Phosphonate lipid tubules II

Co-assembled nanotubes with controlled curvature radius using a hydrogen bond regulation strategy

The design of co-organized nanotube systems with controlled curvature radius that are realized by tilt modulation of co-assembled molecules, induced by the strength of non-covalent interactions in aqueous media, remains a significant challenge. Here, we report success in utilizing a hydrogen bond regulation strategy to stimulate molecular tilt for the formation of nanotubes with controlled curvature radius based on the co-assembly of two kinds of achiral cationic building blocks in aqueous solution. Computations and electron microscopy experiments suggest that the nanotube curvature radius drastically decreases as the tilt angle θ of co-assembled molecules increases with an increase of hydrogen bond strength. Interestingly, a slight change in the co-assembled molecular tilt causes a drastic change in the nanotube curvature radius.

Supramolecular nanotubes based on halogen bonding interactions: cooperativity and interaction with small guests

In this manuscript the formation of a series of self-assembled supramolecular nanotubes (SNTs) governed by noncovalent halogen bonding interactions is studied.

Controlled self-assembly of helical nano-ribbons formed by achiral amphiphiles

Helical nano-ribbons with a large aspect ratio were obtained through the self-assembly of an achiral amphiphile. The symmetry breaking is attributed to the orderly but twisted stacking of terpyridine groups. In addition, the morphology of the assemblies can be tuned by the coordination between terpyridine and Zn(2+) ion.

Emerging chirality in nanoscience

Chirality in nanoscience may offer new opportunities for applications beyond the traditional fields of chirality, such as the asymmetric catalysts in the molecular world and the chiral propellers in the macroscopic world. In the last two decades, there has been an amazing array of chiral nanostructures reported in the literature. This review aims to explore and categorize the common mechanisms underlying these systems. We start by analyzing the origin of chirality in simple systems such as the helical spring and hair vortex. Then, the chiral nanostructures in the literature were categorized according to their material composition and underlying mechanism. Special attention is paid to highlight systems with original discoveries, exceptional structural characteristics, or unique mechanisms.

pH sensitive tubules of a bile acid derivative: a tubule opening by release of wall leaves

Tubules formed by self-assembly of organic molecules have vast potential for nanotechnology applications, and the introduction of sensitivity to stimuli into self-assembly tubules represents a particularly attractive feature. Here we report the preparation and characterization of a molecule obtained by chemical modification of a natural bile acid, a biological surfactant, that self-assembles in pH sensitive tubules in aqueous solutions. The tubules, which are rigid, single-walled and with a diameter of 60 nm, form at pH 8-9 and open up when the pH is increased. The transition is reversible, it occurs in the pH range of 9-10 with an opening mechanism that is remarkably different from those so far proposed in the literature. It involves a release of wall layers similar to leaves, and is determined by a drastic pH-triggered change in the molecular arrangement, which in turn induces a radical modification of the wall curvature. The description of the morphological transformation is provided by means of cryogenic transmission electron microscopy and represents, to our knowledge, the first detailed visualization of pH stimulated tubule opening. UV and circular dichroism spectroscopies are used to investigate the evolution at the molecular level.

Drug-loaded nanoparticles and supramolecular nanotubes formed from a volatile microemulsion with bile salt derivatives

The main objective of this study was to form nanoparticles of a model hydrophobic drug, celecoxib, from a volatile microemulsion stabilized by a bile salt derivative. Nanoparticles were obtained by conversion of the microemulsion nanodroplets with the dissolved drug into solid nanometric particles. The use of bile salt derivatives as the surfactants for the formation of a microemulsion enabled significantly higher loading of the drug in both the microemulsion and nanoparticles, compared with the native bile salt. In addition, superior stability of the particles was achieved with the bile salt derivatives, and drug crystallization was inhibited. Interestingly, differences in particle stability and crystallization inhibition were observed between two bile salt derivatives differing only by one hydroxyl group on the bile salt backbone, indicating the delicate balance of interactions in the system. For one of the derivatives, upon dispersion of the nanoparticles in water, they spontaneously arranged into well-defined elongated nanometric tubules as detected and attested by cryo-TEM. It was found that the drug present in nanoparticles induces formation of the nanotubes.

