The membrane structure and function affected by water

Behavior of water at lipid/water interfaces upon phase transition of the lipid bilayer: Insights from 1D- and 2D-vibrational sum frequency generation spectral calculations from molecular dynamics simulations

We have investigated the structural and dynamical changes of the interfacial water near [1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine] (DMPC) lipid bilayer across various temperatures, ranging from 285 K (gel phase of lipid) to 320 K (liquid phase of lipid), through calculations of one-dimensional (1D) and two-dimensional (2D) vibrational sum frequency generation (VSFG) spectra from molecular dynamics simulations. The 1D-VSFG spectra show a broad positive peak in the hydrogen-bonded region, which means that water molecules are oriented upward toward the lipid bilayer. Although DMPC is a zwitterionic lipid, the negatively charged phosphate group primarily influences the orientation of the water molecules. The absence of a dangling peak in the 1D- and 2D-VSFG spectra shows that the water molecules form hydrogen bonds with the lipid headgroup atoms. The spectral diffusion timescales obtained from the 2D-VSFG metrics of the slope of the nodal line clearly reveal a dynamical crossover and exhibit Arrhenius behavior with different activation energies before and after the melting of the lipid bilayer. Apart from 2D-VSFG, the frequency fluctuation time correlation function also exhibits a dynamical crossover upon melting of the lipid bilayer.

Dehydration of biological membranes in a non-condensing environment

The study of the dehydration process in a cell membrane allows a better understanding of how water is bound to it. While in prior studies, cell dehydration was commonly analyzed under osmotic stress conditions, in the present work, we focus on the dehydration driven by evaporation in a restricted condensing environment. Using a thermogravimetry method, we studied the dehydration of Escherichia coli through isothermal evaporation in the presence of a gas flux. To figure out the loss of mass in this situation, we first evaluated the dynamics of water evaporation of a suspension of multilamellar liposomes. We found that the evaporation of liposomal suspensions composed of individual lipids is constant, although slightly restricted by the presence of liposomes, while the evaporation of liposomal suspensions composed of a mixture of different lipids follows an exponential decay. This is explained considering that the internal pressure at the air-water interface is proportional to the amount of bound water. The evaporation of water from a biomass sample follows this latter behaviour.

Interaction of GC376, a SARS-COV-2 MPRO inhibitor, with model lipid membranes

Partitioning and effect of antiviral GC376, a potential SARS-CoV-2 inhibitor, on model lipid membranes was studied using dynamic light scattering (DLS), UV–VIS spectrometry, Excimer fluorescence, Differential scanning calorimetry (DSC) and Small- and Wide-angle X-ray scattering (SAXS/WAXS). Partition coefficient of GC376 between lipid and water phase was found to be low, reaching K_P = 46.8 ± 18.2. Results suggest that GC376 partitions into lipid bilayers at the level of lipid head-groups, close to the polar/hydrophobic interface. Changes in structural and thermodynamic properties strongly depend on the GC376/lipid mole ratio. Already at lowest mole ratios GC376 induces increase of lateral pressures, mainly in the interfacial region of the bilayer. Hereby, the pre- and main-transition temperature of the lipid system increases, what is attributed to tighter packing of acyl chains induced by GC376. At GC376/DPPC ≥ 0.03 mol/mol we detected formation of domains with different GC376 content resulting in the lateral phase separation and changes in both, main transition temperature and enthalpy. The observed changes are attributed to the response of the system on the increased lateral stresses induced by partitioning of GC376. Obtained results are discussed in context of liposome-based drug delivery systems for GC376 and in context of indirect mechanism of virus replication inhibition.

Dissecting the mechanisms of environment sensitivity of smart probes for quantitative assessment of membrane properties

The plasma membrane, as a highly complex cell organelle, serves as a crucial platform for a multitude of cellular processes. Its collective biophysical properties are largely determined by the structural diversity of the different lipid species it accommodates. Therefore, a detailed investigation of biophysical properties of the plasma membrane is of utmost importance for a comprehensive understanding of biological processes occurring therein. During the past two decades, several environment-sensitive probes have been developed and become popular tools to investigate membrane properties. Although these probes are assumed to report on membrane order in similar ways, their individual mechanisms remain to be elucidated. In this study, using model membrane systems, we characterized the probes Pro12A, NR12S and NR12A in depth and examined their sensitivity to parameters with potential biological implications, such as the degree of lipid saturation, double bond position and configuration (cis versus trans), phospholipid headgroup and cholesterol content. Applying spectral imaging together with atomistic molecular dynamics simulations and time-dependent fluorescent shift analyses, we unravelled individual sensitivities of these probes to different biophysical properties, their distinct localizations and specific relaxation processes in membranes. Overall, Pro12A, NR12S and NR12A serve together as a toolbox with a wide range of applications allowing to select the most appropriate probe for each specific research question.

