Physicochemical Properties Of Lipid Bilayers Research Articles

Cholesterol Effect on the Specific Capacitance of Submicrometric DOPC Bilayer Patches Measured by in-Liquid Scanning Dielectric Microscopy.

The specific capacitance of biological membranes is a key physical parameter in bioelectricity that also provides valuable physicochemical information on composition, phase, or hydration properties. Cholesterol is known to modulate the physicochemical properties of biomembranes, but its effect on the specific capacitance has not been fully established yet. Here we use the high spatial resolution capabilities of in-liquid scanning dielectric microscopy in force detection mode to directly demonstrate that DOPC bilayer patches at 50% cholesterol concentration show a strong reduction of their specific capacitance with respect to pure DOPC bilayer patches. The reduction observed (∼35%) cannot be explained by the small increase in bilayer thickness (∼16%). We suggest that the reduction of the specific capacitance might be due to the dehydration of the polar head groups caused by the insertion of cholesterol molecules in the bilayer. The results reported confirm the potential of in-liquid SDM to study the electrical and physicochemical properties of lipid bilayers at very small scales (down to ∼200 nm here), with implications in fields such as biophysics, bioelectricity, biochemistry, and biosensing.

Physicochemical properties of L-alpha dipalmitoyl phosphatidylcholine large unilamellar vesicles: Effect of hydrophobic block (PLA/PCL) of amphipathic diblock copolymers

In the present work, we show how amphipathic diblock copolymers affect the physicochemical properties of the lipid bilayer of DPPC liposome. Diblock copolymers proposed for this study are focused in the difference between PLA and PCL hydrophobic block, because PLA and PCL differ in their glass transition temperature, where a higher ratio of PLA, lowers the flexibility of the diblock copolymer. On the contrary, a greater proportion of PCL makes the diblock copolymer more flexible. This flexibility difference between hydrophobic block would affect the physicochemical properties of lipid bilayer of DPPC. The difference of rigidity or flexibility of hydrophobic block and their interaction with DPPC large unilamellar vesicles (LUVs) was evaluated at low and high copolymers concentration. The copolymer concentrations used were chosen based on their respective cmc. We measure (a) Thermotropic behavior from GP of Laurdan and fluorescence anisotropy of DPH; (b) Relation between wavelength excitation and generalized polarization of Laurdan; (c) Time-resolved fluorescence anisotropy of DPH; (d) Water outflow through the lipid bilayer and (e) calcein release from DPPC LUVs. Furthermore, large unilamellar vesicles in the absence and in the presence of different copolymers were characterized by size and zeta-potential. The results show that the diblock copolymer at high PLA/PCL ratio, that is, greater rigidity of hydrophobic block produces an increase of the phase transition temperature (Tm). For DPPC LUVs, Tm increase 3.5 °C at low and about 4.5 °C at high copolymers concentration, sensed by Laurdan and DPH fluorescent probes, although the DPPC/copolymers molar ratio for Cop4 is higher than Cop3, Cop2 and Cop1. In addition, we observed a decrease in the polarity of microenvironments in the bilayer and an increase in the order of the acyl chains in the bilayer to a high proportion of PLA. Furthermore, the presence of diblock copolymer with high proportion of PLA, decreases water outflow from DPPC liposome and water efflux is slower; leading to a decrease in calcein release from DPPC liposomes. Our results clearly show that the greater the stiffness of the hydrophobic block, greater degree of packaging of the lipid bilayer, greater the order of the acyl chains, and greater retention of water and calcein inside the liposome. Therefore, the presence of AB-type diblock copolymers with a more rigid hydrophobic block, stabilizes the lipid bilayer and would allow a more controlled release of water, and encapsulated molecules inside of the DPPC liposome.

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 70 °C 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.

Mechanisms of Regulation of Amyloid-Induced Permeability of Model Lipid Membranes by Polyphenols

