Vesicle- vesicle interactions in sonicated dispersions of dipalmitoylphosphatidylcholine

Hydrophobic Characteristic Is Energetically Preferred for Cysteine in a Model Membrane Protein

The naturally occurring amino acid cysteine has often been implicated with a crucial role in maintaining protein structure and stability. An intriguing duality in the intrinsic hydrophobicity of the cysteine side chain is that it exhibits both polar as well as hydrophobic characteristics. Here, we have utilized a cysteine-scanning mutational strategy on the transmembrane β-barrel PagP to examine the membrane depth-dependent energetic contribution of the free cysteine side chain (thiolate) versus the parent residue at an experimental pH of 9.5 in phosphatidylcholine vesicles. We find that introduction of cysteine causes destabilization at several of the 26 lipid-facing sites of PagP that we mutated in this study. The destabilization is minimal (0.5-1.5 kcal/mol) when the mutation is toward the bilayer midplane, whereas it is higher in magnitude (3.0-5.0 kcal/mol) near the bilayer interface. These observations suggest that cysteine forms more favorable interactions with the hydrophobic lipid core as compared to the amphiphilic water-lipid interface. The destabilizing effect is more pronounced when cysteine replaces the interfacial aromatics, which are known to participate in tertiary interaction networks in transmembrane β-barrels. Our observations from experiments involving the introduction of cysteine at the bilayer midplane further strengthen previous views that the free cysteine side chain does possess strongly apolar characteristics. Additionally, the free energy changes observed upon cysteine incorporation show a depth-dependent correlation with the estimated energetic cost of partitioning derived from reported hydrophobicity scales. Our results and observations from the thermodynamic analysis of the PagP barrel may explain why cysteine, despite possessing a polar sulfhydryl group, tends to behave as a hydrophobic (rather than polar) residue in folded protein structures.

Energetics of side-chain partitioning of β-signal residues in unassisted folding of a transmembrane β-barrel protein

The free energy of water-to-interface amino acid partitioning is a major contributing factor in membrane protein folding and stability. The interface residues at the C terminus of transmembrane β-barrels form the β-signal motif required for assisted β-barrel assembly in vivo but are believed to be less important for β-barrel assembly in vitro. Here, we experimentally measured the thermodynamic contribution of all 20 amino acids at the β-signal motif to the unassisted folding of the model β-barrel protein PagP. We obtained the partitioning free energy for all 20 amino acids at the lipid-facing interface (ΔΔG⁰_w,i(φ)) and the protein-facing interface (ΔΔG⁰_w,i(π)) residues and found that hydrophobic amino acids are most favorably transferred to the lipid-facing interface, whereas charged and polar groups display the highest partitioning energy. Furthermore, the change in non-polar surface area correlated directly with the partitioning free energy for the lipid-facing residue and inversely with the protein-facing residue. We also demonstrate that the interface residues of the β-signal motif are vital for in vitro barrel assembly, because they exhibit a side chain–specific energetic contribution determined by the change in nonpolar accessible surface. We further establish that folding cooperativity and hydrophobic collapse are balanced at the membrane interface for optimal stability of the PagP β-barrel scaffold. We conclude that the PagP C-terminal β-signal motif influences the folding cooperativity and stability of the folded β-barrel and that the thermodynamic contributions of the lipid- and protein-facing residues in the transmembrane protein β-signal motif depend on the nature of the amino acid side chain.

