Lyotropic Liquid Crystalline Cubic Phases as Versatile Host Matrices for Membrane-Bound Enzymes

Expanding the Enzymatic Polymerization Landscape by Lipid Mesophase Soft Nanoconfinement

Soft nanoconfinement can increase chemical reactivity in nature and has therefore led to considerable interest in transferring this universal feature to artificial biological systems. However, little is known about the underlying principles of soft nanoconfinement responsible for the enhancement of biochemical reactions. Herein we demonstrate how enzymatic polymerization can be expanded, optimized, and engineered when carried out under soft nanoconfinement mediated by lipidic mesophases. By systematically varying the water content in the mesophase and thus the diameter of the confined water nanochannels, we show higher efficiency, turnover rate, and degrees of polymerization as compared to the bulk aqueous solution, all controlled by soft nanoconfinement effects. Furthermore, we exploit the unique properties of unfreezing soft nanoconfined water to perform the first enzymatic polymerization at -20 °C in pure aqueous media. These results underpin lipidic mesophases as a versatile host system for chemical reactions and promote them as an original and unexplored platform for enzymatic polymerization.

Supramolecular design of CO2-responsive lipid nanomaterials

HYPOTHESIS

Stimuli-responsive materials can innovate in various fields, including food and pharmaceutical sciences. Their response to a specific stimulus can be utilized to release loaded bioactive molecules or sense their presence. The biocompatibility and abundance of CO₂ in the environment make it an exciting stimulus for such applications. We hypothesize the formation of CO₂-responsive self-assemblies of oleyl-amidine in water. Their integration into glycerol-monooleate-based (GMO) dispersions is further thought to form CO₂-switchable liquid crystalline nanoparticles. The switch from an non-charged acetamidine surfactant to its cationic amidinium form triggers curvature changes that ultimately induces phase transitions.

EXPERIMENTS

The CO₂-switchable lipid (E)-N,N-dimethyl-N-((Z)-octadec-9-en1-yl)acetimidamide (OAm) is synthesized and formulated into emulsions and dispersed liquid crystals with GMO. The supramolecular structure and its response to CO₂ are characterized using small angle X-ray scattering, dynamic light scattering, ζ-potential measurements and cryogenic transmission electron microscopy.

FINDINGS

Depending on the composition, OAm is discovered to self-assemble into a variety of CO₂-responsive lyotropic liquid crystalline structures that can be dispersed in excess water. CO₂-triggered colloidal transformations from unstructured OAm-in-water emulsions to direct micelles; dispersed inverse hexagonal phase to direct rod-like micelles, and sponge phase to vesicles are discovered. These structural changes are driven by the reaction of OAm's amidine headgroup with CO₂. The results provide a fundamental understanding of CO₂-triggered functional nanomaterials and may guide their future design into delivery platforms and biosensors.

Lipid-based liquid crystalline materials in electrochemical sensing and nanocarrier technology

Some biologically active substances are unstable and poorly soluble in aqueous media, at the same time exhibiting low bioavailability. The incorporation of these biologically active compounds into the structure of a lipid-based lyotropic liquid crystalline phase or nanoparticles can increase or improve their stability and transport properties, subsequent bioavailability, and applicability in general. The aim of this short overview is (1) to clarify the principle of self-assembly of lipidic amphiphilic molecules in an aqueous environment and (2) to present lipidic bicontinuous cubic and hexagonal phases and their current biosensing (with a focus on electrochemical protocols) and biomedical applications.

The EcCLC antiporter embedded in lipidic liquid crystalline films - molecular dynamics simulations and electrochemical methods

Lipidic-liquid crystalline nanostructures (lipidic cubic phases), which are biomimetic and stable in an excess of water, were used as a convenient environment to investigate the transport properties of the membrane antiporter E. coli CLC-1 (EcCLC). The chloride ion transfer by EcCLC was studied by all-atom molecular dynamics simulations combined with electrochemical methods at pH 7 and pH 5. The cubic phase film was used as the membrane between the chloride donor and receiving compartments and it was placed on the glassy carbon electrode and immersed in the chloride solution. Structural characterization of lipidic mesoscopic systems with and without the incorporation of EcCLC was performed using small-angle X-ray scattering. The EcCLC transported chloride ions more efficiently at more acidic pH, and the resistance of the film decreased at lower pH. 4,4-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) employed as an inhibitor of the protein was shown to decrease the transport efficiency upon hydrolysis to DADS at both pH 7 and pH 5. The molecular dynamics simulations, performed for the first time in lipidic cubic phases for EcCLC, allowed studying the collective movements of chloride ions which can help in elucidating the mechanism of transporting the ions by the EcCLC antiporter. The protein modified lipidic cubic phase film is a convenient and simple system for screening potential inhibitors of integral membrane proteins, as demonstrated by the example of the EcCLC antiporter. The use of lipidic cubic phases may also be important for the further development of new electrochemical sensors for membrane proteins and enzyme electrodes.

A pressure-jump study on the interaction of osmolytes and crowders with cubic monoolein structures

Many vital processes that take place in biological cells involve remodeling of lipid membranes. These processes take place in a milieu that is packed with various solutes, ranging from ions and small organic osmolytes to proteins and other macromolecules, occupying about 30% of the available volume. In this work, we investigated how molecular crowding, simulated with the polymer polyethylene glycol (PEG), and the osmolytes urea and trimethylamine-N-oxide (TMAO) affect the equilibration of cubic monoolein structures after a phase transition from a lamellar state induced by an abrupt pressure reduction. In absence of additives, swollen cubic crystallites form after the transition, releasing excess water over several hours. This process is reflected in a decreasing lattice constant and was monitored with small angle X-ray scattering. We found that the osmotic pressure exerted by PEG and TMAO, which are displaced from narrow inter-bilayer spaces, accelerates the equilibration. When the radius of gyration of the added PEG was smaller than the radius of the water channels of the cubic phase, the effect became more pronounced with increasing molecular weight of the polymers. As the release of hydration water from the cubic structures is accompanied by an increasing membrane curvature and a reduction of the interface between lipids and aqueous phase, urea, which has a slight affinity to reside near membrane surfaces, stabilized the swollen crystallites and slowed down the equilibration dynamics. Our results support the view that cellular solutes are important contributors to dynamic membrane processes, as they can accelerate dehydration of inter-bilayer spaces and promote or counteract membrane curvature.