This work is devoted to the study of the processes of formation and functioning of ion channels by amyloidogenic peptides, pathological aggregation and accumulation of which is a cause of neurodegenerative disorders. The effect of the plant polyphenols phloretin, butein, resveratrol, isoliquiritigenin, 4'-hydroxychalcone, and cardamonine on the pore-forming activity of β-amyloid peptide fragment 25–35 in bilayer lipid membranes from palmitoyl-phosphocholine was studied. It was demonstrated that the introduction of phloretin, butein or isoliquiritigenin in membrane-bathing solutions to a concentration of 20 µM leads to the increase of macroscopic transmembrane currents induced by peptide. At the same time, cardamonine, 4'-hydroxychalcone, and resveratrol have no effect on the activity of β-amyloid peptide fragment 25–35. The comparison of the results of studying the effect of tested polyphenols on electric and elastic properties of model membranes and pore-forming ability of β-amyloid peptide fragment 25–35 allowed it to concluded that there is no connection between the potentiating effect of phloretin, butein, or isoliquiritigenin and changes in the physicochemical properties of lipid bilayers. Results obtained by means of a confocal fluorescent microscopy indicate that the domain organization of the lipid bilayer may play a role in the pore-forming activity of amyloidogenic peptide. The results of electrophysiological measurements obtained for α-synuclein (another protein forming ion-permeable pores) do not contradict the hypothesis of binding of polyphenols, hydroxylated in the 7 position of the A cycle and in the 4'-position of the B cycle, with an open propane fragment with β-layers formed by amyloid peptides.

Role of Lipid Composition, Physicochemical Interactions, and Membrane Mechanics in the Molecular Actions of Microbial Cyclic Lipopeptides.

Several experimental and theoretical studies have extensively investigated the effects of a large diversity of antimicrobial peptides (AMPs) on model lipid bilayers and living cells. Many of these peptides disturb cells by forming pores in the plasma membrane that eventually lead to the cell death. The complexity of these peptide-lipid interactions is mainly related to electrostatic, hydrophobic and topological issues of these counterparts. Diverse studies have shed some light on how AMPs act on lipid bilayers composed by different phospholipids, and how mechanical properties of membranes could affect the antimicrobial effects of such compounds. On the other hand, cyclic lipopeptides (cLPs), an important class of microbial secondary metabolites, have received comparatively less attention. Due to their amphipathic structures, cLPs exhibit interesting biological activities including interactions with biofilms, anti-bacterial, anti-fungal, antiviral, and anti-tumoral properties, which deserve more investigation. Understanding how physicochemical properties of lipid bilayers contribute and determining the antagonistic activity of these secondary metabolites over a broad spectrum of microbial pathogens could establish a framework to design and select effective strategies of biological control. This implies unravelling-at the biophysical level-the complex interactions established between cLPs and lipid bilayers. This review presents, in a systematic manner, the diversity of lipidated antibiotics produced by different microorganisms, with a critical analysis of the perturbing actions that have been reported in the literature for this specific set of membrane-active lipopeptides during their interactions with model membranes and in vivo. With an overview on the mechanical properties of lipid bilayers that can be experimentally determined, we also discuss which parameters are relevant in the understanding of those perturbation effects. Finally, we expose in brief, how this knowledge can help to design novel strategies to use these biosurfactants in the agronomic and pharmaceutical industries.

Ergosterol Turnover in Yeast: An Interplay between Biosynthesis and Transport.

Sterols are important components of biological membranes that determine the physicochemical properties of lipid bilayer and regulate the functioning of membrane proteins. Being insoluble in water, sterols cannot diffuse between the membrane compartments separated by an aqueous phase. For this reason, distribution of sterols across cellular membranes is rather uneven. Membrane-to-membrane transport of sterols occurs mainly in a non-vesicular fashion and is provided by Lam and Osh proteins. In this review, we discuss the consequences of impairments in sterol biosynthesis and transport mostly relying on the studies performed on the model organism Saccharomyces cerevisiae. Despite the fact that molecular mechanisms underlying the functioning of Lam and Osh proteins are well established, the biological roles of these proteins are still unclear, because deletions of corresponding genes do not affect yeast phenotype. At the same time, disruptions in the biosynthesis of ergosterol, the major sterol of S. cerevisiae, lead to either cell death or reduced stress resistance. However, under certain conditions (e.g., mild salt or thermal stresses), a decrease in the ergosterol levels causes an increase in cell resistance. This suggests that the cells possess a mechanism facilitating rapid adjustment of the plasma membrane sterol content. We argue that the biological role of Lam proteins is, in particular, fast optimization of sterol composition of cell membranes.