The effect of the placement and total charge of the basic amino acid clusters on antibacterial organism selectivity and potency

Extensive circular dichroism, isothermal titration calorimetry and induced calcein leakage studies were conducted on a series of antimicrobial peptides (AMPs), with a varying number of Lys residues located at either the C-terminus or the N-terminus to gain insight into their effect on the mechanisms of binding with zwitterionic and anionic membrane model systems. Different CD spectra were observed for these AMPs in the presence of zwitterionic DPC and anionic SDS micelles indicating that they adopt different conformations on binding to the surfaces of zwitterionic and anionic membrane models. Different CD spectra were observed for these AMPs in the presence of zwitterionic POPC and anionic mixed 4:1 POPC/POPG LUVs and SUVs, indicating that they adopt very different conformations on interaction with these two types of LUVs and SUVs. In addition, ITC and calcein leakage data indicated that all the AMPs studied interact via very different mechanisms with anionic and zwitterionic LUVs. ITC data suggest these peptides interact primarily with the surface of zwitterionic LUVs while they insert into and form pores in anionic LUVs. CD studies indicated that these compounds adopt different conformations depending on the ratio of POPC to POPG lipids present in the liposome. There are detectable spectroscopic and thermodynamic differences between how each of these AMPs interacts with membranes, that is position and total charge density defines how these AMPs interact with specific membrane models and thus partially explain the resulting diversity of antibacterial activity of these compounds.

Determining the effect of the incorporation of unnatural amino acids into antimicrobial peptides on the interactions with zwitterionic and anionic membrane model systems

Circular Dichroism (CD), isothermal calorimetry (ITC) and calcein fluorescence leakage experiments were conducted to provide insight into the mechanisms of binding of a series of antimicrobial peptides containing unnatural amino acids (Ac-XF-Tic-Oic-XK-Tic-Oic-XF-Tic-Oic-XK-Tic-KKKK-CONH(2)) to zwitterionic and anionic micelles, SUVs and LUVs; where X (Spacer# 1) is either Gly, β-Ala, Gaba or 6-aminohexanoic acid. It is the intent of this investigation to correlate these interactions with the observed potency and selectivity against several different strains of bacteria. The CD spectra of these compounds in the presence of zwitterionic DPC micelles and anionic SDS micelles are very different indicating that these compounds adopt different conformations on binding to the surface of anionic and zwitterionic membrane models. These compounds also exhibited very different CD spectra in the presence of zwitterionic POPC and anionic mixed 4:1 POPC/POPG SUVs and LUVs, indicating the formation of different conformations on interaction with the two membrane types. This observation is also supported by ITC and calcein leakage data. ITC data suggested these peptides interact primarily with the surface of zwitterionic LUVs and was further supported by fluorescence experiments where the interactions do not appear to be concentration dependent. In the presence of anionic membranes, the interactions appear more complex and the calorimetric and fluorescence data both imply pore formation is dependent on peptide concentration. Furthermore, evidence suggests that as the length of Spacer# 1 increases the mechanism of pore formation also changes. Based on the observed differences in the mechanisms of interactions with zwitterionic and anionic LUVs these AMPs are potential candidates for further drug development.

CD spectroscopy of peptides and proteins bound to large unilamellar vesicles

Circular dichroism (CD) spectroscopy is an essential tool for determining the conformation of proteins and peptides in membranes. It can be particularly useful for measuring the free energy of partitioning of peptides into lipid vesicles. The belief is broadly held that such CD measurements can only be made using sonicated small unilamellar vesicles (SUVs) because light scattering associated with extruded large unilamellar vesicles (LUVs) is unacceptably high. We have examined this issue using several experimental approaches in which a chiral object (i.e., peptide or protein) is placed both on the membrane and outside the membrane. We show that accurate CD spectra can be collected in the presence of LUVs. This is important because SUVs, unlike LUVs, are metastable and consequently unsuitable for equilibrium thermodynamic measurements. Our data reveal that undistorted CD spectra of peptides can be measured at wavelengths above 200 nm in the presence of up to 3 mM LUVs and above 215 nm in the presence of up to 7 mM LUVs. We introduce a simple way of characterizing the effect on CD spectra of light scattering and absorption arising from suspensions of vesicles of any diameter. Using melittin as an example, we show that CD spectroscopy can be used to determine the fractional helical content of peptides in LUVs and to measure their free energy of partitioning of into LUVs.