Experimental observation of double-walled peptide nanotubes and monodispersity modeling of the number of walls

Self-assembled nanoarchitectures based on biological molecules are attractive because of the simplicity and versatility of the building blocks. However, size control is still a challenge. This control is only possible when a given system is deeply understood. Such is the case with the lanreotide acetate, an octapeptide salt that spontaneously forms monodisperse nanotubes when dissolved into pure water. Following a structural approach, we have in the past demonstrated the possibility to tune the diameter of these nanotubes while keeping a strict monodispersity, either by chemical modification of one precise amino acid on the peptide sequence or by changing the size of the counterions. On the basis of these previous studies, we replaced monovalent counterions by divalent ones to vary the number of walls. Indeed, in the present work, we show that lanreotide associated with a divalent counterion forms double-walled nanotubes while keeping the average diameter constant. However, the strict monodispersity of the number of walls was unexpected. We propose that the divalent counterions create an adhesion force that can drive the wall packing. This adhesion force is counterbalanced by a mechanical one that is related to the stiffness of the peptide wall. By taking into account these two opposite forces, we have built a general model that fully explains why the lanreotide nanotubes formed with divalent counterions possess two walls and not more.

Chiral self-assembly of designed amphiphiles: optimization for nanotube formation

Four amphiphiles with L-aspartic acid headgroups (Asp) and a diphenyldiazenyl group (Azo) contained within the hydrophobic tails were designed and synthesized for self-assembly into helically based nanotubes. The amphiphiles of the form R'-{4-[(4-alkylphenyl)diazenyl]phenoxy}alkanoyl-L-aspartic acid (where R' is 10 or 11) varied only in alkyl chain lengths either side of the azo group, having 4, 7, or 10 carbon distal chains and 10 or 11 carbon proximal chains (R-Azo-R'-Asp, where R denotes the number of carbons in the distal chain and R' denotes the number of carbons in the proximal chain). Despite the molecular similarities, distinct differences were identified in the chiral order of the structures self-assembled from hot methanolic aqueous solutions using microscopy and spectroscopic analyses. This was reflected in dominant thermodynamic aggregate morphologies that ranged from amorphous material for 10-Azo-10-Asp, through twisted ribbons (196 ± 49 nm pitch) for 7-Azo-11-Asp, to the desired helically based nanotubes for 4- and 7-Azo-10-Asp (81 ± 11 and 76 ± 6 nm diameters, respectively). Another key variable in the self-assembly of the amphiphiles was the use of a second method to precipitate aggregates from solution at room temperature. This method enabled the isolation of thermodynamically unstable and key transitional structures. Helical ribbons were precursor structures to the nanotubes formed from 4- and 7-Azo-10-Asp as well as the wide, flattened nanotube structures (587 ± 85 nm width) found for 4-Azo-10-Asp. Overall, the results highlighted the interplay of influence of the headgroup and the hydrophobic tail on self-assembly, providing a basis for future rational design of self-assembling amphiphiles.

Polydiacetylenes: supramolecular smart materials with a structural hierarchy for sensing, imaging and display applications

While a large variety of conjugated polymers exist, polydiacetylenes (PDAs) remain a major research area among scientists due to their interesting optical, spectral, electronic, and structural properties. Heavily reviewed in regards to their stimuli responsive properties, much is known about the assortment of sensing and detection capabilities of PDAs. In this article, we look more upon the structural diversities of polydiacetylenes that have been achieved in recent years, particularly from a hierarchical perspective of 1, 2, and 3-dimensional configurations. In addition, we examine how these different dimensional arrangements of PDAs have heralded clear applications in several key areas. Successful integration of these stimuli-responsive "smart" materials into various geometries has required researchers to have a comprehensive understanding of both the fabrication and synthesis processes, as well as the signalling mechanism for the optical, fluorogenic or spectral transitions. The on-going discovery of new PDA formulations continues to provide interesting structural manifestations such as liposomes, tubes, fibres, organic/inorganic incorporated hybrids and composite structures. By highlighting some of the recent conceptual and technological developments, we hope to provide a measure of the current pace in new PDA derivative development as core components in efficient sensor, imaging and display systems.

