFG can provide insights into the antibacterial activity and selectivity of MSI-78. Interestingly, the peptide exhibits a concentration-dependent membrane orientation in the lamellar-phase POPG bilayer and was also found to induce toroidal-type pore formation. The deduced lipid flip-flop from SFG signals observed from lipids also supports MSI-78-induced toroidal-type pore formation.     

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

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

Design,synthesis and characterization of artificial phospholipids as model membrane receptors for specific binding of rabbit C-reactive protein

C-reactive protein (CRP) is an acute phase reactive protein, which has been shown to specifically bind to phosphorylcholine (PC) and phosphorylethanoamine (PE) moieties in the presence of calcium. In order to investigate the effect of steric hindrance on the specific binding of CRP to membranes, we designed and synthesized six phospholipids, each containing a long-arm spacer of 3, 6 or 8 atoms between the head group and hydrophobic tail. By mixing synthesized lipids and natural lipids the ligand-containing monolayers were prepared, which have PC or PE groups protruding out of the membrane surface. To characterize of the synthesized phospholipids, the thickness of the lipid monolayers was measured by surface plasmon resonance (SPR) technique, the phase behavior of the lipid monolayer at air/water interface was studied by pressure–area analysis, and the specific binding of rabbit C-reactive protein to the synthesized lipid containing membranes was studied by imaging ellipsometry.     

Criterion for amino acid composition of defensins and antimicrobial peptides based on geometry of membrane destabilization

Defensins comprise a potent class of membrane disruptive antimicrobial peptides (AMPs) with well-characterized broad spectrum and selective microbicidal effects. By using high-resolution synchrotron small-angle X-ray scattering to investigate interactions between heterogeneous membranes and members of the defensin subfamilies, α-defensins (Crp-4), β-defensins (HBD-2, HBD-3), and θ-defensins (RTD-1, BTD-7), we show how these peptides all permeabilize model bacterial membranes but not model eukaryotic membranes: defensins selectively generate saddle-splay ("negative Gaussian") membrane curvature in model membranes rich in negative curvature lipids such as those with phosphoethanolamine (PE) headgroups. These results are shown to be consistent with vesicle leakage assays. A mechanism of action based on saddle-splay membrane curvature generation is broadly enabling, because it is a necessary condition for processes such as pore formation, blebbing, budding, and vesicularization, all of which destabilize the barrier function of cell membranes. Importantly, saddle-splay membrane curvature generation places constraints on the amino acid composition of membrane disruptive peptides. For example, we show that the requirement for generating saddle-splay curvature implies that a decrease in arginine content in an AMP can be offset by an increase in both lysine and hydrophobic content. This "design rule" is consistent with the amino acid compositions of 1080 known cationic AMPs.     

Effect of Headgroups on Small‐Ion Permeability across Archaea‐Inspired Tetraether Lipid Membranes

This paper examines the effects of four different polar headgroups on small‐ion membrane permeability from liposomes comprised of Archaea‐inspired glycerolmonoalkyl glycerol tetraether (GMGT) lipids. We found that the membrane‐leakage rate across GMGT lipid membranes varied by a factor of ≤1.6 as a function of headgroup structure. However, the leakage rates of small ions across membranes comprised of commercial bilayer‐forming 1‐palmitoyl‐2‐oleoyl‐sn‐glycerol (PO) lipids varied by as much as 32‐fold within the same series of headgroups. These results demonstrate that membrane leakage from GMGT lipids is less influenced by headgroup structure, making it possible to tailor the structure of the polar headgroups on GMGT lipids while retaining predictable leakage properties of membranes comprised of these tethered lipids.     