NMR of molecules interacting with lipids in small unilamellar vesicles

Detailed characterization of protein, peptide or drug interactions with natural membrane is still a challenge. This review focuses on the use of nuclear magnetic resonance (NMR) for the analysis of interaction of molecules with small unilamellar vesicles (SUV). These phospholipid vesicles are often used as model membranes for fluorescence or circular dichroism experiments. The various NMR approaches for studying molecule-lipid association are reviewed. After a brief survey of the SUV characterization, the use of heteronuclear NMR (phosphorous, carbon, fluorine) is described. Applications of proton NMR through transferred nuclear Overhauser effect to perform structural determination of peptide are presented. Special care is finally given to the influence of the kinetic of the interactions for the proton NMR of bound molecules in SUV, which can constitute a good model for the study of dynamical processes at the membrane surface.

The effect of cholesterol on the solubilization of phosphatidylcholine bilayers by the non-ionic surfactant Triton X-100

Most of the studies on the solubilization of model membranes by Triton X-100 (TR) involve one lipid. The aim of the present study was to evaluate the effect of the addition of cholesterol on the solubilization of bilayers made of palmitoyloleoylphosphatidylcholine (POPC) or dipalmitoylphosphatidylcholine (DPPC). Detailed investigation of the kinetics of solubilization of the cholesterol-containing bilayers by TR at different temperatures reveals that: (i) At 4 degrees C, solubilization of both systems is relatively slow. Hence, in order to prevent misleading conclusions from turbidity measurements it is important to monitor the solubilization after steady-state values of optical density (OD) are reached. (ii) Studies of the temperature-induced changes of the aggregates present in mixtures of TR, POPC and cholesterol indicate that the state of aggregation at all temperatures (including 4 degrees C) represents equilibrium. By contrast, for DPPC/cholesterol/TR mixtures "kinetic traps" may occur not only at 4 degrees C but at higher temperatures as well (e.g. 37 degrees C). (iii) The presence of cholesterol in POPC bilayers makes the bilayers more resistant to solubilization at low temperatures, especially at 4 degrees C. As a consequence, the temperature dependence of the TR concentration required for complete solubilization (Dt(sol)) is no longer a monotonically increasing function (as for POPC bilayers) but a bell-shaped function, with a minimum at about 25 degrees C. Inclusion of cholesterol in DPPC bilayers makes the bilayers more resistant to solubilization at all temperatures except 4 degrees C. In this system, we observe a bell-shaped dependence of Dt(sol) on temperature, with a minimum at 37 degrees C. (iv) Both the rate of vesicle size growth and the rate of the solubilization of POPC vesicles are not affected by the inclusion of cholesterol in the bilayers. Similarly, cholesterol did not affect significantly the rate of size growth of DPPC bilayers at all temperatures, but reduced the rate of solubilization at 4 degrees C.

Energetics of vesicle fusion intermediates: comparison of calculations with observed effects of osmotic and curvature stresses

We reported previously the effects of both osmotic and curvature stress on fusion between poly(ethylene glycol)-aggregated vesicles. In this article, we analyze the energetics of fusion of vesicles of different curvature, paying particular attention to the effects of osmotic stress on small, highly curved vesicles of 26 nm diameter, composed of lipids with negative intrinsic curvature. Our calculations show that high positive curvature of the outer monolayer "charges" these vesicles with excess bending energy, which then releases during stalk expansion (increase of the stalk radius, r(s)) and thus "drives" fusion. Calculations based on the known mechanical properties of lipid assemblies suggest that the free energy of "void" formation as well as membrane-bending free energy dominate the evolution of a stalk to an extended transmembrane contact. The free-energy profile of stalk expansion (free energy versus r(s)) clearly shows the presence of two metastable intermediates (intermediate 1 at r(s) approximately 0 - 1.0 nm and intermediate 2 at r(s) approximately 2.5 - 3.0 nm). Applying osmotic gradients of +/-5 atm, when assuming a fixed trans-bilayer lipid mass distribution, did not significantly change the free-energy profile. However, inclusion in the model of an additional degree of freedom, the ability of lipids to move into and out of the "void", made the free-energy profile strongly dependent on the osmotic gradient. Vesicle expansion increased the energy barrier between intermediates by approximately 4 kT and the absolute value of the barrier by approximately 7 kT, whereas compression decreased it by nearly the same extent. Since these calculations, which are based on the stalk hypothesis, correctly predict the effects of both membrane curvature and osmotic stress, they support the stalk hypothesis for the mechanism of membrane fusion and suggest that both forms of stress alter the final stages, rather than the initial step, of the fusion process, as previously suggested.

