Process of destruction of large unilamellar vesicles by a zwitterionic detergent, CHAPS: partition behavior between membrane and water phases

Molecular Transformation for Self-reproducing Vesicles and Underlying Analysis Methods

Closed bilayer membranes of amphiphiles in water, termed vesicles, represent one of the promising models of primitive cellular compartments. Herein, we reviewed studies on the design and construction of vesicle-based cell models capable of sequential growth and division and their underlying analysis methods. We discussed the potential contribution of these studies to the universal understanding of the chemical/physical logics behind the steady reproduction of cellular membranes.

Nanoplasmonic Sensing and Capillary Electrophoresis for Fast Screening of Interactions between Phosphatidylcholine Biomembranes and Surfactants

Nanoplasmonic sensing (NPS), based on localized surface plasmon resonance, with sensors composed of glass covered with golden nanodisks and overlaid with a SiO2 coating was applied in this study. Egg phosphatidylcholine (eggPC), being an easily accessible membrane-forming lipid, was used for preparation of biomimicking membranes. Small unilamellar vesicles with an approximate hydrodynamic diameter of 30 nm, formed by sonication in 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid buffer, were adsorbed within 10 min on the sensor surface either as intact vesicles or as a planar bilayer. The adsorbed biomembrane systems were further utilized for interaction studies with four different well-known surfactants (negatively and positively charged, zwitterionic, and nonionic) and each surfactant was tested at concentrations below and above the critical micelle concentration (CMC). Our results allowed the evaluation of different NPS patterns for every particular supported membrane system, surfactant, and its concentration. The most significant effect on the membrane was achieved upon the introduction of zwitterionic surfactant micelles, which in fact completely solubilized and removed the lipid membranes from the sensor surface. Other surfactant micelles interacted with the membranes and formed mixed structures remaining on the sensor surface. The studies performed at the concentrations below the CMCs of the surfactants showed that different mixed systems were formed. Depending on the supported membrane system and the type of surfactant, the mixed systems indicated different formation kinetics. Additionally, the final water rinse revealed the stability of the formed systems. To investigate the effect of the studied surfactants on the overall surface charge of the biomembrane, capillary electrophoresis (CE) experiments were carried out in parallel with the NPS analysis. The electroosmotic flow mobility of an eggPC-coated fused silica capillary was used to measure the total surface charge of the biomembrane after its treatment with the surfactants. Our results indicated in general good correlation between CE and NPS data. However, some discrepancies were seen while applying either zwitterionic or positively charged surfactants. This confirmed that CE analysis was able to provide additional data about the investigated systems. Taken together, the combination of NPS and CE proved to be an efficient way to describe the nature of interactions between biomimicking membranes and amphiphilic molecules.

The mechanism of detergent solubilization of lipid bilayers

Multiple data are available on the self-assembly of mixtures of bilayer-forming amphiphiles, particularly phospholipids and micelle-forming amphiphiles, commonly denoted detergents. The structure of such mixed assemblies has been thoroughly investigated, described in phase diagrams, and theoretically rationalized in terms of the balance between the large spontaneous curvature of the curvophilic detergent and the curvophobic phospholipids. In this critical review, we discuss the mechanism of this process and try to explain the actual mechanism involved in solubilization. Interestingly, membrane solubilization by some detergents is relatively slow and the common attribute of these detergents is that their trans-bilayer movement, commonly denoted flip-flop, is very slow. Only detergents that can flip into the inner monolayer cause relatively rapid solubilization of detergent-saturated bilayers. This occurs via the following sequence of events: 1), relatively rapid penetration of detergent monomers into the outer monolayer; 2), trans-membrane equilibration of detergent monomers between the two monolayers; 3), saturation of the bilayer by detergents and consequent permeabilization of the membrane; and 4), transition of the whole bilayer to thread-like mixed micelles. When the detergent cannot flip to the inner monolayer, the outer monolayer becomes unstable due to mass imbalance between the monolayers and inclusion of the curvophilic detergent molecules in a flat surface. Consequently, the outer monolayer forms mixed micellar structures within the outer monolayer. Shedding of these micelles into the aqueous solution results in partial solubilization. The consequent leakage of detergent into the liposome results in trans-membrane equilibration of detergent and subsequent micellization through the rapid bilayer-saturation mechanism.

Comparative study of the interaction of CHAPS and Triton X-100 with the erythrocyte membrane

The zwitterionic detergent CHAPS, a derivative of the bile salts, is widely used in membrane protein solubilization. It is a "facial" detergent, having a hydrophilic side and a hydrophobic back. The objective of this work is to characterize the interaction of CHAPS with a cell membrane. To this aim, erythrocytes were incubated with a wide range of detergent concentrations in order to determine CHAPS partition behavior, and its effects on membrane lipid order, hemolytic effects, and the solubilization of membrane phospholipids and cholesterol. The results were compared with those obtained with the nonionic detergent Triton X-100. It was found that CHAPS has a low affinity for the erythrocyte membrane (partition coefficient K=0.06mM(-1)), and at sub-hemolytic concentrations it causes little effect on membrane lipid order. CHAPS hemolysis and phospholipid solubilization are closely correlated. On the other side, binding of Triton X-100 disorders the membrane at all levels, and has independent mechanisms for hemolysis and solubilization. Differential behavior was observed in the solubilization of phospholipids and cholesterol. Thus, the detergent resistant membranes (DRM) obtained with the two detergents will have different composition. The behaviors of the two detergents are related to the differences in their molecular structures, suggesting that CHAPS does not penetrate the lipid bilayer but binds in a flat position on the erythrocyte surface, both in intact and cholesterol depleted erythrocytes. A relevant result for Triton X-100 is that hemolysis is not directly correlated with the solubilization of membrane lipids, as it is usually assumed.

