Large Headgroup Research Articles

Addition of lysophospholipids with large head groups to cells inhibits Shiga toxin binding.

Shiga toxin (Stx), an AB5 toxin, binds specifically to the neutral glycosphingolipid Gb3 at the cell surface before being transported into cells. We here demonstrate that addition of conical lysophospholipids (LPLs) with large head groups inhibit Stx binding to cells whereas LPLs with small head groups do not. Lysophosphatidylinositol (LPI 18:0), the most efficient LPL with the largest head group, was selected for in-depth investigations to study how the binding of Stx is regulated. We show that the inhibition of Stx binding by LPI is reversible and possibly regulated by cholesterol since addition of methyl-β-cyclodextrin (mβCD) reversed the ability of LPI to inhibit binding. LPI-induced inhibition of Stx binding is independent of signalling and membrane turnover as it occurs in fixed cells as well as after depletion of cellular ATP. Furthermore, data obtained with fluorescent membrane dyes suggest that LPI treatment has a direct effect on plasma membrane lipid packing with shift towards a liquid disordered phase in the outer leaflet, while lysophosphoethanolamine (LPE), which has a small head group, does not. In conclusion, our data show that cellular treatment with conical LPLs with large head groups changes intrinsic properties of the plasma membrane and modulates Stx binding to Gb3.

Glycerolipidome responses to freezing- and chilling-induced injuries: examples in Arabidopsis and rice.

BackgroundGlycerolipids are the principal constituent of cellular membranes; remodelling of glycerolipids plays important roles in temperature adaptation in plants. Temperate plants can endure freezing stress, but even chilling at above-zero temperatures can induce death in tropical species. However, little is known about the differences in glycerolipid response to low temperatures between chilling-sensitive and freezing-tolerant plants. Using ESI-MS/MS-based lipidomic analysis, we compared the glycerolipidome of chilling (4 and 10 °C)-treated rice with that of freezing (−6 and −12 °C)-treated Arabidopsis, both immediately after these low-temperature treatments and after a subsequent recovery culture period.ResultsArabidopsis is a 16:3 plant that harbours both eukaryotic and prokaryotic-type lipid synthesis pathways, while rice is an 18:3 plant that harbours only the eukaryotic lipid synthesis pathway. Arabidopsis contains higher levels of galactolipids than rice and has a higher double bond index (DBI). Arabidopsis contains lower levels of high melting point phosphatidylglycerol (PG) molecules and has a lower average acyl chain length (ACL). Marked phospholipid degradation occurred during the recovery culture period of non-lethal chilling treated rice, but did not occur in non-lethal freezing treated Arabidopsis. Glycerolipids with larger head groups were synthesized more in Arabidopsis than in rice at sub-lethal low-temperatures. Levels of phosphatidic acid (PA) and phosphatidylinositol (PI) rose in both plants after low-temperature treatment. The DBI and ACL of total lipids did not change during low-temperature treatment.ConclusionsA higher DBI and a lower ACL could make the membranes of Arabidopsis more fluid at low temperatures. The ability to synthesize glycerolipids containing a larger head group may correlate with low-temperature tolerance. The low-temperature-induced increase of PA may play a dual role in plant responses to low temperatures: as a lipid signal that initiates tolerance responses, and as a structural molecule that, on extensive in large accumulation, could damage the integrity of membranes. Changes in ACL and DBI are responses of plants to long-term low temperature.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0758-8) contains supplementary material, which is available to authorized users.

No benefit on functional outcomes and dislocation rates by increasing head size to 36 mm in ceramic-on-ceramic total hip arthroplasty