Temperature-dependence of the solubilization of dipalmitoylphosphatidylcholine (DPPC) by the non-ionic surfactant Triton X-100, kinetic and structural aspects

Most of the studies on the solubilization of model membranes conducted thus far involved model membranes made of liquid-crystalline phospholipids. Relatively little is known on the influence of temperature and of the phase of the lipid bilayers on their solubilization by detergents. The aim of the present study was to gain knowledge about the temperature and phase dependence of the solubilization of phospholipid bilayers by the non-ionic detergent Triton X-100 (TR). Detailed investigation of the kinetics of the solubilization of dipalmitoylphosphatidylcholine (DPPC), as well as of palmitoyloleoylphosphatidylcholine (POPC) by TR at different temperatures reveals that: (i) solubilization of DPPC is a relatively slow process, especially below Tm. This means that in order to prevent misleading conclusions it is important to monitor the solubilization after a steady state is established. (ii) Both the steady state structure and size of DPPC/TR aggregates and the kinetics of solubilization depend on temperature. (iii) The TR concentration required for solubilization of POPC bilayers is an increasing function of temperature, although no phase change of bilayers occurs in the studied temperature range. (iv) Detailed studies of the temperature-induced changes of the aggregates present in DPPC/TR or POPC/TR mixtures suggest that the state of aggregation at any temperature above 23 degrees C represents equilibrium. By contrast, for DPPC/TR mixtures at 4 degrees C all the processes are very slow, which complicates the interpretation of results obtained through the common practice of studying "rafts" by investigating detergent-resistant membranes.

Photothermal time-resolved imaging of living cells

BACKGROUND AND OBJECTIVES

Thermal effects of laser radiation at cell level play very important role in cell functioning and in many laser applications. The aim of this study was to evaluate a new method of photothermal imaging (PTI) for monitoring short-time nano-scale thermal effects in individual living cells.

STUDY DESIGN/MATERIALS AND METHODS

PTI is based on the irradiation of a cell with a short laser pump pulse (8 nanoseconds, 532 nm) and on registration of the laser-induced local thermal effects using time-resolved phase-contrast imaging with a pulsed probe laser.

RESULTS

PT images of lymphocytes, lympholeukemia cells in vitro were obtained at different laser energies. PTI in time-resolved mode allowed visualizing the structures with size less than diffraction limit (90-nm liposomes). The photodamage process was visualized for a single human leukocyte in suspension.

CONCLUSIONS

PTI in non-invasive mode offered better contrast of living cell image than conventional optical phase-contrast microscopy. The data obtained showed that PTI is in perspective for studies of live non-fluorescent cells.

Interleukin-2-induced small unilamellar vesicle coalescence

Recombinant human interleukin-2 (rhIL-2) was incorporated in liposomes for potential therapeutic applications using a novel process. In this process, rhIL-2 caused the formation of large, unique multilamellar vesicles (MLVs) from small unilamellar vesicles (SUVs) of dimyristoylphosphatidylcholine (DMPC). Vesicle coalescence occurred most rapidly at 19 degrees C, between the pre- and main phase transition temperatures of DMPC, and showed a dependence upon pH (pH <5.5), ionic strength (>50 mM) and the initial size of the unilamellar vesicles (<or=25 nm). Intermediates (partially coalesced vesicles) within the forming multilamellar structures were identified by freeze-fracture electron microscopy and their presence was corroborated by differential scanning calorimetry. Several distinct steps were identified in the coalescence process. In the initial step, rhIL-2 rapidly bound to the DMPC SUVs. This was followed by a pH-dependent conformational change in the protein, as evidenced by an increase in tryptophan fluorescence intensity. The SUVs then aggregated in large clusters that eventually annealed to form closed MLVs. In this process over 90% of the rhIL-2 was bound to and incorporated within the multilamellar structures.

