Neutral Amphiphiles Research Articles

Probing the interfacial structure of aqueous surfactants through helium atom evaporation.

Dissolved helium atoms evaporate from liquids in super-Maxwellian speed distributions because their interactions are too weak to enforce full thermal equilibration at the surface as they are "squeezed" out of solution. The excess speeds of these He atoms reflect their final interactions with solvent and solute molecules at the surfaces of water and other liquids. We extend this observation by monitoring He atom evaporation from salty water solutions coated with surfactants. These surface-active molecules span neutral, anionic, and cationic amphiphiles: butanol, 3-methyl-1-butanol, pentanol, pentanoic acid, pentanoate, tetrabutylammonium, benzyltrimethylammonium, hexyltrimethylammonium, and dodecyltrimethylammonium, each characterized by surface tension measurements. The helium energy distributions, recorded in vacuum using a salty water microjet, reveal a sharp distinction between neutral and ionic surfactant films. Helium atoms evaporate through neutral surfactant monolayers in speed distributions that are similar to a pure hydrocarbon, reflecting the common alkyl chains of both. In contrast, He atoms appear to evaporate through ionic surfactant layers in distributions that are closer to pure salty water. We speculate that the ionic surfactants distribute themselves more loosely and deeply through the top layers of the aqueous solution than do neutral surfactants, with gaps between the surfactants that may be filled with salty water. This difference is supported by prior molecular dynamics simulations and ion scattering measurements of surfactant solutions.

Effect of chemical dispersion on distribution of distillates cuts in various crude oil samples

The present paper reports on the distillation behavior of the neat and surfactants dispersed crude oils monitored through boiling point curves/ASTM curves. The surfactants used were cetyltrimethylammonium bromide (CTAB) as cationic, sodium dodecyl sulfate (SDS) as anionic, and Triton-X as neutral amphiphiles. Three crude oils including light (denoted as RCP-I), middle (denoted as RCN-II) and heavy oils (denoted as RCA-III) were studied using atmospheric pressure distillation. The distillation curves (the boiling temperature versus cumulative volume fraction distilled), at 10% (v/v) intervals, were constructed from the obtained results. The results revealed an increased volatility of the crudes in the presence of the surfactants and the samples yielded lighter products (gasoline and diesel range) when provided with surfactants. The dispersion of the RCA-III with SDS significantly increased the yield of light fraction to 58% and the yield of the residue decreased to 6%. The CTAB was effective in RCN-II crude which increased the yield of light fraction to 49%, and decreased the residue yield to 7%. The Triton X-100 also proved to be more effective in RCN-II crude and gave 48 and 6% yields of light and residue fractions, respectively.

PH Responsiveness of Near-infrared Fluorescent Cyanine Dyes Encapsulated in Self-assemblies Composed of Various Amphiphiles

pH-Responsive near-infrared (NIR) cyanine dyes were encapsulated in self-assemblies consisting of anionic, cationic, and neutral amphiphiles in phosphate buffered saline. Self-assemblies of the neu...

Importance of Molecular Meshing for the Stabilization of Solvophobic Assemblies.

The effect of the methyl groups in neutral gear-shaped amphiphiles (GSAs) on the stability of nanocubes was investigated using a novel C2 v-symmetric GSA, which was synthesized using selective alternate trilithiation of a pentabrominated hexaphenylbenzene derivative. The lack of only one methyl group in the GSA decreased the association constant for the assembly of the nanocube by 3 orders of magnitude. A surface analysis recently developed by the authors (SAVPR: surface analysis with varying probe radii) was carried out for characteristic isomers of the nanocube consisting of C2 v-symmetric GSAs. It was found that the methyl groups near the equator of the nanocube play a significant role in the stabilization of the nanocubes.

An investigation of microscopic precipitation and micellization of sodium deoxycholate in presence of Tween 40

Abstract In this work the behavior of NaDC as a function of solution pH in presence of Tween 40 has been studied. The pH results were modelled in terms of carboxylate soap solution theory by Lucassen and its generalized form by Kralchevsky et al. The results reveal that the composition of the NaDC solution in terms of various soaps changes as a function of Tween 40 concentration. In general, Tween 40 has almost no effect on the alkanoic acid HZ formation, but hinders the formation of acid soap (HZ)2(MZ)1 depending upon Tween 40 concentration. However, neutral soap MZ formation is favoured at very high concentrations of Tween 40. The influence of Tween 40 on the composition of NaDC solution has been attributed to the ability of the former to show enhanced hydrogen bonding. The presence of different precipitates is well correlated with the surface tension variation and micelles are formed prior to HZ precipitation. The study is expected to provide useful insights to understand the correlation between precipitation and micellization of bile salts and is expected to help in devising methods to enhance their biological functioning in presence of neutral amphiphiles like Tween 40 commonly encountered in delivery systems or food stuffs etc.

