Lipid Phase Behavior Research Articles

Evaluating the Impact of the Membrane Thickness on the Function of the Intramembrane Protease GlpG

Cellular membranes exhibit a huge diversity of lipids and membrane proteins that differ in their properties and chemical structure. Cells organize these molecules into distinct membrane compartments characterized by specific lipid profiles and hydrophobic thicknesses of the respective domains. If a hydrophobic mismatch occurs between a membrane protein and the surrounding lipids, there can be functional consequences like reduced protein activity. This phenomenon has been extensively studied for single-pass transmembrane proteins, rhodopsin, and small polypeptides such as gramicidin. Here we investigate the E. coli rhomboid intramembrane protease GlpG as a model to systematically explore the impact of membrane thickness on GlpG activity. We used fully saturated dilauroylpohosphatidylcholine (DLPC) and dimyristoylphosphatidylcholine (DMPC) model lipids and altered membrane thickness by varying the cholesterol content. Physical membrane parameters were determined by 2H and 31P NMR spectroscopy and correlated with GlpG activity measurements in the respective host membranes. Differences in bulk and annular lipids as well as alterations in protein structure in the respective host membranes were determined using molecular dynamics simulations. Our findings indicate that GlpG can influence the membrane thickness in DLPC/cholesterol membranes but not in DMPC/cholesterol membranes. Moreover, we observe that GlpG protease activity is reduced in DLPC membranes at low cholesterol content, which was not observed for DMPC. While a change in GlpG activity can already be due to smallest differences in the lipid environment, potentially enabling allosteric regulation of intramembrane proteolysis, there is no overall correlation to cholesterol-mediated lipid bilayer organization and phase behavior. Additional factors such as the influence of cholesterol on membrane bending rigidity and curvature energy need to be considered. In conclusion, the functionality of α-helical membrane proteins such as GlpG relies not only on hydrophobic matching but also on other membrane properties, specific lipid interaction and the composition of the annular layer.

The phase behavior of skin-barrier lipids: A combined approach of experiments and simulations

Skin barrier function is localized in its outermost layer, the stratum corneum (SC), which is comprised of corneocyte cells embedded in an extracellular lipid matrix containing ceramides (CERs), cholesterol (CHOL), and free fatty acids (FFAs). The unique structure and composition of this lipid matrix are important for skin barrier function. In this study, experiments and molecular dynamics simulation were combined to investigate the structural properties and phase behavior of mixtures containing nonhydroxy sphingosine CER (CER NS), CHOL, and FFA. X-ray scattering for mixtures with varying CHOL levels revealed the presence of the 5.4 nm short periodicity phase in the presence of CHOL. Bilayers in coarse-grained multilayer simulations of the same compositions contained domains with thicknesses of approximately 5.3 and 5.8 nm that are associated with elevated levels, respectively, of CER sphingosine chains with CHOL, and CER acyl chains with FFA chains. The prevalence of the thicker domain increased with decreasing CHOL content. This might correspond to a phase with ∼5.8 nm spacing observed by x-rays (other details unknown) in mixtures with lower CHOL content. Scissoring and stretching frequencies from Fourier transform infrared spectroscopy (FTIR) also indicate interaction between FFA and CER acyl chains and little interaction between CER acyl and CER sphingosine chains, which requires CER molecules to adopt a predominantly extended conformation. In the simulated systems, neighbor preferences of extended CER chains align more closely with the FTIR observations than those of CERs with hairpin ceramide chains. Both FTIR and atomistic simulations of reverse mapped multilayer membranes detect a hexagonal to fluid phase transition between 65 and 80°C. These results demonstrate the utility of a collaborative experimental and simulation effort in gaining a more comprehensive understanding of SC lipid membranes.

