Headgroup Size Research Articles

Aggregation behavior of a chiral long-chain ionic liquid in aqueous solution

The synthesis of a chiral long-chain ionic liquid (IL), S-3-hexadecyl-1-(1-hydroxy-propan-2-yl)-imidazolium bromide ([C 16hpim]Br), is presented. The adsorption and aggregation of this surface active IL in aqueous solution is described. The critical micelle concentration (cmc) measurement suggests that the chiral IL has superior capacity for micelle formation compared to traditional ionic surfactants. The relatively larger hydrophilic head group of the IL results in a larger maximum surface excess concentration ( Γ max) and a smaller minimum molecular cross-sectional area ( A min). Electrical conductivity studies show a small degree of counterion binding to these micelles, which may increase the electrostatic repulsions between the hydrophilic heads of adjacent surfactant molecules. Both factors of the hydrophilic headgroup size and electrostatic repulsion in [C 16hpim]Br micelles lead to a looser packing of the surfactant molecules in the micelles. As a result, a higher micropolarity and smaller mean aggregation number is observed. Moreover, the looser micellar packing of the [C 16hpim]Br molecules results in a unusual upfield shift of the proton NMR signals in the hydrophobic chains after micelle formation. 1H NMR and 2D ROESY spectroscopic analyses confirm a chiral arrangement of the micelles. Chiral IL micelles may have potential applications in the stereochemical recognition of surfaces or of biological structures.

Effect of Headgroup Size, Charge, and Solvent Structure on Polymer−Micelle Interactions, Studied by Molecular Dynamics Simulations

We performed atomistic molecular dynamics simulations of anionic and cationic micelles in the presence of poly(ethylene oxide) (PEO) to understand why nonionic water-soluble polymers such as PEO interact strongly with anionic micelles but only weakly with cationic micelles. Our micelles include sodium n-dodecyl sulfate (SDS), n-dodecyl trimethylammonium chloride (DTAC), n-dodecyl ammonium chloride (DAC), and micelles in which we artificially reverse the sign of partial charges in SDS and DTAC. We observe that the polymer interacts hydrophobically with anionic SDS but only weakly with cationic DTAC and DAC, in agreement with experiment. However, the polymer also interacts with the artificial anionic DTAC but fails to interact hydrophobically with the artificial cationic SDS, illustrating that large headgroup size does not explain the weak polymer interaction with cationic micelles. In addition, we observe through simulation that this preference for interaction with anionic micelles still exists in a dipolar "dumbbell" solvent, indicating that water structure and hydrogen bonding alone cannot explain this preferential interaction. Our simulations suggest that direct electrostatic interactions between the micelle and polymer explain the preference for interaction with anionic micelles, even though the polymer overall carries no net charge. This is possible given the asymmetric distribution of negative charges on smaller atoms and positive charges on larger units in the polymer chain.

Catalytic effect of silver nanoparticle on electron transfer reaction: Reduction of [Co(NH 3) 5Cl](NO 3) 2 by iron(II)

Silver nanoparticle was synthesized by chemical reduction method and stabilized in a series of non-ionic Tween surfactants. Its size and shape vary with the chain length, head group size and concentration of surfactants. The rate of electron transfer reaction between [Co III(NH 3) 5Cl](NO 3) 2 and Mohr's salt (Fe II) was investigated in presence of different sizes of silver nanoparticles. The reaction is second order: first order in each [Co(NH 3) 5Cl](NO 3) 2 and Mohr's salt in presence of silver nanoparticles. Silver nanoparticle enhances the rate about two to three times. The ionic strength does not influence the rate significantly in presence of silver nanoparticle, possibly of significant catalytic effect of nanoparticle. Thermodynamic activation parameters (Δ ‡ H° and Δ ‡ S°) were determined from variable temperature kinetic studies. The negative values of Δ ‡ S° indicate that the reactions proceed through an associative inner sphere mechanism. All kinetic and thermodynamic activation parameters support an electron transfer from Fe II to Co III through the chloro bridge on the surface of silver nanoparticle within the stern layer.

