Monolayer properties of 1,3-diamidophospholipids

Mechanoresponsive Drug Delivery Systems for Vascular Diseases

Mechanoresponsive drug delivery systems (DDS) have emerged as promising candidates to improve the current effectiveness and lower the side effects typically associated with direct drug administration in the context of vascular diseases. Despite tremendous research efforts to date, designing drug delivery systems able to respond to mechanical stimuli to potentially treat these diseases is still in its infancy. By understanding relevant biological forces emerging in healthy and pathological vascular endothelium, it is believed that better-informed design strategies can be deduced for the fabrication of simple-to-complex macromolecular assemblies capable of sensing mechanical forces. These responsive systems are discussed through insights into essential parameter design (composition, size, shape, and aggregation state) , as well as their functionalization with (macro)molecules that are intrinsically mechanoresponsive (e.g., mechanosensitive ion channels and mechanophores). Mechanical forces, including the pathological shear stress and exogenous stimuli (e.g., ultrasound, magnetic fields), used for the activation of mechanoresponsive DDS are also introduced, followed by in vitro and in vivo experimental models used to investigate and validate such novel therapies. Overall, this review aims to propose a fresh perspective through identified challenges and proposed solutions that could be of benefit for the further development of this exciting field.

Two-Dimensional Titanium Dioxide-Surfactant Photoactive Supramolecular Networks: Synthesis, Properties, and Applications for the Conversion of Light Energy

The desire to harness solar energy to address current global environmental problems led us to investigate two-dimensional (2D) core-shell hybrid photocatalysts in the form of a 2D-TiO₂-surfactant, mainly composed of fatty acids. The bulk products, prepared by two slightly different methods, consist of stacked host-guest hybrid sheets held together by van der Waals forces between alkyl carboxylate moieties, favoring the synergistic conjugation of the photophysical properties of the core and the hydrophobicity of the self-assembled surfactant monolayer of the shell. X-ray diffraction and the vibrational characteristics of the products revealed the influence of synthesis strategies on two types of supramolecular aggregates that differ in the core chemical structure, guest conformers of alkyl surfactant tails and type, and the bilayer and monolayer of the structure of nanocomposites. The singular ability of the TiO₂ core to anchor carboxylate leads to commensurate hybrids, in contrast to both layered clay and layered double-hydroxide-based ion exchangers which have been previously reported, making them potentially interesting for modeling the role of fatty acids and lipids in bio-systems. The optical properties and photocatalytic activity of the products, mainly in composites with smaller bandgap semiconductors, are qualitatively similar to those of nanostructured TiO₂ but improve their photoresponse due to bandgap shifts and the extreme aspect-ratio characteristics of two-dimensional TiO₂ confinement. These results could be seen as a proof-of-concept of the potential of these materials to create custom-designed 2D-TiO₂-surfactant supramolecular photocatalysts.

Recent Developments in Nanomaterial-Based Shear-Sensitive Drug Delivery Systems

Nanomaterial-based drug delivery systems (DDSs) increase the efficacy of various therapeutics, and shear stress has been shown to be a robust modulator of payload release. In the past few decades, a deeper understanding has been gained of the effects of flow in the body and its alteration in pathological microenvironments. More recently, shear-responsive nanomaterial DDSs have been developed. Studies on this subject mainly from the last decade are reviewed here, focusing on innovations of the material design and mechanisms of the shear response. The two most popular shear-controlled drug carriers distinguished by different release mechanisms, that is, shear-deformable nanoparticles (NPs) and shear-dissociated NP aggregates (NPAs), are surveyed. The influence of material structures on their properties such as drug loading, circulation time, and shear sensitivity are discussed. The drug development stages, therapeutic effects, limitations, and potential of these DDSs are further inspected. The reviewed research emphasizes the advantages and significance of nanomaterial-based shear-sensitive DDSs in the field of targeted drug delivery. It is also believed that efforts to rationally design nanomaterial DDSs responsive to shear may prompt a new class of diagnostics and therapeutics for signaling and rectifying pathological flows in the body.