Molecular, Crystalline, and Microstructures of Lipids from Astrocaryum Species in Guyana and Their Thermal and Flow Behavior

The phase behavior of lipids extracted from Astrocaryum vulgare (AV) and Astrocaryum aculeatum (AA) pulp and kernels and their microstructural, thermal and flow properties were studied. The lipid profiles, crystal structures, microstructures, thermal stabilities and flow behaviors of these lipids provided important structure–function information that are useful to assess potential applications in the food, cosmetic and pharmaceutical industries. AV and AA fruits were sourced from the lowlands and rainforests, respectively, of Guyana. AV and AA pulp oils (AVP and AAP) were distinguished from each other in composition and unsaturation, with AVP oils being predominated by a di-unsaturated TAG (2-(palmitoyloxy)propane-1,3-diyl dioleate (POO)) and AAP oils predominated by propane-1,2,3-triyl trioleate (OOO); there were unsaturation levels of 65% and 80%, respectively. The main fatty acids in AVP oils were oleic, palmitic and stearic; for AAP, these were oleic, linoleic, palmitic and stearic. The kernel fats of AV and AA were similar in composition and had saturation levels of 80%, being mainly comprised of tri-saturated TAGs propane-1,2,3-triyl tridodecanoate (LLL) and 3-(tetradecanoyloxy)propane-1,2-diyl didodecanoate (LML). The onset of mass loss (T5%on) of AV and AA pulp oils were similar at 328 ± 6 °C, which were 31 °C ± 9 higher compared to that of the kernel fats, which demonstrated similar T5%on = 293 ± 7 °C. AA and AV pulp oils were liquid at room temperature, with melting points of −5 ± 1 °C and 3 ± 1 °C, respectively; both kernel fats were solid at room temperature, packing in β′ (90% of crystals) and β (10% of crystals) polymorphic forms and melting almost identically at 30 ± 1 °C. Pulp oils demonstrated sporadic nucleation at the onset of crystallization with slow growth into rod-shaped crystallites, leading to an approximately 50% degree of crystallization at undercooling of approximately 40K. Nucleation for kernel fats was instantaneous at undercooling of approximately 23K, demonstrating a spherulitic growth pattern incorporating crystalline lamella and a 90% degree of crystallization. Kernel fats and pulp oils demonstrated Newtonian flow behavior and similar dynamic viscosity in the melt, approximately 28.5 mPa·s at 40 °C. The lipid profiles of AVP and AAP oils were dominated by unsaturated TAGs, suggesting potential nutrition and health benefits, particularly compared to other tropical oils with higher saturation levels, such as palm oil. AAP oil in particular is as unsaturated as olive oil, contains high levels of beta carotene and provides a unique flavor profile. The AAK and AVK lipid profiles and phase transformation indicate potential for applications where a high solid fat content and medium-chain fatty acids are required. Their high lauric and myristic acid content makes them similar to industrially important tropical oils (coconut and palm kernel), suggesting their use in similar formulations. The melting point and plasticity of the kernel fats are similar to that of cocoa and shea butters, suggesting use as replacements in cosmetics, foods and confections. There is, however, the need to better understand their nutritional status and effects on health.

Effect of Functional Groups in Lipid Molecules on the Stability of Nanostructured Lipid Carriers: Experimental and Computational Investigations.

Lipid nanoparticles have been used as drug carriers for decades. Many lipid types have been screened for both solid lipid nanoparticles and nanostructured lipid carriers (NLCs). Specifically, for NLCs that are composed of lipids in the solid form mixed with lipids in the liquid form, compatibility of lipid combination and phase behavior play a significant role in the NLC quality. In this study, stearic acid (STA) and cetyl palmitate (CTP) were used as solid lipids, and oleic acid (OLA), isopropyl palmitate (IPP), and caprylic/capric triglycerides were used as liquid lipids. NLCs were prepared at solid:liquid ratios of 50:50, 70:30, and 90:10, respectively. The characteristics and stability of the prepared NLCs were investigated. Laboratory results showed that the solid lipid had a greater influence on the particle size than the liquid lipid. Meanwhile, cetyl palmitate, an ester compound, provides higher NLC stability compared to stearic acid, a carboxylic acid compound. A MARTINI-based coarse-grained molecular dynamics simulation was used to simulate the lipid droplet in water. The distribution of lipid molecules in the droplet was characterized by the polar group density distribution. Different spatial arrangements of the lipid headgroup and lipid molecules, when CTP or STA was used as solid lipids, might contribute to the different stabilities of prepared NLCs. The understanding of mixed lipid systems via simulations will be a significant tool for screening the type of lipids for drug carriers and other pharmaceutical applications.

Secondary Ion Mass Spectrometry of Single Giant Unilamellar Vesicles Reveals Compositional Variability.

