Synthesis and Vesicle Formation from Hybrid Bolaphile/Amphiphile Ion-Pairs. Evidence of Membrane Property Modulation by Molecular Design

Preparation and Characterization of Cationic Vesicles from a New Type of Ion Pair Amphiphile Carbethopendecinium-Dodecylsulfate

The aim of this work was to prepare and characterize a yet undescribed type of catanionic vesicle based on a new ion pair amphiphile, which were formed from carbethopendecinium bromide (also called Septonex) and sodium dodecylsulfate. The vesicles were prepared by mixing an amphiphilic pair together with dioctadecyldimethylammonium chloride and cholesterol at a concentration of 0, 5, 10, 15, 20, 30, 40, and 60 mol %. Final size, polydispersity and stability was studied by dynamic and electrophoretic light scattering. Fluorescence anisotropy, generalized polarization and high-resolution ultrasound spectroscopy were used to determine membrane order and hydration their outer part. The vesicles appeared to be stable in the short term independently of the cholesterol content. From a long-term (28 days) perspective, it was possible to detect a threshold of 15 mol % above which the vesicles were stable. The same cholesterol content threshold also applied to stability at elevated temperatures. Fluorescence measurements showed that the phase transition temperature was located at significantly lower temperatures compared to a similar system composed of hexadecyl-trimethylammonium-dodecyl sulfate. Using differential scanning calorimetry, it was found that this temperature was slightly affected by the cholesterol content and was around 15 °C. The results from ultrasonic spectroscopy together with generalized polarization and anisotropy showed a threshold of 20 mol % cholesterol content, above which the properties of the vesicular membrane, such as its stiffness and hydration, began to change significantly.

A functional and self-assembling octyl-phosphonium-tagged esculetin as an effective siRNA delivery agent

Herein, we document a self-assembling octyl-TPP tagged esculetin (Mito-Esc) as functionally active and as a novel small molecule siRNA delivery vector. While Mito-Esc itself induces selective breast cancer cell death, the amphiphilic nature of Mito-Esc delivers therapeutic siRNAs intracellularly without the need for any excipient to exacerbate the anti-proliferative effects.

Responsive morphology transition from micelles to vesicles based on dynamic covalent surfactants

A dynamic covalent bond is widely used to fabricate stimuli responsive systems due to its reversible molecular recognition properties. In this study, we developed a pH-responsive morphology transition system based on a mixture of a cationic surfactant CTAB and two nonamphiphilic precursors, 4-hydroxybenzaldehyde (HB) and octylamine (OA), at a molar ratio of 100 : 60 : 60 (CTAB/HB/OA). The morphology transition of CTAB/HB/OA was characterized by 1H NMR spectroscopy, Fourier transform infrared spectroscopy, macroscopic appearance observation, dynamic light scattering, and rheological and cryo-TEM measurements. The phase behavior of CTAB/HB/OA solutions underwent transition from a water-like fluid to a transparent gel-like solution and then converted into a turbid low-viscosity solution upon increasing the pH. Upon increasing the pH from 4.93 to 7.99, the morphology was transformed from spherical micelles to wormlike micelles. Upon further increasing the pH to 12.02, the wormlike micelles gradually disappeared with the formation of vesicles. Thus, a morphology transition from micelles to vesicles can be triggered by varying the pH of CTAB/HB/OA solutions. This drastic variation in morphology behavior was attributed to the pH dependent ionization and formation of the anionic surfactant HB-OA-. Besides, over 3 cycles of morphological alternation among spherical micelles, wormlike micelles and vesicles of the CTAB/HB/OA solutions can be obtained by adjusting the pH.

A unique self-assembly-driven probe for sensing a lipid bilayer: ratiometric probing of vesicle to micelle transition

Membrane-driven self-assembly of an amphiphilic pyrene–terpyridine probe efficiently reports on vesicle–micelle transition through ratiometric changes.

Self-Assembly of Bilayer Vesicles Made of Saturated Long Chain Fatty Acids

Saturated long chain fatty acids (sLCFA, e.g., C14:0, C16:0, and C18:0) are potentially the greenest and cheapest surfactants naturally available. However, because aqueous sodium soaps of sLCFA are known to crystallize, the self-assembly of stable bilayer vesicles has not been reported yet. Here, by using such soaps in combination with guanidine hydrochloride (GuHCl), which has been shown recently to prevent crystallization, we were capable of producing stable bilayer vesicles made of sLCFA. The phase diagrams were established for a variety of systems showing that vesicles can form in a broad range of composition and pH. Both solid state NMR and small-angle neutron scattering allowed demonstrating that in such vesicles sLCFA are arranged in a bilayer structure which exhibits similar dynamic and structural properties as those of phospholipid membranes. We expect these vesicles to be of interest as model systems of protocells and minimal cells but also for various applications since fatty acids are potentially substitutes to phospholipids, synthetic surfactants, and polymers.

