Water-soluble Amphiphiles Research Articles

Regulation of Gramicidin Channel Function Solely by Changes in Lipid Intrinsic Curvature.

Membrane protein function is regulated by the lipid bilayer composition. In many cases the changes in function correlate with changes in the lipid intrinsic curvature (c0), and c0 is considered a determinant of protein function. Yet, water-soluble amphiphiles that cause either negative or positive changes in curvature have similar effects on membrane protein function, showing that changes in lipid bilayer properties other than c0 are important—and may be dominant. To further investigate the mechanisms underlying the bilayer regulation of protein function, we examined how maneuvers that alter phospholipid head groups effective “size”—and thereby c0—alter gramicidin (gA) channel function. Using dioleoylphospholipids and planar bilayers, we varied the head groups’ physical volume and the electrostatic repulsion among head groups (and thus their effective size). When 1,2-dioleyol-sn-glycero-3-phosphocholine (DOPC), was replaced by 1,2-dioleyol-sn-glycero-3-phosphoethanolamine (DOPE) with a smaller head group (causing a more negative c0), the channel lifetime (τ) is decreased. When the pH of the solution bathing a 1,2-dioleyol-sn-glycero-3-phosphoserine (DOPS) bilayer is decreased from 7 to 3 (causing decreased head group repulsion and a more negative c0), τ is decreased. When some DOPS head groups are replaced by zwitterionic head groups, τ is similarly decreased. These effects do not depend on the sign of the change in surface charge. In DOPE:DOPC (3:1) bilayers, pH changes from 5→9 to 5→0 (both increasing head group electrostatic repulsion, thereby causing a less negative c0) both increase τ. Nor do the effects depend on the use of planar, hydrocarbon-containing bilayers, as similar changes were observed in hydrocarbon-free lipid vesicles. Altering the interactions among phospholipid head groups may alter also other bilayer properties such as thickness or elastic moduli. Such changes could be excluded using capacitance measurements and single channel measurements on gA channels of different lengths. We conclude that changes gA channel function caused by changes in head group effective size can be predicted from the expected changes in c0.

Fast and slow biomembrane solubilizing detergents: Insights into their mechanism of action

Detergents are water-soluble amphiphiles. Above a critical concentration they self-organize in micelles and in the presence of phospholipids mixed micelles are formed. Much information is available on the structure of these self-assemblies and on the thermodynamics of their formation. The aim of this study was to deepen our understanding of the mechanisms of solubilization. Solubilization of lipid vesicles made of egg phosphatidylcholine (PC) by twenty one commercially available, structurally heterogeneous detergents, has been assessed by a decrease in turbidity of the vesicle suspension. Both steady-state and time-resolved measurements have been performed. The results show that the detergents under study fall into one of two categories, namely fast-solubilizing and slow-solubilizing detergents. This categorization is independent of detergent concentration, i.e. a “slow” cannot be converted into a “fast” surfactant by increasing its bulk concentration. 31P-NMR spectra indicate that slow-acting detergents cause either a gradual, monotonic micellization of bilayers (sodium dodecyl sulphate), or formation of more complex, perhaps non-lamellar, non-micellar intermediates (dodecylmaltoside). In contrast, fast detergents (e.g. Triton X-100) cause lysis and reassembly of vesicles before bulk solubilization takes place. These results support the idea that membrane solubilization by detergents is rapid only when surfactant transbilayer (flipping) motion is easy.

Spontaneous Crystallization at the Air−Water Interface: An Unusual Feature of Gemini Surfactant with a Rigid Spacer

An unusual feature which involves spontaneous crystallization at the air-water interface from aqueous solution was reported for a water-soluble gemini surfactant with xylyl spacer, (p-phenylenedimethylene) bis(dodecyldimethylammonium) dibromide. Polarizing microscope, in situ confocal microscopic Raman spectroscopy, and powder XRD were used to characterize the structure of the crystal and investigate the driving force for nucleation. It was inferred that, besides the surface enrichment of amphiphiles and the intra- and intermolecular interaction of alkyl chains, the pi-pi stacking interaction of benzene rings plays an extraordinary role in promoting nucleation and stabilizing crystal structure. A mechanism for constructing supramolecular architectures in situ at the air-water interface directly from aqueous solution via water-soluble amphiphiles with groups favorable for pi-pi stacking interaction was proposed.