The influence of mild acidity on lysyl-phosphatidylglycerol biosynthesis and lipid membrane physico-chemical properties in methicillin-resistant Staphylococcus aureus

The increased biosynthesis of lysyl-phosphatidylglycerol in Staphylococcus aureus when cultured under conditions of mild acidity and the resultant increased proportion of this lipid in the plasma membrane of the bacterium, alters the physico-chemical properties of lipid bilayers in a manner which is itself dependent upon environmental pH. Clinically relevant strains of S. aureus, both methicillin susceptible and resistant, all exhibited increased lysyl-phosphatidylglycerol biosynthesis in response to mild environmental acidity, albeit to differing degrees, from ∼30% to ∼55% total phospholipid. Polar lipid extracts from these bacteria were analysed by 31P NMR and reconstituted into vesicles and monolayers, which were characterised by zeta potential measurements and Langmuir isotherms respectively. A combination of increased lysyl-phosphatidylglycerol content and mild environmental acidity were found to synergistically neutralise the charge of the membranes, in one instance altering the zeta potential from −56mV to +21mV, and induce closer packing between the lipids. Challenge of reconstituted S. aureus lipid model membranes by the antimicrobial peptide magainin 2 F5W was examined using monolayer subphase injection and neutron diffraction, and revealed that ionisation of the headgroup α-amine of lysyl-phosphatidylglycerol at pH 5.5, which reduced the magnitude of the peptide-lipid interaction by 80%, was more important for resisting peptide partitioning than increased lipid content alone. The significance of these results is discussed in relation to how colonising mildly acidic environments such as human mucosa may be facilitated by increased lysyl-phosphatidylglycerol biosynthesis and the implications of this for further biophysical analysis of the role of this lipid in bacterial membranes.

Effect of preparation protocol on physicochemical features and biointeractions of pegylated liposomes

Poly(ethylene glycol) (PEG) stabilized liposomes are widely used in drug delivery research because these surface grafted polymers stabilize liposomes in blood stream. However, hydrophilic polymers also affect lipid membrane organization and it is fundamental to investigate their effect on the physicochemical properties of lipid bilayers as well as on their interaction with biological membrane. PEG monolaurate was included in liposomes formulated with dimyristoyl-sn-glycero-3-phosphocholine and one of two cationic gemini surfactants that differ each other for the configuration of a stereogenic center on the polar headgroup. These formulations previously showed good efficiency as drug and DNA delivery systems. The effect of the presence of different amounts of a PEG-lipid and of the liposome preparation protocol was investigated by turbidimetry, dynamic laser light scattering, differential scanning calorimetry and electrophoretic mobility. Flow cytometry and laser scanning confocal microscopy on murine (C6) and on human glioblastoma (LN229) cell lines were also carried out to evaluate the influence of pegylation and of the protocol of preparation on the interaction of liposomes with the biological milieu.

Lipidomic approach for stratification of acute myeloid leukemia patients

The pathogenesis and progression of many tumors, including hematologic malignancies is highly dependent on enhanced lipogenesis. De novo fatty-acid synthesis permits accelerated proliferation of tumor cells by providing membrane components but these may also alter physicochemical properties of lipid bilayers, which can impact signaling or even increase drug resistance in cancer cells. Cancer type-specific lipid profiles would permit us to monitor and interpret actual effects of lipid changes, potential fingerprints of individual tumors to be explored as diagnostic markers. We have used the shotgun MS approach to identify lipid patterns in different types of acute myeloid leukemia (AML) patients that either show no karyotype change or belong to t(8;21) or inv16 types. Differences in lipidomes of t(8;21) and inv(16) patients, as compared to AML patients without karyotype change, presented mostly as substantial modulation of ceramide/sphingolipid synthesis. Furthermore, between the t(8;21) and all other patients we observed significant changes in physicochemical membrane properties. These were related to a marked alteration in lipid saturation levels. The discovered differences in lipid profiles of various AML types improve our understanding of the pathobiochemical pathways involved and may serve in the development of diagnostic tools.

Novel benzanthrone probes for membrane and protein studies

The applicability of a series of novel benzanthrone dyes to monitoring the changes in physicochemical properties of lipid bilayer and to differentiating between the native and aggregated protein states has been evaluated. Based on the quantitative parameters of the dye-membrane and dye-protein binding derived from the fluorimetric titration data, the most prospective membrane probes and amyloid tracers have been selected from the group of examined compounds. Analysis of the red edge excitation shifts of the membrane- and amyloid-bound dyes provided information on the properties of benzanthrone binding sites within the lipid and protein matrixes. To understand how amyloid specificity of benzanthrones correlates with their structure, quantitative structure activity relationship (QSAR) analysis was performed involving a range of quantum chemical molecular descriptors. A statistically significant model was obtained for predicting the sensitivity of novel benzanthrone dyes to amyloid fibrils.