Self-assembly of copper(II) ion-mediated nanotube and its supramolecular chiral catalytic behavior

Self-assembly of several low-molecular-weight L-glutamic acid-based gelators, which individually formed helical nanotube or nanofiber structures, was investigated in the presence of Cu(2+) ion. It was found that, when Cu(2+) was added into the system, the self-assembly manner changed significantly. Only in the case of bolaamphiphilic glutamic acid, N,N'-hexadecanedioyl-di-L-glutamic acid (L-HDGA), were the hydrogel formation as well as the nanotube structures maintained. The addition of Cu(2+) ion caused a transition from monolayer nanotube of L-HDGA to a multilayer nanotube with the thickness of the tubular wall about 10 nm. For the other amphiphiles, the gel was destroyed and nanofiber structures were mainly formed. The formed Cu(2+)-containing nanostructures can function as an asymmetric catalyst for Diels-Alder cycloaddition between cyclopentadiene and aza-chalcone. In comparison with the other Cu(2+)-containing nanostructures, the Cu(2+)-mediated nanotube structure showed not only accelerated reaction rate, but enhanced enantiomeric selectivity. It was suggested that, through the Cu(2+) mediated nanotube formation, the substrate molecules could be anchored on the nanotube surfaces and produced a stereochemically favored alignment. When adducts reacted with the substrate, both the enantiomeric selectivity and the reaction rate were increased. Since the Cu(2+)-mediated nanotube can be fabricated easily and in large amount, the work opened a new way to perform efficient chiral catalysis through the supramolecular gel.

Helical nanostructures self-assembled from optically active phthalocyanine derivatives bearing four optically active binaphthyl moieties: effect of metal-ligand coordination on the morphology, dimension, and helical pitch of self-assembled nanostructures

(R)- and (S)-enantiomers of optically active metal free tetrakis[11,12:13,14-di(1',2'-naphtho)-1,4,7,10,15,18-hexaoxacycloeicosa-2,11,13-trieno]-phthalocyanine and their zinc complexes, (R)- and (S)-H(2)Pc (1) and (R)- and (S)-ZnPc (2), were prepared from the tetramerization of corresponding phthalonitriles, (R)- and (S)-2,3-(4',5'-dicyanobenzo)-11,12:13,14-di(1',2'-naphtho)-1,4,7,10,15,18-hexaoxacycloeicosa-2,11,13-triene, in the absence and presence of Zn(OAc)(2).2H(2)O template, respectively, promoted by organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Their self-assembly behavior in the absence and presence of 4,4'-bipyridine has been comparatively investigated by electronic absorption and circular dichroism (CD) spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) technique, and X-ray photoelectron spectroscopy (XPS). The metal free phthalocyanine self-assembles into highly ordered fibrous nanostructures (ca. 3 microm length, 70 nm width, and 125 nm helical pitch) with left-handed and right-handed helicity for (R)-1 and (S)-1, respectively, through the hierarchical manner via one-dimensional helices with chirality determined by the optically active binaphthyl side chains. In contrast, self-assembly of the phthalocyaninato zinc analogue leads to the formation of nanoparticles. However, in the presence of 4,4'-bipyridine, additionally formed metal-ligand Zn-N(4,4'-bipyridine) coordination bonds between the nitrogen atoms of additive 4,4'-bipyridine molecule and the zinc center of (R)- and (S)-2 molecules together with pi-pi interaction and chiral discrimination of chiral side chains induce a right-handed and left-handed helical arrangement in a stack of (R)- and (S)-2 molecules, respectively, which further hierarchically packs into highly ordered fibrous nanostructures of average tens of micrometers in length, 30 nm width, and 106 nm helical pitch with the same helicity to the stack, revealing the effect of metal-ligand coordination bonding interaction on the morphology, dimension, handedness, and helical pitch of self-assembled nanostructures.