Luminescent supramolecular soft nanostructures from amphiphilic dinuclear Re(I) complexes.

Luminescent metallo-surfactants based on highly emissive dinuclear Re(I) complexes have been synthesized combining the peculiar photophysical behaviour of this class of neutral hydrophobic complexes with new properties imparted by hydrophilic chains anchored on the coordinated chromophoric ligand. In solution, the resulting neutral amphiphiles tend to self-assembly in soft structures. The aggregation properties have been thoroughly investigated in dioxane-water mixtures, where all the complexes assembly in globular-like supramolecular architectures with well-defined size (hydrodynamic diameter = 200-400 nm). The morphology of these nano-objects has been completely characterized with Dynamic Light Scattering (DLS) analysis, Scanning Transmission Electron Microscopy (STEM) and cryo-TEM to determine the size, polydispersity, and stability of the nanoparticles in relationship with the structure of the metallo-surfactants. The photophysical properties of both the isolated metal complexes and their aggregates have been investigated by means of UV-Vis absorption, steady-state and time-resolved emission spectroscopy. Noteworthy, the self-assembly properties of the reported luminescent rhenium metallo-amphiphiles can be modulated by solvent polarity. Even more importantly, such aggregation process yielded a small hypsochromic shift of the emission energy accompanied by a sizeable elongation of the excited-state lifetime and an enhancement of the photoluminescence quantum yield, reaching a remarkably high value of 0.20 despite the air-equilibrated aqueous condition. The presented findings endorse novel possibilities for the efficient use of soft-nanostructures based on metallo-amphiphiles in dual (electron and optical microscopy) bio-imaging applications and theranostics where the non-covalent nature of the intermolecular interactions would offer the powerful and unique possibility to reversibly assemble and disassemble imaging agents.

Synthesis and characterization of neutral and anionic cellulosic amphiphiles

Polymer design and selection are crucial to the development of amorphous solid dispersions (ASD) for solubilization of otherwise poorly water-soluble drugs. The matrix polymer is required to interact strongly at the molecular level with the drug to prevent recrystallization, but must also be able to release the drug at an adequate rate upon entering the absorptive portion of the digestive tract. Herein we report versatile syntheses of a non-ionic, water-soluble cellulosic polymer family, cellulose trioxodecanoates, containing a hydrophilic oligo(ethylene oxide) side chain. This series of cellulose derivatives is designed for both adequate stabilization of amorphous drugs with high crystallization tendency, and timely release of those drugs. Alternatively, these polymers can be rendered anionic by also appending a pH-responsive ω-carboxyalkanoate group. Detailed structural information and structure–property relationship characterization of these amphiphilic polymers are described, which will permit evaluation of these materials as ASD polymers for enhancement of drug solubility and bioavailability.

Hydrophobic radicals embedded in neutral surfactants for dynamic nuclear polarization of aqueous environments at 9.4 Tesla.

We show how large DNP enhancements of NMR signals can be obtained from several hydrophobic radicals that are solubilised in aqueous environments by a variety of biologically compatible neutral amphiphiles. In particular we show that the bi-radical TEKPOL can be incorporated into micelles formed by the surfactant polysorbate 80 (Tween-80), where we obtain large DNP enhancements (∼60) at 9.4 T and ∼100 K.

Synthesis and Comparative Physical-chemical Characterisation of Neutral and Cationic Amphiphiles Using RP-HPLC

A series of norbornane containing amphiphiles was synthesized, their lipophilicity corresponding to neutral and cationic forms was then investigated using reverse phase HPLC (High Performance Liquid Chromatography). This series of amphiphiles incorporated varied lipophilic chain length and also varied distances between the polar/cationic head group from the norbornane scaffold. Our investigation included studying the impact of the stationary phase as a replication of a membrane for both cationic and neutral amphiphiles. The choice of stationary phase was shown to be a very important consideration for this type of measurement. In this connection, C18, Cyano and Polar columns were all investigated, the cyano column was observed to be the optimal stationary phase for the comparison of both charged and neutral amphiphiles. Keywords: Amphiphiles, cationic lipid, capacity factor, lipophilicity, high performance liquid chromatography.

