Effect of arbutin on the dipole potential and area per lipid of ester and ether phosphatidylcholine and phosphatidyl ethanolamine monolayers

The role of hydrophobic patches of de novo designed MSI-78 and VG16KRKP antimicrobial peptides on fragmenting model bilayer membranes

Antimicrobial peptides (AMPs) with cell membrane lysing capability are considered potential candidates for the development of the next generation of antibiotics. Designing novel AMPs requires an in-depth understanding of the mechanism of action of the peptides. In this work, we used various biophysical techniques including ³¹P solid-state NMR to examine the interaction of model membranes with amphipathic de novo-designed peptides. Two such peptides, MSI-78 and VG16KRKP, were designed with different hydrophobicity and positive charges. The model lipid membranes were constituted by mixing lipids of varying degrees of 'area per lipid' (APL), which directly affected the packing properties of the membrane. The observed emergence of the isotropic peak in ³¹P NMR spectra as a function of time is a consequence of the fragmentation of the membrane mediated by the peptide interaction. The factors such as the charges, overall hydrophilicity of the AMPs, as well as lipid membrane packing, contributed to the kinetics of membrane fragmentation. Furthermore, we anticipate the designed AMPs follow the carpet and toroidal pore mechanisms when lysing the cell membrane. This study highlights the significance of the effect of the overall charges and the hydrophobicity of the novel AMPs designed for antimicrobial activity.

Differential sensitivity of pHLIP to ester and ether lipids

pH (low) insertion peptide (pHLIP) is a polypeptide from the third transmembrane helix of bacteriorhodopsin. The pH-dependent membrane insertion of pHLIP has been conveniently exploited for translocation of cargo molecules and as a novel imaging agent in cancer biology due to low extracellular pH in cancer tissues. Although the application of pHLIP for imaging tumor and targeted drug delivery is well studied, literature on pHLIP-membrane interaction is relatively less studied. Keeping this in mind, we explored the differential interaction of pHLIP with ester and ether lipid membranes utilizing fluorescence and CD spectroscopy. We report, for the first time, higher binding affinity of pHLIP toward ether lipid relative to ester lipid membranes. There results gain relevance since Halobacterium halobium (source of bacteriorhodopsin) is enriched with ether lipids. In addition, we monitored the difference in microenvironment around pHLIP tryptophans utilizing red edge excitation shift and observed increased motional restriction of water molecules in the interfacial region in ether lipid membranes. These changes were accompanied with increase in helicity of pHLIP in ether lipid relative to ester lipid membranes. Our results assume further relevance since ether lipids are upregulated in cancer cells and have emerged as potential biomarkers of various diseases including cancer.

V-Shaped Molecular Configuration of Wax Esters of Jojoba Oil in a Langmuir Film Model

The aim of the present work was to understand the interfacial properties of a complex mixture of wax esters (WEs) obtained from Jojoba oil (JO). Previously, on the basis of molecular area measurements, a hairpin structure was proposed as the hypothetical configuration of WEs, allowing their organization as compressible monolayers at the air-water interface. In the present work, we contributed with further experimental evidence by combining surface pressure (π), surface potential (Δ V), and PM-IRRAS measurements of JO monolayers and molecular dynamic simulations (MD) on a modified JO model. WEs were self-assembled in Langmuir films. Compression isotherms exhibited π_lift-off at 100 Ų/molecule mean molecular area ( A_lift-off) and a collapse point at π_c ≈ 2.2 mN/m and A_c ≈ 77 Ų/molecule. The Δ V profile reflected two dipolar reorganizations, with one of them at A > A_lift-off due to the release of loosely bound water molecules and another one at A_c < A < A_lift-off possibly due to reorientations of a more tightly bound water population. This was consistent with the maximal SP value that was calculated according to a model that considered two populations of oriented water and was very close to the experimental value. The orientation of the ester group that was assumed in that calculation was coherent with the PM-IRRAS behavior of the carbonyl group with the C═O oriented toward the water and the C-O oriented parallel to the surface and was in accordance with their orientational angles (∼45 and ∼90°, respectively) determined by MD simulations. Taken together, the present results confirm a V shape rather than a hairpin configuration of WEs at the air-water interface.