How to measure and analyze tryptophan fluorescence in membranes properly, and why bother?

Tryptophan fluorescence is a powerful tool for studying protein structure and function, especially membrane-active proteins and peptides. It is arguably the most frequently used tool for examining the interactions of proteins and peptides with vesicular unilamellar model membranes. However, high light scattering associated with vesicular membrane systems presents special challenges. Because of their reduced light scattering compared to large unilamellar vesicles (LUV), small unilamellar vesicles (SUV) produced by sonication are widely used membrane models. Unfortunately, SUV, unlike LUV, are metastable and consequently unsuitable for equilibrium thermodynamic measurements. We present simple and easily implemented experimental procedures for the accurate determination of tryptophan (Trp) fluorescence in either LUV or SUV. Specifically, we show that Trp spectra can be obtained in the presence of up to 6 mM LUV that are virtually identical to spectra obtained in buffer alone, which obviates the use of SUV. We show how the widths and peak positions of such spectra can be used to evaluate the heterogeneity of the membrane conformation and penetration of peptides. Finally, we show how to use a reference fluorophore for the correction of intensity measurements so that the energetics of peptide partitioning into membranes can be accurately determined.

The transverse location of the fluorescent probe trans-parinaric acid in lipid bilayers

The transverse location of trans-parinaric acid in spherical vesicles made up from dipalmitoylphosphatidylcholine has been investigated by the differential quenching of the probe fluorescence by 5- and 16-doxylstearic acid derivatives. The quenching data are interpreted in terms of a local fluorophore concentration factor. In this way it was found that the polyene of t-PnA is located within the inner part of the bilayer (presumably aligned with the bilayer lipids), both in the gel and in the liquid crystalline phases.

Fusion activity of an amphiphilic polypeptide having acidic amino acid residues: generation of fusion activity by alpha-helix formation and charge neutralization

A sequential polypeptide, poly(Glu-Aib-Leu-Aib) (Aib represents 2-aminoisobutyric acid), was synthesized and the pH-dependence of fusogenic activity of the polypeptide was studied. The polypeptide was designed to take amphiphilic structure upon the formation of alpha-helix. Circular dichroism spectra of the polypeptide showed a negative Cotton effect with double minima, indicative of an alpha-helical conformation. The alpha-helix content was increased with lowering pH and/or increasing the ionic strength. It was found that the polypeptide induces remarkable leakage of calcein from egg-yolk phosphatidylcholine (EYPC) vesicles loaded in the inner aqueous phase with lowering pH and/or increasing ionic strength. The polypeptide caused fusion of EYPC liposomes and dipalmitoylphosphatidylcholine liposomes more strongly with decreasing pH. Moreover, two distinct increases of fusogenic activity of the polypeptide were observed near pH 6.0 and below pH 4.5. The former corresponds to the midpoint of pH-dependent change in helical content of the polypeptide and the latter the pKa of the gamma-carboxyl group of glutamic acid. These results indicate that elevation of the fusogenic activity of the polypeptide is related to the increase in two factors, alpha-helix content and hydrophobicity.

