Temperature-dependent self-assembly and mixing behavior of symmetrical single-chain bolaamphiphiles

Stimuli-responsive viscosity modifiers

Stimuli responsive viscosity modifiers entail an important class of materials which allow for smart material formation utilizing various stimuli for switching such as pH, temperature, light and salinity. They have seen applications in the biomedical space including tissue engineering and drug delivery, wherein stimuli responsive hydrogels and polymeric vessels have been extensively applied. Applications have also been seen in other domains like the energy sector and automobile industry, in technologies such as enhanced oil recovery. The chemistry and microstructural arrangements of the aqueous morphologies of dissolved materials are usually sensitive to the aforementioned stimuli which subsequently results in rheological sensitivity as well. Herein, we overview different structures capable of viscosity modification as well as go over the rheological theory associated with classical systems studied in literature. A detailed analysis allows us to explore correlations between commonly discussed models such as molecular packing parameter, tube reptation and stress relaxation with structural and rheological changes. We then present five primary mechanisms corresponding to stimuli responsive viscosity modification: (i) packing parameter modification via functional group conditioning and (ii) via dynamic bond formation, (iii) mesh formation by interlinking of network nodes, (iv) viscosity modification by chain conformation changes and (v) viscosity modification by particle jamming. We also overview several recent examples from literature that employ the concepts discussed to create novel classes of intriguing stimuli responsive structures and their corresponding rheological properties. Furthermore, we also explore systems that are responsive to multiple stimuli which can provide enhanced functionality and versatility by providing multi-level and precise actuation. Such systems have been used for programmed site-specific drug delivery.

Beyond Sol-Gel: Molecular Gels with Different Transitions

The existence of sol–gel transitions is one of the most manifest properties of molecular gels. These transitions reflect their nature since they correspond to the association or dissociation of low weight molecules through non-covalent interactions to form the network constitutive of the gel. Most described molecular gels undergo only one gel-to-sol transition upon heating, and the reverse sol-to-gel transition upon cooling. It has been long observed that different conditions of formation could lead to gels with different morphologies, and that gels can undergo a transition from gel to crystals. However, more recent publications report molecular gels which exhibit additional transitions, for instance gel-to-gel transitions. This review surveys the molecular gels for which, in addition to sol–gel transitions, transitions of different nature have been reported: gel-to-gel transitions, gel-to-crystal transition, liquid–liquid phase separations, eutectic transformations, and synereses.

Investigating bolalipids as solubilizing agents for poorly soluble drugs: Effects of alkyl chain length on solubilization and cytotoxicity

Synthetic single-chain bolalipids with symmetrical headgroups have shown potential in various pharmaceutical applications, such as the stabilization of liposome bilayers. Despite their amphiphilic character, synthetic bolalipids have not yet been investigated for their suitability as solubilizing agents for poorly soluble drug compounds. In this study, three synthetic single-chain bolalipids with increasing alkyl chain lengths (C22, C24 and C26) were investigated. All three bolalipids were able to achieve an increased solubility of the model drug, mefenamic acid, by approximately 180% in a pH 7.4 buffer compared to only a 102-105% increase achieved by sodium dodecyl sulfate (SDS) or the non-ionic surfactant pegylated hydroxystearate (PEG-HS). Subsequently, interfacial activity of bolalipids and their ability to destabilize liposomal bilayers were investigated. The C22 bolalipid exhibited a consistently lower interfacial activity, which was consistent with its significantly lower cytotoxicity in the macrophage-like cell line, J774. A1, compared to C24 and C26 counterparts. The mean IC₅₀ values of the bolalipids tested (0.035-0.093 mM) were approximately 4-100-fold lower than that of SDS (0.401 mM) or PEG-HS (0.922 mM), with the mechanism of toxicity linked to increased cell membrane permeability, as is expected for surfactants. In summary, evidence from this study shows that decreasing the length of the bolalipid alkyl linker from C26 to C22 resulted in a significantly decreased cytotoxicity with no loss in drug solubilization efficiency.