BackgroundCeramic-on-ceramic (COC) total hip arthroplasty (THA) has gained popularity since improvements in wear characteristics and longevity. Whether large ceramic femoral heads (≥36 mm) have increased postoperative range of motion (ROM) and a lower dislocation rate is not clear. This study aimed to compare functional outcomes and early complications between large-head (≥36 mm) and smaller-head (≤32 mm) COC prostheses with a minimum follow-up of 12 months. MethodsA total of 95 consecutive uncemented COC THAs were performed in 90 patients between January 2012 and July 2013. Of these, 49 patients (smaller-head group) received third generation and 41 patients (large-head group) received fourth generation COC prostheses. Harris hip score (HHS), Western Ontario and McMaster Universities Arthritis index (WOMAC), and ROM of the hip pre- and post-operatively were compared, as well as the presence of early complications. ResultsPostoperative HHSs (88.4 vs. 89.3, p = 0.34) and WOMAC scores (12.0 vs. 11.0, p = 0.111) were not different between the groups. Postoperative flexion ROM was lower in the smaller-head group (98.8° vs. 106.1°, p < 0.001), but there were no differences in extension, abduction, adduction, internal rotation, and external rotation. One patient in each group reported a grinding noise. There was one dislocation (1.9%) in the smaller-head group, and none in the large-head group (p = 0.371). No infections or loosening of the components occurred. ConclusionsLarge-head COC articulation provided better flexion, but functional outcomes and early complications are similar to the smaller-head COC.

Effect of Phosphatidic Acid on Biomembrane: Experimental and Molecular Dynamics Simulations Study.

We consider the impact of phosphatidic acid (namely, 1,2-dioleoyl-sn-glycero-3-phosphate, DOPA) on the properties of a zwitterionic (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) bilayer used as a model system for protein-free cell membranes. For this purpose, experimental measurements were performed using differential scanning calorimetry and the Langmuir monolayer technique at physiological pH. Moreover, atomistic-scale molecular dynamics (MD) simulations were performed to gain information on the mixed bilayer's molecular organization. The results of the monolayer studies clearly showed that the DPPC/DOPA mixtures are nonideal and the interactions between lipid species change from attractive, at low contents of DOPA, to repulsive, at higher contents of that component. In accordance with these results, the MD simulations demonstrated that both monoanionic and dianionic forms of DOPA have an ordering and condensing effect on the mixed bilayer at low concentrations. For the DOPA monoanions, this is the result of both (i) strong electrostatic interactions between the negatively charged oxygen of DOPA and the positively charged choline groups of DPPC and (ii) conformational changes of the lipid acyl chains, leading to their tight packing according to the so-called "umbrella model", in which large headgroups of DPPC shield the hydrophobic part of DOPA (the conical shape lipid) from contact with water. In the case of the DOPA dianions, cation-mediated clustering was observed. Our results provide a detailed molecular-level description of the lipid organization inside the mixed zwitterionic/PA membranes, which is fully supported by the experimental data.

Formation of an ordered phase by ceramides and diacylglycerols in a fluid phosphatidylcholine bilayer — Correlation with structure and hydrogen bonding capacity

Ceramides and diacylglycerols are lipids with a large hydrophobic part (acyl chains and long-chain base) whereas their polar function (hydroxyl group) is small. They need colipids with large head groups to coexist in bilayer membranes. In this study, we have determined how saturated and unsaturated ceramides and acyl-chain matched diacylglycerols form ordered domains in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers as a function of bilayer concentration. The formation of ordered domains was determined from lifetime analysis of trans-parinaric acid. Ceramides formed ordered domains with equal average tPA lifetime at lower bilayer concentration when compared to acyl-chain matched diacylglycerols. This was true for both saturated (16:0) and mono-unsaturated (18:1) species. This finding suggested that hydrogen bonding among ceramides contributed to their more efficient ordered phase formation, since diacylglycerols do not form similar hydrogen bonding networks. The role of hydrogen bonding in ordered domain formation was further verified by using palmitoyl ceramide analogs with 2N and 3OH methylated long-chain bases. These analogs do not form hydrogen bonds from the 2NH or the 3OH, respectively. While methylation of the 3OH did not affect ordered phase formation compared to native palmitoyl ceramide, 2NH methylation markedly attenuated ceramide ordered phase formation. We conclude that in addition to acyl chain length, saturation, molecular order, and lack of large head group, also hydrogen bonding involving the 2NH is crucial for efficient formation of ceramide-rich domains in fluid phosphatidylcholine bilayers.

A GH57 4-α-glucanotransferase of hyperthermophilic origin with potential for alkyl glycoside production.

