Partitioning of octyl glucoside between octyl glucoside/phosphatidylcholine mixed aggregates and aqueous media as studied by isothermal titration calorimetry

Lipid Scrambling Induced by Membrane-Active Substances

The functional roles of the lipid asymmetry of biomembranes are attracting increasing attention. This study characterizes the activity of surfactants to induce transmembrane flip-flop of lipids and thus "scramble" this asymmetry. Detergent-induced lipid scrambling of liposomes mimicking the charge asymmetry of bacterial membranes with 20 mol % of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol in the outer leaflet only was quantified by ζ-potential measurements for octaethylene glycol dodecyl ether (C₁₂EO₈), octyl glucoside (OG), and dodecyl maltoside. Membrane leakage was separately measured by the fluorescence lifetime-based calcein leakage assay and the onset of the membrane-to-micelle transition by isothermal titration calorimetry. Partition coefficients and partial molar areas were obtained as well. For the quickly membrane-permeant C₁₂EO₈ and OG, leakage proceeds at a rather sharp threshold content in the membrane, which is well below the onset of solubilization and little dependent on incubation time; it is accompanied by fast lipid scrambling. However, unlike leakage, flip-flop is a relaxation process that speeds up gradually from taking weeks in the detergent-free membrane to minutes or less in the leaking membrane. Hence, after 24 h of incubation, 10 mol % of C₁₂EO₈ or 50 mol % of OG in the membrane suffice for virtually complete lipid scrambling, whereas leakage remains below 10% for up to 14 mol % of C₁₂EO₈ and 88 mol % of OG. There is thus a concentration window in which lipid scrambling proceeds without leakage. This implies that lipid scrambling must be considered a possible mode of action of antimicrobial peptides and other membrane-active drugs or biomolecules. A related, detergent-based protocol for scrambling the lipid asymmetry of liposomes and maybe cells without compromising their overall integrity would be a very valuable tool to study functions of lipid asymmetry.

"Staying Out" Rather than "Cracking In": Asymmetric Membrane Insertion of 12:0 Lysophosphocholine

Interactions between detergents and model membranes are well described by the three-stage model: saturation and solubilization boundaries divide bilayer-only, bilayer-micelle coexistence, and micelle-only ranges. An underlying assumption of the model is the equilibration of detergent between the two membrane leaflets. However, many detergents partition asymmetrically at room temperature due to slow flip-flop, such as sodium dodecyl sulfate (SDS) and lysolipids. In this work, we use isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) to investigate the solubilization of unilamellar POPC vesicles by 12:0 lysophosphocholine (12:0 LPC). Flip-flop of 12:0 LPC occurs beyond the time scale of our experiments, which establish a characteristic nonequilibrated state with asymmetric distribution: 12:0 LPC partitions primarily into the outer leaflet. Increasing asymmetry stress in the membrane does not lead to membrane failure, i.e., "cracking in" as seen for alkyl maltosides and other surfactants; instead, it reduces further membrane insertion which leads to the "staying out" of 12:0 LPC in solution. At above the critical micellar concentration of 12:0 LPC in the presence of the membrane, micelles persist and accommodate further LPC but take up lipid from vesicles only very slowly. Ultimately, solubilization proceeds via the micellar mechanism (Kragh-Hansen et al., 1995). With a combination of demicellization and solubilization experiments, we quantify the molar ratio partition coefficient (0.6 ± 0.1 mM^-1) and enthalpy of partitioning (6.1 ± 0.3 kJ·mol^-1) and estimate the maximum detergent/lipid ratio reached in the outer leaflet (<0.13). Despite the inapplicability of the three-stage model to 12:0 LPC at room temperature, we are able to extract quantitative information from ITC solubilization experiments and DLS that are important for the understanding of asymmetry-dependent processes such as endocytosis and the gating of mechanosensitive channels in vitro.

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.

Monitoring detergent-mediated solubilization and reconstitution of lipid membranes by isothermal titration calorimetry

The solubilization and reconstitution of biological or liposomal membranes by detergents and biomolecules with detergent-like properties play a major role for technical applications (e.g., the isolation of membrane proteins) and biological phenomena (of, e.g., amphiphilic peptides). It is therefore important to know and understand the amounts of a given detergent required for the onset and completion of membrane solubilization and the detergent-lipid interactions in general. Lipid-detergent systems can form a variety of aggregate structures, which can be grouped into two pseudophases (lamellae and micelles) so that solubilization can be approximately described as a phase transition. Here we present a protocol for establishing the phase diagram and a detailed thermodynamic description of a lipid-detergent system based on isothermal titration calorimetry (ITC). The protocol can also be used to detect additive-induced membrane destabilization, permeabilization, domain formation and lipid-dependent transitions between rod-like and spherical micelles. A minimal protocol consisting of all sample preparation procedures and a single solubilization experiment can be accomplished within 2 days; a more extensive series comprising both solubilization and reconstitution experiments requires several days to a few weeks, depending on the number of titrations performed.