Cancer chemotherapy administered by activated carbon particles and liposomes

In cancer chemotherapy, a specific drug delivery system is expected since many anticancer drugs show toxicity against not only cancer cells but also against normal tissues. The dosage form comprising anticancer drugs adsorbed on activated carbon particles or encapsulated in liposomes is developed as a drug-delivery system which enhances the therapeutic efficacy and reduces the adverse effects. The dosage forms are versatile in size and electric charge, so that large amounts of the drugs are distributed to the "targeted" organs or tissues and lesser amounts are distributed to the whole body. The dosage forms are designed to release the drugs slowly for a long time at local sites. Through this process, practical use of the dosage forms as an anticancer drug carrier results in an enhancement of anticancer efficacy on the local lesion and a decrease of systemic toxicity.

Effect of monovalent cations on polyvalent cation-induced fusion of phosphatidylserine small unilamellar vesicles

Fluorescence internal contents mixing assay was used to monitor the fusion of phosphatidylserine (PS) small unilamellar vesicles, initiated by metal ions (Ca2+, La3+ and Tb3+), at various concentrations of monovalent cations (Li+, Na+ and K+). The influence of ionic strength (0.02-1.0 M) on the threshold concentration of "fusogenic" cations required to induce fusion was measured. The threshold concentrations increased monotonically (1 mM at 0.1 M to 3.1 mM at 1 M) with the increasing ionic strength of the solution for Ca2+, but remained unchanged for both La3+ and Tb3+. Changes in the ionic strength of the encapsulated solution did not alter the threshold concentrations for all the ions studied, in the range 0.02-0.3 M. The results are analyzed in terms of competitive binding between the monovalent ions and the "fusogenic" ions (Ca2+, Tb3+ and La3+). It is shown that there is a critical value for calcium bound-PS, below which no massive fusion occurs. Bound and free fractions of PS are calculated based on the Gouy-Chapman model, taking activities rather than concentrations of metal ions into account. Our experiments also show that monovalent ions alone do not induce fusion even at high concentrations.

Lipid miscibility and size increase of vesicles composed of two phosphatidylcholines

The size increase of small unilamellar vesicles composed of binary mixtures either of saturated fatty acid phosphatidylcholines with different chain lengths or of saturated and unsaturated phosphatidylcholines was found to depend on the miscibility properties of the lipid components. No size increase was detected in vesicles formed by two miscible phosphatidylcholines. In vesicles composed of two lipids which are partially immiscible in the gel state, a size increase was observed at temperatures which mainly overlapped the range of temperatures of the lipid phase transition. The rate of size increase of vesicles composed of two lipids which are immiscible in the gel state was faster than that of vesicles composed of two partially immiscible phosphatidylcholines, and the process occurred not only at the temperature ranges of the lipid phase transition, but also when both lipids were in the gel state. The vesicle size increase process occurred without the mixing of the internal content of the vesicles. A model is proposed in which the presence of 'fractures' between membrane regions of different fluidity and/or lipid composition controls the rate of this process.

Salt-induced aggregation and fusion of dioctadecyldimethylammonium chloride and sodium dihexadecylphosphate vesicles

Small dioctadecyldimethylammonium chloride (DODAC) vesicles prepared by sonication fuse upon addition of NaCl as detected by several methods (electron microscopy, trapped volume determinations, temperature-dependent phase transition curves, and osmometer behavior. In contrast, small sodium dihexadecyl phosphate (DHP) vesicles mainly aggregate upon NaCl addition as shown by electron microscopy and the lack of osmometer behavior. Scatter-derived absorbance changes of small and large DODAC or DHP vesicles as a function of time after salt addition were obtained for a range of NaCl or amphiphile concentration. These changes were interpreted in accordance with a phenomenological model based upon fundamental light-scattering laws and simple geometrical considerations. Short-range hydration repulsion between DODAC (or DHP) vesicles is possibly the main energy barrier for the fusion process.

