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Membrane plasticity induced by myo-inositol derived archaeal lipids: chemical synthesis and biophysical characterization

Archaeal membrane lipids have specific structures that allow Archaea to withstand extreme conditions of temperature and pressure. In order to understand the molecular parameters that govern such resistance, the synthesis of 1,2-di-O-phytanyl-sn-glycero-3-phosphoinositol (DoPhPI), an archaeal lipid derived from myo-inositol, is reported. Benzyl protected myo-inositol was first prepared and then transformed to phosphodiester derivatives using a phosphoramidite based-coupling reaction with archaeol. Aqueous dispersions of DoPhPI alone or mixed with DoPhPC can be extruded and form small unilamellar vesicles, as detected by DLS. Neutron, SAXS, and solid-state NMR demonstrated that the water dispersions could form a lamellar phase at room temperature that then evolves into cubic and hexagonal phases with increasing temperature. Phytanyl chains were also found to impart remarkable and nearly constant dynamics to the bilayer over wide temperature ranges. All these new properties of archaeal lipids are proposed as providers of plasticity and thus means for the archaeal membrane to resist extreme conditions.

Molecular Rearrangements in Protomembrane Models Probed by Laurdan Fluorescence

Lipid membranes are a key component of living systems and have been essential to the origin of life. One hypothesis for the origin of life assumes the existence of protomembranes with ancient lipids formed by Fischer-Tropsch synthesis. We determined the mesophase structure and fluidity of a prototypical decanoic (capric) acid-based system, a fatty acid with a chain length of 10 carbons, and a lipid system consisting of a 1:1 mixture of capric acid with a fatty alcohol of equal chain length (C10 mix). To shed light on the mesophase behavior and fluidity of these prebiotic model membranes, we employed Laurdan fluorescence spectroscopy, which reports on the lipid packing and fluidity of membranes, supplemented by small-angle neutron diffraction data. The data are compared with data of the corresponding phospholipid bilayer systems of the same chain length, 1,2-didecanoyl-sn-glycero-3-phosphocholine (DLPC). We demonstrate that the prebiotic model membranes capric acid and the C10 mix show formation of stable vesicular structures needed for cellular compartmentalization at low temperatures only, typically below 20 °C. They reveal the fluid-like lipid dynamic properties needed for optimal physiological function. High temperatures lead to the destabilization of the lipid vesicles and the formation of micellar structures.

Incorporation and localisation of alkanes in a protomembrane model by neutron diffraction

Protomembranes at the origin of life were likely composed of short-chain lipids, readily available on the early Earth. Membranes formed by such lipids are less stable and more permeable under extreme conditions, so a novel membrane architecture was suggested to validate the accuracy of this assumption. The model membrane includes the presence of a layer of alkanes in the mid-plane of the protomembrane in between the two monolayer leaflets and lying perpendicular to the lipid acyl chains. Here, we investigated such a possibility experimentally for membranes formed by the short-chain phospholipid 1,2-didecanoyl-sn-glycero-3-phophocholine, including or not the alkanes eicosane, squalane or triacontane by means of neutron membrane diffraction and contrast variation. We found strong indications for incorporation of two of the three alkanes in the membrane mid-plane through the determination of neutron scattering length density profiles with hydrogenated vs deuterated alkanes and membrane swelling at various relative humidities indicating a slightly increased bilayer thickness when the alkanes are incorporated into the bilayers. The selectivity of the incorporation points out the role of the length of the n-alkanes with respect to the capacity of the membrane to incorporate them.

Evidence by neutron diffraction of molecular compounds in triarylamine tris-amide organogels and in their hybrid thermoreversible gels with PVC

We report on neutron diffraction experiments performed on organogels prepared from triarylamine tris-amide (TATA), as well as on their ternary thermoreversible gels made up with poly[vinyl chloride] (PVC). Three different solvents together with their deuterated counterparts have been used; tetrachloroethane, wherein TATA fibrils display ohmic conductivity, bromobenzene and o-dichlorobenzene. The TATA crystal structure differs in the three solvents. Most importantly, the difference in the diffraction patterns whether hydrogenous solvents or deuterated solvents are used demonstrate the occurrence of molecular compounds. Tentative unit cells are presented. These results are also discussed in the light of the current views on the solvent role in the gelation process.

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.