Counterion, Temperature, and Time Modulation of Nanometric Chiral Ribbons from Gemini-Tartrate Amphiphiles

Amphiphile supramolecular assemblies result from the cooperative effects of multiple weak interactions between a large number of subcomponents. As a result, prediction of and control over the morphologies of such assemblies remains difficult to achieve. Here, we described the fine-tuning of the shape, size, and morphology transitions of twisted and helical membranes formed by non-chiral dicationic n-2-n gemini amphiphiles complexed with chiral tartrate anions. We have reported that such systems express the chirality of the tartrate components at a supramolecular level and that the mechanism of the chiral induction by counterions involves specific anion cation recognition and the induction of conformationally labile chirality in the cations. Here, we demonstrate that the morphologies and dimensions of twisted and helical ribbons, as well as tubules, can be controlled and that interconversion between these structures can be induced upon modifying temperature, upon introducing small amounts of additives, or slightly modifying molecular structure. Specifically, electron microscopy, IR spectroscopy, and small-angle X-ray scattering show that (i) varying the hydrophobic chain length or adding gemini having bromide counterions (1%) or the opposite enantiomer (10%) leads to an increase of the diameter of membrane tubules from 33 to 48.5 nm; (ii) further addition (1.5%) of gemini bromide or a slight increase in temperature induces a transition from tubules to twisted ribbons; (iii) the twist pitch of the ribbons can be continuously tuned by varying enantiomeric excess; and (iv) it was also observed that the morphologies of these ribbons much evolve with time. Such unprecedented observations over easy tuning of the chiral supramolecular structures are clearly related to the original feature that the induction of chirality is solely due the counterions, which are much more mobile than the amphiphiles.

Photopolymerization of and patterned fluorescence imaging with a bispyrenyl group-containing diacetylene

Photopolymerization of a diacetylene monomer having terminal pyrene groups afforded formation of polydiacetylene nanoparticles in aqueous solvent along with fluorescence quenching of pyrene moieties.

Vancomycin-induced morphological transformation of self-assembled amphiphilic diacetylene supramolecules

Reversible ribbon-sphere microstructural transformation of dipeptide-containing diacetylene supramolecules was observed by specific ligand-receptor interactions.

Self-assembling structures of long-chain sugar-based amphiphiles influenced by the introduction of double bonds

Nine phenyl glucoside or galactoside amphiphiles possessing a saturated or unsaturated long alkyl-chain group as the self-assembling unit of a highly organized molecular architecture were synthesized. Their self-assembly properties were investigated by using energy-filtering TEM (EF-TEM), SEM, CD, XRD, and FT-IR techniques. Compound 2, possessing one cis double bond in the lipophilic portion, exhibited twisted helical fibers, which formed a bilayered structure with a 3.59 nm period, while 3 exhibited helical ribbons and left-handed nanotubular structures with 150-200 nm inner diameters and a wall thickness of approximately 20 nm. Very interestingly, 4, possessing three cis double bonds, exhibited a nanotubular structure with an inner diameter of approximately 70 nm and a d spacing value of 4.62 nm. On the other hand, 7, possessing two trans double bonds in the lipophilic region, exhibited crystal- or plate-like structures, which formed a bilayer structure with a d spacing value of 3.93 nm. These results indicate that the self-assembly properties are strongly dependent on the type of double bond. Furthermore, 8 and 9, with the galactopyranose moiety, revealed helical ribbon and well-defined double helical fiber structures, respectively. These findings support the view that the orientation of the intermolecular hydrogen-bonding interaction between the sugar moieties plays a critical role in producing the nanotubular structures. According to CD and powder XRD experiments, the relatively strong intermolecular hydrogen-bonding interaction of the glucopyranoside moiety in 3 and 4 provided a highly ordered chiral packing structure. Even though these compounds formed a weak hydrophobic interaction between lipophilic groups, it led to the formation of the nanotubular structure.

Nanoscale ripples in self-assembled lipid tubules

Self-assembled cylindrical tubules of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (DC(8,9)PC) have been studied by atomic force microscopy in both the height and amplitude modes. Nanoscale ripple structures in the cylindrical lipid tubules are clearly resolved in amplitude mode images. The periodicity of the ripples is found to be 200 +/- 30 nm for tubules with diameters in the range from 200 to 650 nm. The angle of the ripples with respect to the equator of the tubules shows a bimodal distribution with centers at approximately 28 degrees and approximately 5 degrees.