Effect of the addition of alkanols of different topology to dipalmitoyl-phosphatidylcholine vesicles in the presence of gramicidin

In the present work, we analyze the effect of incorporation of the nonanol family (e.g., 1-Nonanol (1-N), 5-Nonanol (5-N), and 2,6-Dimethyl-4-Heptanol (2,6-DH)) into DPPC LUVs in the presence of different gramicidin concentrations. The principal aim of this work is to study the effect of alkanols solubilization on the physicochemical properties of lipid bilayers in the presence of peptide trans-membrane channels, that is, the effects of nonanol family in the interface of lipid–peptide region, considering that the study provides the analysis of a ternary system by direct excitation as well as by Fluorescence Resonance Energy Transfer. Fluorescence measurements were carried out at 20°C after direct excitation of the extrinsic probe or by Fluorescence Resonance Energy Transfer (FRET) from the tryptophan group of gramicidin. Alkanol incorporation decreases with increasing gramicidin content and branching of the additives. 1-N generates most important changes in the inner part of the bilayer, where it produces an increase in bulk acyl chain mobility. Similarly, 1-N significantly modifies the properties of the hydrophilic–hydrophobic interface region sensed by Laurdan, increasing the polarity of the probe microenvironment and/or increasing the relaxation time of interfacial water molecules. On the other hand, 1-N produces a decrease in PDA fluorescence lifetime, a result that can be explained by a significant amount of water entrance to the inner part of the bilayer. The same behavior was observed when pseudo-first-order quenching rate constants by oxygen were measured. 1-N produces an increase in mobility/solubility of the oxygen in the lipid membrane, an effect that is more noticeable in the deep region of the bilayer sensed by PDA, in the absence and in the presence of 2mol% of Gr. 1-N incorporation produces a greater reduction in GP value than 5-N and 2,6-DH when Laurdan was excited by FRET. These results show that 1-N has the greatest effect in the lipidic domains near the gramicidin channel. On the other hand, excimer–monomer ratios of PDA obtained by FRET show that 1-N reduces the lateral mobility of acyl chains near the lipid–gramicidin interface when gramicidin concentration in the lipid bilayer increases. This effect is more noticeable than that obtained by direct irradiation of the probe in the presence of 5-N and 2,6-DH. On the other hand, the addition of the three alkanols in the presence of Gr produces a noticeable increase in the water permeability, particularly for 1-N. In this context, we propose a scheme that represents the effect of 1-nonanol on the water outflow in DPPC LUVs in the absence and in the presence of Gr.

Novel Benzanthrone Aminoderivatives for Membrane Studies

The potential of novel benzanthrone aminoderivatives to trace the changes in physicochemical properties of lipid bilayer has been evaluated. Binding of the dyes to the lipid bilayers composed of zwitterionic phospholipid phosphatidylcholine (PC) and its mixtures with anionic phospholipid cardiolipin (CL) and cholesterol (Chol) was followed by significant quantum yield increase with small blue shift of emission maximum. Analysis of partition coefficients of the dyes under study showed that all aminobenzanthrones possess high lipid-associating ability. The dyes A8 and AM2 proved to be sensitive to the variations in membrane chemical composition responding to the changes in bilayer hydration induced by CL and Chol.

Adding to Hans Kuhn's thesis on the emergence of the genetic apparatus: Of the Darwinian advantage to be neither too soluble, nor too insoluble, neither too solid, nor completely liquid

A scenario of the origin of the genetic apparatus is described where the surface and physico-chemical properties of lipid bilayers and multilayers of vesicles play a crucial role. Peptides, nucleic acids and lipids are ‘collaborating’ to bring about a first successful genetic apparatus. Lipidic vesicles acquire new properties through hosting nucleic acids that are transiently but covalently linked to lipophilic peptides. These peptides anchor the associated nucleic acids into the surface of lipidic vesicles. In the interior of such vesicles, within the lipidic bilayer, peptidyl transfers occur that are reminiscent of modern-day ribosomal peptidyl transfer reactions. One can expect that the growing peptides eventually acquire, stepwise and essentially arbitrarily, new functions different from anchoring nucleic acids, such as specific aminoacylation of nucleic acids, template-assisted nucleic acid synthesis, nucleotide deoxygenation and fatty acid synthesis.

The computer as a laboratory for the physical chemistry of membranes

A mini-review is given of some recent advances in the use of computer-simulation approaches to the study of physico-chemical properties of lipid bilayers and biological membranes. The simulations are based on microscopic molecular interaction models as well as random-surface models of fluid membranes. Particular emphasis is put on those properties that are controlled by the many-particle character of the lamellar membrane, i.e. correlations and fluctuations in density, composition and large-scale conformational structure. It is discussed how dynamic membrane heterogeneity arises and how it is affected by various molecular species interacting with membranes, such as cholesterol, drugs, insecticides, as well as polypeptides and integral membrane proteins. The influence of bending rigidity and osmotic-pressure gradients on large-scale membrane conformation and topology is described.