Characterisation of a synthetic Archeal membrane reveals a possible new adaptation route to extreme conditions

It has been proposed that adaptation to high temperature involved the synthesis of monolayer-forming ether phospholipids. Recently, a novel membrane architecture was proposed to explain the membrane stability in polyextremophiles unable to synthesize such lipids, in which apolar polyisoprenoids populate the bilayer midplane and modify its physico-chemistry, extending its stability domain. Here, we have studied the effect of the apolar polyisoprenoid squalane on a model membrane analogue using neutron diffraction, SAXS and fluorescence spectroscopy. We show that squalane resides inside the bilayer midplane, extends its stability domain, reduces its permeability to protons but increases that of water, and induces a negative curvature in the membrane, allowing the transition to novel non-lamellar phases. This membrane architecture can be transposed to early membranes and could help explain their emergence and temperature tolerance if life originated near hydrothermal vents. Transposed to the archaeal bilayer, this membrane architecture could explain the tolerance to high temperature in hyperthermophiles which grow at temperatures over 100 °C while having a membrane bilayer. The induction of a negative curvature to the membrane could also facilitate crucial cell functions that require high bending membranes.

Structural Characterization of Natural Yeast Phosphatidylcholine and Bacterial Phosphatidylglycerol Lipid Multilayers by Neutron Diffraction

Eukaryotic and prokaryotic cell membranes are difficult to characterize directly with biophysical methods. Membrane model systems, that include fewer molecular species, are therefore often used to reproduce their fundamental chemical and physical properties. In this context, natural lipid mixtures directly extracted from cells are a valuable resource to produce advanced models of biological membranes for biophysical investigations and for the development of drug testing platforms. In this study we focused on single phospholipid classes, i.e. Pichia pastoris phosphatidylcholine (PC) and Escherichia coli phosphatidylglycerol (PG) lipids. These lipids were characterized by a different distribution of their respective acyl chain lengths and number of unsaturations. We produced both hydrogenous and deuterated lipid mixtures. Neutron diffraction experiments at different relative humidities were performed to characterize multilayers from these lipids and investigate the impact of the acyl chain composition on the structural organization. The novelty of this work resides in the use of natural extracts with a single class head-group and a mixture of chain compositions coming from yeast or bacterial cells. The characterization of the PC and PG multilayers showed that, as a consequence of the heterogeneity of their acyl chain composition, different lamellar phases are formed.

Graphite intercalation compound (GIC) crystal monochromators for cold neutron instruments: Characterization of KC24 by time-of-flight neutron diffraction

Graphite intercalation compounds (GICs) are a group of layered materials that are suitable as monochromators for cold neutrons. KC₂₄ is a particularly interesting compound in this regard as it features a large c-axis lattice spacing of 8.74 Å, high reflectivity, and the possibility to produce large crystals with mosaicity that matches the beam divergence of cold neutron guides. GICs can be synthesized with different levels of intercalation, known as the stage of the compounds. Each stage displays a specific d-spacing. Impure GIC-monochromators containing multiple stages produce mixing of neutron wavelengths, which complicates data analysis on neutron reflectometers. We discuss the implications of GIC crystal purity and stage contamination for neutron reflectometry and show how GIC crystals can be characterized by time-of-flight neutron diffraction providing an efficient and quantifiable measure of the reflected wavelength spectrum. This allows taking into account multiple wavelength contaminations and ascertains the robustness of reflectometry measurements.

Cation-Zwitterionic Lipid Interactions Are Affected by the Lateral Area per Lipid

Interactions of the divalent cations Ca²⁺ and Mg²⁺ with the zwitterionic lipid bilayers prepared of a fully saturated dipalmitoylphosphatidylcholine (DPPC) or a di-monounsaturated dioleoylphosphatidylcholine (DOPC) were studied by using the neutron scattering methods and molecular dynamics simulations. The effect on the bilayer structural properties confirms the direct interactions in all cases studied. The changes are observed in the bilayer thickness and lateral area. The extent of these structural changes, moreover, suggests various mechanisms of the cation-lipid interactions. First, we have observed a small difference when studying DPPC bilayers in the gel and fluid phases, with somewhat larger effects in the former case. Second, the hydration proved to be a factor in the case of DOPC bilayers, with the larger effects in the case of less hydrated systems. Most importantly, however, there was a qualitative difference between the results of the fully hydrated DOPC bilayers and the others examined. These observations then prompt us to suggest an interaction model that is plausibly governed by the lateral area of lipid, though affected indirectly also by the hydration level. Namely, when the interlipid distance is small enough to allow for the multiple lipid-ion interactions, the lipid-ion-lipid bridges are formed. The bridges impose strong attractions that increase the order of lipid hydrocarbon chains, resulting in the bilayer thickening. In the other case, when the interlipid distance extends beyond a limiting length corresponding to the area per lipid of ∼65 Ų, Mg²⁺ and Ca²⁺ continue to interact with the lipid groups by forming the separate ion-lipid pairs. As the interactions proposed affect the lipid membrane structure in the lateral direction, they may prove to play their role in other mechanisms lying within the membrane multicomponent systems and regulating for example the lipid-peptide-ion interactions.