Bending and Radial Deformation of Lipid Tubules on Self-Assembled Thiol Monolayers

Lipid tubules represent a hollow, cylindrical supramolecular structure formed by rolled-up lipid bilayers. We find that the lipid tubules of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine can be bent into a loopike shape by the shrinking contact line of droplets on self-assembled monolayers (SAMs) of 1-dodecanethiol. The persistence length of individual lipid tubules is estimated to be approximately 41 microm. The radial deformation of the lipid tubules on SAMs is studied under applied load using atomic force microscope. The stiffness of the tubules in the radial direction is found to increase when the number of the lipid bilayers in the tubule wall increases.

Cocklebur-shaped colloidal dispersions

Unique cocklebur-shaped colloidal dispersions were prepared using a combination of a nanoextruder applied to the aqueous solution containing methyl methacrylate (MMA) and n-butyl acrylate (n-BA) with azo-bis-isobutyronitrile (AIBN) or potassium persulfate (KPS) initiators and stabilized by a mixture of sodium dioctyl sulfosuccinate (SDOSS) and 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DCPC) phospholipid. Upon extrusion and heating to 75 degrees C, methyl methacrylate/n-butyl acrylate (MMA/nBA) colloidal particles containing tubules pointing outward were obtained as a result of DCPC phospholipids present at the particle surfaces. The same cocklebur-shaped particles were obtained when classical polymerization was used without a nanoextruder under similar compositional and thermal conditions, giving a particle size of 159 nm. However, when Ca(2+) ions are present during polymerization, cocklebur morphologies are disrupted. Because DCPC tubules undergo a transition at 38 degrees C, such cocklebur morphologies may offer numerous opportunities for devices with stimuli-responsive characteristics.

Lipid nanotubes and microtubes: experimental evidence for unsymmetrical monolayer membrane formation from unsymmetrical bolaamphiphiles

Unsymmetrical bolaamphiphiles, omega- [N-beta-D-glucopyranosylcarbamoyl] alkanoic acids, with even-numbered oligomethylene chains (12, 14, 16, 18, and 20 carbons) self-assembled in water to form lipid nano- and microtubes. The tubular assemblies were separated by centrifugation and examined by transmission electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy to study the molecular packing within the tubular membranes. The nanotubes encapsulated the staining reagent phosphotungstate, which revealed them to be hollow cylinders up to several hundred micrometers long with 30-43-nm outer diameters and 14-29-nm inner diameters. By comparing the membrane stacking periodicity obtained from powder X-ray diffraction analysis of the dehydrated tubes with the molecular packing within single crystals, we found that the nanotubes consist of an unsymmetrical monolayer lipid membrane (MLM) in which the molecules are packed in a parallel fashion. This suggests that the inner surface of the nanotubes is covered with carboxy headgroups and the outer surface with 1-glucosamide headgroups. The inner diameters of the lipid nanotubes could be controlled in the range 17.7-22.2 nm in steps of approximately 1.5 nm/two carbons by varying the oligomethylene spacer length. The microtubes had three types of molecular arrangements. The first type was a symmetrical MLM in which the molecules were packed in an antiparallel fashion. The other two types had unsymmetrical MLM stacking with head-to-head and head-to-tail motifs. Increasing the number of oligomethylene spacers stabilized the unsymmetrical MLM structure in both nano- and microtubes.

Chiral tubule self-assembly from an achiral diynoic lipid

Tubules possessing microm-scale chiral substructure self-assemble from an achiral isomer of the tubule-forming diynoic phosphatidylcholine, 1,2-bis(10,12-tricosadiynoyl)sn-glycero-3-phosphocholine [DC(8,9)PC], showing that molecular chirality is not essential for tubule formation. CD spectroscopy shows that these structures' helical sense of handedness instead originates in a spontaneous cooperative chiral symmetry-breaking process. We conclude that the chiral symmetry-breaking must originate in the unusual feature common to the chiral and achiral tubule-forming molecules, the diynes centered in their hydrocarbon tails.