4-α-Glucanotransferase (GTase) enzymes (EC 2.4.1.25) modulate the size of α-glucans by cleaving and reforming α-1,4 glycosidic bonds in α-glucans, an essential process in starch and glycogen metabolism in plants and microorganisms. The glycoside hydrolase family 57 enzyme (GTase57) studied in the current work catalyzes both disproportionation and cyclization reactions. Amylose was converted into cyclic amylose (with a minimum size of 17 glucose monomers) as well as to a spectrum of maltodextrins, but in contrast to glycoside hydrolase family 13 cyclodextrin glucanotransferases (CGTases), no production of cyclodextrins (C6-C8) was observed. GTase57 also effectively produced alkyl-glycosides with long α-glucan chains from dodecyl-β-D-maltoside and starch, demonstrating the potential of the enzyme to produce novel variants of surfactants. Importantly, the GTase57 has excellent thermostability with a maximal activity at 95 °C and an activity half-life of 150 min at 90 °C which is highly advantageous in this manufacturing process suggesting that enzymes from this relatively uncharacterized family, GH57, can be powerful biocatalysts for the production of large head group glucosides from soluble starch.

An effective dispersant for oil spills based on food-grade amphiphiles.

Synthetic dispersants such as Corexit 9500A were used in large quantities (∼2 million gallons) to disperse the oil spilled in the ocean during the recent Deepwater Horizon event. These dispersant formulations contain a blend of surfactants in a base of organic solvent. Some concerns have been raised regarding the aquatic toxicity and environmental impact of these formulations. In an effort to create a safer dispersant, we have examined the ability of food-grade amphiphiles to disperse (emulsify) crude oil in seawater. Our studies show that an effective emulsifier is obtained by combining two such amphiphiles: lecithin (L), a phospholipid extracted from soybeans, and Tween 80 (T), a surfactant used in many food products including ice cream. Interestingly, we find that L/T blends show a synergistic effect, i.e., their combination is an effective emulsifier, but neither L or T is effective on its own. This synergy is maximized at a 60/40 weight ratio of L/T and is attributed to the following reasons: (i) L and T pack closely at the oil-water interface; (ii) L has a low tendency to desorb, which fortifies the interfacial film; and (iii) the large headgroup of T provides steric repulsions between the oil droplets and prevents their coalescence. A comparison of L/T with Corexit 9500A shows that the former leads to smaller oil droplets that remain stable to coalescence for a much longer time. The smaller size and stability of crude oil droplets are believed to be important to their dispersion and eventual microbial degradation in the ocean. Our findings suggest that L/T blends could potentially be a viable alternative for the dispersion of oil spills.

Molecular Modeling of Lipid Membrane Curvature Induction by a Peptide: More than Simply Shape

Molecular dynamics simulations of an amphipathic helix embedded in a lipid bilayer indicate that it will induce substantial positive curvature (e.g., a tube of diameter 20 nm at 16% surface coverage). The induction is twice that of a continuum model prediction that only considers the shape of the inclusion. The discrepancy is explained in terms of the additional presence of specific interactions described only by the molecular model. The conclusion that molecular shape alone is insufficient to quantitatively model curvature is supported by contrasting molecular and continuum models of lipids with large and small headgroups (choline and ethanolamine, respectively), and of the removal of a lipid tail (modeling a lyso-lipid). For the molecular model, curvature propensity is analyzed by computing the derivative of the free energy with respect to bending. The continuum model predicts that the inclusion will soften the bilayer near the headgroup region, an effect that may weaken curvature induction. The all-atom predictions are consistent with experimental observations of the degree of tubulation by amphipathic helices and variation of the free energy of binding to liposomes.