Effects of albumin and erythrocyte membranes on spread monolayers of lung surfactant lipids

Dipalmitoyl phosphatidylcholine (DPPC), one of the main constituents of lung surfactant is mainly responsible for reduction of surface tension to near 0 mN/m during expiration, resisting alveolar collapse. Other unsaturated phospholipids like palmitoyloleoyl phosphatidylglycerol (PG), palmitoyloleoyl phosphatidylcholine (POPC) and neutral lipids help in adsorption of lung surfactant to the air-aqueous interface. Lung surfactant lipids may interact with plasma proteins and hematological agents flooding the alveoli in diseased states. In this study, we evaluated the effects of albumin and erythrocyte membranes on spread films of DPPC alone and mixtures of DPPC with each of PG, POPC, palmitoyloleoyl phosphatidylethanolamine (PE), cholesterol (CHOL) and palmitic acid (PA) in 9:1 molar ratios. Surface tension-area isotherms were recorded using a Langmuir-Blodgett (LB) trough at 37 degrees C with 0.9% saline as the sub-phase. In the presence of erythrocyte membranes, DPPC and DPPC+PA monolayers reached minimum surface tensions of 7.3+/-0.9 and 9.6+/-1.4 mN/m, respectively. Other lipid combinations reached significantly higher minimum surface tensions >18 mN/m in presence of membranes (Newman Keul's test, p<0.05). The relative susceptibility to membrane inhibition was [(DPPC+PG, 7:3)=(DPPC+PG, 9:1)=(DPPC+POPC)=(DPPC+PE)=(DPPC+CHOL)]>[(DPPC+PA)=(DPPC)]. The differential response was more pronounced in case of albumin with DPPC and DPPC+PA monolayers reaching minimum surface tensions less than 2.4 mN/m in presence of albumin, whereas DPPC+PG and DPPC+POPC reached minimum surface tensions of around 20 mN/m in presence of albumin. Descending order of susceptibility of the spread monolayers of lipid mixtures to albumin destabilization was as follows: [(DPPC+PG, 7:3)=(DPPC+PG, 9:1)=(DPPC+POPC)]>[(DPPC+PE)=(DPPC+CHOL)]>[(DPPC+PA)=(DPPC)] The increase in minimum surface tension in presence of albumin and erythrocyte membranes was accompanied by sudden increases in compressibility at surface tensions of 15-30 mN/m. This suggests a monolayer destabilization and could be indicative of phase transitions in the mixed lipid films due to the presence of the hydrophobic constituents of erythrocyte membranes.     

“Doubly Selective” Antimicrobial Polymers: How Do They Differentiate between Bacteria?

We have investigated how doubly selective synthetic mimics of antimicrobial peptides (SMAMPs), which can differentiate not only between bacteria and mammalian cells, but also between Gram‐negative and Gram‐positive bacteria, make the latter distinction. By dye‐leakage experiments on model vesicles and complementary experiments on bacteria, we were able to relate the Gram selectivity to structural differences of these bacteria types. We showed that the double membrane of E. coli rather than the difference in lipid composition between E. coli and S. aureus was responsible for Gram selectivity. The molecular‐weight‐dependent antimicrobial activity of the SMAMPs was shown to be a sieving effect: while the 3000 g mol^?1 SMAMP was able to penetrate the peptidoglycan layer of the Gram‐positive S. aureus bacteria, the 50000 g mol^?1 SMAMP got stuck and consequently did not have antimicrobial activity.     

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

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

Hydrophobic/superhydrophobic oxidized metal surfaces showing negligible contact angle hysteresis

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

Inverse-phosphocholine lipids: a remix of a common phospholipid

Zwitterionic inverse-phosphocholine (iPC) lipids contain headgroups with an inverted charge orientation relative to phosphocholine (PC) lipids. The iPC lipid headgroup has a quaternary amine adjacent to the bilayer interface and a phosphate that extends into the aqueous phase. Neutral iPC lipids with ethylated phosphate groups (CPe) and anionic iPC lipids nonethylated phosphate groups (CP) were synthesized. The surface potential of CPe liposomes remains negative across a broad pH range and in the presence of up to 10 mM Ca(2+). CP liposomes aggregate in the presence of Ca(2+), but at a slower rate than other anionic lipids. Hydrolysis of CP lipids by alkaline phosphatases generates a cationic lipid. CPe liposomes release encapsulated anionic carboxyfluorescein (CF) 20 times faster than PC liposomes and release uncharged glucose twice as fast as PC liposomes. As such, iPC lipids afford a unique opportunity to investigate the biophysical and bioactivity-related ramifications of a charge inversion at the bilayer surface.     