Electrochemistry of TEMPO in the Aqueous Liquid/Vapor Interfacial Region: Measurements of the Lateral Mobility and Kinetics of Surface Partitioning

A new method is described to simultaneously determine the kinetics of surface partitioning and the lateral diffusion constant of redox active amphiphiles. It concerns water-soluble amphiphiles for which the surface adsorption equilibrium constant and the solution diffusion constant are measured independently. The method involves cyclic voltammetric experiments carried out at the air/water interface with microband electrodes aligned with the plane of the water surface. Typically, 100 nm wide, 1.0 cm long microband electrodes are fabricated by the vacuum vapor deposition of gold films on glass. The front face of the electrode substrates are coated with impermeable, dimensionally stable, polymer barrier films with thickness L in the range of approximately 0.1-1.0 microm. Fracturing such gold-coated glass substrates exposes gold microbands. The recorded voltammetric current sensitively depends on the barrier film thickness, the surfactant surface diffusion constant, Dsurf, and its rate constant of desorption, kdes. For a given surfactant, such as the nitroxyl piperidine free radical TEMPO featured in this report, large currents are observed with microband electrodes that do not carry a barrier film (L = 0). This is because the surfactant surface population diffusing along the air/water interface can be directly electro-oxidized at the edge of the microband. Smaller currents are measured in the presence of a barrier film, since, in those instances, the surface population may contribute to the voltammetric current only via a mechanism involving surfactant desorption from the water surface into bulk, where it contributes to the three-dimensional solution diffusion processes. The quantitative interpretation of the voltammetric experiments was made possible with finite element simulations with FEMLAB. These produce a set of calibration curves, Dsurf versus log kdes, for each value of the barrier film thickness. The intersection of the calibration curves determines the unique values of Dsurf and kdes. For TEMPO, Dsurf = 4.4 +/- 1.2 x 10(-5) cm2/s and kdes >/= 2 x 10(4) s(-1). Surfactant desorption rate constants of this magnitude have not been previously experimentally accessible. Since, in our earlier report (Wu, D. G.; Malec, A. D.; Head-Gordon, M.; Majda, M. J. Am. Chem. Soc. 2005, 27, 4490-4496), we showed that TEMPO is not immersed in water and that it diffuses along the interface hydrogen-bonded to just one or two water molecules, its Dsurf value approximates the water diffusion constant in the aqueous liquid-vapor interfacial region.

Formation of hybrid monolayers of alkylammonium cations and a clay mineral at an air–water interface: clay as an inorganic stabilizer for water-soluble amphiphiles

Smectite-type clay particles dispersed in an aqueous subphase stabilize a monolayer of water-soluble alkylammonium cations at an air–water interface. When a chloroform solution of the alkylammonium salt is spread onto the clay dispersion, the ammonium cations are adsorbed on the clay particles electrostatically at the interface to form a floating hybrid monolayer. Atomic force microscopy (AFM) observation of the film confirms hybridization of the ammonium cations with the clay particles.

Phase transition in adsorbed monolayers of 2-hydroxyethyl laurate at the air-water interface

Kinetics of adsorption of a sparingly water soluble amphiphile, 2-hydroxyethyl laurate (2-HEL), at the air-water interface is followed by the measurement of surface pressure ( π) with time ( t). Simultaneous change in the surface morphology of the adsorbed monolayers is investigated with Brewster angle microscopy (BAM). A cusp point in each of the π– t curves indicates the start of a phase transition. Highly ordered condensed phase domains having stripes in their structures are observed to grow during the phase transition. One end of the separate lines of stripes gather to a point on the circular peripheral line of the domain like a vertex of the boojum. However, another end of the separate lines disappear at the peripheral line keeping the stripe width almost constant. After the completion of the phase transition, surface pressure starts to increase again and reaches a final equilibrium value after a long time. Spread monolayers of the same amphiphile can be obtained by the usual solvent spreading technique and compared with the adsorbed monolayers. The critical surface pressures necessary to cause the phase transition are almost the same for both adsorbed and spread monolayers. Although the shapes of the domains are circular for both cases, the size of those in the spread monolayers is considerably smaller exhibiting no internal stripes than those in adsorbed monolayers.