Adsorption and Desorption of Nonionic Surfactants on Silica from Toluene Studied by ATR-FTIR

The adsorption and desorption kinetics and equilibrium surface coverages for a range of poly(ethylene glycol) monoalkyl ether nonionic surfactants (C(n)E(m)s) from toluene onto silica were studied using attentuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) coupled with a constant-flow liquid cell. The initial adsorption rate appears to be transport controlled, and broadly independent of molecular structure, whereas equilibrium adsorbed amounts are dependent on surfactant head group size (which controls both surface interactions and the surface-bulk partitioning). Desorption kinetic data are fitted to a simple two-step model based upon Langmuir-type kinetics. The results are interpreted in terms of a simple model comprising two modes of surfactant adsorption, each with a characteristic adsorbed amount and desorption rate constant; a possible physical basis for this observation is presented. Trace water levels appear to have a strong impact on the equilibrium surface coverages in these systems, possibly by influencing both surfactant interactions and solubility.

Surfactant-Based Photorheological Fluids: Effect of the Surfactant Structure

The effect of the surfactant structure on the mechanical and structural properties of surfactant based photorheological fluids are presented in this paper. Cetyltrimethylammonium bromide (CTABr) mixed with trans-o-methoxycinnamic acid in a basic environment can form photosensitive systems. The driving force is the ability of surfactant molecules to form wormlike micelles in the presence of the anionic photosensitive additive. Taking into account that slight changes in the surfactant monomer's structure can induce drastic modifications of the micellar aggregate features, the role the of the nature of the counterion (in the CTAX type surfactants) or the headgroup size (CTRABr type surfactants) and its influence on the mechanical properties of surfactant based photorheological fluids using trans-o-methoxycinnamic acid (trans-OMCA) as additive were investigated. Rheological studies reported in this paper show that the viscosity of these systems drastically varies only by changing the nature of the surfactant counterion. Moreover, by increasing the bulk simply by replacing the three methyl groups with three ethyl groups in the surfactant headgroup moiety, the viscosity drastically decreases. Highly photosensitive PR fluids can be further obtained using cetyltrimethylammonium trans-o-methoxycinnamate (CTAOMC) as surfactant at neutral pH. In addition to the complete rheological characterization carried out by means of the application of both a steady shear and a dynamic shear stress, a 1H NMR and NOESY study was also performed.

Electrostatic Screening and Charge Correlation Effects in Micellization of Ionic Surfactants

We have used atomistic simulations to study the role of electrostatic screening and charge correlation effects in self-assembly processes of ionic surfactants into micelles. Specifically, we employed grand canonical Monte Carlo simulations to investigate the critical micelle concentration (cmc), aggregation number, and micellar shape in the presence of explicit sodium chloride (NaCl). The two systems investigated are cationic dodecyltrimethylammonium chloride (DTAC) and anionic sodium dodecyl sulfate (SDS) surfactants. Our explicit-salt results, obtained from a previously developed potential model with no further adjustment of its parameters, are in good agreement with experimental data for structural and thermodynamic micellar properties. We illustrate the importance of ion correlation effects by comparing these results with a Yukawa-type surfactant model that incorporates electrostatic screening implicitly. While the effect of salt on the cmc is well-reproduced even with the implicit Yukawa model, the aggregate size predictions deviate significantly from experimental observations at low salt concentrations. We attribute this discrepancy to the neglect of ion correlations in the implicit-salt model. At higher salt concentrations, we find reasonable agreement of the Yukawa model with experimental data. The crossover from low to high salt concentrations is reached when the electrostatic screening length becomes comparable to the headgroup size.