Hydration in Lipid Monolayers: Correlation of Water Activity and Surface Pressure

In order to give a physical meaning to each region of the membrane we define the interphase as the region in a lipid membrane corresponding to the polar head groups imbibed in water with net different properties than the hydrocarbon region and the water phase. The interphase region is analyzed under the scope of thermodynamics of surface and solutions based on the definition of Defay-Prigogine of an interphase and the derivation that it has in the understanding of membrane processeses in the context of biological response. In the view of this approach, the complete monolayer is considered as the lipid layer one molecule thick plus the bidimensional solution of the polar head groups inherent to it (the interphase region). Surface water activity appears as a common factor for the interaction of several aqueous soluble and surface active proteins with lipid membranes of different composition. Protein perturbation can be measured by changes in the surface pressure of lipid monolayers at different initial water surface activities. As predicted by solution chemistry, the increase of surface pressure is independent of the particle nature that dissolves. Therefore, membranes give a similar response in terms of the determined surface states given by water activity independent of the protein or peptide.

Effect of dipole potential variations on the surface charge potential of lipid membranes

When the dipole potential of dimyristoylphosphatidylcholine (DMPC) monolayers was decreased, either by the insertion of phloretin or by the elimination of carbonyl groups at the interphase, the surface charge potential was displaced to lower negative values. At low ionic strength, the decrease of the negative charge density can be ascribed to a different exposure of the phosphate to water, as there is a good correlation to an increase in the area per lipid. At high ionic strength, the magnitude of the changes in the zeta potential produced by the effects on the dipole potential was found to be dependent on the type of anions present in the subphase. Differences between Cl- and ClO4- were ascribed to the adsorption of anions according to their different hydrations and polarizabilities. The influence of a low dipole potential on the anion adsorption can be ascribed to a less positive image charge at the membrane interior, resulting from an increase in the hydrocarbon core permittivity. This is congruent with the neutralization of interfacial dipoles and the area increase, as well as with the decrease in packing of the hydrocarbon groups. Phloretin did not cause changes in the dipole potential of dimyristoylphosphatidylethanolamine (DMPE), and in consequence, no effects on the zeta potential were measured. It is concluded that changes in the inner water/hydrocarbon plane affect the electrostatic potential measured in the outer plane of the polar headgroup region.

Structural and functional properties of hydration and confined water in membrane interfaces

The scope of the present review focuses on the interfacial properties of cell membranes that may establish a link between the membrane and the cytosolic components. We present evidences that the current view of the membrane as a barrier of permeability that contains an aqueous solution of macromolecules may be replaced by one in which the membrane plays a structural and functional role. Although this idea has been previously suggested, the present is the first systematic work that puts into relevance the relation water-membrane in terms of thermodynamic and structural properties of the interphases that cannot be ignored in the understanding of cell function. To pursue this aim, we introduce a new definition of interphase, in which the water is organized in different levels on the surface with different binding energies. Altogether determines the surface free energy necessary for the structural response to changes in the surrounding media. The physical chemical properties of this region are interpreted in terms of hydration water and confined water, which explain the interaction with proteins and could affect the modulation of enzyme activity. Information provided by several methodologies indicates that the organization of the hydration states is not restricted to the membrane plane albeit to a region extending into the cytoplasm, in which polar head groups play a relevant role. In addition, dynamic properties studied by cyclic voltammetry allow one to deduce the energetics of the conformational changes of the lipid head group in relation to the head-head interactions due to the presence of carbonyls and phosphates at the interphase. These groups are, apparently, surrounded by more than one layer of water molecules: a tightly bound shell, that mostly contributes to the dipole potential, and a second one that may be displaced by proteins and osmotic stress. Hydration water around carbonyl and phosphate groups may change by the presence of polyhydroxylated compounds or by changing the chemical groups esterified to the phosphates, mainly choline, ethanolamine or glycerol. Thus, surface membrane properties, such as the dipole potential and the surface pressure, are modulated by the water at the interphase region by changing the structure of the membrane components. An understanding of the properties of the structural water located at the hydration sites and the functional water confined around the polar head groups modulated by the hydrocarbon chains is helpful to interpret and analyze the consequences of water loss at the membranes of dehydrated cells. In this regard, a correlation between the effects of water activity on cell growth and the lipid composition is discussed in terms of the recovery of the cell volume and their viability. Critical analyses of the properties of water at the interface of lipid membranes merging from these results and others from the literature suggest that the interface links the membrane with the aqueous soluble proteins in a functional unit in which the cell may be considered as a complex structure stabilized by water rather than a water solution of macromolecules surrounded by a semi permeable barrier.