Giant unilamellar vesicles (GUVs) are a widely used model system to interrogate lipid phase behavior, study biomembrane mechanics, reconstitute membrane proteins, and provide a chassis for synthetic cells. It is generally assumed that the composition of individual GUVs is the same as the nominal stock composition; however, there may be significant compositional variability between individual GUVs. Although this compositional heterogeneity likely impacts phase behavior, the function and incorporation of membrane proteins, and the encapsulation of biochemical reactions, it has yet to be directly quantified. To assess heterogeneity, we use secondary ion mass spectrometry (SIMS) to probe the composition of individual GUVs using non-perturbing isotopic labels. Both 13C- and 2H-labeled lipids are incorporated into a ternary mixture, which is then used to produce GUVs via gentle hydration or electroformation. Simultaneous detection of seven different ion species via SIMS allows for the concentration of 13C- and 2H-labeled lipids in single GUVs to be quantified using calibration curves, which correlate ion intensity to composition. Additionally, the relative concentration of 13C- and 2H-labeled lipids is assessed for each GUV via the ion ratio 2H-/13C-, which is highly sensitive to compositional differences between individual GUVs and circumvents the need for calibration by using standards. Both quantification methods suggest that gentle hydration produces GUVs with greater compositional variability than those formed by electroformation. However, both gentle hydration and electroformation display standard deviations in composition (n = 30 GUVs) on the order of 1-4 mol %, consistent with variability seen in previous indirect measurements.

Unusual phosphatidylcholine lipid phase behavior in the ionic liquid ethylammonium nitrate

HypothesisThe forces that govern lipid self-assembly ionic liquids are similar to water, but their different balance can result in unexpected behaviour. ExperimentsThe self-assembly behaviour and phase equilibria of two phospholipids, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), in the most common protic ionic liquid, ethylammonium nitrate (EAN) have been investigated as function of composition and temperature by small- and wide-angle X-ray scattering (SAXS/WAXS) and small-angle neutron scattering (SANS). FindingsBoth lipids form unusual self-assembly structures and show complex and unexpected phase behaviour unlike that seen in water; DSPC undergoes a gel Lβ to crystalline Lc phase transition on warming, while POPC forms worm-like micelles L1 upon dilution. This surprising phase behaviour is attributed to the large size of the EAN ions that solvate the lipid headgroup compared to water changing amphiphile packing. Weaker H-bonding between EAN and lipid headgroups also contributes. These results provide new insight for the design of lipid based nanostructured materials in ionic liquids with atypical properties.

Channel-lipid interactions drive the emergence of ion channel clustering, channel-channel cooperativity, and activation hysteresis.

The activation of voltage gated ion channels has been intensely investigated for decades and it is now so well characterized that we have an atomically-detailed description of the major conformational changes taking place at the level of the single molecule. However, this accurate picture cannot accommodate recent experimental observations that highlight a dramatic and so far unsuspected collective behavior: voltage gated ion channels in physiological membranes form clusters and gate cooperatively. Here we present a quantitative theory that bridges these two vastly different length scales and reconciles these seemingly conflicting views. This novel conceptual framework, which is rooted in the statistical physics of immiscible fluids, accounts for the formation of lipid rafts and for the bias they entail on the conformation of the residing ion channels. First, we use tens-of-microseconds MARTINI simulations to show that annular lipids have a distinct compositional bias, which is different for the resting and the activated states. We then use these simulations to estimate channel-lipid interactions and thus parametrize a mesoscopic model. This allows us to explore the spatial organization of ion channels and their response (in the time domain) to changes in the transmembrane potential. We find that channels embedded in membranes close to a miscibility transition develop attractive long-range interactions that give rise to clustering effects. Strikingly, the lipid phase behavior is itself affected by the activation state of the ion channels. Finally, we observed strong activation cooperativity and long time-scales hysteresis effects which can potentially explain the modal gating phenomena observed in many single channel recordings.

Examining compositional variability of giant unilamellar vesicles via secondary ion mass spectrometry.