Gel-to-gel non-variant transition of an organogel caused by polymorphism from nanotubes to crystallites

An amide based gelator forms gels in trans-decalin. Below concentrations of 1 wt% the gels melt at temperatures varying with concentration. Above a concentration of 1 wt%, upon heating, the gel transforms into an opaque gel at an invariant temperature, and melts at higher temperature. The gel-to-gel transition is evidenced by several techniques: DSC, rheology, NMR, OM and turbidimetry. The phase diagram with the domain of the existence of both morphs was mapped by these techniques. Optical and electronic microscopy studies show that the first gel corresponds to the self-assembled nanotubes while the second gel is formed by crystalline fibers. The fibers are crystalline, as shown by the presence of Bragg peaks in the scattering curves. Both morphs correspond to a different H-bonding pattern as shown by FTIR. The first gel forms at a higher cooling rate, is metastable and transforms slowly into the second one. The second gel is stable. It forms at a low cooling rate, or by thermal annealing or aging of the first gel.

Tuning the Thickness of a Biomembrane by Stapling Diamidophospholipids with Bolalipids

In a biological membrane, proteins require specific lipids of distinctive length and chain saturation surrounding them. The active tuning of the membrane thickness therefore opens new possibilities in the study and manipulation of membrane proteins. Here, we introduce the concept of stapling phospholipids to different degrees of interdigitation depth by mixing 1,3-diamidophospholipids with single-chain bolalipids. The mixed membranes were studied by calorimetric assays, electron microscopy, X-ray, and infrared measurements to provide a complete biophysical characterization of membrane stapling. The matching between the diamidophospholipids and the bolalipids can be so strong as to completely induce a new phase that is more stable than the gel phase of the individual components.

A Technical Introduction to Transmission Electron Microscopy for Soft-Matter: Imaging, Possibilities, Choices, and Technical Developments

With a significant role in material sciences, physics, (soft matter) chemistry, and biology, the transmission electron microscope is one of the most widely applied structural analysis tool to date. It has the power to visualize almost everything from the micrometer to the angstrom scale. Technical developments keep opening doors to new fields of research by improving aspects such as sample preservation, detector performance, computational power, and workflow automation. For more than half a century, and continuing into the future, electron microscopy has been, and is, a cornerstone methodology in science. Herein, the technical considerations of imaging with electrons in terms of optics, technology, samples and processing, and targeted soft materials are summarized. Furthermore, recent advances and their potential for application to soft matter chemistry are highlighted.

The Use of Artificial Gel Forming Bolalipids as Novel Formulations in Antimicrobial and Antifungal Therapy

The alarming growth of multi-drug resistant bacteria has led to a quest for alternative antibacterial therapeutics. One strategy to circumvent the already existing resistance is the use of photodynamic therapy. Antimicrobial photodynamic therapy (aPDT) involves the use of non-toxic photosensitizers in combination with light and in situ oxygen to generate toxic radical species within the microbial environment which circumvents the resistance building mechanism of the bacteria. Hydrogels are used ubiquitously in the biological and pharmaceutical fields, e.g., for wound dressing material or as drug delivery systems. Hydrogels formed by water-insoluble low-molecular weight gelators may potentially provide the much-needed benefits for these applications. Bolalipids are a superior example of such gelators. In the present work, two artificial bolalipids were used, namely PC-C32-PC and Me2PE-C32-Me2PE, which self-assemble in water into long and flexible nanofibers leading to a gelation of the surrounding solvent. The aim of the study was to create stable hydrogel formulations of both bolalipids and to investigate their applicability as a novel material for drug delivery systems. Furthermore, methylene blue-a well-known photosensitizer-was incorporated into the hydrogels in order to investigate the aPDT for the treatment of skin and mucosal infections using a custom designed LED device.

Stimuli Responsive, Self-Sustainable, and Self-Healable Functionalized Hydrogel with Dual Gelation, Load-Bearing, and Dye-Absorbing Properties

The motivation for designing low-molecular-weight gelators with self-healing characteristics originates from elegant examples in biology such as vines of the genus Aristolochia whose internal secondary growth exhibits rapid self-healing in their stems. In the present work, we had explored the stimuli-responsive dual gelation characteristics for the ester-functionalized surfactant (4-(2-(hexadecyloxy)-2-oxoethyl)-4-methylmorpholin-4-ium bromide, C₁₆EMorphBr) in aqueous medium at 7.20% (w/v) critical gel concentration and pH 7.4. The hydrogel provides an excellent platform to study dynamic phase behavior within a supramolecular network as it exhibits transformation from a fibrillar opaque hydrogel to a transparent hydrogel upon heating. Molecular interactions, arrangement within the supramolecular framework, and mechanical properties of the hydrogels were characterized using Fourier transform infrared, small-angle neutron scattering, rheological analysis, and tensile strength and cyclic loading-unloading tests. The fibrillar opaque gel has been characterized for its morphology using scanning electron microscopy, field emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The self-sustained, self-healable porous fibrillar opaque xerogel was further explored for selectively absorbing anionic dyes and for its load-bearing characteristics. We conclude a perspective on designing a new-age gelator that can open entirely new avenues in environmental protection and wearable "smart" devices.