Dynamic Domains in Polymersomes: Mixtures of Polyanionic and Neutral Diblocks Respond More Rapidly to Changes in Calcium than to pH

Chemical triggering of membrane domain dynamics is of broad relevance to cell signaling through lipid bilayers and might also be exploited in application of phase-separated vesicles. Here we describe the morphodynamics and remixing kinetics of spotted polymersomes made with mixtures of polyanionic and neutral amphiphiles plus calcium. Addition of the calcium chelator EDTA to vesicle dispersions produced a decrease in domain size within minutes, whereas increasing the pH with NaOH led to the viscous fingering of domains and decreased domain size over hours. Although the latter suggests that the charge of the polyanion contributes to domain formation, the remixing of more negative chains at high pH is surprising. Domain roughening at high pH is also accelerated by EDTA, which highlights the dominance of cross-bridging. Importantly, even though vesicles were perturbed only externally, the inner and outer leaflets remain coupled throughout, consistent with molecular dynamics simulations and suggestive of an order-disorder transition that underlies the remixing kinetics.

Spots and stripes

Cation binding is shown to trigger mesoscale domain formation within assembled mixtures of neutral and anionic polymer amphiphiles. The spotted and striped particles could be used for drug delivery or biomedical sensing.

Spotted vesicles, striped micelles and Janus assemblies induced by ligand binding.

Selective binding of multivalent ligands within a mixture of polyvalent amphiphiles provides, in principle, a mechanism to drive domain formation in self-assemblies. Divalent cations are shown here to crossbridge polyanionic amphiphiles that thereby demix from neutral amphiphiles and form spots or rafts within vesicles as well as stripes within cylindrical micelles. Calcium and copper crossbridged domains of synthetic block copolymers or natural lipid (PIP2, phosphatidylinositol-4,5-bisphosphate) possess tunable sizes, shapes, and/or spacings that can last for years. Lateral segregation in these ‘responsive Janus assemblies’ couples weakly to curvature and proves restricted within phase diagrams to narrow regimes of pH and cation concentration that are centered near the characteristic binding constants for polyacid interactions. Remixing at high pH is surprising, but a theory for Strong Lateral Segregation (SLS) shows that counterion entropy dominates electrostatic crossbridges, thus illustrating the insights gained into ligand induced pattern formation within self-assemblies.

Listeria monocytogenes Phosphatidylinositol-Specific Phospholipase C: Kinetic Activation and Homing in on Different Interfaces

The phosphatidylinositol-specific phospholipase C (PI-PLC) from Listeria monocytogenes forms aggregates with anionic lipids leading to low activity. The specific activity of the enzyme can be enhanced by dilution of the protein or by addition of both zwitterionic and neutral amphiphiles (e.g., diheptanoylphosphatidylcholine or Triton X-100) or 0.1-0.2 M inorganic salts. Activation by amphiphiles occurs with both micellar (phosphatidylinositol dispersed in detergents) and monomeric [dibutroylphosphatidylinositol (diC(4)PI)] phosphotransferase substrates and inositol 1,2-(cyclic)-phosphate (cIP), the phosphodiesterase substrate. The presence of zwitterionic and neutral amphiphiles (to which the protein binds weakly) dilutes the surface concentration of the interfacial anionic substrate and thereby reduces the level of enzyme-phospholipid particle aggregation. Zwitterionic amphiphiles also can bind directly to the protein and enhance catalysis since they enhance both diC(4)PI and cIP hydrolysis. In contrast to activation by amphiphiles, the rate enhancement by salt occurs for only the phosphotransferase step of the reaction. Added salt has a synergistic effect with zwitterionic phospholipids, leading to high specific activities for PI cleavage with only moderate dilution of the anionic substrate in the interface. This kinetic activation correlates with weakening of strong PI-PLC hydrophobic interactions with the interface as monitored by a decrease in the maximum monolayer surface pressure for insertion of the protein. Several point mutations of surface hydrophobic residues (W49A, L51A, L235A, and F237W) can dramatically alter the unusual kinetics of this secreted enzyme. The high affinity of PI-PLC for anionic phospholipids along with a strong hydrophobic interaction, which gives rise to the unusual kinetic behavior, is considered in terms of how it might contribute to the role of this phospholipase in L. monocytogenes infectivity.