Giant unilamellar vesicles (GUVs) are a widely used model system to interrogate lipid phase behavior, study biomembrane mechanics, reconstitute membrane proteins, and provide a chassis for synthetic cells. However, there may be significant compositional variability between individual GUVs, either due to lipid sorting during film hydration or from vesicle budding following GUV formation. Although this compositional heterogeneity likely impacts phase behavior, the function and incorporation of membrane proteins, and the encapsulation of biochemical reactions, it has yet to be directly quantified. To assess heterogeneity, we use secondary ion mass spectrometry (SIMS) to probe the composition of individual GUVs using non-perturbing isotopic labels. Both 13C- and 2H-labeled lipids are incorporated into a ternary POPC/DSPC/CHOL mixture, which is then used to produce GUVs via gentle hydration or electroformation. Simultaneous detection of seven different ion species via SIMS allows for the concentration of 13C- and 2H-labeled lipids in a single GUVs to be quantified using calibration curves, which correlate ion intensity to composition. Additionally, the relative concentration of 13C- and 2H-labeled lipids is assessed for each GUV via the ion ratio 2H−/13C−, which is highly sensitive to compositional changes between individual GUVs and circumvents the need for calibration using standards. Both quantification methods suggest that gentle hydration produces GUVs with greater compositional variability than those formed by electroformation. However, both gentle hydration and electroformation display compositional variability on the order of several mole percent.

Fundamental Aspects of Lipid-Based Excipients in Lipid-Based Product Development.

Poor aqueous solubility of drugs is still a foremost challenge in pharmaceutical product development. The use of lipids in designing formulations provides an opportunity to enhance the aqueous solubility and consequently bioavailability of drugs. Pre-dissolution of drugs in lipids, surfactants, or mixtures of lipid excipients and surfactants eliminate the dissolution/dissolving step, which is likely to be the rate-limiting factor for oral absorption of poorly water-soluble drugs. In this review, we exhaustively summarize the lipids excipients in relation to their classification, absorption mechanisms, and lipid-based product development. Methodologies utilized for the preparation of solid and semi-solid lipid formulations, applications, phase behaviour, and regulatory perspective of lipid excipients are discussed.

Changes in levels of omega-O-acylceramides and related processing enzymes of sun-exposed and sun-protected facial stratum corneum in differently pigmented ethnic groups.

IntroductionWe report on the differences in ceramide composition and levels of omega‐O‐acylceramide processing enzymes of sun‐exposed and sun‐protected facialstratum corneum (SC) among Albino African, Black African and Caucasian women living in South Africa.MethodsTape strippings were taken from the sun‐exposed cheek and the sunprotected postauricular site (PA). In two subsets proteomic (n = 18) and lipidomic (n = 24) analysis were performed using mass‐spectrometry‐based shotgun platforms.ResultsNo significant differences in total ceramide levels or ceramide subtypes were found between the Black African and Caucasian women in either the cheek or PA samples. Compared to the other two groups the levels of total ceramide as well as selected omega‐O‐acylceramide species were increased in Albino Africans. On the cheek, ceramide (CER) EOS, EOH along with CER AS were increased relative to the Caucasian women, while CER EOP and EOdS were elevated relative to the Black African women. Moreover, on the PA site CER EOP and EOdS were elevated compared with the Black African women and CER EOdS in Caucasians. Decreasesin masslevels of 12R‐LOX and eLOX3 were observed on cheeks compared with the PA sites in all ethnic groups. On the PA sites 12R‐LOX was particularly lower in the Albino Africans compared with the Black African and Caucasian women. On the cheeks mass levels of SDR9C7 was also lower in the Albino Africans.ConclusionThe mass levels of the ceramides were similar between Black African and Caucasian women. However, elevated total ceramides and excessively elevated selected omega‐O‐acylceramides were apparent in the Albino African women. The findings in the Albino African women were unexpected as these participants suffer from impaired skin barrier function. However, the elevated levels omega‐O‐acylceramides can contribute to barrier insufficiency by directly impacting SC lipid phase behaviour and/or secondly elevated omegaO‐acylceramide levels may indicate a reduced attachment of ceramides to the corneocyte lipid envelope and reduced corneocyte maturation that can also impair the barrier. Indeed, differences in the mass levels of omega‐O‐acylceramide processing enzymes were observed for 12R‐LOX and SDR9C7 for the Albino Africans. This indicates a corneocyte lipid scaffold disorder in this population.

Machine learning platform for determining experimental lipid phase behaviour from small angle X-ray scattering patterns by pre-training on synthetic data

Lipid membranes are vital in a wide range of biological and biotechnical systems; underpinning functions from modulation of protein activity to drug uptake and delivery. Rapid in situ determination of phase behaviour is a key experimental objective.