Influence of a single ether bond on assembly, orientation, and miscibility of phosphocholine lipids at the air-water interface

How does a small change in the structure of a phospholipid affect its supramolecular assembly? In aqueous suspensions, the substitution of one ester linkage in DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) by an ether linkage alters its phase behaviour completely. To unravel the effect of replacing a phospholipid's ester linkage by an ether linkage in lipid monolayers, we characterized pure monolayers of the model lipid DPPC and its sn-2 ether analogue PHPC (1-palmitoyl-2-O-hexadecyl-sn-glycero-3-phosphocholine) as well as mixtures of both by measurements of surface pressure-molecular area (π-Amol) isotherms. In addition, we used infrared reflection absorption spectroscopy (IRRAS) to study lipid condensation, lipid chain orientation, headgroup hydration, and lipid miscibility in all samples. Mixed monolayers consisting of DPPC and PHPC were studied further using epifluorescence microscopy. Our results indicate a strong influence of the sn-2 ether linkage on headgroup hydration and ordering effects in the regions of the apolar chains and the headgroups. Both effects could originate from changes in glycerol conformation. Furthermore, we observed a second plateau in the π-Amol isotherms of DPPC/PHPC mixtures and analysis of the mixed π-Amol isotherms reveals a non-ideal mixing behaviour of both lipids which may be caused by conformational differences in their headgroups.

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.

Artificial Phospholipids and Their Vesicles

Phospholipids are at the heart and origin of life on this planet. The possibilities in terms of phospholipid self-assembly and biological functions seem limitless. Nonetheless, nature exploits only a small fraction of the available chemical space of phospholipids. Using chemical synthesis, artificial phospholipid structures become accessible, and the study of their biophysics may reveal unprecedented properties. In this article, the recent advances by our work group in the field of chemical lipidology are summarized. The family of diamidophospholipids is discussed in detail from monolayer characterization to the formation of faceted vesicles, culminating in the template-free self-assembly of phospholipid cubes and the possible applications of vesicle origami in modern personalized medicine.

Against the rules: pressure induced transition from high to reduced order

Envisioning the next generation of drug delivery nanocontainers requires more in-depth information on the fundamental physical forces at play in bilayer membranes. In order to achieve this, we combine chemical synthesis with physical-chemical analytical methods and probe the relationship between a molecular structure and its biophysical properties. With the aim of increasing the number of hydrogen bond donors compared to natural phospholipids, a phospholipid compound bearing urea moieties has been synthesized. The new molecules form interdigitated bilayers in aqueous dispersions and self-assemble at soft interfaces in thin layers with distinctive structural order. At lower temperatures, endothermic and exothermic transitions are observed during compression. The LC1 phase is dominated by an intermolecular hydrogen bond network of the urea moieties leading to a very high chain tilt of 52°. During compression and at higher temperatures, presumably this hydrogen bond network is broken allowing a much lower chain tilt of 35°. The extremely different monolayer thicknesses violate the two-dimensional Clausius-Clapeyron equation.

Physical stimuli-responsive liposomes and polymersomes as drug delivery vehicles based on phase transitions in the membrane

This paper reviews liposomes with crystalline phase and polymersomes exhibiting crystalline and thermotropic liquid crystalline phases in the membrane. Intriguing morphologies of vesicles are described, including spherical, ellipsoidal and faceted vesicles, produced by a large variety of amphiphilic molecules and polymers with nematic phase, smectic phase or crystalline phase. It is highlighted how the phase transitions and the phase grain boundaries could be used ingeniously to destabilize the vesicular structure and to achieve cargo-release under the action of external stimulation. These liposomes and polymersomes are responsive to physical stimuli, such as temperature variation, shear stress, light illumination, and magnetic and electric fields. These stimuli-responsive properties make them promising candidates as new smart drug delivery systems.