Effect of trehalose on the contributions to the dipole potential of lipid monolayers

The dipole potential and the area changes induced by trehalose on dimyristoyl phosphatidylcholine (DMPC), 1,2-di-O-tetradecyl-sn-glycero-3-phosphocholine (dietherPC), dimyristoyl phosphatidylethanolamine (DMPE), 1,2-di-O-tetradecyl-sn-glycero-3-phosphoethanolamine (dietherPE) monolayers have been studied at different temperatures. The insertion of trehalose into DMPC monolayers in the fluid and gel states requires of the presence of carbonyl groups. The area increase observed at 0.15M trehalose is congruent with the decrease in the dipole potential. However, in dietherPC, in which trehalose does not affect the area, a decrease in the dipole potential is also observed. This is interpreted as a result of the displacement of water from the phosphate groups exposed to the aqueous phase. In DMPE, trehalose also decreases the dipole potential without affecting the area of saturated monolayers and in dietherPE no effect on dipole potential and area was observed. It is concluded that the spacer effect of trehalose depends on the specific interaction with CO, which is modulated by the strength of the interaction of the PO groups with lateral NH groups. However, it is not the only contribution to the dipole potential decrease.

Superficially active water in lipid membranes and its influence on the interaction of an aqueous soluble protease

The purpose of this paper is to demonstrate that the interaction of an aqueous soluble enzyme with lipid membranes is influenced by the lipid composition of the interphase. The results show that the interaction of an aqueous soluble protease, Rennet from Mucor miehei, depends on the exposure of the carbonyl and phosphate groups at the membrane interphase. The changes produced by the protease on the surface pressure of monolayers of dimyristoylphosphatidylcholine (DMPC); dioleoylphosphatidylcholine (DOPC); diphytanoylphosphatidylcholine (DPhPC); dipalmitoylphosphatidylcholine (DPPC); di-O-tetradecylphosphatidyl-choline [D(ether)PC]; dimyristoylphosphatidylethanolamine (DMPE); di-O-tetradecyl-phosphatidylethanolamine [D(ether)PE] were measured at different initial surface pressures. The meaning of the DeltaPi vs. Pi curves was interpreted in the light of the concept of interphase given by Defay and Prigogine [R. Defay, I. Prigogine, Surface Tension and Adsorption, John Wiley & Sons, New York, 1966, pp. 273-277] considering the interphase as a bidimensional solution of polar head groups. With this approach, and based on reported evidences that carbonyls and phosphates are the main hydration sites of the lipid membranes, it is suggested that the mechanism of interaction of aqueous soluble protein involves water beyond the hydration shell. At high surface pressure, only water strongly bound to carbonyl and phosphate groups is present and the interaction is not occurring. In contrast, at low surface pressures, the protease-membrane interaction is a function of acyl chain for different polar groups. This is interpreted, as a consequence of the changes in the interfacial tension produced by the displacement of water confined between the hydrated head groups.

Arbutin blocks defects in the ripple phase of DMPC bilayers by changing carbonyl organization

The effect of arbutin, a 4-hydroxyphenyl-beta-glucopyranoside, on dimyristoylphosphatidylcholine (DMPC) bilayers was studied by turbidimetry, EPR and FTIR spectroscopies. The disruption of DMPC multilamellar vesicles (MLV's) with monomyristoylphosphatidylcholine (lysoPC), a product of hydrolysis of phospholipase A(2) (PLA(2)), is more efficient at 18 degrees C, where DMPC MLV's are known to be in the ripple P(beta') phase, than at 10 degrees C (L(beta') flat gel phase). Disruption at 18 degrees C was inhibited by increasing concentrations of arbutin in the solution. This inhibition was correlated with the disappearance of the ripple phase in MLV's when arbutin is present. Shifts in FTIR carbonyl bands caused by arbutin or by temperature changes allow us to propose a model. It is interpreted that the changes in the water-hydrocarbon interface caused by arbutin, forcing a reaccommodation of the carbonyl groups, eliminate the topological defects in the lattice due to mismatches among regions with different area per lipid where lysoPC can insert.