PH-Dependent Phase Behavior and Stability of Cationic Lipid–mRNA Nanoparticles

Lipid nanoparticles (LNPs) containing mRNA can deliver genetic material to cells for use as vaccines or protein replacement therapies. We characterized the effect of solution pH on cationic LNPs containing green fluorescent protein (EGFP) mRNA and their transfection efficiency. We compared the structural and colloidal properties of mRNA LNPs with LNPs not containing mRNA and mRNA free in solution. We used a combination of biophysical technique to build a picture of the structure of the lipids and mRNA across pH and temperature in the form of an empirical phase diagram (EPD). A combination of Fourier-transform infrared (FTIR) spectroscopy and differential scanning calorimetry was used to investigate lipid phase behavior.The mRNA–LNPs transition from an inverse hexagonal phase at pH values below the pKa of the cationic lipid to a lamellar phase above the pKa. At higher temperatures the mRNA–LNPs also transitioned from an inverse hexagonal phase to a lamellar phase indicating the inverse hexagonal phase is more thermodynamically favorable. Based on circular dichroism, the mRNA within the LNP has more A form structure at pH values below the lipid pKa than above it.Optical density, zeta potential and dynamic light scattering measurements were used to probe the colloidal stability of the mRNA–LNPs. The particles were larger and more prone to aggregation below the pKa. A stability study was performed to relate the biophysical characteristics to the storage of the particles in solution at 4 and 25 °C. mRNA–LNPs had the highest transfection efficiency and stability at pH values below the pKa. However, there was a trade-off between the stability and aggregation propensity since at very low pH the particles were most prone to aggregation. We performed kinetic experiments to show that the time scale of the pH-dependent phase behavior is slow (6 hour transition) and the transition from lamellar to inverse hexagonal phases is irreversible. This suggests that the lamellar phase is less stable and kinetically trapped. Our findings deepen our structural understanding of mRNA–LNPs and will aid the development of related formulations.

Bicontinuous inverted cubic phase stabilization as an index of antimicrobial and membrane fusion peptide activity

Some antimicrobial peptides (AMPs) and membrane fusion-catalyzing peptides (FPs) stabilize bicontinuous inverted cubic (QII) phases. Previous authors proposed a topological rationale: since AMP-induced pores, fusion intermediates, and QII phases all have negative Gaussian curvature (NGC), peptides which produce NGC in one structure also do it in another. This assumes that peptides change the curvature energy of the lipid membranes. Here I test this with a Helfrich curvature energy model. First, experimentally, I show that lipid systems often used to study peptide NGC have NGC without peptides at higher temperatures. To determine the net effect of an AMP on NGC, the equilibrium phase behavior of the host lipids must be determined. Second, the model shows that AMPs must make large changes in the curvature energy to stabilize AMP-induced pores. Peptide-induced changes in elastic constants affect pores and QII phase differently. Changes in spontaneous curvature affect them in opposite ways. The observed correlation between QII phase stabilization and AMP activity doesn't show that AMPs act by lowering pore curvature energy. A different rationale is proposed. In theory, AMPs could simultaneously stabilize QII phase and pores by drastically changing two particular elastic constants. This could be tested by measuring AMP effects on the individual constants. I propose experiments to do that. Unlike AMPs, FPs must make only small changes in the curvature energy to catalyze fusion. It they act in this way, their fusion activity should correlate with their ability to stabilize QII phases.

Lipid Vesicles and Other Polymolecular Aggregates—From Basic Studies of Polar Lipids to Innovative Applications