CD1a-autoreactive T cells recognize natural skin oils that function as headless antigens

CD1a autoreactive T cells are common in human blood and skin, but the search for natural autoantigens has been confounded by background T cell responses to CD1 proteins and self lipids. After capturing CD1a-lipid complexes, we gently eluted ligands, while preserving unliganded CD1a for testing lipids from tissues. CD1a released hundreds of ligands of two types. Inhibitory ligands were ubiquitous membrane lipids with polar headgroups, whereas stimulatory compounds were apolar oils. CD1a autoantigens naturally accumulate in epidermis and sebum, where they were identified as squalene and skin waxes. T cell activation by skin oils suggests that headless mini-antigens nest within CD1a and displace non-antigenic resident lipids with large head groups. Oily autoantigens naturally coat the skin's surface, pointing to a new mechanism of barrier immunity.

Isolevuglandin-modified phosphatidylethanolamine is metabolized by NAPE-hydrolyzing phospholipase D

Lipid aldehydes including isolevuglandins (IsoLGs) and 4-hydroxynonenal modify phosphatidylethanolamine (PE) to form proinflammatory and cytotoxic adducts. Therefore, cells may have evolved mechanisms to degrade and prevent accumulation of these potentially harmful compounds. To test if cells could degrade isolevuglandin-modified phosphatidylethanolamine (IsoLG-PE), we generated IsoLG-PE in human embryonic kidney 293 (HEK293) cells and human umbilical cord endothelial cells and measured its stability over time. We found that IsoLG-PE levels decreased more than 75% after 6 h, suggesting that IsoLG-PE was indeed degraded. Because N-acyl phosphatidylethanolamine-hydrolyzing phospholipase D (NAPE-PLD) has been described as a key enzyme in the hydrolysis of N-acyl phosphatidylethanoamine (NAPE) and both NAPE and IsoLG-PE have large aliphatic headgroups, we considered the possibility that this enzyme might also hydrolyze IsoLG-PE. We found that knockdown of NAPE-PLD expression using small interfering RNA (siRNA) significantly increased the persistence of IsoLG-PE in HEK293 cells. IsoLG-PE competed with NAPE for hydrolysis by recombinant mouse NAPE-PLD, with the catalytic efficiency (V(max)/K(m)) for hydrolysis of IsoLG-PE being 30% of that for hydrolysis of NAPE. LC-MS/MS analysis confirmed that recombinant NAPE-PLD hydrolyzed IsoLG-PE to IsoLG-ethanolamine. These results demonstrate that NAPE-PLD contributes to the degradation of IsoLG-PE and suggest that a major physiological role of NAPE-PLD may be to degrade aldehyde-modified PE, thereby preventing the accumulation of these harmful compounds.

Anomalies in Solution Behavior of an Alkyl Aminopolycarboxylic Chelating Surfactant

The solution behavior of a DTPA (diethylenetriamine pentaacetic acid)-based chelating surfactant, 4-C12-DTPA, has been studied by tensiometry and NMR diffusometry. In the absence of metal ions, the eight donor atoms in the headgroup are titrating, and the charge of the headgroup can thus be tuned by altering the pH. 4-C12-DTPA changes from cationic at very low pH, over a number of zwitterionic species as the pH is increased, and eventually becomes anionic at high pH. Around the isoelectric point, the chelating surfactant precipitated. The solution properties, studied above the solubility gap, were found strongly pH dependent. When increasing the amount of negative charges in the headgroup, by increasing the pH, the adsorption efficiency was reduced and the cmc was increased. An optimum in surface tension reduction was found at pH 5, due to a proper balance between protonated and dissociated groups. Anomalies between surface tension measurements and NMR diffusometry in determination of cmc revealed a more complex relation between surface tension, surface coverage, and cmc than usually considered, which is not in line with the common interpretation of the Gibbs adsorption equation. At some of the investigated pH levels, measurements of bulk pH could confirm the location of cmc, due to the increased protonation of micelles compared to monomers in solution. The adsorption of monomers to the air-water interface showed unusually slow time dependence, evident from decreasing surface tension for several hours. This is explained by rearrangements of the large head groups to reduce the headgroup area and increase the packing parameter.