Structure of Nonionic Surfactant (Glycerol α-Monomyristate) Micelles in Organic Solvents: A SAXS Study

In this paper we present the study of reverse micellar aggregates formed by glycerol alpha-monomyristate (C(14)G(1)) in different organic solvents. We have investigated the structure (mainly shape and size) of the monomyristin reverse micelles depending on alkyl chain length of the oils, temperature, and the surfactant concentration. Moreover, we present how addition of trace polar additives such as water or glycerol and the hydrophilic size of the surfactant affect the structure of the micelles. Monomyristin could not form any self-organized structure in nonpolar media at room temperature because of its smaller headgroup size and tends to separate from the nonpolar media. In dilute regions this surfactant forms a solid dispersion, which upon heating transforms into an isotropic reverse micellar solution. For the structural characterization of the reverse micelles formed at elevated temperatures, we have performed small-angle X-ray scattering (SAXS) measurements, and the data were analyzed utilizing the generalized indirect Fourier transformation (GIFT) method. The SAXS data have shown that the size of the reverse micelles increases with the alkyl chain length of oils and surfactant concentration but decreases with temperature. A dramatic growth to the micelles could be achieved when the hydrophilic size of the surfactant is increased. Furthermore, addition of trace water or glycerol induces a significant change in the structure of micelles in terms of a cross-section and maximum length. The added water tends to form a water pool in the micellar core and results in swollen micelles. Model-free cross-section analysis of the SAXS data provided complementary information on the structural parameters. Thus, the present study has highlighted the possible ways of controlling the structure, mainly the shape and the size of the inverse micelles.

Superior Activity of Structurally Deprived Enzyme−Carbon Nanotube Hybrids in Cationic Reverse Micelles

In the present work, we report the superior activity of hydrophobically adsorbed enzymes onto single-walled carbon nanotubes (SWNTs) in the reverse micelles of cationic surfactants. Horseradish peroxidase and soybean peroxidase adsorbed onto SWNTs endure a notable loss in secondary structure and catalytic activity. This structurally and functionally deformed enzyme-SWNT when confined in CTAB reverse micelles showed approximately 7-9-fold enhancement in activity compared to that was in water and also importantly approximately 1500-3500 times higher activity than that of the enzymes in aqueous-organic biphasic mixtures. The activation observed for this nanobiocomposite is due to the (i) possible localization of enzyme-SWNT hybrid at the micellar interface; (ii) facile transport of substrates across the microscopic interface of reverse micelles; and (iii) greater local concentration of substrates at the augmented interfacial space in the presence of SWNT. This interfacial localization of the SWNT-protein hybrid was tested using FITC-tagged protein (BSA) by fluorescence spectroscopy. FTIR and CD spectroscopy established that the enzyme notably loses its native structure as it gets adsorbed onto the CNTs. However, this loss in the secondary structure is neither aggravated nor recovered when the enzyme-SWNT resides at the reverse micellar interface. So, localization of the surface-active peroxidase-CNT hybrids at the interface is the main reason for significant enzyme activation. The generality of the activation of the enzyme-CNT hybrid by reverse micelles was tested using amphiphiles with varying headgroup sizes, where an overall enhancement in activity was observed with an increase in headgroup size. Activation of this nanobiocomposite would find utmost importance in material science as the activity of structurally deprived enzyme in reverse micelles surpassed (approximately 1.7-fold) even the activity of the native enzyme in water.

Biophysical Characterization Of Phosphatidyl Alcohols In Model Membranes- Effects Of Headgroup Size

The polymorphic phase behavior of different phospholipids depends strongly on fine details of their head group and backbone structures. Studies of phospholipid head group modification offers an attractive strategy in elucidating how structural elements of the head group effect membrane properties. We are investigating phosphatidyl alcohols with saturated fatty acyl chains and their interactions with phospholipids and sterols. Molecules of interest are dipalmitoyl glycerophospholipids with methanol, ethanol, propanol and butanol as different headgroups. These molecules can be formed from e.g., corresponding phosphatidylcholines due to the transphosphatidylation activity of phospholipase D. Phosphatidyl ethanol is of physiological relevance since it is increasingly formed in cells of chronic alcoholics. Thermodynamic properties such as phase transition temperatures, heat capacity of transitions and transition cooperativity have been studied with high sensitivity differential scanning calorimetry (DSC). The miscibility and effects of cholesterol on mixtures of these negatively charged phospholipids with high Tm and low Tm phospholipids have been determined using DSC, and with fluorescence spectroscopy using domain-selective lipid probes. The relative acyl-chain order in the bilayer membranes has been assessed using steady-state DPH anisotropy. We will report on the phase behavior of these molecules in simple and more complex bilayer systems. The formation, by the dipalmitoyl phosphatidyl alcohols, of sterol-enriched or ordered domains in fluid bilayer membranes will also be determined. Comparison of the properties of these phosphatidyl alcohols with molecules having closely related structures will help in understanding the relationships between the chemical structures, physical properties and functional properties of the phospholipid molecules in model as well as biological membranes.