Peptide-carrier interaction: induction of liposome fusion and aggregation by insulin

As the roles of peptidic agents in therapy expand, the need for gaining the knowledge for formulating peptides and/or polypeptides becomes increasingly urgent. In an attempt to study various approaches to formulating peptidic agents for therapeutic applications, we investigated the interactions between drug carriers and peptides, using liposomes and insulin as a model. The fusion and aggregation properties of dipalmitoylphosphatidylcholine (DPPC) small, unilamellar liposomes, on the binding of insulin was studied by the techniques of resonance energy transfer of fluorescent labeled lipids, electron microscopy, and right-angle scattering. Within 1 h of adding insulin to DPPC liposomes at 25 degrees C, the average size of the liposomes increased from 239 to 361 A in diameter. There was no further increase in the size of the liposomes after the fused liposomes reached this size. However, the aggregation of the fused liposomes continued to increase for several hours after the insulin-induced fusion stopped. Our results suggest that insulin induces the aggregation of newly fused liposomes, when the temperature is below the gel----liquid crystalline phase-transition temperature (Tc) of the liposomes. The aggregation of fused liposomes is markedly affected by not only the zinc content of insulin but also the pH and ionic strength of the solution. The results of the present study demonstrate that an amphyphilic molecule, such as insulin, could induce the fusion of liposomes via hydrophobic interaction and facilitate liposome aggregation via hydrophilic interaction. Thus, when entrapping insulin by small, unilamellar liposomes, care should be taken to avoid fusion and aggregation.

Gramicidin induced aggregation and size increase of phosphatidylcholine vesicles

To investigate the role of membrane proteins in the fusion process, linear hydrophobic polypeptide gramicidin was used as fusogenic agent in small unilamellar vesicles (SUV) constituted of saturated lecithins. It was found that gramicidin, externally added to a suspension of vesicles, induces a reversible vesicles aggregation. When incorporated into the bilayer, gramicidin induces increase in vesicle size. The vesicle size increase was monitored by column chromatography and transmission electron microscopy. The process of vesicle size increase occurs only when the lipid membrane is in the gel state. A maximum is observed in the kinetics at a temperature of approx. 25 degrees C lower than the phase transition temperature of lipids. Higher rates of vesicle size increase are obtained as the lipid chain length increases. The process is accompanied by a release of internal vesicle content and by membrane lipid mixing.

Effect of cholesterol on vesicle bilayer geometry of choline plasmalogen and comparison with dialkyl-, alkylacyl- and diacyl-glycerophosphocholines

Small unilamellar vesicles containing alkenylacyl-, alkylacyl-, dialkyl- or diacyl-glycerophosphocholine were prepared by sonication. Their size was determined from the average internal volume after chromatography on Sepharose 2B and from 31P-NMR linewidths. Alkenylacyl glycerophosphocholine (choline plasmalogen) was found to form the largest vesicles. By addition of 30 mol% cholesterol, the size of plasmalogen vesicles, but not of those containing the alkyl and acyl analogue lipids, was significantly increased. The presence of 50 mol% sterol led to highly increased vesicle sizes of alkylacyl, dialkyl and diacyl-glycerophosphocholine. Mixtures of plasmalogens with 50 mol% cholesterol did not form unilamellar vesicles upon sonication. Bilayer thickness and surface area per phospholipid molecule were determined by small angle X-ray scattering and measurement of partial specific volumes. There is little difference between alkenylacyl glycerophosphocholine and the corresponding diacyl-analog, whereas bilayers consisting of dioleoyl glycerophosphocholine are significantly thinner. Correspondingly their molecular surface area is by about 8% larger than that of the mixed-chain diradyl glycerophosphocholine, since the partial molar volumes are similar for all vesicles tested.