Synthesis and Biophysical Characterization of an Odd-Numbered 1,3-Diamidophospholipid

Nanomedicine suffers from low drug delivery efficiencies. Mechanoresponsive vesicles could provide an alternative way to release active compounds triggered by the basic physics of the human body. 1,3-Diamidophospholipids with C16 tails proved to be an effective building block for mechanoresponsive vesicles, but their low main phase transition temperature prevents an effective application in humans. As the main phase transition temperature of a membrane depends on the fatty acyl chain length, we synthesized a C17 homologue of a 1,3-diamidophospholipid: Rad-PC-Rad. The elevated main phase transition temperature of Rad-PC-Rad allows mechanoresponsive drug delivery at body temperature. Herein, we report the biophysical properties of Rad-PC-Rad monolayer and bilayer membranes. Rad-PC-Rad is an ideal candidate for advancing the concept of physically triggered drug release.

Vesicle Origami: Cuboid Phospholipid Vesicles Formed by Template-Free Self-Assembly

Phospholipid liposomes are archetypical self-assembled structures. To minimize the surface tension, the vesicles typically are spherical. Deciphering the bilayer code, the basic physical interactions between phospholipids would allow these molecules to be utilized as building blocks for novel, non-spherical structures. A 1,2-diamidophospholipid is presented that self-assembles into a cuboid structure. Owing to intermolecular hydrogen bonding, the bilayer membranes form an exceptionally tight subgel packing, leading to a maximization of flat structural elements and a minimization of any edges. These conditions are optimized in the geometrical structure of a cube. Surprisingly, the lateral surface pressure in the membrane is only one third of the value typically assumed for a bilayer membrane, questioning a long-standing rule-of-thumb.

Structures of malonic acid diamide/phospholipid composites and their lipoplexes

As a continuation of previous work, the self-assembly process of cationic lipid formulations in the presence and absence of DNA was investigated with respect to binary lipid mixtures suitable as polynucleotide carrier systems. The lipid blends consist of one malonic-acid-based cationic lipid with a varying alkyl chain pattern, either N-{6-amino-1-[N-(9Z)-octadec-9-enylamino]-1-oxohexan-(2S)-2-yl}-N'-{2-[N,N-bis(2-aminoethyl)amino]ethyl}-2-hexadecylpropandiamide () or N-[6-amino-1-oxo-1-(N-tetradecylamino)hexan-(2S)-2-yl]-N'-{2-[N,N-bis(2-aminoethyl)amino]ethyl}-2-hexadecylpropandiamide (), and one neutral co-lipid, either 1,2-di-[(9Z)-octadec-9-enoyl]-sn-glycero-3-phosphocholine (DOPE) or 1,2-di-(hexadecanoyl)-sn-glycero-3-phosphocholine (DPPC). Although the cationic lipids exhibit only slight differences in their structure, the DNA transfer efficiency varies drastically. Therefore, self-assembly was studied in 3D systems by small- and wide-angle X-ray scattering (SAXS and WAXS) and transmission electron microscopy (TEM) as well as in 2D systems by infrared reflection-absorption spectroscopy (IRRAS) on Langmuir films. The investigated lipid mixtures show quite different self-assembly in the absence of DNA, with varying structures from vesicles (/DOPE; /DOPE) and tubes (/DOPE) to discoid structures (/DPPC; /DPPC). Twisted ribbons and sheets, which were stabilized due to hydrogen-bond networks, were found in all investigated lipid mixtures in the absence of DNA. The addition of DNA leads to the formation of lamellar lipoplexes for all the investigated lipid compositions. The lipoplexes differ in crucial parameters, such as the lamellar repeat distance and the spacing between the DNA strands, indicating differences in the binding strength between DNA and the lipid composition. The formation of associates with an ideal charge density might emerge as a key parameter for efficient DNA transfer. Furthermore, the structures observed for the different lipid compositions in the absence of DNA prepare the way for other applications besides gene therapy.