Nanofiber Formation and Polymerization of Bolalipids with Diacetylene-Modified Single Alkyl Chains

The nanofiber formation in aqueous suspension of two classes of symmetric single-chain bolaamphiphiles with different polar headgroups and a diacetylene-modified alkyl chain with a length of 32, 34, and 36 C atoms was investigated by differential scanning calorimetry, transmission electron microscopy, and small-angle neutron scattering. As observed before for other bolalipids with phosphocholine (PC) and dimethyl-phosphoethanolamine (Me₂PE) headgroups, the molecules form fibers when suspended in water at low temperatures but disassemble into micellar-like aggregates upon heating. The introduction of a diacetylene group in the middle of the long chain leads to a perturbation of chain packing so that this fiber-micelle transition occurs at lower temperature compared to the other bolalipids having unmodified alkyl chains. The aim of our project was the introduction of diacetylene groups into alkyl chains to be able to polymerize the fibers at low temperature. This should enhance the fiber stability and prevent the disassembly into micellar aggregates at higher temperature. Polymerization of aggregates containing diacetylene-modified bolaamphiphiles can be easily traced by UV/vis spectroscopy as colored products are formed. We found that polymerization of bolaamphiphiles with PC headgroups leads to a breakdown of most fibers into micellelike aggregates, and only some longer fibers segments are still detectable. In contrast, the use of Me₂PE headgroups improves polymerizability and length of the polymerized fibers. The compound with 36 C atoms in the chain could be polymerized at low temperatures, and the fibers remained stable at least up to a temperature of 60 °C. This shows that the perturbation of the chain packing due to the diacetylene groups in the chains can be overcome by elongation of the chains, so that thermostable fibers with a diameter of the length of the bolalipid molecule can be successfully formed.

Thermo-Switchable de Novo Ionic Liquid-Based Gelators with Dye-Absorbing and Drug-Encapsulating Characteristics

An ionic liquid-based surfactant with ester functionality self-aggregates in an aqueous medium and forms ionogels at 8.80% (w/v) concentration at physiological pH. The ionogel exhibited a remarkable change in its appearance with temperature from fibrillar opaque to transparent because of the dynamic changes within its supramolecular structure. This gel-to-gel phase transition occurs below the melting point of the solid ionic liquid. The ionogels were investigated using turbidity, differential scanning calorimetry, scanning electron microscopy (SEM), field emission SEM (FE-SEM), inverted microscopy, transmission electron microscopy imaging, Fourier transform infrared spectroscopy, and rheological measurements. The fibrillar opaque ionogel and transparent ionogel were studied for their ability to absorb dyes (methyl orange and crystal violet) and to encapsulate drugs (diclofenac sodium and imatinib mesylate).

Hierarchical Assembly of l-Phenylalanine-Terminated Bolaamphiphile with Porphyrin Show Tunable Nanostructures and Photocatalytic Properties