Computational Studies of Substrate Binding and Conformational Change in the Glycine Betaine Symporter BetP

The glycine betaine symporter BetP is an important protein for regulation of osmotic pressure in the microbe Corynebacterium glutamicum, a bacterium used extensively in biotechnology. BetP responds to changes in external osmolality by regulation of its transport activity. The recently solved X-ray crystallographic structure of this protein reveals that it is a homotrimer and that each monomer possesses its own substrate binding pocket. Available structural data for several secondary transporters suggest that these proteins may adopt one of several structurally-distinct states, namely outward- and inward-facing state conformations, as well as a so-called occluded state. Since the monomers in the BetP X-ray structure do not show any apparent substrate pathways, they are likely to represent an occluded state. To identify structural features of alternate states, which may have relevance for a range of secondary transporters, we constructed 3D models of outward- and inward-facing states of BetP using secondary transporters of known structure as templates and validated the modelling results through the rigid-body fitting of these models to low-resolution cryo-EM maps. In addition, to address several other remaining questions, including: the location of glycine betaine and Na+ binding sites; the effect of the headgroup size and net charge of lipid molecules; and the importance of the trimeric state of the protein, we have performed all-atom molecular dynamics simulations of BetP. Finally, we combined the results of structural and simulation studies with those from sequence analysis of BCCT transporters in order to identify structural and functional roles for several important residues. The results of our computational studies may lead to a better understanding of key events in the transport cycle and they are being validated experimentally.

Formation of Block Liposomes is a General Phenomenon of Charged Membranes

We have recently reported on the discovery of block liposomes (BLs), a new class of chain-melted (liquid) vesicles, formed in mixtures of curvature-stabilizing hexadecavalent cationic lipid MVLBG2, DOPC and water (Zidovska et al., Langmuir, 2008). BLs consist of distinctly shaped, yet connected spheres, pears, tubes, or rods. Unlike typical liposome systems, where spherical vesicles, tubular vesicles, and cylindrical micelles are separated on the macroscopic scale, within a BL, shapes are separated on the nanometer scale. We carried out structural studies of BLs with differential-contrast-microscopy (DIC) and cryo-transmission-electron-microscopy (cryo-TEM) and identified membrane charge and spontaneous membrane curvature as key parameters controlling the BL-formation. BL-formation was believed to be a special capacity of MVLBG2, a newly synthesized highly charged (16+) lipid (Ewert et al., JACS, 2006) with giant dendrimer-like headgroup leading to a conical molecular shape resulting into high spontaneous membrane curvature, when incorporated into lipid bilayer. In this work we report formation of BLs for other cationic lipids, demonstrating that BL-formation is a general phenomenon of all charged membranes. We carried out systematic study of binary lipid mixtures comprised of DOPC and a cationic lipid, varying the headgroup size and charge of cationic lipid from 1+, 3+ to 5+. We find that all cationic lipids form BLs on the micrometer and nanometer scale. We have also found that pure DOPC forms BLs in presence of monovalent salt, which is known to cause zwitterionic DOPC to become negatively charged. This latter finding confirms that BL-formation is a general capacity of charged membranes independent of the charge nature. Block liposomes may find a range of applications in chemical and nucleic acid delivery and as building blocks in the design of templates for hierarchical structures. Funding by DOE DE-FG-02-06ER46314, NIH GM-59288, NSF DMR-0503347.