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.

Triton X-100 partitioning into sphingomyelin bilayers at subsolubilizing detergent concentrations: effect of lipid phase and a comparison with dipalmitoylphosphatidylcholine

We examined the partitioning of the nonionic detergent Triton X-100 at subsolubilizing concentrations into bilayers of either egg sphingomyelin (SM), palmitoyl SM, or dipalmitoylphosphatidylcholine. SM is known to require less detergent than phosphatidylcholine to achieve the same extent of solubilization, and for all three phospholipids solubilization is temperature dependent. In addition, the three lipids exhibit a gel-fluid phase transition in the 38-41 degrees C temperature range. Experiments have been performed at Triton X-100 concentrations well below the critical micellar concentration, so that only detergent monomers have to be considered. Lipid/detergent mol ratios were never <10:1, thus ensuring that the solubilization stage was never reached. Isothermal titration calorimetry, DSC, and infrared, fluorescence, and (31)P-NMR spectroscopies were applied in the 5-55 degrees C temperature range. The results show that, irrespective of the chemical nature of the lipid, DeltaG degrees of partitioning remained in the range of -27 kJ/mol lipid in the gel phase and of -30 kJ/mol lipid in the fluid phase. This small difference cannot account for the observed phase-dependent differences in solubilization. Such virtually constant DeltaG degrees occurred as a result of the compensation of enthalpic and entropic components, which varied with both temperature and lipid composition. Consequently, the observed different susceptibilities to solubilization cannot be attributed to differential binding but to further events in the solubilization process, e.g., bilayer saturability by detergent or propensity to form lipid-detergent mixed micelles. The data here shed light on the relatively unexplored early stages of membrane solubilization and open new ways to understand the phenomenon of membrane resistance toward detergent solubilization.

Heat evolution of micelle formation, dependence of enthalpy, and heat capacity on the surfactant chain length and head group

Micelle formation by many surfactants is endothermic at low temperatures but exothermic at high temperatures. In this respect, dissociation of micelles (demicellization) is similar to dissolving hydrocarbons in water. However, a remarkable difference between the two processes is that dissolving hydrocarbons is isocaloric at about 25 degrees C, almost independently of the hydrocarbon chain length, whereas the temperature (T*) at which demicellization of different surfactants is athermal varies over a relatively large range. We have investigated the temperature dependence of the heat of demicellization of three alkylglucosides with hydrocarbon chains of 7, 8, and 9 carbon atoms. At about 25 degrees C, the heat of demicellization of the three studied alkylglucosides varied within a relatively small range (DeltaH=-7.8+/-0.4 kJ/mol). The temperature dependence of DeltaH(demic) indicates that within the studied temperature range the heat capacity of demicellization (DeltaC(P,demic)) is about constant. The value of DeltaC(P,demic) exhibited an apparently linear dependence on the surfactant's chain length (DeltaC(P,demic)/n(CH(2))=47+/-7 kJ/mol K). Our interpretation of these results is that (i) the transfer of the head groups from micelles to water is exothermic and (ii) the temperature dependence of the heat associated with water-hydrocarbon interactions is only slightly affected by the head group. This implies that the deviation of the value of T* from 25 degrees C results from the contribution of the polar head to the overall heat of demicellization. Calorimetric studies of other series of amphiphiles will have to be conducted to test whether the latter conclusion is general.

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

The process of vesicle destruction by zwitterionic detergent, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), was examined to clarify the vesicle-micelle transition mechanism. The physicochemical properties including turbidity, apparent particle size, Cl(-) permeability, electron spin resonance (ESR) spectroscopic parameters, and freeze-fracture electron microscopy were investigated. The concentration of CHAPS was analyzed using HPLC to determine the partition coefficient during the solubilization process. The data obtained revealed that maximum turbidity and apparent particle size were found at the effective ratio (R(e)) of 0.21 and 0.49, respectively. With a further increase in CHAPS concentration, turbidity and particle size abruptly decreased, suggesting the formation of mixed micelles. The partition coefficient changed throughout the solubilization process. In the presence of low concentrations of CHAPS, CHAPS partitioned into vesicles without destruction of membrane bilayers. When the R(e)<0.04, the partition coefficient was independent of the detergent concentration with value of 24 M(-1). At R(e) greater than 0.05, the membrane barrier abruptly decreased. At 0.04</=R(e)<0.21, the gradual increase in the partition coefficient accounted for the occurrence of larger vesicles. In range of 0.21</=R(e)<0.52, the abrupt increase in the partition behavior was possibly attributed to the structural change of mixed vesicles to mixed micelles. Furthermore, the ESR results showed that the incorporation of CHAPS into vesicles led to an increase in membrane fluidity near the polar head, and a decrease near the end of the acyl chain. ESR spectra of 5-doxylstearic acid in CHAPS-containing micelles were anisotropic, indicating that the steroidal structure of CHAPS was responsible for the micelles possessing an orderly arrangement of hydrocarbon chains.