[World constructed by self-organization of some amphiphils--with a focus on vesicle formation--]

The world constructed by self-organization of some amphiphils was discussed on the basis of micelle formation, vesicle formation, and oriented-nano-wire formation. First, the micelle formation of a both water- and oil- soluble surfactant, Aerosol OT, was discussed. Solution states of micelles and monomer were discussed on the basis of thermodinamic and NMR spectroscopic analyses of micelle formation. Next, micelle-vesicle transition was discussed. It was proposed that the phospholipid LUV formation by removing detergents and destruction by adding detergents occurred via 4 stages. The 4 stage model instead of the 3 stage model could not only elucidate the complicated phenomena observed during micelle-vesicle transition, but predicted the size and properties of the vesicles formed by detergent removal from mixed micelles. Next, the vesicle formation of a fatty acid with a single hydrophobic chain different from phospholipid, which has two hydorophobic chains, was discussed. The vesicle formation was strongly affected by the presence of preformed vesicles and the size was biased on the preformed vesicles. It was shown there exist two pass ways in the process of micelle-vesicle transition by pH jump. One is fission of the preformed vesicles after transfer of monomers from newly added oleate micelles and the other is transition from the mixed micelles after partial solubilization by the oreate micelles. Then, the vesicle formation of HCO-10, which has 3 hydrophobic chains, the mixed vesicle formation of phosphatidylethanolamine and lysophosphtidylcholine, which can not form vesicles, and the phospholipid vesicle formation and destruction by removing and adding PEG-lipid, were discussed. Lastly, oriented nano wire formation of mulamyldipeptid-conjugated lipids with ca 5 nm of diameter was discussed.

Membrane interactions of novicidin, a novel antimicrobial peptide: phosphatidylglycerol promotes bilayer insertion

Novicidin is an antimicrobial peptide derived from ovispirin, a cationic peptide which originated from the ovine cathelicidin SMAP-29. Novicidin, however, has been designed to minimize the cytotoxic properties of SMAP-29 and ovisipirin toward achieving potential therapeutic applications. We present an analysis of membrane interactions and lipid bilayer penetration of novicidin, using an array of biophysical techniques and biomimetic membrane assemblies, complemented by Monte Carlo (MC) simulations. The data indicate that novicidin interacts minimally with zwitterionic bilayers, accounting for its low hemolytic activity. Negatively charged phosphatidylglycerol, on the other hand, plays a significant role in initiating membrane binding of novicidin, and promotes peptide insertion into the interface between the lipid headgroups and the acyl chains. The significant insertion into bilayers containing negative phospholipids might explain the enhanced antibacterial properties of novicidin. Overall, this study highlights two distinct outcomes for membrane interactions of novicidin, and points to a combination between electrostatic attraction to the lipid/water interface and penetration into the subsurface lipid headgroups region as important determinants for the biological activity of novicidin.

Membrane solubilisation and reconstitution by octylglucoside: comparison of synthetic lipid and natural lipid extract by isothermal titration calorimetry

We have studied the solubilisation and reconstitution of lipid membranes composed of either synthetic phosphatidylcholine or Escherichia. coli polar lipid extract by the non-ionic detergent octylglucoside. For both lipid systems, composition-dependent transformations of unilamellar vesicles into micelles or vice versa were followed by high-sensitivity isothermal titration calorimetry. Data obtained over a range of detergent and lipid concentrations could be rationalised in terms of a three-stage phase separation model involving bilayer, bilayer/micelle coexistence, and micellar ranges, yielding the detergent/lipid phase diagrams and the bilayer-to-micelle partition coefficients of both detergent and lipid. The most notable difference between the lipids investigated was a substantial widening of the bilayer/micelle coexistence range for E. coli lipid, which was due to an increased preference of the detergent and a decreased affinity of the lipid for the micellar phase as compared with the bilayer phase. These effects on the bilayer-to-micelle partition coefficients could be explained by the high proportion in E. coli membranes of lipids possessing negative spontaneous curvature, which hampers both their transfer into strongly curved micellar structures as well as the insertion of detergent into condensed bilayers.