Demands related to clean energy and environmental protection promote the development of novel supramolecular assemblies for photocatalysis. Because of the distinctive aggregation behaviors, bolaamphiphiles with two hydrophilic end groups could be theoretically the right candidates for the fabrication of high-performance photocatalysis. However, photocatalytic applications based on bolaamphiphilic assemblies were still rarely investigated. Especially, the relationship between diverse assembled nanostructures and the properties for different applications is urgently needed to be studied. Herein, we demonstrate that using the hierarchical assembly of bolaamphiphiles could correctly induce the porphyrin supramolecular architectures with much better photocatalytic performances than the aggregations containing 450 times of the porphyrin molecules, even though both molecular structures as well as the J-aggregations of porphyrin building blocks are same in two different systems. Thus, the co-assembly of l-phenylalanine terminated bolaamphiphile (Bola-F) and the porphyrin containing four hydroxyl groups (tetrakis-5,10,15,20-(4-hydroxyphenyl)porphyrin) can form microtube in methanol and forms fibers/spheres in methanol/water mixture. For catalyzing the photodegradation of rhodamine B, the small amount of J-aggregated porphyrin within Bola-F microtubes show much better photocatalytic performance comparing with that of huge porphyrin J-aggregations in fibers/spheres. The supramolecular assemblies as well as the photocatalysis were thoroughly characterized by different spectroscopies and electron microscopy. It is demonstrated that the co-assembly with bolaamphiphiles could inhibit the energy transfer of porphyrin aggregation and subsequently benefit the electron transfer and corresponding photocatalysis under photo-irradiation. This work is not only useful for further understanding the hierarchically supramolecular assembly but also provides a new strategy for making novel functional supramolecular architectures based on the assembly of bolaamphiphiles and porphyrins.

(Cryo)Transmission Electron Microscopy of Phospholipid Model Membranes Interacting with Amphiphilic and Polyphilic Molecules

Lipid membranes can incorporate amphiphilic or polyphilic molecules leading to specific functionalities and to adaptable properties of the lipid bilayer host. The insertion of guest molecules into membranes frequently induces changes in the shape of the lipid matrix that can be visualized by transmission electron microscopy (TEM) techniques. Here, we review the use of stained and vitrified specimens in (cryo)TEM to characterize the morphology of amphiphilic and polyphilic molecules upon insertion into phospholipid model membranes. Special emphasis is placed on the impact of novel synthetic amphiphilic and polyphilic bolalipids and polymers on membrane integrity and shape stability.

The cmc-value of a bolalipid with two phosphocholine headgroups and a C24 alkyl chain: Unusual binding properties of fluorescence probes to bolalipid aggregates

Bolalipids with a long alkyl chain and two phosphocholine polar groups self-assemble in water into two different types of aggregate structures, namely helical nanofibers at low temperature and two types of micellar aggregates at higher temperature. We tried to determine the critical aggregation concentration (cac) or critical micellar concentration (cmc) of the bolalipid tetracosane-1,24-bis(phosphocholine) (PC-C24-PC) by using different fluorescent probes. The use of pyrene or pyrene derivatives as fluorophores failed, whereas the probes 1,8-ANS and particularly bis-ANS gave consistent results. The structure of the bolalipid aggregates obviously hinders partitioning or binding of pyrene derivatives into the micellar interior, whereas 1,8-ANS and bis-ANS can bind to the surface of the aggregate structures. The observed large increase in fluorescence intensity of bis-ANS indicates that binding to the hydrophobic surface of the aggregates leads to a reduction of the dye mobility. However, binding of bis-ANS is relatively weak, so that the determination of a cac/cmc-value is difficult. Simulations of the intensity curves for PC-C24-PC lead to estimates of the cac/cmc-value of 0.3-1.0×10^-6M, depending on the structure of the aggregates. Single molecule fluorescence correlation spectroscopy was used to determine the mobility of bis-ANS as a function of concentration of PC-C24-PC. The dye diffusion time and the molecular brightness are lower at low bolalipid concentration, when only free dye is present, and increase at higher concentration when bis-ANS is bound to the aggregates. The experimental cac/cmc-values are higher than those estimated, using an incremental method for the change in Gibbs free energy for micellization with n-alkyl-phosphocholines with only one polar group as a comparison. Apparently, for PC-C24-PC in micellar or fibrous aggregates, more CH₂ groups are exposed to water than in a conventional micelle of an n-alkyl-phosphocholine.

A unique thermo-induced gel-to-gel transition in a pH-sensitive small-molecule hydrogel

For a hydrogel based on a zwitterionic dendritic surfactant, we report an apparently unprecedented reversible temperature-induced gel-to-gel phase transition below the melting point of its alkyl chains, where the supramolecular self-assembly of surfactant molecules underwent a dramatic transformation from low-temperature surfactant bilayers to high-temperature entangled surfactant worm-like micelles.

