Structures of Fibrous Supramolecular Assemblies Constructed by Amino Acid Surfactants: Investigation by AFM, SANS, and SAXS

Solution‐Phase Magnetic Mechanistic Study of Ni‐Seamed Pyrogallol[4]arene Nanocapsules Reveal Presence of Novel Cylindrical and Spherical Species

The magnetic properties of nickel‐seamed C‐pyrogallol[4]arene (PgC₃Ni) hexamers and dimers are studied for the first time in solution. The combination of small‐angle neutron scattering and superconducting quantum interference device magnetometer measurements of the solution species reveal their paramagnetic and weakly antiferromagnetic behaviour. Surprisingly, the magnetic results indicated the presence of an unprecedented 13 Å‐radius species, larger than both the dimeric and hexameric nanocapsules with both octahedral and square‐planar metal centers. To confirm the presence of this novel species, we performed a mechanistic study of PgC₃Ni as a function of temperature and solvent and deduced the presence of two additional new species: a) an 11 Å cylinder with Ni atoms seaming the tubular framework and b) an 8 Å‐radius sphere with non‐interacting Ni centers located within the internal cavity. Select parameters that shift the equilibrium towards desired species are also identified.

Formation and phase transition of hydrogel in a zwitterionic/anionic surfactant system

Hydrogel was formed in a mixture of the zwitterionic surfactant HDPS (with a saturated C₁₆tail) and anionic surfactant SDS, and could easily be switched between gel and sol by dual stimulus–response processes employing temperature and salt.

Capturing Nanoscale Structure in Network Gels by Microemulsion Polymerization

Small-angle neutron scattering and turbidity were used to probe nanoscale structure in bicontinuous microemulsions before and after polymerization. Difficulties in capturing nanoscale structure by polymerizing microemulsions have persisted with the use of thermal initiation. Bicontinuous microemulsion polymerization with a reactive surfactant monomer and cross-linker was done with only a 20% increase in repeat distance. This small increase represents better than an order of magnitude advance over previous attempts, exhibiting hundreds to thousands percent increases. Both the network gel and the precursor microemulsion were transparent and devoid of microphase separation.

Versatile supramolecular gelators that can harden water, organic solvents and ionic liquids

We developed novel supramolecular gelators with simple molecular structures that could harden a broad range of solvents: aqueous solutions of a wide pH range, organic solvents, edible oil, biodiesel, and ionic liquids at gelation concentrations of 0.1-2 wt %. The supramolecular gelators were composed of a long hydrophobic tail, amino acids and gluconic acid, which were prepared by liquid-phase synthesis. Among seven types of the gelators synthesized, the gelators containing L-Val, L-Leu, and L-Ile exhibited high gelation ability to various solvents. These gelators were soluble in aqueous and organic solvents, and also in ionic liquids at high temperature. The gelation of these solvents was thermally reversible. The microscopic observations (TEM, SEM, and CLSM) and small-angle X-ray scattering (SAXS) measurements suggested that the gelator molecules self-assembled to form entangled nanofibers in a large variety of solvents, resulting in the gelation of these solvents. Molecular mechanics and density functional theory (DFT) calculations indicated the possible molecular packing of the gelator in the nanofibers. Interestingly, the gelation of an ionic liquid by our gelator did not affect the ionic conductivity of the ionic liquid, which would provide an advantage to electrochemical applications.

Morphological characterization of self-assembled peptide nucleic acid amphiphiles