Physicochemical properties and antioxidant activity of gamma-oryzanol-loaded liposome formulations for topical use

The objective of this study is to prepare the gamma-oryzanol-loaded liposomes and investigate their physicochemical properties and antioxidant activity intended for cosmetic applications. Liposomes, Composing phosphatidylCholine (PC) and Cholesterol (Chol), CHAPS or sodium taurocholate (NaTC) were prepared by sonication method. Gamma-oryzanol-loaded liposomes were prepared by using 3, 5 and 10% gamma-oryzanol as an initial concentration. The formulation factors in a particular type and composition of lipid and initial drug loading on the physicochemical properties (i.e., particle size, zeta potential, entrapment efficiency, drug release) and antioxidant activity were studied. The particle sizes of bare liposomes were in nanometer range. The gamma-oryzanol-loaded liposomes in formulations of PC/CHAPS and PC/NaTC liposomes were smaller than PC/Chol liposomes. The incorporation efficiency of 10% gamma-oryzanol-loaded PC/Chol liposomes was less than gamma-oryzanol-loaded PC/CHAPS liposomes and PC/NaTC liposomes allowing higher in vitro release rate due to higher free gamma-oryzanol in buffer solution. The antioxidant activity of gamma-oryzanol-loaded liposomes was not different from pure gamma-oryzanol. Both gamma-oryzanol-loaded PC/CHAPS liposomes and PC/NaTC liposomes were showed to enhance the antioxidant activity in NHF cells. gamma-oryzanol-loaded PC/Chol liposomes demonstrated the lowest cytotoxicity in NHF cells. It was conceivably concluded that liposomes prepared in this study are suitable for gamma-oryzanol incorporation without loss of antioxidant activity.

Interactions of surfactants with lipid membranes

Surfactants are surface-active, amphiphilic compounds that are water-soluble in the micro- to millimolar range, and self-assemble to form micelles or other aggregates above a critical concentration. This definition comprises synthetic detergents as well as amphiphilic peptides and lipopeptides, bile salts and many other compounds. This paper reviews the biophysics of the interactions of surfactants with membranes of insoluble, naturally occurring lipids. It discusses structural, thermodynamic and kinetic aspects of membrane-water partitioning, changes in membrane properties induced by surfactants, membrane solubilisation to micelles and other phases formed by lipid-surfactant systems. Each section defines and derives key parameters, mentions experimental methods for their measurement and compiles and discusses published data. Additionally, a brief overview is given of surfactant-like effects in biological systems, technical applications of surfactants that involve membrane interactions, and surfactant-based protocols to study biological membranes.

Additive action of two or more solutes on lipid membranes

A wide variety of biological processes, pharmaceutical applications, and technical procedures is based on the combined action of two or more soluble compounds to perturb, permeabilize, or lyse biological membranes. Here we present a general model describing the additive action of solutes on the properties of membranes or micelles. The onset and completion of membrane solubilization induced by two surfactants (lauryl maltoside, with nonyl maltoside, octyl glucoside, or CHAPS, respectively) are very well described by our model on the basis of their individual partition coefficients, cmc's, and critical mole ratios R e sat and R e sol as detected by isothermal titration calorimetry. This suggests that the thermodynamic phase transition is governed by a single parameter (e.g., spontaneous curvature) in spite of the complexity of structural changes. Such surfactant mixtures show unique features such as nonlinear solubilization boundaries and concentration-dependent effective partition coefficients. Other phenomena such as membrane leakage are predicted to obey additive action if the solutes act via the same mechanism (e.g., toroidal pore formation) but deviate from the model in the case of independent, synergistic, or antagonistic action.

[Mechanism of micelle-vesicle transformation and control of vesicular sizes and properties]

The mechanism of vesicle-to-micelle or micelle-to-vesicle transition was studied in order to control sizes and fluidities of vesicles during periods of preparation. Dependence of particle sizes measured by quasi-elastic light scattering, turbidities, fluidity parameters monitored by ESR spectroscopy, and morphological changes of mixed aggregates of egg yolk phosphatidylcholine (EPC) and a detergent (octylglucoside (OG) or sodium cholate (Na-chol)) on detergent concentration provided a model of vesicle destruction. It possessed three phase transition points, and proceeded in a stepwise fashion: vesicles, small particles containing large amounts of detergents (SUV(*)), intermediate structures, and mixed micelles. Vesicle formation on removal of detergents from micelles proceeded oppositely. Micelle-vesicle transition mechanism was common to all detergents examined. The feature of the mechanism was the presence of SUV(*). Next, SUV(*) was prepared by adding appropriate amount of a detergent to small unilamellar vesicles obtained by sonication. Time-dependent size growth of the SUV(*) was remarkable in the case of OG-containing SUV(*), but was insignificant in the case of Na-chol-containing SUV(*), suggesting the size determining step to be the stage of the SUV(*). The tendency to produce large or small vesicles from micelles was related to the absence or presence, respectively, of a net charge in the detergent molecule. The fluidities of EPC micelles containing small amounts of a detergent possessing a steroidal structure (e.g., Na-chol or CHAPS) were significantly smaller than the corresponding values of a detergent without a steroidal structure (e.g., OG), suggesting a method of control of orderliness of hydrocarbon chains in EPC vesicles.