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

Solid phase extraction with rotating cigarette filter for determination of bisphenol A in source and drinking water: computational and analytical studies

An ultrasound assisted solid phase extraction method using rotating cigarette filter is developed herein to preconcentrate and determine trace amount of bisphenol in source and drinking water. Qualitative and quantitative measurements were performed using high-performance liquid chromatography coupled with ultra violet detector. Sorbent-analyte interactions were thoroughly investigated computationally and experimentally using molecular dynamics simulations; and attenuated total reflectance Fourier transform infrared spectroscopy, and Raman spectroscopy, respectively. Various extraction parameters were investigated and optimized. Under the optimal conditions, the results were linear in a low scale range of 0.01-55 ng/mL with correlation coefficient of 0.9941 and a low limit of detection (0.04 ng/mL, signal/noise = 3:1). A good precision (intra-day relative standard deviation ≤ 6.05%, inter-day relative standard deviation ≤ 7.12%) and recovery (intra-day ≥ 98.41%, inter-day ≥ 98.04%)) are obtained. Finally, the proposed solid phase extraction method offered a low cost, simple, fast, and sensitive analytical method to determine trace amount of bisphenol A in source and drinking water samples with chromatographic detection.

Correlation between the hydration of acyl chains and phosphate groups in lipid bilayers: Effect of phase state, head group, chain length, double bonds and carbonyl groups

This paper demonstrates by means of FTIR/ATR analysis that water molecules intercalate at different extents in the acyl chain region of lipid membranes in correlation with the hydration of the phosphate groups. This correlation is sensible to the chain length, the presence of double bonds and the phase state of the lipid membrane. The presence of carbonyl groups CO modifies the profile of hydration of the two regions as observed from the comparison of DMPC and 14:0 Diether PC. The different water populations in lipid interphases would give arrangements with different free energy states that could drive the interaction of biological effectors with membranes.

Functional Hydration Behavior: Interrelation between Hydration and Molecular Properties at Lipid Membrane Interfaces

Water is an abundant commodity and has various important functions. It stabilizes the structure of biological macromolecules, controls biochemical activities, and regulates interfacial/intermolecular interactions. Common aspects of interfacial water can be obtained by overviewing fundamental functions and properties at different temporal and spatial scales. It is important to understand the hydrogen bonding and structural properties of water and to evaluate the individual molecular species having different hydration properties. Water molecules form hydrogen bonds with biomolecules and contribute to the adjustment of their properties, such as surface charge, hydrophilicity, and structural flexibility. In this review, the fundamental properties of water molecules and the methods used for the analyses of water dynamics are summarized. In particular, the interrelation between the hydration properties, determined by molecules, and the properties of molecules, determined by their hydration properties, are discussed using the lipid membrane as an example. Accordingly, interesting water functions are introduced that provide beneficial information in the fields of biochemistry, medicine, and food chemistry.

Interaction of semiochemicals with model lipid membranes: A biophysical approach

Unravelling the chemical language of insects has been the subject of intense research in the field of chemical ecology for the past five decades. Insect communication is mainly based on chemosensation due to the small body size of insects, which limits their ability to produce or perceive auditory and visual signals, especially over large distances. Chemicals involved in insect communication are called semiochemicals. These volatiles and semivolatiles compounds allow to Insects to find a mate, besides the oviposition site in reproduction and food sources. Actually, insect olfaction mechanism is subject to study, but systematic analyses of the role of neural membranes are scarce. In the present work we evaluated the interactions of α-pinene, benzaldehyde, eugenol, and grandlure, among others, with a lipid membrane model using surface pressure experiments and Monte Carlo computational analysis. This allowed us to propose a plausible membranotropic mechanism of interaction between semiochemicals and insect neural membrane.

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.

Structural and thermodynamic properties of water-membrane interphases: significance for peptide/membrane interactions

Water appears as a common intermediary in the mechanisms of interaction of proteins and polypeptides with membranes of different lipid composition. In this review, how water modulates the interaction of peptides and proteins with lipid membranes is discussed by correlating the thermodynamic response and the structural changes of water at the membrane interphases. The thermodynamic properties of the lipid-protein interaction are governed by changes in the water activity of monolayers of different lipid composition according to the lateral surface pressure. In this context, different water populations can be characterized below and above the phase transition temperature in relation to the CH₂ conformers' states in the acyl chains. According to water species present at the interphase, lipid membrane acts as a water state regulator, which determines the interfacial water domains in the surface. It is proposed that those domains are formed by the contact between lipids themselves and between lipids and the water phase, which are needed to trigger adsorption-insertion processes. The water domains are essential to maintain functional dynamical properties and are formed by water beyond the hydration shell of the lipid head groups. These confined water domains probably carries information in local units in relation to the lipid composition thus accounting for the link between lipidomics and aquaomics. The analysis of these results contributes to a new insight of the lipid bilayer as a non-autonomous, responsive (reactive) structure that correlates with the dynamical properties of a living system.