Peptide nucleic acid amphiphiles (PNAA) are a promising set of materials for sequence-specific separation of nucleic acids from complex mixtures. To implement PNAA in micellar separations, the morphology and size of PNAA micelles in the presence and absence of a sodium dodecyl sulfate (SDS) cosurfactant have been studied by small-angle X-ray scattering and dynamic light scattering. We find that a 6-mer PNAA with a 12-carbon n-alkane tail forms ellipsoidal micelles (a = 5.15 nm; b = 3.20 nm) above its critical micelle concentration (CMC) of 110.9 microM. On addition of a stoichiometric amount of complementary DNA, PNAA hybridizes to DNA, suppressing the formation of PNAA micelles. At a ratio of 19:1 SDS/PNAA (total concentration = 20 mM), spherical micelles are formed with outer radius Rs = 2.67 nm, slightly larger than spherical micelles of pure SDS. Capillary electrophoresis studies show that PNAA/DNA duplexes do not comicellize with SDS micelles. No such effects are observed using noncomplementary DNA. The shape and size of the PNAA micelles is also verified by dynamic light scattering (DLS) studies. These results provide an interesting case study with competing electrostatic, hydrophobic, and hydrogen-bonding interactions in micellar systems and make possible the use of PNAA in micellar separations of DNA oligomers.

Self-Assembly in a Catanionic Mixture with an Aminoacid-Derived Surfactant: From Mixed Micelles to Spontaneous Vesicles

The aqueous self-assembly of a novel lysine-derived surfactant with a gemini-like architecture, designated here as 12-Lys-12, has been experimentally investigated for the amphiphile alone in water and in a mixture with dodecyltrimethylammonium bromide (DTAB). The neat surfactant forms interesting micrometer-sized rigid tubules in the dilute region, resulting in very viscous solutions. For the catanionic mixture with DTAB, various single and multiphase regions were identified (up to a total surfactant concentration of 1.5 wt %) by means of combined polarizing light microscopy, cryo-TEM, and NMR. In the DTAB-rich side, for a mixing molar ratio in the range 2 < DTAB/12-Lys-12 < 4, a region of stable, unilamellar vesicles can be found. Furthermore, it was found that upon addition of 12-Lys-12 to pure DTAB solutions, the mixed micelles grow and beyond a given mixing ratio, vesicles assemble and coexist with small micelles. The transition is not continuous, since there is a narrow mixing range where phase separation occurs. Self-diffusion measurements and cryo-TEM imaging show that the average vesicle radius is on the order of 30-40 nm.

Surfactant assemblies and their various possible roles for the origin(s) of life

A large number of surfactants (surface active molecules) are chemically simple compounds that can be obtained by simple chemical reactions, in some cases even under presumably prebiotic conditions. Surfactant assemblies are self-organized polymolecular aggregates of surfactants, in the simplest case micelles, vesicles, hexagonal and cubic phases. It may be that these different types of surfactant assemblies have played various, so-far underestimated important roles in the processes that led to the formation of the first living systems. Although nucleic acids are key players in the formation of cells as we know them today (RNA world hypothesis), it is still unclear how RNA could have been formed under prebiotic conditions. Surfactants with their self-organizing properties may have assisted, controlled and compartimentalized some of the chemical reactions that eventually led to the formation of molecules like RNA. Therefore, surfactants were possibly very important in prebiotic times in the sense that they may have been involved in different physical and chemical processes that finally led to a transformation of non-living matter to the first cellular form(s) of life. This hypothesis is based on four main experimental observations: (i) Surfactant aggregation can lead to cell-like compartimentation (vesicles). (ii) Surfactant assemblies can provide local reaction conditions that are very different from the bulk medium, which may lead to a dramatic change in the rate of chemical reactions and to a change in reaction product distributions. (iii) The surface properties of surfactant assemblies that may be liquid- or solid-like, charged or neutral, and the elasticity and packing density of surfactant assemblies depend on the chemical structure of the surfactants, on the presence of other molecules, and on the overall environmental conditions (e. g. temperature). This wide range of surface characteristics of surfactant assemblies may allow a control of surface-bound chemical reactions not only by the charge or hydrophobicity of the surface but also by its "softness". (iv) Chiral polymolecular assemblies (helices) may form from chiral surfactants. There are many examples that illustrate the different roles and potential roles of surfactant assemblies in different research areas outside of the field of the origin(s) of life, most importantly in investigations of contemporary living systems, in nanotechnology applications, and in the development of drug delivery systems. Concepts and ideas behind many of these applications may have relevance also in connection to the different unsolved problems in understanding the origin(s) of life.