Lipid/water interface of galactolipid bilayers in different lyotropic liquid-crystalline phases

In this study, carried out using computational methods, the organisation of the lipid/water interface of bilayers composed of galactolipids with both α-linolenoyl acyl chains is analysed and compared in three different lyotropic liquid-crystalline phases. These systems include the monogalactosyldiglyceride (MGDG) and digalactosyldiglyceride (DGDG) bilayers in the lamellar phase, the MGDG double bilayer during stalk phase formation and the inverse hexagonal MGDG phase. For each system, lipid-water and direct and water-mediated lipid-lipid interactions between the lipids of one bilayer leaflet and those of two apposing leaflets at the onset of new phase (stalk) formation, are identified. A network of interactions between DGDG molecules and its topological properties are derived and compared to those for the MGDG bilayer.

Order vs. Disorder: Cholesterol and Omega-3 Phospholipids Determine Biomembrane Organization

Lipid structural diversity strongly affects biomembrane chemico-physical and structural properties in addition to membrane-associated events. At high concentrations, cholesterol increases membrane order and rigidity, while polyunsaturated lipids are reported to increase disorder and flexibility. How these different tendencies balance in composite bilayers is still controversial. In this study, electron paramagnetic resonance spectroscopy, small angle neutron scattering, and neutron reflectivity were used to investigate the structural properties of cholesterol-containing lipid bilayers in the fluid state with increasing amounts of polyunsaturated omega-3 lipids. Either the hybrid 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine or the symmetric 1,2-docosahexaenoyl-sn-glycero-3-phosphocholine were added to the mixture of the naturally abundant 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and cholesterol. Our results indicate that the hybrid and the symmetric omega-3 phospholipids affect the microscopic organization of lipid bilayers differently. Cholesterol does not segregate from polyunsaturated phospholipids and, through interactions with them, is able to suppress the formation of non-lamellar structures induced by the symmetric polyunsaturated lipid. However, this order/disorder balance leads to a bilayer whose structural organization cannot be ascribed to either a liquid ordered or to a canonical liquid disordered phase, in that it displays a very loose packing of the intermediate segments of lipid chains.

Water at charged interfaces

The ubiquity of aqueous solutions in contact with charged surfaces and the realization that the molecular-level details of water-surface interactions often determine interfacial functions and properties relevant in many natural processes have led to intensive research. Even so, many open questions remain regarding the molecular picture of the interfacial organization and preferential alignment of water molecules, as well as the structure of water molecules and ion distributions at different charged interfaces. While water, solutes and charge are present in each of these systems, the substrate can range from living tissues to metals. This diversity in substrates has led to different communities considering each of these types of aqueous interface. In this Review, by considering water in contact with metals, oxides and biomembranes, we show the essential similarity of these disparate systems. While in each case the classical mean-field theories can explain many macroscopic and mesoscopic observations, it soon becomes apparent that such theories fail to explain phenomena for which molecular properties are relevant, such as interfacial chemical conversion. We highlight the current knowledge and limitations in our understanding and end with a view towards future opportunities in the field.

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.

Profile and Content of Residual Alkaloids in Ten Ecotypes of Lupinus mutabilis Sweet after Aqueous Debittering Process

The evaluation of the level of alkaloids in edible Lupinus species is crucial from a food safety point of view. Debittering of lupin seeds has a long history; however, the control of the level of alkaloids after processing the seeds is typically only evaluated by changes in the bitter taste. The aim of this study was to evaluate the profile and residual levels of quinolizidine alkaloids (QA) in (Lupinus mutabilis Sweet) after aqueous debittering process. Samples from 10 ecotypes from different areas of Peru were analyzed before and after the process. Based on results obtained by gas chromatography and mass spectrometry, from eight alkaloids identified before the debittering process, only small amounts of lupanine (avg. 0.0012 g/100 g DM) and sparteine (avg. 0.0014 g/100 g DM) remained in the seeds after the debittering process, and no other alkaloids were identified. The aqueous debittering process reduced the content of alkaloids to levels far below the maximal level allowed by international regulations (≤ 0.2 g/kg DM).