Structural Studies of Surfactants at the Oil−Water Interface by Neutron Reflectometery

We report information regarding the structure at the interface between hexadecane and an aqueous solution of trimethyl tetradecyl ammonium bromide (C14TAB) with appropriate deuterium labeling. We also report the role of the headgroup size and the charge on structures at the oil-water interface by comparing the data for C14TAB to that for a deuterated trimethyl tetradecyl ammonium sulfate (C14TAS) solution at the oil-water interface. As the charge on the counterion increases, this results in a reduction in the limiting area per molecule and an increase in the adsorbed amount at the oil-water interface.

Lipid-engineered Escherichia coli Membranes Reveal Critical Lipid Headgroup Size for Protein Function

Escherichia coli membranes have a substantial bilayer curvature stress due to a large fraction of the nonbilayer-prone lipid phosphatidylethanolamine, and a mutant (AD93) lacking this lipid is severely crippled in several membrane-associated processes. Introduction of four lipid glycosyltransferases from Acholeplasma laidlawii and Arabidopsis thaliana, synthesizing large amounts of two nonbilayer-prone, and two bilayer-forming gluco- and galacto-lipids, (i) restored the curvature stress with the two nonbilayer lipids, and (ii) diluted the high negative lipid surface charge in all AD93 bilayers. Surprisingly, the bilayer-forming diglucosyl-diacylglycerol was almost as good in improving AD93 membrane processes as the two nonbilayer-prone glucosyl-diacylglycerol and galactosyl-diacylglycerol lipids, strongly suggesting that lipid surface charge dilution by these neutral lipids is very important for E. coli. Increased acyl chain length and unsaturation, plus cardiolipin (nonbilayer-prone) content, were probably also beneficial in the modified strains. However, despite a correct transmembrane topology for the transporter LacY in the diglucosyl-diacylglycerol clone, active transport failed in the absence of a nonbilayer-prone glycolipid. The corresponding digalactosyl-diacylglycerol bilayer lipid did not restore AD93 membrane processes, despite analogous acyl chain and cardiolipin contents. Chain ordering, probed by bis-pyrene lipids, was substantially lower in the digalactosyl-diacylglycerol strain lipids due to its extended headgroup. Hence, a low surface charge density of anionic lipids is important in E. coli membranes, but is inefficient if the headgroup of the diluting lipid is too large. This strongly indicates that a certain magnitude of the curvature stress is crucial for the bilayer in vivo.

Solubilization of lipid bilayers by myristyl sucrose ester: effect of cholesterol and phospholipid head group size

The solubilization of biological membranes by detergents has been used as a major method for the isolation and purification of membrane proteins and other constituents. Considerable interest in this field has resulted from the finding that different components can be solubilized selectively. Certain membrane constituents are incorporated into small micelles, whereas others remain in the so-called detergent-resistant membrane domains that are large enough to be separated by centrifugation. The detergent-resistant fractions contain an elevated percentage of cholesterol, and thus its interaction with specific lipids and proteins may be key for membrane organization and regulation of cellular signaling events. This report focuses on the solubilization process induced by the sucrose monoester of myristic acid, β- d-fructofuranosyl-6-O-myristyl-α- d-glucopyranoside (MMS), a nonionic detergent. We studied the effect of the head group and the cholesterol content on the process. 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and dioctadecyl-dimethyl-ammonium chloride (DODAC) vesicles were used, and the solubilization process was followed using Laurdan (6-dodecanoyl-2-dimethylaminonaphthalene) generalized polarization (GP) measurements, carried out in the cuvette and in the 2-photon microscope. Our results indicate that: (i) localization of the MMS moieties in the lipid bilayer depends on the characteristics of the lipid polar head group and influences the solubilization process. (ii) Insertion of cholesterol molecules into the lipid bilayer protects it from solubilizaton and (iii) the microscopic mechanism of solubilization by MMS implies the decrease in size of the individual liposomes.