Synthesis of silica spheres with neutral and ionic amphiphiles and their interaction with photosensitive spiropyrans

A study was made on the influence of ionic and non-ionic amphiphiles, i.e., CTAB and 1-(2-aminoethyl)-2-heptadecyl-2-imidazoline ( 1), on the pore hierarchy and morphology of siliceous particles synthesized with. The amphiphile ( 1) leads to formation of spherical silica particles of about 250 nm diameter, while the ionic surfactant (CTAB) led to porous silica spheres of about 750 nm diameter. A single-particle analysis using High Resolution Electron Microscopy (HREM) and Optical Density Transforms (ODT) show the inner pore structure of the particles synthesized with CTAB and low molecular weight alcohols co-surfactants (ethanol and propanol) is oriented along the sphere radius, while silica particles synthesized with ( 1) have a random-like pore structure. Both systems were used as supports for adsorption of a photosensitive spiropyran (i.e., Spiro-6) type compound (1’,3’,3’-trimethyl-6-nitrospiro-(2H-1-benzopyran- 2,2’indoline)) and their interaction with the silica surface was characterized by IR spectroscopy, showing a weak interaction with respect to other silicates (lamellar clays). These effects are potentially interesting for applications of biomarkers, stable photosensitive materials, solid-phase organic synthesis and dyes. The UV–vis reflectance spectra of the materials series impregnated with Spiro-6 indicated that siliceous IMP-type materials (i.e., prepared with CTAB and low molecular weight alcohols: Pr-1, Pr-2, Pr-3 and Et-1, Et-2) contained a small shoulder at 460 nm of the less conjugated protonated merocyanine form, together with a broad band at 517 nm, which corresponds to the merocyanine form. The incorporation of aluminum into the silica framework results in a single broad band at 435 nm corresponding to the protonated merocyanine form. Thus, the acidic sites created by aluminum incorporation into the silica spheres seem to favor the formation of protonated merocyanine instead of the basic merocyanine molecule.

Synthesis and characterization of hexa-tailed cryptand based amphiphiles: Spontaneous formation of giant vesicular microcapsule with efficient and long-term dye encapsulation

A cryptand with six secondary amino groups has been derivatized by reacting with acid chlorides of different chain lengths (C 7, C 10 and C 18) to get three cryptand based hexa-tailed neutral amphiphiles (L 1–L 3). The cavity of the cryptand head group accommodates two Cu(II) ion giving another set of three amphiphiles. These amphiphiles and its copper complexes can aggregate spontaneously as giant vesicular microcapsules in 10% ethanolic water medium. In all cases vesicles formed are mostly unilamellar in nature. Vesicular microcapsules prepared from L 1–L 3 can encapsulate hydrophilic dye 5(6)-carboxyfluorescein (CF). Stability of microcapsules, CF encapsulation efficiencies and release rates were dependent on the hydrophobic chain length of the amphiphiles. Results show the permeability of the L 2 and L 3 bilayer is lower than that of phosphatidylcholine vesicles and the loading capacity is ∼3 times greater. Microcapsules have been characterized by optical microscopy, freeze fracture scanning electron microscopy and confocal fluorescence microscopy.

Understanding the Activation of Listeria monocytogenes Phosphatidylinositol‐Specific Phospholipase C

The phosphatidylinositol-specific phospholipase C (PI-PLC) from Listeria monocytogenes (a monomer in solution) shows unusual kinetic properties compared to other well-studied phospholipases: (i) increased specific activity with decreasing protein concentration, (ii) activation of the phosphotransferase step by salts, and (iii) activation of both the interfacial phosphotransferase and water-soluble phosphodiesterase steps by zwitterionic and neutral amphiphiles. A variety of biophysical studies (fluorescence, NMR, monolayer, vesicle binding) of enzyme/lipid complexes coupled with kinetics have allowed us to propose a model that accounts for these features. The enzyme binds tightly to anionic surfaces and much more weakly to a zwitterionic interface. The tight binding can be reduced by adding KCl at concentrations that activate the enzyme. In the crystal structure of the enzyme, many basic residues are clustered on the bottom side of TIM-barrel far away from the opening to the active site. These cause the enzyme to adopt a non-productive orientation on negatively charged membranes that leads to a reversible clustering of anionic lipids and vesicle aggregation. An increased surface concentration of zwitterionic / neutral amphiphiles along with the salt disperses the anionic substrate, shields charges on the protein, and enhances productive encounters of the protein with substrate molecules. This model has been tested by examining the behavior of enzyme with citraconylated lysines and mutants of neutral surface residues at the rim of the active site. The unusual kinetic behavior of this PI-PLC also appears to contribute to the escape of L. monocytogenes from primary vacuoles during infection. Supported by NIH GM 60418.