Preparation of detergent-lipase complexes utilizing water-soluble amphiphiles in single aqueous phase and catalysis of transesterifications in homogeneous organic solvents

A novel method of preparing detergent-enzyme complexes that can be employed in organic media was developed utilizing newly synthesized water-soluble nonionic gemini-type detergents, N,N-bis(3- d-gluconamidopropyl)-3-(dialkyl- l-glutamatecarbonyl)propanamides (BIG2CnCA: n = 10,12,14,16,18) and N,N-bis(3- d-lactonamidopropyl)-3-(dialkyl- l-glutamatecarbonyl)propanamides (BIL2CnCA: n = 16,18), and nonionic twin-headed detergents, N,N-bis(3- d-gluconamidopropyl)alkanamides (BIG1Cn: n = 12,14,16,18,Δ 9). This method simply entails mixing a selected enzyme with an appropriate detergent in an aqueous solution followed by lyophilization, and it offers the advantages of enhanced enzymatic activity in organic solvents and eliminates both enzyme loss and the necessity for an organic solvent in the preparation stage. Using various modified lipases originating from Aspergillus niger (Lipase A), Candida rugosa (Lipase C), Pseudomonas cepacia (Lipase P), and porcine pancreas (PPL), prepared using the novel method and detergents, including conventional synthesized nonionic detergents such as dialkyl N- d-glucona- l-glutamates (2CnGE: n = 12,18Δ 9) and octanoyl- N-methylglucamide (MEGA-8), enantioselective transesterifications of 6-methyl-5-hepten-2-ol (sulcatol) and 2,2-dimethyl-1,3-dioxolane-4-methanol (solketal) with a vinyl or isopropenyl carboxylate were carried out in an organic solvent. The modified lipase activity was influenced by both the lipases and the structure of the detergents. The value for the hydrophile-lipophile balance (HLB) of the detergent provided a means of correlating the structure and the obtained modified lipase activity. For detergents of the same class with a HLB value of approximately 9 and 12, the highest activity was obtained for Lipase A and Lipase P, and Lipase C and PPL, respectively. Among detergents of the same HLB value tested, the gemini-type detergents possessing the most bulky head and tail were most effective as a modifier for lipases of all types. The preparation and reaction conditions for these novel gemini-type detergent-modified lipases were optimized using BIG2C 12CA (HLB = 9.4) by studying the effect of the detergent/lipase ratio and the nature of organic solvents on the complex formation. The high enzymatic activities of the BIG2C 12CA-modified lipases were independent of the solubility of the lipases in organic solvents, unlike in the case of 2CnGE-modified lipases prepared using the conventional suspension system.

Preparation of Detergent-Lipase Complexes Utilizing Water-Soluble Amphiphiles in Single Aqueous Phase and Catalysis of Transesterifications in Homogeneous Organic Solvents.

A novel method of preparing detergent-enzyme complexes that can be employed in organic media was developed utilizing newly synthesized water-soluble nonionic gemini-type detergents, N,N-bis(3-D-gluconamidopropyl)-3-(dialkyl-L-glutamatecarbonyl)propanamides (BIG2CnCA: n = 10,12,14,16,18) and N,N-bis(3-D-lactonamidopropyl)-3-(dialkyl-L-glutamatecarbonyl)propanamides (BIL2CnCA: n = 16,18), and nonionic twin-headed detergents, N,N-bis(3-D-gluconamidopropyl)alkanamides (BIG1Cn: n = 12,14,16,18,delta9). This method simply entails mixing a selected enzyme with an appropriate detergent in an aqueous solution followed by lyophilization, and it offers the advantages of enhanced enzymatic activity in organic solvents and eliminates both enzyme loss and the necessity for an organic solvent in the preparation stage. Using various modified lipases originating from Aspergillus niger (Lipase A), Candida rugosa (Lipase C), Pseudomonas cepacia (Lipase P), and porcine pancreas (PPL), prepared using the novel method and detergents, including conventional synthesized nonionic detergents such as dialkyl N-D-glucona-L-glutamates (2CnGE: n = 12,18delta9) and octanoyl-N-methylglucamide (MEGA-8), enantioselective transesterifications of 6-methyl-5-hepten-2-ol (sulcatol) and 2,2-dimethyl-1,3-dioxolane-4-methanol (solketal) with a vinyl or isopropenyl carboxylate were carried out in an organic solvent. The modified lipase activity was influenced by both the lipases and the structure of the detergents. The value for the hydrophile-lipophile balance (HLB) of the detergent provided a means of correlating the structure and the obtained modified lipase activity. For detergents of the same class with a HLB value of approximately 9 and 12, the highest activity was obtained for Lipase A and Lipase P, and Lipase C and PPL, respectively. Among detergents of the same HLB value tested, the gemini-type detergents possessing the most bulky head and tail were most effective as a modifier for lipases of all types. The preparation and reaction conditions for these novel gemini-type detergent-modified lipases were optimized using BIG2C12CA (HLB = 9.4) by studying the effect of the detergent/lipase ratio and the nature of organic solvents on the complex formation. The high enzymatic activities of the BIG2C12CA-modified lipases were independent of the solubility of the lipases in organic solvents, unlike in the case of 2CnGE-modified lipases prepared using the conventional suspension system.