Aggregation behaviour of a single-chain, phenylene-modified bolalipid and its miscibility with classical phospholipids

In the present work, we describe the synthesis of a single-chain, phenylene-modified bolalipid with two phosphocholine headgroups, PC-C18pPhC18-PC, using a Sonogashira cross-coupling reaction as a key step. The aggregation behaviour was studied as a function of temperature using transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and small angle neutron scattering (SANS). We show that our new bolalipid self-assembles into nanofibres, which transform into flexible nanofibres at 27 °C and further to small elongated micelles at 45 °C. Furthermore, the miscibility of the bolalipid with bilayer-forming phosphatidylcholines (DMPC, DPPC, and DSPC) was investigated by means of DSC, TEM, FTIR, and small angle X-ray scattering (SAXS). We could show that the PC-C18pPhC18-PC is partially miscible with saturated phosphatidylcholines; however, closed lipid vesicles with an increased thermal stability were not found. Instead, bilayer fragments and disk-like aggregates are formed.

Antimicrobial Nanoplexes meet Model Bacterial Membranes: the key role of Cardiolipin

Antimicrobial resistance to traditional antibiotics is a crucial challenge of medical research. Oligonucleotide therapeutics, such as antisense or Transcription Factor Decoys (TFDs), have the potential to circumvent current resistance mechanisms by acting on novel targets. However, their full translation into clinical application requires efficient delivery strategies and fundamental comprehension of their interaction with target bacterial cells. To address these points, we employed a novel cationic bolaamphiphile that binds TFDs with high affinity to form self-assembled complexes (nanoplexes). Confocal microscopy revealed that nanoplexes efficiently transfect bacterial cells, consistently with biological efficacy on animal models. To understand the factors affecting the delivery process, liposomes with varying compositions, taken as model synthetic bilayers, were challenged with nanoplexes and investigated with Scattering and Fluorescence techniques. Thanks to the combination of results on bacteria and synthetic membrane models we demonstrate for the first time that the prokaryotic-enriched anionic lipid Cardiolipin (CL) plays a key-role in the TFDs delivery to bacteria. Moreover, we can hypothesize an overall TFD delivery mechanism, where bacterial membrane reorganization with permeability increase and release of the TFD from the nanoplexes are the main factors. These results will be of great benefit to boost the development of oligonucleotides-based antimicrobials of superior efficacy.

Aromatic-Aromatic Interactions Enable α-Helix to β-Sheet Transition of Peptides to Form Supramolecular Hydrogels

Isolated short peptides usually are unable to maintain their original secondary structures due to the lack of the restriction from proteins. Here we show that two complementary pentapeptides from a β-sheet motif of a protein, being connected to an aromatic motif (i.e., pyrene) at their C-terminal, self-assemble to form β-sheet like structures upon mixing. Besides enabling the self-assembly to result in supramolecular hydrogels upon mixing, aromatic-aromatic interactions promote the pentapeptides transform from α-helix to β-sheet conformation. As the first example of using aromatic-aromatic interactions to mimic the conformational restriction in a protein, this work illustrates a bioinspired way to generate peptide nanofibers with predefined secondary structures of the peptides by a rational design using protein structures as the blueprint.

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of molecules, including low molecular weight ones. Finally, we provide a perspective on supramolecular hydrogelators. We hope that this review will serve as an updated introduction and reference for researchers who are interested in exploring supramolecular hydrogelators as molecular biomaterials for addressing the societal needs at various frontiers.

Self-assembly of triangular amphiphiles into diverse nano/microstructures and release behavior of the hollow sphere

Amphiphilic triangular derivatives containing imine bond have been designed and used as building blocks for the construction of soft colloidal materials. The acylhydrazone derivative was found to form various nano/microstructures in different solvents. The microspheres, flower-like, and hollow spheres were formed in the polar solvents, while organogels with microporous structures were formed in nonpolar solvents. However, the simple Schiff base amphiphile without amide group did not form any ordered structures in the tested solvents. Therefore, besides the π-π stacking between the aromatic core and the van der Waals interactions between the alkyl chains, the hydrogen bonding between the acylhydrazone moieties played an important role in promoting the formation of various organized structures. More interestingly, the hollow sphere structure which was formed in THF could be used to encapsulate and release the rhodamine B. It was found that rhodamine B was encapsulated within their interiors under neutral conditions and released in an acidic condition due to the breakage of the imine bond.