Special properties of monolayer formed by a novel Gemini amphiphile at air–water interface

A novel Gemini amphiphile, N, N′-bis-(4-decyloxy-2-hydroxybenzylidene)-ethane-1,2-diamine, was synthesized and characterized. This amphiphile is capable of forming stable monolayer at air–water interface. The limiting area has a minimum when the pH value of subphase is 12. The unique behavior is attributed to the formation of intramolecular hydrogen bond between hydroxyl groups. The existence of this hydrogen bond was supported by the FTIR results. With the addition of copper ion into subphase, the limiting area showed a decreasing value, which is quite different from that of the conventional neutral amphiphiles. XPS and UV–vis were also carried out to investigate the effect of copper ion on molecular area.

Supramolecular assemblies of DNA with neutral nucleoside amphiphiles

A neutral uridine-based amphiphile is described which condenses plasmid DNA. AFM studies show that the three distinct structural components of the amphiphile (i.e, nucleobase, alkyl chains, and poly(ethylene glycol)) are required for the formation of DNA-amphiphile supramolecular assemblies on a mica surface.

A Comparative Study of Cryptand-Based (Triple-Head and Triple-Tail) Amphiphiles at the Air−Water Interface

Two cryptands have been derivatized by reacting with acid chlorides of different alkyl chain lengths (between C4 and C18) to get a set of cryptand-based (triple-head and triple-tail) neutral amphiphiles. The cavity of either cryptand headgroup can accommodate a first-row transition metal ion forming stable cryptates. The free as well as the Cu(II)-complexed amphiphiles are studied at the air−water interface in a Langmuir−Blodgett trough. They form stable monolayers at the air−water interface, which can be transferred onto different substrates (viz., glass, fluorite, and quartz). They form Y-type Langmuir−Blodgett films. The amphiphile L5 crystallizes in the monoclinic space group P21/c with a = 10.091(3) A, b = 29.994(8) A, c = 18.174(5) A, Z = 4, Rf = 0.1048, Rwf = 0.2660, and GOF = 0.925. The amphiphile L6 crystallizes in the trigonal space group P31c with a = 15.118(2) A, b = 15.118(2) A, c = 11.734(2) A, Z = 2, Rf = 0.033, Rwf = 0.062, and GOF = 0.813. The solid-state structures of both the amphiphile...

Dependence of Cardiac Cell Ca2+Permeability on Sialic Acid-containing Sarcolemmal Gangliosides

The specific removal of negatively-charged sialic acid by neuraminidase produces a large increase in cardiac myocyte Ca uptake (17.3±1.1 mmol Ca/kg dry weight) and marked cell contracture. Importantly, the insertion of the negatively-charged amphiphile dodecyl sulfate in the sarcolemma eliminates the increased calcium uptake and preserves contractile function. In the present study, we further examine the role of sialic acid–Ca interaction and, specifically, the role of gangliosides (sialic acid-containing glycolipids) in cardiac cells» Ca permeability. Neonatal cell culture and adult ventricular myocytes were used. The major findings of this study are: (1) while dodecyl sulfate inhibits cellular calcium uptake and contracture development induced by sialic acid removal, cationic and neutral amphiphiles are without effect. (2) Ca channel blockers (nifedipine and protamine) and the Na/Ca exchange inhibitor Ni do not modify the effect of sialic acid removal. (3) A non-classical-channel related whole-cell current appears and increases after 21±2.2 min treatment with 0.02 U/ml neuraminidase (n=4). Incubation with neuraminidase in the presence of dodecyl sulfate greatly delays the appearance of these currents to 44.4±6.1 min (n=4). (4) The use of a specific probe for GM1 ganglioside, the cholera toxin B subunit (3μg/ml), induces a moderate but clear increase in cellular Ca (1.63±0.3 mmol Ca kg dry weight;n=8). However, this increase was not modified by treatment with dodecyl sulfate. (5) Finally, 50 mU/ml endoglycoceramidase, an enzyme which specifically cleaves the link between the sialic acid-containing oligosaccharide and ceramide of gangliosides, induced a significant increase in Ca uptake (4.4±0.9 mmol Ca kg dry weight,n=4). These results indicate the importance of negatively charged sialic acid-containing gangliosides in the maintenance of cardiac cell physiological Ca permeability. The increase in Ca uptake induced by the removal of sialic acid seems to be mediated by development of a Ca “leak” via other than classical cation channels.