Lipid vesicles (liposomes) are a unique and fascinating type of polymolecular aggregates, obtained from bilayer-forming amphiphiles—or mixtures of amphiphiles—in an aqueous medium. Unilamellar vesicles consist of one single self-closed bilayer membrane, constituted by the amphiphiles and an internal volume which is trapped by this bilayer, whereby the vesicle often is spherical with a typical desired average diameter of either about 100 nm or tens of micrometers. Functionalization of the external vesicle surface, basically achievable at will, and the possibilities of entrapping hydrophilic molecules inside the vesicles or/and embedding hydrophobic compounds within the membrane, resulted in various applications in different fields. This review highlights a few of the basic studies on the phase behavior of polar lipids, on some of the concepts for the controlled formation of lipid vesicles as dispersed lamellar phase, on some of the properties of vesicles, and on the challenges of efficiently loading them with hydrophilic or hydrophobic compounds for use as delivery systems, as nutraceuticals, for bioassays, or as cell-like compartments. Many of the large number of basic studies have laid a solid ground for various applications of polymolecular aggregates of amphiphilic lipids, including, for example, cubosomes, bicelles or—recently most successfully—nucleic acids-containing lipid nanoparticles. All this highlights the continued importance of fundamental studies. The life-saving application of mRNA lipid nanoparticle COVID-19 vaccines is in part based on year-long fundamental studies on the formation and properties of lipid vesicles. It is a fascinating example, which illustrates the importance of considering (i) details of the chemical structure of the different molecules involved, as well as (ii) physical, (iii) engineering, (iv) biological, (v) pharmacological, and (vii) economic aspects. Moreover, the strong demand for interdisciplinary collaboration in the field of lipid vesicles and related aggregates is also an excellent and convincing example for teaching students in the field of complex molecular systems.

Effect of pH on the phase behavior of DMPC bilayers.

We have studied the effect of acidic pH on the phase behavior of the zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) using differential scanning calorimetry and x-ray scattering. Dispersions of DMPC in HCl solutions of pH = 4 and 3 behave identical to dispersions in water. The main transition temperature increases sharply and the pre-transition disappears at lower pH. An untilted gel phase is observed at pH = 2 and 1, in contrast to the tilted gel phase found at higher pH. The relatively large periodicity of the untilted gel phase, in comparison to that of the tilted gel phase occurring near neutral pH, clearly demonstrates the simultaneous charging and dehydration of the headgroups as the pH approaches the pK of the phosphate group. Headgroup dehydration at low pH also leads to the formation of DMPC crystallites and the inverted hexagonal phase at low and high temperatures, respectively, after a few days of incubation. These results show the significant effect of acidic pH on the phase behavior of zwitterionic lipids.

Bipolar Imidazolium-Based Lipid Analogues for Artificial Archaeosomes.

Archaeal lipids have harvested biomedical and biotechnological interest because of their ability to form membranes with low permeability and enhanced temperature and pressure stability. Because of problems in isolating archaeal lipids, chemical synthesis appears to be a suitable means of producing model lipids that mimic the biological counterparts. Here, we introduce a new concept: we synthesized bipolar alkylated imidazolium salts of different chain lengths (BIm10-32) and studied their structure and lyotropic phase behavior. Furthermore, mixtures of the bolalipid analogues with phospholipid model biomembranes of diverse complexity were studied. DSC, fluorescence and FTIR spectroscopy, confocal fluorescence microscopy, DLS, SAXS, and TEM were used to reveal changes in lipid phase behavior, fluidity, the lipid's conformational order, and membrane morphology over a wide range of temperatures and for selected pressures. It could be shown that the long-chain BImN32 can form monolayer sheets. Integrated in phospholipid membranes, it reveals a fluidizing effect. Here, the two polar head groups, connected by a long alkyl chain, enable the integration into the bilayer. Interestingly, addition of BImN32 to fluid DPPC liposomes increased the lipid packing markedly, rendering the membrane system more stable at higher temperatures. The membrane system is also stable against compression as indicated by the high-pressure stability of the system, mimicking an archaeal lipid-like behavior. BImN32 incorporation into raft-like anionic model biomembranes led to marked changes in lateral membrane organization, topology, and fusogenicity of the membrane. Overall, it was found that long-chain imidazolium-based bolalipid analogues can help adjust membrane's biophysical properties, while the imidazolium headgroup provides the ability for crucial electrostatic interaction for vesicle fusion or selective interaction with membrane-related signaling molecules and polypeptides in a synthetically tractable manner. The results obtained may help to develop new approaches for rational design of extremophilic bolalipid-based liposomes for various applications, including delivery of drugs and vaccines.

Investigation of the Action of Peptides on Lipid Membranes.