The headgroup (a)symmetry strongly determines the aggregation behavior of single-chain phenylene-modified bolalipids and their miscibility with classical phospholipids

In the present work, we describe the synthesis of two single-chain phenylene-modified bolalipids, namely PC-C17pPhC17-PC and PC-C17pPhC17-OH, with either symmetrical (phosphocholine) or asymmetrical (phosphocholine and hydroxyl) headgroups using a Sonogashira cross-coupling reaction as key step. The temperature-dependent aggregation behavior of both bolalipids in aqueous suspension was studied using transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, small angle neutron scattering (SANS), and X-ray scattering. We show that different headgroup symmetries lead to a change in the aggregation behavior: Whereas PC-C17pPhC17-PC forms nanofibers with a diameter of 5.7 nm that transform into small ellipsoidal micelles at 23 °C, the PC-C17pPhC17-OH self-assembles into lamellae with bolalipid molecules in an antiparallel orientation up to high temperatures. Furthermore, the mixing behavior of both bolalipids with bilayer-forming phospholipids (DPPC and DSPC) was studied by means of DSC and TEM. The aim was to stabilize bilayer membranes formed of phospholipids in order to improve these mixed lipid vesicles for drug delivery purposes. We show that the symmetrical PC-C17pPhC17-PC is miscible with DPPC and DSPC; however, closed lipid vesicles are not observed, and elongated micelles and bilayer fragments are found instead. In contrast, the asymmetrical PC-C17pPhC17-OH shows no miscibility with phospholipids at all.

Self-assembly of different single-chain bolaphospholipids and their miscibility with phospholipids or classical amphiphiles

A variety of bolalipids with a single long alkyl chain and two identical headgroups self-assemble in aqueous solutions into helical entangled nanofibers leading to the formation of a hydrogel. An increase in temperature usually leads to the break-up of the fiber structure into micellar aggregates. In this paper the question is addressed whether bolalipids of different lengths or different headgroup structures can form mixed fibers. Also, the stability of the fiber aggregation of bolalipids in mixtures with phospholipids forming lamellar bilayers is discussed. Here, the question whether single-chain bolalipids can be incorporated into phospholipid bilayers to stabilize bilayer membranes is important, as possibly lipid vesicles used for drug delivery can be improved. Finally, the stability of the fiber aggregate against solubilisation by common surfactants was studied. The paper addresses the question which type of aggregate structure dominates the self-assembly of bipolar and monopolar amphiphiles in aqueous suspension.

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.

Fusion of bolaamphiphile micelles: a method to prepare stable supported biomimetic membranes

Supported biomimetic membranes (SBMs) on solid substrates have been commonly prepared from vesicle-forming double-tail lipids, such as zwitterionic phospholipids, using the method of vesicle fusion. Here we report on the preparation of SBMs on silica surfaces via a similar process of "micelle fusion" from a cationic single-tail bolaamphiphile GLH-20 that forms spherical and elongated thread-like micelles in solution. We demonstrate that, in contrast to zwitterionic phospholipids, GLH-20 self-assembles into a stable contiguous SBM at both low and high ionic strengths. The cationic charge of GLH-20 promotes the formation of a stable SBM through enhanced double-layer interactions with the negatively charged silica surface. It is also shown that spinach aquaporin PM-28 was successfully incorporated within bolaamphiphile SBM in a manner similar to SBMs prepared by vesicle/proteoliposome fusion; thereby the inherent curvature of the micelle surface does not inhibit protein reconstitution. The results suggest that SBMs based on charged bolaamphiphiles might be an attractive platform for applications such as water purification and biosensors, where the stability and low defect rate of SBMs in diverse conditions are crucial for achieving desired performance.

Monte Carlo study of the self-assembly of achiral bolaform amphiphiles into helical nanofibers

It is shown by coarse-grained off-lattice Monte Carlo simulations that a geometrically induced frustration of the parallel arrangement of rigid achiral bolaform amphiphiles can cause chirality in self-assembled nanostructures. The amphiphilic molecules are represented as rigid linear chains of 8 equally sized hydrophobic spheres (tail) and a hydrophilic sphere (head) at each end. The hydrophilic and hydrophobic spheres differ in size. A very simple interaction scheme consisting of only hard-core repulsion between all spheres and square-well attraction between hydrophobic spheres is sufficient for self-assembly into helical fibers for molecules with head/tail diameter ratios ranging from 1.3 to 1.8.