Penetration of milk-derived antimicrobial peptides into phospholipid monolayers as model biomembranes

Three antimicrobial peptides derived from bovine milk proteins were examined with regard to penetration into insoluble monolayers formed with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DPPG). Effects on surface pressure (Π) and electric surface potential (ΔV) were measured, Π with a platinum Wilhelmy plate and ΔV with a vibrating plate. The penetration measurements were performed under stationary diffusion conditions and upon the compression of the monolayers. The two type measurements showed greatly different effects of the peptide-lipid interactions. Results of the stationary penetration show that the peptide interactions with DPPC monolayer are weak, repulsive, and nonspecific while the interactions with DPPG monolayer are significant, attractive, and specific. These results are in accord with the fact that antimicrobial peptides disrupt bacteria membranes (negative) while no significant effect on the host membranes (neutral) is observed. No such discrimination was revealed from the compression isotherms. The latter indicate that squeezing the penetrant out of the monolayer upon compression does not allow for establishing the penetration equilibrium, so the monolayer remains supersaturated with the penetrant and shows an under-equilibrium orientation within the entire compression range, practically.

Biological activity of antibacterial peptides matches synergism between electrostatic and non electrostatic forces

Substitution of Ala 108 and Ala 111 in the 107-115 human lysozyme (hLz) fragment results in a 20-fold increased anti-staphylococcal activity while its hemolytic activity becomes significant (30%) at very high concentrations. This analog displays an additional positive charge near the N-terminus (108) and an extra Trp residue at the center of the molecule (111), indicating that this particular amino acid sequence improves its interaction with the bacterial plasma membrane. In order to understand the role of this arrangement in the membrane interaction, studies with model lipid membranes were carried out. The interactions of peptides, 107-115 hLz and the novel analog ([K(108)W(111)]107-115 hLz) with liposomes and lipid monolayers were evaluated by monitoring the changes in the fluorescence of the Trp residues and the variation of the monolayers surface pressure, respectively. Results obtained with both techniques revealed a significant affinity increase of [K(108)W(111)]107-115 hLz for lipids, especially when the membranes containing negatively charged lipids, such as phosphatidylglycerol. However, there is also a significant interaction with zwitterionic lipids, suggesting that other forces in addition to electrostatic interactions are involved in the binding. The analysis of adsorption isotherms and the insertion kinetics suggest that relaxation processes of the membrane structure are involved in the insertion process of novel peptide [K(108)W(111)]107-115 hLz but not in 107-115 hLz, probably by imposing a reorganization of water at the interphases. In this regard, the enhanced activity of peptide [K(108)W(111)]107-115 hLz may be explained by a synergistic effect between the increased electrostatic forces as well as the increased hydrophobic interactions.

Relaxation processes in the adsorption of surface layer proteins to lipid membranes

The present work evaluates the kinetics of the interaction of S-layer protein from Lactobacillus brevis with lipid monolayers by measuring the changes in the surface pressure as a function of time for different lipid compositions and at different lateral compressions. At high surface pressures, or at high cholesterol ratios, in which membrane rigidity and surface polarity are increased, the kinetics can be described by a pure diffusional process. At low pressures or in the absence of cholesterol, the kinetics of protein interaction can be interpreted as a consequence of a relaxation process of the membrane structure coupled to diffusion. As the less packed monolayers are more hydrated, the relaxation processes at low initial surface pressures could be ascribed to changes in water organization in the membrane. These observations denote that kinetic insertion of proteins can be modulated by components that modify the hydration state of the interface.

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.

Plant fructans in stress environments: emerging concepts and future prospects

Plants are sessile and sensitive organisms known to possess various regulatory mechanisms for defending themselves under stress environments. Fructans are fructose-based polymers synthesized from sucrose by fructosyltransferases (FTs). They have been increasingly recognized as protective agents against abiotic stresses. Using model membranes, numerous in vitro studies have demonstrated that fructans can stabilize membranes by direct H-bonding to the phosphate and choline groups of membrane lipids, resulting in a reduced water outflow from the dry membranes. Inulin-type fructans are flexible random-coiled structures that can adopt many conformations, allowing them to insert deeply within the membranes. The devitrification temperature (T(g)) can be adjusted by their varying molecular weights. In addition, above T(g) their low crystallization rates ensure prolonged membrane protection. Supporting, in vivo studies with transgenic plants expressing FTs showed fructan accumulation and an associated improvement in freezing and/or chilling tolerance. The water-soluble nature of fructans may allow their rapid adaptation as cryoprotectants in order to give optimal membrane protection. One of the emerging concepts for delivering vacuolar fructans to the extracellular space for protecting the plasma membrane is vesicle-mediated, tonoplast-derived exocytosis. It should, however, be noted that natural stress tolerance is a very complex process that cannot be explained by the action of a single molecule or mechanism.