Mixed Micellar Aggregates of Nonionic and Anionic Surfactants with Short Hydrophobic Tails: A Microcalorimetric Study

Apparent molar relative enthalpies were measured for the nonionic ethoxylated surfactant CH(3)-(CH(2))(5)-(OCH(2)-CH(2))(5)OH (C(6)E(5)) in aqueous solution at constant molality of the ionic surfactant CH(3)-(CH(2))(5)-SO(-)(3)Na(+)(C(6)SNa) at 25 degrees C. The experimental data obtained by a stepwise dilution process allowed evaluation of the C(6)E(5) first interaction parameter at several constant molalities of C(6)SNa. The C(6)E(5) critical micelle composition as a function of the C(6)SNa molality was also estimated. The experimental calorimetric data, together with the mixed micelles composition computed in the past by some of us [Ciccarelli et al., Langmuir 14, 7130 (1998)], allowed computation of the Deltah(Mic) of micellization. The experimental data are compared to those predicted by the ideal solution model and regular solution model of mixed micellization. From a calorimetric study performed on the water-hexanol-C(6)SNa and water-penthaethylene glycol-C(6)SNa model systems, it can be argued that the interactions among the hydrophilic heads in the C(6)E(5)-C(6)SNa mixed micelles prevail on the contribution of the hydrophobic tails in ruling the enthalpic properties of the system.

Phase boundaries in mixtures of membrane-forming amphiphiles and micelle-forming amphiphiles

The phase behavior of mixtures of phospholipids and detergents in aqueous solutions is an issue of basic importance for understanding the solubilization and reconstitution of biological membranes. We review the existing knowledge on the compositionally induced reversible transformation of phospholipid bilayers into lipid-detergent mixed micelles. First, we describe the experimental protocols used for preparation of such mixtures and emphasize the scope and limitations of the various techniques used for evaluation of the microstructures of the self-assembled amphiphiles in the mixture. Subsequently, we interpret the existing data in terms of the spontaneous curvature of the amphiphiles and the finite size of the mixed micelles. These considerations lead to a general description of the phase behavior, which forms the basis for a rational approach to solubilization and reconstitution experiments.

The vesicle-to-micelle transition of phosphatidylcholine vesicles induced by nonionic detergents: effects of sodium chloride, sucrose and urea

The vesicle-to-micelle transition of egg phosphatidylcholine LUVs induced by octylglucoside was studied in buffers with 0-4 M sodium chloride, sucrose or urea. We used both light scattering and fluorescent probes to follow the lipid-detergent complexes in these buffers. The vesicle-to-micelle transition process was fundamentally the same in each solute. However, the detergent-to-lipid ratio required for micelle formation shifted in ways that depended on the aqueous solute. The partitioning of octylglucoside between the vesicles and the aqueous phase was primarily determined by the change in its critical micelle concentration (cmc) induced by each solute. Specifically, the cmc decreased in high salt and sucrose buffers but increased in high concentrations of urea. Cmc for two additional nonionic detergents, decyl- and dodecyl-maltoside, and three zwittergents (3-12, 3-14 and 3-16) were determined as a function of concentration for each of the solutes. In all cases NaCl and sucrose decreased the solubility of the detergents, whereas urea increased their solubilities. The effects clearly depended on acyl chain length in urea-containing solutions, but this dependence was less clear with increasing NaCl and sucrose concentrations. The contributions of these solutes to solubility and to interfacial interactions in the bilayers, pure and mixed micelles are considered.

Titration calorimetry of surfactant-membrane partitioning and membrane solubilization

The interaction of surfactants with membranes has been difficult to monitor since most detergents are small organic molecules without spectroscopic markers. The development of high sensitivity isothermal titration calorimetry (ITC) has changed this situation distinctly. The insertion of a detergent into the bilayer membrane is generally accompanied by a consumption or release of heat which can be measured fast and reliably with modern titration calorimeters. It is possible to determine the full set of thermodynamic parameters, i.e., the partitioning enthalpy, the partitioning isotherm, the partition coefficient, the free energy, and the entropy of transfer. The application of ITC to the following problems is described: (i) measurement of the critical micellar concentration (CMC) of pure detergent solutions; (ii) analysis of surfactant-membrane partitioning equilibria, including asymmetric insertion; and (iii) membrane-surfactant phase diagrams. Finally, the thermodynamic parameters derived for non-ionic detergents are discussed and the affinity for micelle formation is compared with membrane incorporation.