Effect of emulsifier type and concentration, aqueous phase volume and wax ratio on physical, material and mechanical properties of water in oil lipsticks

Water-in-oil emulsions in lipsticks could have the potential to improve moisturizing properties and deliver hydrophilic molecules to the lips. The aims of this work were (i) to investigate the effect of emulsifier type (polymer vs. monomer, and saturated vs. unsaturated chain) and concentration on droplet size and (ii) to investigate the effect of wax ratio (carnauba wax, microcrystalline wax, paraffin wax and performalene) and aqueous phase volume on material properties (Young's modulus, point of fracture, elastic modulus and viscous modulus). Emulsion formation was achieved using a high shear mixer. Results showed that the saturated nature of the emulsifier had very little effect on droplet size, neither did the use of an emulsifier with a larger head group (droplet size ~18-25μm). Polyglycerol polyricinoleate (PGPR) resulted in emulsions with the smallest droplets (~3-5μm), as expected from previous studies that show that it produces a thick elastic interface. The results also showed that both Young's modulus and point of fracture increase with increasing percentage of carnauba wax (following a power law dependency of 3), but decrease with increasing percentage of microcrystalline wax, suggesting that the carnauba wax is included in the overall wax network formed by the saturated components, whereas the microcrystalline wax forms irregular crystals that disrupt the overall wax crystal network. Young's modulus, elastic modulus and viscous modulus all decrease with increasing aqueous phase volume in the emulsions, although the slope of the decrease in elastic and viscous moduli is dependent on the addition of solid wax, as a result of strengthening the network. This work suggests the potential use for emulsions in lipstick applications, particularly when PGPR is used as an emulsifier, and with the addition of solid wax, as it increases network strength.

Pressure Reentrant Assembly: Direct Simulation of Volumes of Micellization

Surfactants exhibit maxima in their critical micelle concentrations upon application of hydrostatic pressure, which is attributable to changes in their volumes of micellization from positive to negative values with increasing pressure. We present a direct molecular simulation analysis of the volumes of micellization of an anionic, cationic, and nonionic surfactant in aqueous solution at pressures up to 2500 bar. Excellent agreement with experiment is observed. A Kirkwood-Buff theory analysis based on proximal solvent distributions permits the breakdown of the volumes of micellization into constituent surfactant headgroup and tailgroup contributions. Although the micellization volume crossover is analogous to the transfer of an alkane from water to its pure liquid, significant differences are observed, including lower compressibilities of micelle volumes compared to that of the alkane liquid, negative partial compressibilites for anionic sulfated surfactant monomers, and large nonionic ethoxy headgroup contributions to the micellization volume.

The nanostructure of surfactant–DNA complexes with different arrangements

The nanostructure of DNA with different cationic surfactant has been studied in order to elucidate the detailed arrangement concerning the position of DNA and surfactant domains in the complexes. Also, the orientation of the DNA cylinders in the thin films of the complexes was investigated. Attention was directed on the preparation methods of the complexes and to how the different surfactant structure affects the compaction of the DNA. The cationic surfactant–DNA complexes were investigated by X-ray scattering, polarized light microscopy and elemental microanalysis. It was observed that the molecular organization of the complexes between DNA and cationic surfactant corresponds to a hexagonal structure with different packing arrangements. The nanostructure of the complexes depends on the hydrophobic/hydrophilic balance of the cationic surfactant. In particular the use of arginine derived surfactants, with a large polar head group able to interact not only by electrostatics but also by hydrogen bonding, allows for the formation of more compact structures. The results suggest that the smaller the lattice parameter the more compact and stable is the complex implying slower DNA release.

Thermodynamic properties of new gluconamide-based cationic surfactants in aqueous solution: Experimental and modeling approaches

