Transmembrane gradient driven phase transitions within vesicles: lessons for drug delivery

AbstractPhase transitions in closed vesicles, i.e., microenvironments defined by the size of the vesicle, its contents, and permeability of its membrane are becoming increasingly important in several scientific disciplines including catalysis, growth of small crystals, cell function studies, and drug delivery. The membrane composed from lipid bilayer is in general impermeable to ions and larger hydrophilic ions. Ion transport can be regulated by ionophores while permeation of neutral and weakly hydrophobic molecules can be controlled by concentration gradients. Some weak acids or bases, however, can be transported through the membrane due to various gradients, such as electrical, ionic (pH) or specific salt (chemical potential) gradients. Upon permeation of appropriate species and reaction with the encapsulated species precipitation may occur in the vesicle interior. Alternatively, these molecules can also associate with the leaflets of the bilayer according to the transmembrane potential. Efficient liposomal therapeutics require high drug to lipid ratios and drug molecules should have, especially when associated with long circulating liposomes, low leakage rates. In this article we present very efficient encapsulation of two drugs via their intraliposomal precipitation, characterize the state of encapsulated drug within the liposome and try to fit the experimental data with a recently developed theoretical model. Nice agreement between a model which is based on chemical potential equilibration of membrane permeable species with experimental data was observed. The high loading efficiencies, however, are only necessary but not sufficient condition for effective therapies. If adequate drug retention within liposomes, especially in the case of long-circulating ones, is not achieved, the therapeutic index decreases substantially. Anticancer drug doxorubicin precipitates in the liposome interior in a form of gel with low solubility product and practically does not leak out in blood circulation in the scale of days. With an antibiotic, ciprofloxacin, the high loading efficacy and test tube stability is not reproduced in in vitro plasma leakage assays and in vivo. We believe that the reasons are higher solubility product of precipitated drug in the liposome, larger fraction of neutral molecules due closer pK values of the drug with the pH conditions in the solutions and high membrane permeability of this molecule. High resolution cryoEM shows that encapsulated anticancer agent doxorubicin is precipitated in the form of bundles of parallel fibers while antibiotic ciprofloxacin shows globular precipitate. Doxorubicin gelation also causes the change of vesicle shape

Tuning of Morphology by Chirality in Self-Assembled Structures of Bis(Urea) Amphiphiles in Water

We present the synthesis and self‐assembly of a chiral bis(urea) amphiphile and show that chirality offers a remarkable level of control towards different morphologies. Upon self‐assembly in water, the molecular‐scale chiral information is translated to the mesoscopic level. Both enantiomers of the amphiphile self‐assemble into chiral twisted ribbons with opposite handedness, as supported by Cryo‐TEM and circular dichroism (CD) measurements. The system presents thermo‐responsive aggregation behavior and combined transmittance measurements, temperature‐dependent UV, CD, TEM, and micro‐differential scanning calorimetry (DSC) show that a ribbon‐to‐vesicles transition occurs upon heating. Remarkably, chirality allows easy control of morphology as the self‐assembly into distinct aggregates can be tuned by varying the enantiomeric excess of the amphiphile, giving access to flat sheets, helical ribbons, and twisted ribbons.Supramolecular & Biomaterials Chemistr

Mechanism of formation of multilayered 2D crystals of the Enzyme IIC-mannitol transporter

AbstractWe have recently reported the crystallization by reconstitution into lipid bilayer structures of Enzyme IICmtl, the transmembrane C-domain of the mannitol transporter from E. coli. The projected structure was determined to a resolution of 0.5 nm [J. Mol. Biol. 287 5 (1999) 845]. However, further investigation proved that these crystals were multilamellar stacks instead of 2D crystals, and therefore were unsuitable for three-dimensional structural analysis by electron crystallography. Understanding the crystallogenesis of these crystals could reveal the mechanism of formation of multilayers. In the present study, cryo-electron microscopy (cryo-EM) and turbidimetry are used to study the successive steps of reconstitution of Enzyme IICmtl into phospholipid-containing structures and its crystallization under different conditions. Our experimental approach enabled us to distinguish the separate steps of reconstitution and crystallization. The salt concentration especially influenced the nature of the vesicles, either half open unilamellar or aggregated multilamellar, formed during reconstitution of Enzyme IICmtl. The presence of DOPE and DOPC and the temperature influenced the type of lipid structures that were formed during the crystallization phase of Enzyme IICmtl. Cryo-EM showed that protein crystallization is closely associated with the formation of isotropic lipid (cubic) phases. We believe that DOPE is responsible for the formation of these lipid cubic phases, and that crystallization is driven by exclusion of protein from these phases and its concentration into the lamellar phases. This mechanism is inextricably associated with the formation of multilayers