Silsesquioxane-Based Amphiphiles

A novel type of amphiphilic spherosilsesquioxane derivative, 1-(1,ω-propylenemethoxy)oligo(ethylene oxide)-3,5,7,9,11,13,15-heptahydridopentacyclo[9.5.13,9.15,15.17,13]octasiloxane has been prepared from (HSiO3/2)8 and allyl-functional oligo(ethyleneoxide) (Mn = 750 g/mol) by hydrosilylation. The monosubstituted octahydridosilsesquioxane was characterized by 1H, 13C, and 29Si NMR spectroscopy, IR, and MALDI-TOF mass spectroscopy as well as elemental analysis. Surface tension measurements of the water-soluble amphiphile show a cmc in the range of 6 × 10-4 mol/L. Aggregation of the uncondensed amphiphile leads to micellar and vesicular structures that can be cross-linked to liposome-like silica particles at elevated pH.

Structural Polymorphism of Poly(ethylene oxide)−Poly(propylene oxide) Block Copolymers in Nonaqueous Polar Solvents

The solution properties of water-soluble amphiphiles in nonaqueous polar solvents are important in the elucidation of the effects of solvent quality on self-assembly and also in practical applications where the use of water as a solvent is undesirable. We studied the self-assembly of a poly(ethylene oxide)−poly(propylene oxide) (PEO−PPO) block copolymer (Pluronic P105: EO37PO58EO37) in formamide (as selective solvent for the PEO block) and present here results on the binary concentration−temperature phase diagram and on the microstructure. In addition to formamide-rich and polymer-rich solution regions, four “gel” regions with different microstructures, stable over a wide temperature range (from 20 °C to more than 90 °C), have been identified and characterized by small-angle X-ray scattering (SAXS). The PEO−PPO block copolymer in formamide exhibits a thermoreversible transition from a micellar solution to a micellar cubic gel (of Pm3n crystallographic structure) at 25−35 wt % polymer concentrations. At h...

Amphiphile-induced phosphatidylserine exposure in human erythrocytes.

Nonionic and anionic water-soluble amphiphiles were shown to increase strongly the binding of fluorescein isothiocyanate-conjugated annexin V (FITC-annexin V) in human erythrocytes pretreated with the aminophospholipid translocase (APLT) inhibitor n-ethylmaleimide (NEM). At high sublytic amphiphile-concentrations the binding of FITC-annexin V, monitored in a flow cytometer, was time- and temperature-dependent and occurred heterogeneously in the cell population, with 43-81% of cells being stained above background following incubation for 60 minutes at 37 degrees C. The increased FITC-annexin V binding apparently indicates an increased flop rate of phosphatidylserine (PS) to the outer membrane leaflet. When the NEM-pretreatment was omitted, the FITC-annexin V binding was markedly, but not completely, reduced. In erythrocytes incubated with a zwitter-ionic amphiphile, a small increase in FITC-annexin V binding was detected, while cationic amphiphiles did not induce an increased FITC-annexin V binding. The potency of amphiphiles to induce PS exposure was not related to the type of shape alteration or vesiculation induced. Our results indicate a significant role of the charge status of a membrane intercalated amphiphile for its capability to induce PS exposure.