PH effects on drug interactions with lipid bilayers by liposome electrokinetic chromatography

Liposome electrokinetic chromatography (LEKC) provides convenient and rapid methods for studying drug interactions with lipid bilayers using liposomes as a pseudostationary phase. LEKC was used to determine the effects of pH on the partitioning of basic drugs into liposomes composed of zwitterionic phosphatidylcholine (PC), anionic phosphatidylglycerol (PG), and cholesterol, which mimic the composition of natural cell membranes. An increase in pH results in a smaller degree of ionization of the basic drugs and consequently leads to a lower degree of interaction with the negatively charged membranes. From the LEKC retention data, the fractions of drugs distributed in the bulk aqueous and the liposome phase were determined at various pH values. Finally, lipid mediated shifts in the ionization constants of drugs were examined.     

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

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

Analysis of liposomal membrane composition using Raman tweezers

We have developed a methodology for the analysis of liposomal membranes and their contents using near-IR Raman spectroscopy on liposomes held in an optical trap. We were able to detect a variety of membrane components including lipids, cholesterol, and small molecule solutes such as ethanol, DMSO and hexafluoroisopropanol. The methodology is able to distinguish between solutes that equilibrate across the liposomal membrane from those that partition selectively into the lipid bilayer.     

Phase Diagram for a Lysyl-Phosphatidylglycerol Analogue in Biomimetic Mixed Monolayers with Phosphatidylglycerol: Insights into the Tunable Properties of Bacterial Membranes

Ion pairing between the major phospholipids of the Staphylococcus aureus plasma membrane (phosphatidylglycerol – PG and lysyl-phosphatidylglycerol – LPG) confers resistance to antimicrobial peptides and other antibiotics. We developed 3adLPG, a stable synthetic analogue which can substitute for the highy-labile native LPG, in biophysical experiments examining the membrane-protecting role of lipid ion pairing, in S. aureus and other important bacteria. Here we examine the surface charge and lipid packing characteristics of synthetic biomimetic mixtures of DPPG and DP3adLPG in Langmuir monolayers, using a combination of complementary surface-probing techniques such as infrared reflection-absorption spectroscopy and grazing-incidence x-ray diffraction. The resultant phase diagram for the ion paired lipids sheds light on the mixing behavior of lipids in monolayer models of resistant phenotype bacterial membranes, and provides a platform for future biophysical studies.     

Glycolipid membranes through atomistic simulations: effect of glucose and galactose head groups on lipid bilayer properties

Though glycolipids are involved in a multitude of cellular functions, the understanding of their atom-scale properties in lipid membranes has remained very limited due to the lack of atomistic simulations. In this work, we employ extensive simulations to characterize one-component membranes comprised of glycoglycerolipids, focusing on two common glyco head groups, namely glucose and galactose. The properties of these two glycoglycerolipid bilayers are compared in a systematic manner with membranes consisting of phosphatidylcholine (PC) or phosphatidylethanolamine (PE) lipids, whose structures aside from the head group are identical with those of the two glycolipids. We find that the glycolipid systems are characterized by a substantial number of hydrogen bonds in the head group region, leading to membrane packing that is stronger than in a PC but less significant than that in a PE bilayer. The role played by the glyco head group is especially evident in the electrostatic membrane potential, which is particularly large in the glycolipid membranes. For the same reason, the interfacial forces near glycolipid bilayers are significantly different from those found in PC and PE bilayers, affecting, e.g., the ordering of water close to the membrane. These effects are particularly important for the case of galactose, an important component in thylacoids.