Thermodynamic characterization of the pretrasition of unilamellar dipalmitoyl-phosphatidylcholine vesicles

High-sensitivity differential scanning calorimetry of small unilamellar vesicles (SUV) made by sonication of dipalmitoylphosphatidylcholine (DPPC) reveals that the gel-liquid crystalline transition at 37 C is preceded by a pretransition at 28 C that is relatively slow (t1/2 approximately equal to 2-4 min) and has an enthalpy change of ca. 0.2 kcal/mol. On incubation at 4 C, these SUV fuse spontaneously into large unilamellar vesicles (LUV). LUV also exhibit both a pretransition and a main-phase transition. The temperature of the main transition (Tm) and the enthalpy change of both the pretransition and main transition of these fused vesicles are similar to those of large multilamellar vesicles (MLV). The enthalpy change associated with the transition at 28 C decreases in SUV in a manner directly correlated to the decrease in the apparent enthalpy change of the 37 C main transition, indicating that the smaller (low temperature) transition is indeed a pretransition that is an inherent property of SUV. Therefore, unilamellar vesicles of DPPC appear to exhibit a pretransition at a temperature that varies from 28 C for the small vesicles to 35 C for the much larger vesicles.

Solubilization of phospholipids by detergents. Structural and kinetic aspects

Most amphiphiles in biological membranes including phospholipids, steroids, and membrane proteins are insoluble amphiphiles and would form liquid crystals or insoluble precipitates alone in aqueous media. Detergents are soluble amphiphiles and above a critical concentration and temperature form micelles of various sizes and shapes. Much of the recent progress in studying the insoluble amphiphiles is due to the formation of thermodynamically stable isotropic solutions of these compounds in the presence of detergents. This process, which is commonly denoted as "solubilization,' involves transformation of lamellar structures into mixed micelles. The information available to date on the solubilization of phospholipids, which constitute the lipid skeleton of biomembranes, by the common detergents is discussed in this review, both with respect to the kinetics of this process and the structure of the various phospholipid-detergent mixed micelles formed. It is hoped that this discussion will lead to somewhat more useful, although still necessarily fairly empirical, approaches to the solubilization of phospholipids by detergents.

Interaction of alpha-lactalbumin with dimyristoylphosphatidylcholine vesicles. III. Influence of the temperature and of the lipid-to-protein molar ratio on the complex formation

We investigated the interaction between alpha-lactalbumin and sonicated dimyristoylphosphatidylcholine at pH 4 and different temperatures. (1) At 23 degrees C and lipid-to-protein molar ratios below 170, the interaction results in a disruption of the original vesicles to form smaller complex particles. By the sedimentation velocity method we determined for this particle a molar mass of (1.05 +/- 0.16) X 10(6) g X mol-1. The lipid-to-protein molar ratio within the complex particle is 70/1, as earlier estimated. It follows that there are approximately 1200 lipid and 17 alpha-lactalbumin molecules per particle. At molar ratios above 170, alpha-lactalbumin strongly associates with the vesicles. In this case the vesicle entity remains. The ability of alpha-lactalbumin to break up the vesicles at this temperature is determined by the number of protein molecules which are required in the complex particle. (2) By means of fluorescence polarization of the lipophilic probe 1,6-diphenyl-1,3,5-hexatriene and energy transfer of the tryptophan groups of the protein to 1,3-(1,1'-dipyrenyl)propane located in the hydrocarbon region of the vesicles, it is shown that with increasing temperature above 25 degrees C, complexes of decreasing internal lipid-to-protein molar ratio are formed. However, by electron microscopy we show that the overall size of these complexes remains approximately the same, i.e., bars with dimensions 70 X 220 A. A temperature-reversible transformation occurs between these complexes, which cannot be isolated by gel chromatography. In contrast, the complex of molar ratio 70/1 remains stable at lower temperatures.

Photon correlation spectroscopy study on the stability of small unilamellar DPPC vesicles

The growth in size of dipalmitoylphosphatidylcholine small unilamellar vesicles (SUV) below Tm has been studied by photon correlation spectroscopy and differential scanning calorimetry. We see an initial fast rise of the hydrodynamic diameter of the vesicles followed by a slower increase. We assign the slow component of the size change to fusion of SUV. The order of the kinetics appears to be higher than first order. The estimated half lifetime of the fusion is approximately 67 h. The diameters for the fast and slow processes at t = O are 756 and 256 A, respectively, while as t leads to infinity the diameters increase to 1,570 and 733 A, respectively.