Lipid and protein composition of exovesicles released from human erythrocytes following treatment with amphiphiles

Human erythrocytes were treated with different water-soluble amphiphiles (detergents) at sublytic concentrations, whereafter released exovesicles and treated cells were isolated. Lipid analyses showed that exovesicles had a lower cholesterol/phospholipid ratio and a higher phosphatidylserine/phospholipid ratio compared to parent cells. Protein analyses revealed that exovesicles were, relative to their total protein content, depleted in spectrin, actin and band 6 protein and enriched in band 3 protein and acetylcholinesterase. Exovesicles contained all major glycoproteins. By using a radiolabeled amphiphile ([ 14C]cetyltrimethylammonium bromide) it was shown that the amphiphile/phospholipid ratio was similar in the vesicle membrane and in the parent cell membrane. This indicates that no significant segregation of the intercalated amphiphile between the exovesicle membrane and the parent cell membrane occurs during the vesiculation process. It is suggested that the redistributions of membrane lipids and proteins during the vesiculation process are secondary to the detachment of the cytoskeletal network from the membrane.

Anomalous dose-response relationship due to association colloidal behaviour of soluble amphiphilic compounds

Water-soluble amphiphiles may, above a certain concentration, form colloidal aggregates (micelles or liquid crystals). If such aggregates are formed, there may be a competition between the micelles or the liquid crystals and the biological target for the monomelic form of the amphiphile. This may reduce the number of monomers available for the biological target, which in turn may be reflected in concentration-dependent processes. Therefore, caution should be taken when examining dose-response with soluble amphiphiles forming micelles at low concentrations.

The use of monolayers for simple and quantitative analysis of lipid-drug interactions exemplified with dibucaine and substance P

The interaction between lipids and water soluble amphiphiles was investigated by means of a monolayer technique, monitoring the area increase at constant surface pressure. The area increase could be quantitated and binding isotherms at different surface pressures were measured. A comparison of dibucaine binding to monolayers and bilayers showed that a surface pressure of 32 mN/m best represents the packing density in a lipid bilayer ( Seelig, 1987). Binding isotherms measured for charged dibucaine and substance P (SP) were analyzed by means of two different models. If electrostatic effects were ignored the binding of dibucaine and SP showed biphasic Scatchard plots. If, however, electrostatic effects were taken into account by means of the Gouy-Chapman theory, the insertion of both amphiphiles was best described in terms of a partitioning into the monolayer lipids. The hydrophobic binding constant was K P = 660 ± 80 M −1 for charged dibucaine inserting into coarse liposomes or monolayers at 32 mN/m ( Seelig et al., 1986 ) and 1–1.8 M −1 for SP inserting into monolayers at 32 mN/m ( Seelig and Macdonald, 1989).

Lyotropic Liquid Crystals (Myelin Forms) From Some Sterols and Water-Soluble Amphiphiles

Abstract The formation of myelin tubes, a variety of lyotropic liquid crystals easily observable with a microscope, has been studied in different systems involving sterols (or some metabolically or phylogenetically related substances) and various water-soluble amphiphiles. Different factors influencing myelin tube formation have been investigated: temperature, concentration of the amphiphile solution, and the chemical structure of both partners of the association. The prominent position of cholesterol among numerous related substances is emphasized.

Surfactant-induced liposome fusion: molecular mechanisms and biotechnological applications.

Membrane fusion is an essential event in many important physiological and pathological processes, such as neurotransmitter release, endocrine secretion, certain viral infections, etc. The molecular mechanism of membrane fusion remains still unknown; early studies suggested that amphipathic molecules could be involved1; some recent evidence2,3 supports earlier suggestions of the involvement of osmotic phenomena in membrane fusion; changes in water activity at the cell-water interface have also been invoked4. In our studies, we have examined in particular the possible role of water-soluble amphiphiles (i.e. surfactants), in the fusion process: a whole series of hydro-phyllic and lipophyllic surface active reagents have been tested for fusogenic activity by other authors5.