The micellization processes of 2-(alkyldimethylammonio)ethylgluconamide bromides (CnGAB), dodecyltrimethylammonium bromide (DTAB) and dodecylethyldimethylammonium bromide (DEDAB) were studied using solution conductivity and isothermal titration calorimetry (ITC). The critical micelle concentrations (CMC), the degree of counterion binding (β), the enthalpies (ΔHm) and entropies (ΔSm) of micellization as well as the contributions of the headgroups to Gibbs free energies (ΔGm0(hy)) were calculated. Approximate relative partial enthalpies (L2) were evaluated as functions of concentration from the calorimetric curves. In comparison with DTAB, the hydrophobicity of C12GAB is reduced by an extent corresponding to the shortening of DTAB’s alkyl chain by one methylene group. The micelles of C12GAB are hydrated by one methylene group deeper than those of DTAB. The free energy of the transfer of the headgroup from the bulk solution to the micelle is more favorable for DTAB than C12GAB. This effect is approximately equal to the ΔGm0(CH2) contribution. In contrast to the usual ionic surfactant behavior, the degree of counterions binding for CnGABs decreases with increasing alkyl chain length. This is supposed to be a consequence of screening of the charged layer of the micelle by the large, hydrated and hydrogen-bonded headgroups from the bromide anions in bulk solution. The computational modeling made it possible to relate the surfactants’ molecular properties to the interactions the surfactants enter into, suggesting that, due to the strongly polar character of their headgroup, CnGABs form small premicellar aggregates.

A PM-IRRAS Investigation of Monorhamnolipid Orientation at the Air–Water Interface

The rhamnolipid biosurfactants have been considered as possible "green" alternatives to synthetic surfactants due to their greater compatibility with the environment and excellent surface active properties. In order to understand the molecular orientation of rhamnolipids at the air-water interface, a new monorhamnolipid with two octadecyl chains, Rha-C18-C18, has been studied at the air-water interface with polarization modulated-infrared reflection absorption spectroscopy (PM-IRRAS). Since rhamnolipids possess a carboxylic acid, and hence exhibit pH-dependent properties, their water surface orientation is studied in solutions of pH 2, 5, and 8. Rhamnolipids have also been reported to form strong complexes with Pb(2+); thus, the effect of the presence of Pb(2+) on molecular orientation at the interface is also investigated. PM-IRRA spectra indicate an increase in alkyl chain order and a decrease in alkyl chain tilt angle as the surface pressure of the monolayer increases, with pH-independent tilt angles ranging from 63° to 45°. Molecular modeling using Spartan provides insight into the cause of this large tilt angle as being due to the nature of the monorhamnolipid packing at the air-water interface as dictated by its large hydrophilic headgroup.

Cholesteryl Phosphocholine – A Study on Its Interactions with Ceramides and Other Membrane Lipids

We prepared cholesteryl phosphocholine (CholPC) by chemical synthesis and studied its interactions with small (ceramide and cholesterol) and large headgroup (sphingomyelin (SM) and phosphatidylcholine) colipids in bilayer membranes. We established that CholPC could form bilayers (giant uni- and multilamellar vesicles, as well as extruded large unilamellar vesicles) with both ceramides and cholesterol (initial molar ratio 1:1). The extruded bilayers appeared to be fluid, although highly ordered, even when the ceramide had an N-linked palmitoyl acyl chain. In binary systems containing CholPC and either palmitoyl SM or 1,2-dipalmitoyl-sn-glycero-3-phospholine, CholPC markedly destabilized the gel phase of the respective large headgroup lipid. In 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, CholPC was much less efficient than cholesterol in ordering the acyl chains. In complex bilayers containing POPC and cholesterol or palmitoyl ceramide, CholPC appeared to prefer interacting with the small headgroup lipids over POPC. When the degree of order in CholPC/PCer bilayers was compared to Chol/PSM bilayers, CholPC/PCer bilayers were more disordered (based on DPH anisotropy). This finding may result from different headgroup orientation and dynamics in CholPC and PSM. Our results overall can be understood if one takes into account the molecular shape of CholPC (large polar headgroup and modest size hydrophobic part) when interpreting molecular interactions between small and large headgroup colipids. The results are also consistent with the proposed umbrella model" for explaining cholesterol/colipid interactions.