Examining the Contributions of Lipid Shape and Headgroup Charge on Bilayer Behavior

To better understand bilayer property dependency on lipid electrostatics and headgroup size, we use atomistic molecular dynamics simulations to study negatively charged and neutral lipid membranes. We compare the negatively charged phosphatidic acid (PA), which at physiological pH and salt concentration has a negative spontaneous curvature, with the negatively charged phosphatidylglycerol (PG) and neutrally charged phosphatidylcholine (PC), both of which have zero spontaneous curvature. The PA lipids are simulated using two different sets of partial charges for the headgroup and the varied charge distribution between the two PA systems results in significantly different locations for the Na+ ions relative to the water/membrane interface. For one PA system, the Na+ ions are localized around the phosphate group. In the second PA system, the Na+ ions are located near the ester carbonyl atoms, which coincides with the preferred location site for the PG Na+ ions. We find that the Na+ ion location has a larger effect on bilayer fluidity properties than lipid headgroup size, where the Alipid and acyl chain order parameter values are more similar between the PA and PG bilayers that have Na+ ions located near the ester groups than between the two PA bilayers.

Spreading behaviour of a series of amphiphilic [3 : 3] [60]fullerene hexakisadducts: a systematic study

The spreading behaviour at the air–water interface of a series of [60]fullerene [3 : 3] hexakisadducts has been studied as a function of the hydrophobic–hydrophilic balance of the amphiphiles. More precisely, by changing both the nature and the size of the polar headgroup, by varying the nature of the chemical linker and/or spacer, by modifying the nature and/or the length of the spacer, it has been possible to perform a systematic study. The latter was carried out not only according to the pH of the aqueous subphase but also to its temperature highlighted various behaviours which in particular allowed to us to conclude that the optimal chemical design for this type of amphiphilic molecules is reached with the couples amine polar head/ester linker (Hexa-4) or carboxylic acid polar head/amide linker (Hexa-5).

Wormlike micelles in poly(oxyethylene) surfactant solution: Growth control through hydrophilic-group size variation

Viscoelastic micellar solutions are formed in poly(oxyethylene) cholesteryl ether (ChEO m , m = 15 , 30) aqueous solutions on addition of tri(ethyleneglycol) mono n-dodecyl ether (C 12EO 3). The steady-shear and dynamic rheological behavior of the systems is characteristic of wormlike micellar solution. In either system, the plateau modulus ( G 0 ) and relaxation time ( τ) are found to increase with increasing cosurfactant mixing fractions. The plateau modulus of the ChEO 30–C 12EO 3 system at the maximum viscosity region is found to be higher than that in the ChEO 15–C 12EO 3 system at the maximum viscosity region, whereas for the relaxation time the opposite relation is found. The maximum viscosities obtained in the two systems are of the same order of magnitude. In the ChEO 30–C 12EO 3 system, the maximum viscosity is obtained at a higher cosurfactant mixing fraction than that in the ChEO 15–C 12EO 3 system. It is concluded that decreasing the head-group size of the hydrophilic surfactant favors micellar growth. Monolaurin, another hydrophobic surfactant known to induce growth in some systems, is found to cause phase separation before significant micellar growth occurs in ChEO m solutions, although the effect of head-group size of ChEO m is found to be similar to the ChEO m –C 12EO 3 systems.

Electrostatic Contribution to the Surface Pressure of Charged Monolayers Containing Polyphosphoinositides

Structural and functional studies of lateral heterogeneity in biological membranes have underlined the importance of membrane organization in biological function. Most inquiries have focused on steric determinants of membrane organization, such as headgroup size and acyl-chain saturation. This manuscript reports a combination of theory and experiment that shows significant electrostatic contributions to surface pressures in monolayers of phospholipids where the charge spacing is smaller than the Bjerrum length. For molecules with steric cross sections typical of phospholipids in the cell membrane (∼50 Å 2), only polyphosphoinositides achieve this threshold. The most abundant such lipid is phosphatidylinositol bisphosphate, which has between three and four charged groups at physiological conditions. Theory and experiment show that surface pressure increases linearly with phosphatidylinositol bisphosphate net charge and reveal crossing of high and low ionic strength pressure-area isotherms, due to opposing effects of ionic strength in compressed and expanded monolayers. Theory and experiment show that electrostatic effects are negligible for monolayers of univalent lipids, emphasizing the unique importance of electrostatic effects for lateral organization of polyphosphoinositides. Quantitative differences between theory and experiment suggest that attractive interactions between polyphosphoinositides, possibly mediated by hydrogen bonding, can lessen the effect of electrostatic repulsions.