Aminosilane/oleic acid vesicles as model membranes of protocells

Oleic acid vesicles represent good models of membrane protocells that could have existed in prebiotic times. Here, we report the formation, growth polymorphism, and dynamics of oleic acid spherical vesicles (1-10 μm), stable elongated vesicles (>50 μm length; 1-3 μm diameter), and chains of vesicles (pearl necklaces, >50 μm length; 1-3 μm diameter) in the presence of aminopropyl triethoxysilane and guanidine hydrochloride. These vesicles exhibit a remarkable behavior with temperature: spherical vesicles only are observed when keeping the sample at 4 °C for 2 h, and self-aggregated spherical vesicles occur upon freezing/unfreezing (-20/20 °C) samples. Rather homogeneous elongated vesicles are reformed upon heating samples at 80 °C. The phenomenon is reversible through cycles of freezing/heating or cooling/heating of the same sample. Deuterium NMR evidences a chain packing rigidity similar to that of phospholipid bilayers in cellular biomembranes. We expect these bilayered vesicles to be surrounded by a layer of aminosilane oligomers, offering a variant model for membrane protocells.

Binding of a protein or a small polyelectrolyte onto synthetic vesicles

Catanionic vesicles were prepared by mixing nonstoichiometric amounts of sodium bis(2-ethylhexyl) sulfosuccinate and dioctyldimethylammonium bromide in water. Depending on the concentration and mole ratios between the surfactants, catanionic vesicular aggregates are formed. They have either negative or positive charges in excess and are endowed with significant thermodynamic and kinetic stability. Vesicle characterization was performed by dynamic light scattering and electrophoretic mobility. It was inferred that vesicle size scales in inverse proportion with its surface charge density and diverges as the latter quantity approaches zero and/or the mole ratio equals unity. Therefore, both variables are controlled by the anionic/cationic mole ratio. Small-angle X-ray scattering, in addition, indicates that vesicles are unilamellar. Selected anionic vesicular systems were reacted with poly-L-lysine hydrobromide or lysozyme. Polymer binding continues until complete neutralization of the negatively charged sites on the vesicles surface is attained, as inferred by electrophoretic mobility. Lipoplexes are formed as a result of significant electrostatic interactions between cationic polyelectrolytes and negatively charged vesicles.

Interplay of molecular hydrogelators and SDS affords responsive soft matter systems with tunable properties

The gelation efficiency of low molecular weight bolaamphiphilic hydrogelators 1 and 2 is influenced by the presence of SDS micelles. Similarly, the critical micellar concentration value of SDS is reduced in the presence of the studied molecular hydrogelators. Rheological measurements indicate that the strength of the hydrogels can be modulated with SDS, the gels becoming weaker in the presence of micelles. This behavior has been rationalized with the help of NMR studies using diffusion measurements and NOE correlations. The results obtained clearly point to the formation of mixed micelles composed of SDS and the hydrogelators. In the case of 1, the gelator:SDS ratio in the mixed micelles has been estimated from solubility studies to be ca. 1:2.5. Electron microscopy reveals that when SDS is present, the morphology of the xerogels is modified in its appearance at the micrometer scale but fibers with diameter in the nanometer range are observed in all the cases. The interplay between the surfactant and the gelators provides with new possibilities for the modulation of both gel and micelle formation. Examples are shown to highlight the potential usefulness of this type of interconnected system. In one case the release of a gel entrapped dye is modulated by the presence of SDS and sodium chloride. In another example, an intricate system that responds to a temperature excursion by irreversible micelle disassembly is shown.

Spontaneous formation of vesicles by sodium 2-dodecylnicotinate in water

The surface activity and aggregation behavior of a synthesized nicotinic acid based anionic surfactant, sodium 2-dodecylnicotinate, were studied in aqueous solution. The self-assembly formation was investigated by use of a number of techniques, including surface tension and conductivity measurements, fluorescence spectroscopy, dynamic light scattering measurement, gel permeation chromatography, and microscopy. The amphiphile exhibits two breaks in the surface tension vs concentration plot, indicating stepwise aggregate formation and thus producing two values of the aggregation concentration. Stepwise aggregation of the amphiphile was further confirmed by steady-state fluorescence spectroscopy using pyrene as a probe molecule, and also the micropolarity of the aggregates was determined. The rigidity of the microenvironment was estimated by determining steady-state fluorescence anisotropy using 1,6-diphenyl-1,3,5-hexatriene as a fluorescence probe molecule. The average hydrodynamic radius and size distribution of the aggregate suggest formation of larger aggregates in aqueous solution. The formation of vesicles in water was established by conductivity measurement and a dye entrapment experiment. The entrapment of a small solute and the release capability have also been examined to demonstrate these bilayers form enclosed vesicles. Transmission electron micrographs revealed the existence of closed vesicles and closed tubules in aqueous solution. Therefore, for the first time, it has been observed that this simple single-chain nicotinic acid based amphiphile spontaneously assembles to vesicles in aqueous solution.