Structural Behavior of Aggregate Assemblies of Cationic Surfactants and Their Mixtures with Triblock Polymers

Small angle neutron scattering (SANS) technique has been employed to study the structural aspects of micellar system of cationic surfactants viz. alkyltriphenylphosphonium bromide (C12-, C14-, C16TPB) and hexadecylpyridinium chloride (C16PyCl) with triblock polymers (L64, F68, and F127). SANS data analysis reveals the prolate ellipsoidal shape of mixed micelles and increase in the micellar size upon addition of triblock polymers (L64, F68, and F127). The influence of effective size of the head group segment on the growth of micelles of HTPB (larger head group) has also been compared with that of HPyCl (smaller head group). A proportionate micellar growth of cationic surfactants has been found with increase in the length of tail segment of cationic surfactants. The observed mixed micellar growth in mixed systems is also accounted on the basis of simultaneous increase in the hydrophobicity of both the components in the mixed system. Results from the present study enlightened the effect of variation in head group segment and hydrophobicity on the structural aspects of mixed micellar system.

Tuning the aggregation behaviour of single-chain bolaamphiphiles in aqueous suspension by changes in headgroup asymmetry.

The self-assembly process in aqueous suspension of two new asymmetrical single-chain bolaamphiphiles, namely 32-{[hydroxy(2-hydroxyethoxy)phosphinyl]oxy}dotriacontane-1-yl-{2-[N-(3-dimethylaminopropyl)-N,N-dimethylammonio]ethylphosphate} (DMAPPC-C32-POH) and 32-hydroxydotriacontane-1-yl-{2-[N-(3-dimethylaminopropyl)-N,N-dimethylammonio]ethylphosphate} (DMAPPC-C32-OH), was studied as a function of temperature using transmission electron microscopy, differential scanning calorimetry, FT-IR-spectroscopy, small angle neutron and small angle X-ray scattering to determine whether the asymmetry of the molecule induces the formation of types of aggregates other than the well characterized helical nanofibres of structurally similar symmetrical single-chain bolaamphiphiles with identical headgroups. DMAPPC-C32-POH in acetate buffer at pH 5 can still form nanofibres, i.e. the asymmetry does not induce the formation of other aggregate structures. However, the fibres display a tendency to break more easily and to form irregular, circular structures. This is also reflected by the rheological properties of the suspension that reveal decreased strain resistance at pH 5. In aqueous suspensions at pH 10, where the headgroups of the molecule are negatively charged, only short fibre segments are formed and no gel formation occurs. At higher temperature these fibres convert into micellar aggregates as observed before for symmetrical bolalipids with large headgroups. In contrast, in aqueous suspensions of DMAPPC-C32-OH, a bolalipid where the size difference of the headgroups is much larger, lamellar structures are formed at pH 10 where the headgroup of the molecule is zwitterionic. At low temperature, the molecules are packed in an orthorhombic lattice with interdigitated chains and a repeat distance between lamellae of 6.2 nm is observed. An increase in temperature leads to a lamellar phase with hexagonal packing of the chains. The chains become liquid-crystalline only at very high temperature above 90 °C. At low pH, when the headgroup of the molecule becomes positively charged, some short elongated micellar aggregates are seen besides sheet-like structures. A temperature increase leads to a similar sequence of transformations of the chain packing until formation of a liquid-crystalline lamellar phase at a temperature close to 90 °C. The results show that the aggregation behaviour of single-chain bolaamphiphiles can not only be tuned by changes in chain length or size of both headgroups but also by the difference in headgroup size and charge in asymmetric bolaamphiphiles.

Time-Resolved Fluorescence Quenching Studies of Sodium Lauryl Ether Sulfate Micelles

Aggregation numbers (NAg) of micelles of the commercial anionic detergent sodium lauryl ether sulfate (SLES), with an average of two ethylene oxide subunits, were determined at 30 and 40o C by the time-resolved fluorescence quenching method with pyrene as the fluorescent probe and the N-hexadecylpyridinium ion as the quencher. The added-salt dependent growth of SLES micelles (γ = 0.11-0.15, where γ is the slope of a plot of log aggregation number vs. log [Yaq] and [Yaq] is the sodium counterion concentration free in the intermicellar aqueous phase) is found to be significantly lower than that of sodium alkyl sulfate micelles (γ ca. 0.25), a difference attributed to the larger headgroup size of SLES. The I1/I3 vibronic intensity ratio and the rate constant for intramicellar quenching of pyrene show that the pyrene solubilization microenvironment and the intramicellar microviscosity are insensitive to micelle size or the presence of added salt.