Asymmetric Structural Features in Single Supported Lipid Bilayers Containing Cholesterol and G M1 Resolved with Synchrotron X-Ray Reflectivity

The cell membrane comprises numerous protein and lipid molecules capable of asymmetric organization between leaflets and liquid-liquid phase separation. We use single supported lipid bilayers (SLBs) to model cell membranes, and study how cholesterol and asymmetrically oriented ganglioside receptor G M1 affect membrane structure using synchrotron x-ray reflectivity. Using mixtures of cholesterol, sphingomyelin, and 1,2-dioleoyl- sn-glycero-3-phosphocholine, we characterize the structure of liquid-ordered and liquid-disordered SLBs in terms of acyl-chain density, headgroup size, and leaflet thickness. SLBs modeling the liquid-ordered phase are 10 Å thicker and have a higher acyl-chain electron density (〈 ρ chain〉 = 0.33 e −/Å 3) compared to SLBs modeling the liquid-disordered phase, or pure phosphatidylcholine SLBs (〈 ρ chain〉 = 0.28 e −/Å 3). Incorporating G M1 into the distal bilayer leaflet results in membrane asymmetry and thickening of the leaflet of 4–9 Å. The structural effect of G M1 is more complex in SLBs of cholesterol/sphingomyelin/1,2-dioleoyl- sn-glycero-3-phosphocholine, where the distal chains show a high electron density (〈 ρ chain〉 = 0.33 e −/Å 3) and the lipid diffusion constant is reduced by ∼50%, as measured by fluorescence microscopy. These results give quantitative information about the leaflet asymmetry and electron density changes induced by receptor molecules that penetrate a single lipid bilayer.

Temperature-Dependent Self-Assembly and Mixing Behavior of Symmetrical Single-Chain Bolaamphiphiles

The temperature-dependent self-assembly and the mixing behavior of symmetrical single-chain bolaamphiphiles with different polymethylene chain lengths and different headgroup structures were investigated in water by differential scanning calorimetry (DSC), cryo transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). The even-numbered polymethylene-1,omega-bis(phosphocholines) (PC-C n-PC) are known to form nanofibers composed of stretched molecules with an all- trans alkyl chain conformation (Drescher, S.; Meister, A.; Blume, A.; Karlsson, G.; Almgren, M.; Dobner, B. Chem.Eur. J. 2007, 13, 5300-5307). The odd-numbered analogues were synthesized to study a possible even-odd effect of these bolaamphiphiles during their aggregation in water. In addition to these bolaamphiphiles with phosphocholine headgroups, a new series of polymethylene-1,omega-bis(phosphodimethylethanolamines) (Me2PE-Cn-Me2PE) with smaller headgroup sizes was synthesized. These bolaamphiphiles show an additional fiber-fiber transition when the alkyl chain length exceeds 26 carbon atoms. The mixing behavior of both types of bolaamphiphiles indicates that differences in the alkyl chain length up to six carbon atoms are tolerated within the fiber structure. The mixing of two Me2PE-Cn-Me2PE or PC-Cn-PC type bolaamphiphiles with different alkyl chain lengths offers the possibility to adjust the temperature, where the cross-linking of the fibers is disrupted and where the fibers break apart. As a consequence, temperature switchable hydrogels are obtained that can be fine-tuned for drug delivery applications. The comparison with dotriacontane-1,32-diyl-bis[2-(methylammonio)-ethylphosphate] (MePE-C32-MePE), a new bolaamphiphile with even smaller phosphomonomethylammonio headgroups, illustrates the importance of the headgroup size for the aggregation behavior. This bolaamphiphile self-assembles exclusively into lamellar structures, and this aggregate type persists in mixtures with the fiber forming Me2PE-C32-Me2PE.