Calorimetric and incoherent neutron scattering methods were employed to investigate the action of magainin 2 and PGLa peptides on the phase behavior and molecular dynamics of lipids mimicking cytoplasmic membranes of Gram-negative bacteria. The impact of the peptides, tested individually and cooperatively by differential scanning calorimetry, presented a broadened peak, sometimes with a second shoulder, depicting the phase transition temperature around 21 °C. Neutron scattering revealed a small but significant variation of the membrane dynamics due to the peptides in both in-plane and out-of-plane directions. Although we did not find a clear hint for synergy in the interplay of the two peptides, the calorimetric and neutron data give compatible results in terms of a decrease of the enthalpy due to the presence of the peptides, which destabilize the membrane. The dynamics in the two directions was differentiated when the individual peptides were added to the membranes, but the impact was smaller when both peptides were added together.

Structural Evidence for a Reinforcing Response and Retention of Hydration During Confinement of Cartilage Lipids

Lipids have an important role in the complex lubrication of articulating joints, however changes in lipid phase behavior that occur owing to mechanical confinement are not well understood. Here, a surface force-type apparatus has been combined with neutron reflectometry to measure confinement-induced changes in the structure of lipids, the major surface-active component of the lubricant in articulating joints. The same incompressible state was accessed under low uniaxial stress (1 bar), irrespective of whether the lipids had started out unconfined above or below the Lα phase transition, and irrespective of whether they were fully or partially hydrated. In this incompressible state, the lipid component had thickened indicating extension and rearrangement of the lipid chains in response to the applied stress. The small amount of water remaining between each lipid bilayer was found to be similar for all chain lengths and starting phases. This represents the first structural evidence of the tightly bound water layer at the headgroups, which is required for hydration lubrication under load.

Effect of different derivatives of paraffin waxes on crystallization of eutectic mixture of cocoa butter-coconut oil

Paraffin wax is a mixture of numerous unbranched hydrocarbons used frequently for various purposes: to improve the shelf life of products containing lipid system and develop more shiny products. However, because of its complex nature, the effect of such molecular structure on the solid phase behavior of lipids is hardly unstated. Hence in our study, we focus on understanding the impact of derivatives of paraffin wax on the lipid system. In the current work, three unbranched derivatives of paraffin wax: Eicosane C (20), Pentacosane C (25) and Triacontane C (30) were selected as additives. These n-alkanes are specifically added to the eutectic mixture of cocoa butter (CB) and coconut oil (CO) (ECB-CO) to observe the effect on thermal, morphological, rheological properties and crystallization kinetics with respect to the carbon chain length. Results from our study illustrate that melting and crystallization temperature, storage modulus and solid fat content (SFC) increases after the addition of 1 wt% of C (20), C (25). In contrast, there is a phase separation for 1 wt% C (30). Further similar study with addition of n-alkanes to pure CB and CO reveals that the interaction of n-alkanes with ECB-CO is dominated by the interaction of n-alkanes with CO instead of CB. Therefore, our findings provide insight into the effect of addition of n-alkanes having different carbon chain length and their respective concentration on crystallization process of CB and CO. This will definitely help to design the processes for products containing such model systems.

Influence of the Molecular Weight and the Presence of Calcium Ions on the Molecular Interaction of Hyaluronan and DPPC.

Hyaluronan is an essential physiological bio macromolecule with different functions. One prominent area is the synovial fluid which exhibits remarkable lubrication properties. However, the synovial fluid is a multi-component system where different macromolecules interact in a synergetic fashion. Within this study we focus on the interaction of hyaluronan and phospholipids, which are thought to play a key role for lubrication. We investigate how the interactions and the association structures formed by hyaluronan (HA) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) are influenced by the molecular weight of the bio polymer and the ionic composition of the solution. We combine techniques allowing us to investigate the phase behavior of lipids (differential scanning calorimetry, zeta potential and electrophoretic mobility) with structural investigation (dynamic light scattering, small angle scattering) and theoretical simulations (molecular dynamics). The interaction of hyaluronan and phospholipids depends on the molecular weight, where hyaluronan with lower molecular weight has the strongest interaction. Furthermore, the interaction is increased by the presence of calcium ions. Our simulations show that calcium ions are located close to the carboxylate groups of HA and, by this, reduce the number of formed hydrogen bonds between HA and DPPC. The observed change in the DPPC phase behavior can be attributed to a local charge inversion by calcium ions binding to the carboxylate groups as the binding distribution of hyaluronan and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine is not changed.