Surfactants Possessing Multiple Polar Heads. A Perspective on their Unique Aggregation Behavior and Applications

Surfactants containing more than one head group are known to exhibit a wide range of interesting properties as they undergo aggregation in water. The correlation between the molecular structure of these surfactants and their properties (for example, critical micellar concentration, aggregation number, morphology, counterion dissociation, fractional charge, etc.) can provide useful information to define the structure-activity relationship. The influence of the number of head groups on the surfactant aggregation is further evident from interesting interfacial behavior, seen in biological applications. This Perspective highlights recent trends in surfactant aggregation effects and focuses on emerging challenges in the field.

Vesicle and stable monolayer formation from simple "click" chemistry adducts in water

Click chemistry has been successfully extended into the field of molecular design of novel amphiphatic adducts. After their syntheses and characterizations, we have studied their aggregation properties in aqueous medium. Each of these adducts forms stable suspensions in water. These suspensions have been characterized by dynamic light scattering (DLS) studies and transmission electron microscopy (TEM). The presence of inner aqueous compartments in such aggregates has been demonstrated using dye (methylene blue) entrapment studies. These aggregates have been further characterized using X-ray diffraction (XRD), which indicates the existence of bilayer structures in them. Therefore, the resulting aggregates could be described as vesicles. The temperature-induced order-to-disorder transitions of the vesicular aggregates and the accompanying changes in their packing and hydration have been examined using high-sensitivity differential scanning calorimetry, fluorescence anisotropy, and generalized polarization measurements using appropriate membrane-soluble probe, 1,6-diphenylhexatriene, and Paldan, respectively. The findings of these studies are consistent with each other in terms of the apparent phase transition temperatures. Langmuir monolayer studies confirmed that these click adducts also form stable monolayers on buffered aqueous subphase at the air-water interface.

Understanding membranes through the molecular design of lipids

Lipids are amphiphilic molecules that are composed of hydrophilic and hydrophobic regions. A typical membranous aggregate (vesicles, water-filled lipid nanospheres) is formed upon the self-organization of lipids in water from a diverse collection of amphiphiles producing a dynamic supramolecular structure that shows phase behavior and ordering as required for specific biological functions. The determination of various physical properties of lipid aggregates is the key to determining structure-function relationships. Over the years, we have designed and synthesized a wide variety of lipid molecular systems for the investigation of their membrane-forming properties and have used them for purposes such as gene delivery and enzyme activation. In this feature article, we focus on our work on various types of lipids including ion-paired amphiphiles, cholesterol-based lipids, aromatic lipids, macrocyclic lipids containing disulfide tethers, cationic dimeric lipids, and so forth. The emphasis is on experimental design and bottom-line conclusions.

Pharmaceutical applications for catanionic mixtures

Mixtures of oppositely charged surfactants, so called catanionic mixtures, are a growing area of research. These mixtures have been shown to form several different types of surfactant aggregates, such as micelles of various forms and sizes, and lamellar structures, such as vesicles. In this review, a short introduction to the field of catanionic mixtures is presented and the pharmaceutical possibilities offered by such mixtures are reviewed. There are several interesting ideas on how to apply catanionic mixtures to improve the delivery of, for example, drug compounds and DNA, or for HIV treatment.

Coarse-grained molecular dynamics simulation of the aggregation properties of multiheaded cationic surfactants in water

The aggregation property of multiheaded surfactants has been investigated by constant pressure molecular dynamics (MD) simulation in aqueous medium. The model multiheaded surfactants contain more than one headgroup (x = 2, 3, and 4) for a single tail group. This increases the hydrophilic charge progressively over the hydrophobic tail which has dramatic consequences in the aggregation behavior. In particular, we have looked at the change in the aggregation property such as critical micellar concentration (cmc), aggregation number, and size of the micelles for the multiheaded surfactants in water. We find with increasing number of headgroups of the multiheaded surfactants that the cmc values increase and the aggregation numbers as well as the size of the micelles decrease. These trends are in agreement with the experimental findings as reported earlier with x = 1, 2, and 3. We also predict the aggregation properties of multiheaded surfactant with four headgroups (x = 4) for which no experimental studies exist yet.