Apolar Polyisoprenoids Located in the Midplane of the Bilayer Regulate the Response of an Archaeal-Like Membrane to High Temperature and Pressure

Archaea are known to inhabit some of the most extreme environments on Earth. The ability of archaea possessing membrane bilayers to adapt to high temperature (>85°C) and high pressure (>1,000 bar) environments is proposed to be due to the presence of apolar polyisoprenoids at the midplane of the bilayer. In this work, we study the response of this novel membrane architecture to both high temperature and high hydrostatic pressure using neutron diffraction. A mixture of two diether, phytanyl chain lipids (DoPhPC and DoPhPE) and squalane was used to model this novel architecture. Diffraction data indicate that at high temperatures a stable coexistence of fluid lamellar phases exists within the membrane and that stable coexistence of these phases is also possible at high pressure. Increasing the amount of squalane in the membrane regulates the phase separation with respect to both temperature and pressure, and also leads to an increase in the lamellar repeat spacing. The ability of squalane to regulate the ultrastructure of an archaea-like membrane at high pressure and temperature supports the hypothesis that archaea can use apolar lipids as an adaptive mechanism to extreme conditions.

Anesthetics significantly increase the amount of intramembrane water in lipid membranes

The potency of anesthesia was directly linked to the partitioning of the drug molecules in cell membranes by Meyer and Overton. Many molecules interact with lipid bilayers and lead to structural and functional changes. It remains an open question which change in membrane properties is responsible for a potential anesthetic effect or if anesthetics act by binding to direct targets. We studied the effect of ethanol, diethyl ether and isoflurane on the water distribution in lipid bilayers by combining all-atom molecular dynamics simulations and neutron diffraction experiments. The simulations show strong membrane-drug interactions with partitioning coefficients of 38%, 92% and 100% for ethanol, diethyl ether and isoflurane, respectively, and provide evidence for an increased water partitioning in the membrane core. The amount of intramembrane water molecules was experimentally determined by selectively deuterium labeling lipids, anesthetic drug and water molecules in neutron diffraction experiments. Four additional water molecules per lipid were observed in the presence of ethanol. Diethyl ether and isoflurane were found to significantly increase the amount of intramembrane water by 25% (8 water molecules). This increase in intramembrane water may contribute to the non-specific interactions between anesthetics and lipid membranes.

Lipid Phase Separation Induced by the Apolar Polyisoprenoid Squalane Demonstrates Its Role in Membrane Domain Formation in Archaeal Membranes

Archaea synthesize methyl-branched, ether phospholipids, which confer the archaeal membrane exceptional physicochemical properties. A novel membrane organization was proposed recently to explain the thermal and high pressure tolerance of the polyextremophilic archaeon Thermococcus barophilus. According to this theoretical model, apolar molecules could populate the midplane of the bilayer and could alter the physicochemical properties of the membrane, among which is the possibility to form membrane domains. We tested this hypothesis using neutron diffraction on a model archaeal membrane composed of two archaeal diether lipids with phosphocholine and phosphoethanolamine headgroups in the presence of the apolar polyisoprenoid squalane. We show that squalane is inserted in the midplane at a maximal concentration between 5 and 10 mol % and that squalane can modify the lateral organization of the membrane and induces the coexistence of separate phases. The lateral reorganization is temperature- and squalane concentration-dependent and could be due to the release of lipid chain frustration and the induction of a negative curvature in the lipids.

Behavior of Hydrated Lipid Bilayers at Cryogenic Temperatures

Neutron diffraction was used to study the behavior of water present in phospholipid multilamellar stacks from 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) at cryogenic temperatures. Evidence was found for the existence of a highly viscous phase of water that exists between 180 and 220 K based on the observation that water can leave the intermembrane space at these low temperatures. Similar measurements are described in the literature for purple membrane (PM) samples. From a comparison with results from this natural membrane by using the same flash-cooling protocol, it is found that in the case of pure lipid samples, less water is trapped and the water flows out at lower temperatures. This suggests that the water is less hindered in its movements than in the PM case. It is shown that at least the Lβ′-phase of DMPC can be trapped likely by flash cooling; upon heating to about 260 K, it transforms to another phase that was not fully characterized.