Vesicle Origami and the Influence of Cholesterol on Lipid Packing

The artificial phospholipid Pad-PC-Pad was analyzed in 2D (monolayers at the air/water interface) and 3D (aqueous lipid dispersions) systems. In the gel phase, the two leaflets of a Pad-PC-Pad bilayer interdigitate completely, and the hydrophobic bilayer region has a thickness comparable to the length of a single phospholipid acyl chain. This leads to a stiff membrane with no spontaneous curvature. Forced into a vesicular structure, Pad-PC-Pad has faceted geometry, and in its extreme form, tetrahedral vesicles were found as predicted a decade ago. Above the main transition temperature, a noninterdigitated Lα phase with fluid chains has been observed. The addition of cholesterol leads to a slight decrease of the main transition temperature and a gradual decrease in the transition enthalpy until the transition vanishes at 40 mol % cholesterol in the mixture. Additionally, cholesterol pulls the chains apart, and a noninterdigitated gel phase is observed. In monolayers, cholesterol has an ordering effect on liquid-expanded phases and disorders condensed phases. The wavenumbers of the methylene stretching vibration indicate the formation of a liquid-ordered phase in mixtures with 40 mol % cholesterol.

Impact of Structural Differences in Galactocerebrosides on the Behavior of 2D Monolayers

The molecular interactions of three biologically important galactocerebrosides have been studied in monolayers formed at the soft air/water interface as 2D model membranes. Highly surface-sensitive techniques as GIXD (grazing incidence X-ray diffraction), IRRAS (infrared reflection-absorption spectroscopy), and BAM (Brewster angle microscopy) have been used. The study reveals that small differences in the chemical structure have a relevant impact on the physical-chemical properties and intermolecular interactions. The presence of a 2-d-hydroxyl group in the fatty acid favored for GalCer C24:0 (2-OH) monolayers a higher hydration state of the headgroup at low lateral pressures (<25 mN/m) and a higher condensation effect above 30 mN/m. An opposite behavior was recorded for GalCer C24:0 and GalCer C24:1, for which the intermolecular interactions are defined by the weakly hydrated but strong H-bonded interconnected head groups. Additionally, the 15-cis-double bond in the fatty acid chain (nervonic acid) of GalCer C24:1 stabilized the LE phase but did not disturb the packing parameters of the LC phase as compared with the saturated compound GalCer C24:0.

Bilayer properties of 1,3-diamidophospholipids

A series of 1,3-diamido phosphocholines was synthesized, and their potential to form stable bilayers was investigated. Large and giant unilamellar vesicles produced from these new lipids form a wide variety of faceted liposomes. Factors such as cooling rates and the careful choice of the liposome preparation method influence the formation of facets. Interdigitation was hypothesized as a main factor for the stabilization of facets and effectively monitored by small-angle X-ray scattering measurements.

Grazing incidence X-ray diffraction studies of condensed double-chain phospholipid monolayers formed at the soft air/water interface

The use of highly brilliant synchrotron light sources in the middle of the 1980s for X-ray diffraction has revolutionized the research of condensed monolayers. Since then, monolayers gained popularity as convenient quasi two-dimensional model systems widely used in biophysics and material science. This review focuses on structures observed in one-component phospholipid monolayers used as simplified two-dimensional models of biological membranes. In a monolayer system the phase transitions can be easily triggered at constant temperature by increasing the packing density of the lipids by compression. Simultaneously the monolayer structure changes are followed in situ by grazing incidence X-ray diffraction. Competing interactions between the different parts of the molecule are responsible for the different monolayer structures. These forces can be modified by chemical variations of the hydrophobic chain region, of the hydrophilic head group region or of the interfacial region between chains and head groups. Modifications of monolayer structures triggered by changes of the chemical structure of double-chain phospholipids are highlighted in this paper.

Exit-strategies - smart ways to release phospholipid vesicle cargo

This highlight describes recent trends in fundamental phospholipid research towards possible future drug delivery technology. In particular it focuses on synthetic phospholipids and their vesicular constructs and describes selected "smart" ways to release cargo from liposomes. Various chemical and physical release triggers are discussed such as temperature changes, application of ultrasound, enzyme degradation, changes in pH, redox reactions, photochemical reactions, as well as the effects of shear stress on vesicles.