Dendritic nano- and microhydrogels fabricated by triethoxysilyl focal dendrons

Nano- and microhydrogels were fabricated in water by first to third generation triethoxysilyl focal poly(amido amine) dendrons with hexyl spacer. The focal points of dendrimers were hydrolyzed and covalent-bonded through the simple sol-gel process at an acidic or basic catalytic condition. The growth of aggregates and the following gel formation were determined by rapid increase and convergent steady value in light scattering of dendron solutions. The sol-gel reaction was also confirmed from the disappearance of an infrared absorption band of Si-O(CH) stretching vibration mode (1080 cm) and the appearance of Si-O-Si stretching bands (1136 and 1049 cm). The resultant gels were transparent and rather fluid. Transmission electron microscopic images of the gels showed three-dimensional dendritic growing of fine fibrils. The nanogel nuclei grew up favourably to nanogels in acidic conditions and to microgels in basic conditions, and the growth was more remarkable at higher generation of dendrimers. At high concentration of dendrimer, macrogels with fiber-like texture were formed. It was supported that siloxane-linked focal groups constructed main chains and branches of fibrils, and dendron side chains coated polysiloxane backbones. The hydrogels emitted fluorescence, which was stronger at base-catalyzed condition than at acid-catalyzed condition. This indicates that crowded circumstances or large amount of fluorescence-inducing moieties intensify the fluorescence. Fluorescent images of such architectures were visualized on a fluorescent microscope.

Fibrous assemblies and water gelation in mixtures of lysine with sodium alkyl sulfates

Thermoresponsive, pH-sensitive fibrous structures and gels are formed in aqueous mixtures of the amino acid lysine with oppositely charged sodium alkyl sulfate surfactants. The formation of these assemblies depends on the chain length of the surfactant, which is varied between 8 and 16, the chirality and degree of protonation of the amino acid, and the molar ratio of these species. Self-assembly of the fibers occurs when specific lysine enantiomers are in solution and for pH conditions in which the majority of the amine groups are protonated (i.e., at near-equimolar amounts of HCl and lysine). Racemic mixtures of lysine do not form fibers with sodium dodecyl sulfate. Micelles are the fiber precursors, and the fibers, which can be hundreds of micrometers long, entangle to form gels. With increasing temperature, the gels melt, the fibers dissolve, and a single micellar phase forms. The micelles elongate with decreasing pH when the acid concentration is greater than equimolar with respect to lysine, and they shrink with increasing temperature.

Influence of pH on the Micelle-to-Vesicle Transition in Aqueous Mixtures of Sodium Dodecyl Benzenesulfonate with Histidine

Small unilamellar vesicles (approximately 100 nm in diameter) form spontaneously in aqueous mixtures of histidine and sodium dodecyl benzenesulfonate. By manipulating pH, a gradual transition from micelles to vesicles to bilayers to precipitate is observed. The self-assembly of vesicles occurs over a wide range of compositions when the solution pH is lower than 6.0, the pKa of the imidazole moiety on the histidine molecule. This phenomenon is likely the result of attractive interactions between the negatively charged benzenesulfonate headgroups and the positively charged imidazole group in the amino acid. Similar results are obtained when imidazole salt itself is used.

A small-angle neutron scattering study of cholic acid-based organogel systems

Small-angle neutron scattering measurements were performed on some cholic acid-based gel systems in order to gain detailed information about the network structure. The presence of thin fibers with a radius of about 10-20 A was found for various gelators. Two types of interaction between different sorts of fibers were demonstrated, depending on the molecular structure of the gelator. The first type involves the presence of microcrystalline knots with a dimension of about 100-200 A between the fibers. Upon heating, this network gradually disintegrates. The second type involves loose entanglements between flattened fibers. The occurrence of these types of interaction is related to the length of the alkyl tail attached to cholic acid.