Structural Characterization of an Archaeal Lipid Bilayer as a Function of Hydration and Temperature

Archaea, the most extremophilic domain of life, contain ether and branched lipids which provide extraordinary bilayer properties. We determined the structural characteristics of diether archaeal-like phospholipids as functions of hydration and temperature by neutron diffraction. Hydration and temperature are both crucial parameters for the self-assembly and physicochemical properties of lipid bilayers. In this study, we detected non-lamellar phases of archaeal-like lipids at low hydration levels, and lamellar phases at levels of 90% relative humidity or more exclusively. Moreover, at 90% relative humidity, a phase transition between two lamellar phases was discernible. At full hydration, lamellar phases were present up to 70ᵒC and no phase transition was observed within the temperature range studied (from 25 °C to 70 °C). In addition, we determined the neutron scattering length density and the bilayer's structural parameters from different hydration and temperature conditions. At the highest levels of hydration, the system exhibited rearrangements on its corresponding hydrophobic region. Furthermore, the water uptake of the lipids examined was remarkably high. We discuss the effect of ether linkages and branched lipids on the exceptional characteristics of archaeal phospholipids.

Effect of ergosterol on the interlamellar spacing of deuterated yeast phospholipid multilayers

Sterols regulate several physico-chemical properties of biological membranes that are considered to be linked to function. Ergosterol is the main sterol molecule found in the cell membranes of yeasts and other fungi. Like the cholesterol found in mammalian cells, ergosterol has been proposed to have an ordering and condensing effect on saturated phospholipid membranes. The effects of cholesterol have been investigated extensively and result in an increase in the membrane thickness and the lipid acyl chain order. Less information is available on the effects of ergosterol on phospholipid membranes. Neutron Diffraction (ND) was used to characterize the effect of ergosterol on lipid multilayers prepared with deuterated natural phospholipids extracted from the yeast Pichia pastoris. The data show that the effect of ergosterol on membranes prepared from the natural phospholipid extract rich in unsaturated acyl chains, differs from what has been observed previously in membranes rich in saturated phospholipids. In contrast to cholesterol in synthetic phospholipid membranes, the presence of ergosterol up to 30 mol % in yeast phospholipid membranes only slightly altered the multilayer structure. In particular, only a small decrease in the multilayer d-spacing was observed as function of increasing ergosterol concentrations. This result highlights the need for further investigation to elucidate the effects of ergosterol in biological lipid mixtures.

Application of small-angle neutron diffraction to the localization of general anesthetics in model membranes

We set out to explore the applicability of small-angle neutron diffraction (SAND) to the localization of biomembrane components by studying the general anesthetic n-decane in a model lipid bilayer system composed of dioleoyl-phosphocholine (DOPC). Samples in the form of planar membrane multilayers were hydrated by varied mixtures of deuterated and protonated water, and examined by the means of SAND. Neutron scattering length density (NSLD) profiles of the system were then reconstructed from the experimental data. We exploited the significantly different neutron scattering properties of hydrogen and deuterium atoms via labeling in addition to water contrast variation. Enhancing the signals from particular components of bilayer system led to a set of characteristic membrane profiles and from their comparison we localized n-decane molecules unequivocally in the bilayer's hydrocarbon chain region.

The impact of deuteration on natural and synthetic lipids: A neutron diffraction study

The structural investigation of cellular membranes requires access to model systems where the molecular complexity is representative of the cellular environment and that allow for the exploitation of structural techniques. Neutron scattering, and in particular neutron diffraction can provide unique and detailed information on the structure of lipid membranes. However, deuterated samples are desirable to fully exploit this powerful method. Recently, the extraction of lipids from microorganisms grown in deuterated media was demonstrated to be both an attracting route to obtain complex lipid mixtures resembling the composition of natural membranes, and to producing deuterated molecules in a very convenient way. A full characterization of these deuterated extracts is hence pivotal for their use in building up model membrane systems. Here we report the structural characterization of lipid extracts obtained from Pichia pastoris by means of neutron diffraction measurements. In particular, we compare the structure of membranes extracted from yeast cells grown in a standard culture medium and in a corresponding deuterated culture medium. The results show that the different molecular composition of the deuterated and protiated lipid extracts induce different structural organization of the lipid membranes. In addition, we compare these membranes composed of extracted yeast lipids with stacked bilayers prepared from synthetic lipid mixtures.