Sugar-derived tricatenar catanionic surfactant: self-assembly and aggregation behavior in the cationic-rich side of the system

The aggregation behavior of the cationic-rich side of a sugar-based tricatenar catanionic mixture was investigated in water, and it was shown that the excess of cationic sugar-based surfactant enhanced vesicle stability as well as encapsulation properties. Moreover, when the system was diluted, the vesicular solution collapsed into a lamellar phase, whereas, when it was concentrated, no major impact on the shape and stability of the aggregates was observed. We also showed that both an increase in temperature and the addition of salt induced reversible vesicle aggregation, which appeared to be salt-specific, following the direct order of the Hofmeister series. A proper adjustment of these parameters should then enable better control of the shape, stability, and even encapsulation ability of the aggregates formed by these tricatenar cationic/anionic mixtures.

Sugar-derived tricatenar catanionic surfactant: synthesis, self-assembly properties, and hydrophilic probe encapsulation by vesicles

A new sugar-derived tricatenar catanionic surfactant (TriCat) was developed to obtain stable vesicles that could be exploited for drug encapsulation. The presence of the sugar moiety led to the formation of highly hydrophilic stoichiometric catanionic surfactant systems. The three hydrophobic chains permitted vesicles to form spontaneously. The self-assembly properties (morphology, size, and stability) of TriCat were examined in water and in buffer solution. Encapsulation studies of a hydrophilic probe, arbutin, commonly used in cosmetics for its whitening properties, were performed to check the impermeability of the vesicle bilayer. The enhancement of hydrophobic forces by the three chains of TriCat prevented surfactant equilibrium between the bilayer and the solution and enabled the probe to be retained in the aqueous cavity of the vesicles for at least 30 h. Thus, the present study suggests that this tricatenar catanionic surfactant could be a promising delivery system for hydrophilic drugs.

Organized Assemblies in Bolaamphiphile/Oppositely Charged Conventional Surfactant Mixed Systems

Five ionic bolaamphiphiles were synthesized and the aggregation behavior of bola single systems and bola/oppositely charged conventional surfactant mixed systems was studied. Small spherical vesicles were formed in all these mixed systems revealed by transmission electron microscopy (TEM). Variation of the structure of the hydrophobic chain of bolaamphiphiles has great influences on the vesicle formation ability. Vesicles were also found in the single system of a carboxylate bolaamphiphile, which was attributed to the hydrolysis of the bolaamphiphile. The results of FT-IR and X-ray diffraction (XRD) showed that bolaamphiphiles spanned through the vesicle membranes in these mixed systems. Super thermostability of the vesicles in this kind of mixed system was also investigated.

Glucose Encapsulation in Catanionic Vesicles and Kinetic Study of the Entrapment/Release Processes in the Sodium Dodecyl Benzene Sulfonate/Cetyltrimethylammonium Tosylate/Water System

The sodium dodecyl benzene sulfonate (SDBS)/cetyltrimethyl-ammonium tosylate (CTAT)/water ternary system has previously been shown to give rise to the formation of vesicles for well-defined compositions requiring an excess of one of the surfactants over the other. Two types of vesicular systems can thus be obtained (named V(+) and V(-), depending on the nature of the excess surfactant, i.e., CTAT or SDBS, respectively). In addition, the pure ion-pair amphiphile (IPA) can be obtained after removing the counterions. These different systems (V(+), V(-), and IPA) were investigated in regards to their ability to susbtantially retain a hydrophilic probe such as glucose. The influence of the initial glucose and surfactant concentrations was studied, and dialysis experiments were conducted with a view to determine the kinetics of glucose entrapment and release by the vesicles and, thus, to assess the possibility of a long-term encapsulation. The results indicate that all the vesicular systems studied here are characterized by quite a high permeability of the amphiphilic bilayer. However, SDBS-rich (V(-)) and IPA vesicles proved to be less permeable than CTAT-rich (V(+)) vesicles.

Properties of the amphiphilic films in mixed cationic/anionic vesicles: a comprehensive view from a literature analysis

The so-called 'catanionic' vesicles are made from mixtures of cationic and anionic surfactants. They are attracting much interest because they form spontaneously and they can be obtained from a variety of surfactants, either commercially available or issued from original synthesis. A distinction can be made between the properties of simple surfactant mixtures and of ion pair amphiphiles (IPA), in which the counterions have been removed. We have drawn up in this paper, an inventory of the different vesicular systems which have been described in the literature, insisting on the specific features associated with these two categories of systems. We have collected here especially, information concerning the phase behaviors, the microscopic composition of the vesicular particles, their structural and size determinations, the dynamic aspects (including the micelle-vesicle transition), the theoretical predictions from thermodynamic models and the entrapment of probe molecules. We discuss the potential of catanionic vesicles as delivery systems and we show that a full understanding of their entrapment/release properties will call for much more experimental work with well defined protocols. We also point out some unsolved questions concerning the role of the excess surfactant in the stabilization of the particles and the conditions required to obtain a favourable curvature of the surfactant film.