Orthogonal self-assembly of surfactants and hydrogelators:towards new nanostructures

Self-assembly of small molecular components holds great promises as a bottom-up approach for nano-objects, but functionality of the resulting nanostructures can by far not compete with the sophisticated systems provided by nature. Surfactants, for instance, can lead to a great diversity of aggregates and mesophases (micelles, vesicles, cubic phases...), but with a level of complexity and functionality that still remains limited. Just like in nature, to increase the level of complexity in self-assembling systems, a straightforward approach consists in making use of multiple components that can display orthogonal self-assembly –i.e. the independent formation of two supramolecular structures each with their own characteristic within a single system. More precisely, we have associated surfactants with low-molecular weight hydrogelators: these molecules, based on cyclohexyl-tris-amino acid, can also self- assemble in one direction through the establishment of H-bonds, leading to the formation of a fiber network and consequently macroscopic gels. Work on mixing behavior of surfactants and various gelators have shown the independent formation of a fibrillar network with encapsulated spherical micelles, Figure 1. In order to produce even more complex nanostructures, this approach has been extended to worm-like micelles that can lead to viscoelastic gels, due to their entanglement. Interestingly, the formation of interpenetrating networks, with original and tuneable rheological properties, has been evidenced by cryo-TEM [1]. Screening of various gelators with vesicle-forming surfactants also revealed that most combinations display orthogonal self assembly, Figure 1. Vesicles were indeed successfully incorporated in a highly responsive network of fibers, without any significant disturbance of these two supramolecular structures. By mean of fluorescent spectroscopy, the stability of these encapsulated vesicles with respect to fusion and leakage has also been investigated. This last example has been exploited to successfully develop liposomes with an encapsulated self-assembling hydrogel (“gellosomes”) [1]. The high responsive character of the gelator makes it very interesting as a mimic of cytoskeleton and it is expected that this new type of nanostructure might be of great interest in drug delivery.1. A. M. A. Brizard, M. C. A. Stuart, K. J. C. van Bommel, A. Friggeri, M. R. de Jong, J. H. van Esch. Angewandte Chemie int. ed. 47, (2008), 2063.<br/

Orthogonal self-assembly of surfactants and hydrogelators: towards new nanostructures

Self-assembly of small molecular components holds great promises as a bottom-up approach for nano-objects, but functionality of the resulting nanostructures can by far not compete with the sophisticated systems provided by nature. Surfactants, for instance, can lead to a great diversity of aggregates and mesophases (micelles, vesicles, cubic phases...), but with a level of complexity and functionality that still remains limited. Just like in nature, to increase the level of complexity in self-assembling systems, a straightforward approach consists in making use of multiple components that can display orthogonal self-assembly –i.e. the independent formation of two supramolecular structures each with their own characteristic within a single system. More precisely, we have associated surfactants with low-molecular weight hydrogelators: these molecules, based on cyclohexyl-tris-amino acid, can also self- assemble in one direction through the establishment of H-bonds, leading to the formation of a fiber network and consequently macroscopic gels. Work on mixing behavior of surfactants and various gelators have shown the independent formation of a fibrillar network with encapsulated spherical micelles, Figure 1. In order to produce even more complex nanostructures, this approach has been extended to worm-like micelles that can lead to viscoelastic gels, due to their entanglement. Interestingly, the formation of interpenetrating networks, with original and tuneable rheological properties, has been evidenced by cryo-TEM [1]. Screening of various gelators with vesicle-forming surfactants also revealed that most combinations display orthogonal self assembly, Figure 1. Vesicles were indeed successfully incorporated in a highly responsive network of fibers, without any significant disturbance of these two supramolecular structures. By mean of fluorescent spectroscopy, the stability of these encapsulated vesicles with respect to fusion and leakage has also been investigated. This last example has been exploited to successfully develop liposomes with an encapsulated self-assembling hydrogel (“gellosomes”) [1]. The high responsive character of the gelator makes it very interesting as a mimic of cytoskeleton and it is expected that this new type of nanostructure might be of great interest in drug delivery. 1. A. M. A. Brizard, M. C. A. Stuart, K. J. C. van Bommel, A. Friggeri, M. R. de Jong, J. H. van Esch. Angewandte Chemie int. ed. 47, (2008), 2063