Correlation of membrane/water partition coefficients of detergents with the critical micelle concentration

The membrane/water partition coefficients, K, of 15 electrically neutral (non-charged or zwitterionic) detergents were measured with phospholipid vesicles by using isothermal titration calorimetry, and were compared to the corresponding critical micellar concentrations, cmc. The detergents measured were oligo(ethylene oxide) alkyl ethers (C(m)EO(n) with m = 10/n = 3, 7 and m = 12/n = 3.8); alkylglucosides (octyl, decyl); alkylmaltosides (octyl, decyl, dodecyl); diheptanoylphosphatidylcholine; Tritons (X-100, X-114) and CHAPS. A linear relation between the free energies of partitioning into the membrane and micelle formation was found such that K. CMC approximately 1. The identity K. CMC = 1 was used to classify detergents with respect to their membrane disruption potency. "Strong" detergents are characterized by K. CMC < 1 and solubilize lipid membranes at detergent-to-lipid ratios X(b) < 1 (alkylmaltosides, tritons, heptaethylene glycol alkyl ethers). "Weak" detergents are characterized by K. CMC > 1 and accumulate in the membrane- to detergent-to-lipid ratios X(b) > 1 before the bilayer disintegrates (alkylglucosides, pentaethylene glycol dodecyl ether).

A "release" protocol for isothermal titration calorimetry

Isothermal titration calorimetry (ITC) has become a standard method for investigating the binding of ligands to receptor molecules or the partitioning of solutes between water and lipid vesicles. Accordingly, solutes are mixed with membranes (or ligands with receptors), and the subsequent heats of incorporation (or binding) are measured. In this paper we derive a general formula for modeling ITC titration heats in both binding and partitioning systems that allows for the modeling of the classic incorporation or binding protocols, as well as of new protocols assessing the release of solute from previously solute-loaded vesicles (or the dissociation of ligand/receptor complexes) upon dilution. One major advantage of a simultaneous application of the incorporation/binding and release protocols is that it allows for the determination of whether a ligand is able to access the vesicle interior within the time scale of the ITC experiment. This information cannot be obtained from a classical partitioning experiment, but it must be known to determine the partition coefficient (or binding constant and stochiometry) and the transfer enthalpy. The approach is presented using the partitioning of the nonionic detergent C12EO7 to palmitoyloleoylphosphatidylcholine vesicles. The release protocol could also be advantageous in the case of receptors that are more stable in the ligand-saturated rather than the ligand-depleted state.

The heat of transfer of lipid and surfactant from vesicles into micelles in mixtures of phospholipid and surfactant

We study the heat associated with the transformation of vesicles into micelles in mixtures of bilayer-forming phospholipids and micelle-forming surfactants. We subdivide the total heat evolution deltaQ(coex) within the range of coexistence of vesicles and micelles into three contributions related to the transition of dN(D)m-b molecules of surfactant and dN(L)m-b molecules of lipid from micelles to vesicles and to the extraction of dN(D)m-w molecules of surfactant from micelles to the aqueous solution, so that deltaQ(coex) = deltaH(D)m-w x dN(D)m-w + deltaH(D)m-b x dN(D)m-b + deltaH(L)m-b x dN(L)m-b where deltaH(D)m-w, deltaH(L)m-b, and deltaH(D)m-b are the respective molar "transfer" enthalpies. We design a method for the evaluation of all three molar enthalpies, from isothermal calorimetric titrations conducted according to two different protocols of titration of lipid-surfactant mixtures. In the first protocol the mixture is titrated with an aqueous solution of pure lipid vesicles, and in the second the mixture is titrated with an aqueous solution of pure surfactant. Titration of the mixed systems by a buffer solution serves to verify the results obtained under these protocols. In addition to the values of molar enthalpies, our method yields the cmc value of the pure surfactant. We apply our method to investigating the heat evolution in mixtures of egg yolk phosphatidylcholine and the nonionic surfactant octylglucoside in a phosphate-buffered saline solution at 28 degrees C. These studies gave the following values: deltaH(D)m-w = -1732 cal/mol, deltaH(L)m-b = -592 cal/mol, deltaH(D)m-b = 645 cal/mol, and cmc = 23.5 mM. We discuss the possible physical insight of these values and the perspectives of applications of the proposed method.