Methods of Measuring Mitochondrial Potassium Channels: A Critical Assessment

In this paper, the techniques used to study the function of mitochondrial potassium channels are critically reviewed. The majority of these techniques have been known for many years as a result of research on plasma membrane ion channels. Hence, in this review, we focus on the critical evaluation of techniques used in the studies of mitochondrial potassium channels, describing their advantages and limitations. Functional analysis of mitochondrial potassium channels in comparison to that of plasmalemmal channels presents additional experimental challenges. The reliability of functional studies of mitochondrial potassium channels is often affected by the need to isolate mitochondria and by functional properties of mitochondria such as respiration, metabolic activity, swelling capacity, or high electrical potential. Three types of techniques are critically evaluated: electrophysiological techniques, potassium flux measurements, and biochemical techniques related to potassium flux measurements. Finally, new possible approaches to the study of the function of mitochondrial potassium channels are presented. We hope that this review will assist researchers in selecting reliable methods for studying, e.g., the effects of drugs on mitochondrial potassium channel function. Additionally, this review should aid in the critical evaluation of the results reported in various articles on mitochondrial potassium channels.

Design of Swollen Lipidic Cubic Phase to Increase Transcutaneous Penetration of Biomacromolecules

Lipidic cubic phase (LCP) is a self-assembled system composed of lipids with interpenetrated aqueous channels, and its potential in drug delivery systems has been investigated. Although LCP was shown to improve transcutaneous penetration of hydrophilic molecules of up to 1203 Da so far, the transcutaneous delivery of larger molecules such as proteins has not been achieved. This is likely because proteins are usually larger than the aqueous channels of LCP (∼37.2 Å in diameter), which limits the molecular diffusion in LCP. In this report, we overcome this issue by adding N-octyl-β-d-glucopyranoside to glyceryl monooleate-water-based LCP to give swollen LCP (SLCP), which has larger aqueous channel diameters (∼65.6 Å). First, we systemically evaluated the effect of swelling on drug diffusion in LCP/SLCP. The release kinetics of various peptides and proteins whose sizes ranged from 9.14 to 55.28 Å in diameter were evaluated, and the diffusion coefficients (D) were calculated by the Fickian diffusion model. As expected, all peptides and proteins diffused faster in SLCP than in LCP. A more detailed analysis revealed a negative linear relationship between log D and the ratio of the radius of gyration of the proteins to the aqueous channel radius, indicating that swelling of a cubic nanostructure is an effective strategy to enhance D. Next, the skin penetration of proteins encapsulated in LCP and SLCP was evaluated. The skin penetration of ovalbumin (42.9 kDa), for example, was enhanced by SLCP but not by LCP, and a positive correlation between D and the amount of skin penetration was found. Collectively, this study provides an effective measure for designing LCP systems that enhance transcutaneous penetration of biomacromolecules.

Cryogenic activity and stability of benzaldehyde lyase enzyme in lipidic mesophases-nanoconfined water

Phytantriol-based lipidic mesophases (LMs) are introduced as a platform for cryoenzymology, which relies on the presence of liquid water in LMs at subzero temperatures. After incorporation into LMs, the model enzyme Benzaldehyde lyase (BAL) shows high cryogenic stability and activity. In contrast, BAL in bulk solution undergoes significant secondary structural transitions caused by low temperatures (cold denaturation), demonstrating the potential of this approach to enable in meso cryoenzymology.

Lipid-based mesophases as matrices for nanoscale reactions

Lipidic mesophases are versatile bioorganic materials that have been effectively employed as nanoscale matrices for membrane protein crystallization, drug delivery and as food emulsifiers over the last 30 years. In this review, the focus is upon studies that have employed non-lamellar lipid mesophases as matrices for organic, inorganic and enzymatic reactions. The ability of lipidic mesophases to incorporate hydrophilic, amphiphilic and hydrophobic molecules, together with the high interfacial area of the lipidic cubic and inverse hexagonal phases has been exploited in heterogeneous catalysis as well as for enzyme immobilization. The unique nanostructure of these mesophases is the driving force behind their ability to act as templates for synthesis, resulting in the creation of highly ordered polymeric and inorganic materials with complex geometries.

Preparation and characterization of β-casein stabilized lipopeptide lyotropic liquid crystal nanoparticles for delivery of doxorubicin

A kind of lyotropic liquid crystal nanoparticle (LLC NPs) has been designed and prepared. LLC NPs are dSMO/OA/β-casein/water quaternary systems, and their cubic or hexagonal microstructures have been characterized by cryogenic transmission electron microscopy (cryo-TEM) and small angle X-ray scattering (SAXS). The phase transition of LLC NPs takes place with ratio and pH adjustments. The properties, such as cytotoxicity, stability, drug encapsulation and release ability, have been investigated with MTT assay, cryo-TEM and UV-Vis spectroscopy. The results showed that LLC NPs were nontoxic to cells and stable to enzymatic degradation. Hydrophilic drug doxorubicin hydrochloride (DOX·HCl) could be effectively encapsulated in LLC NPs and its release rate could be regulated by pH. It was concluded that LLC NPs are potential nanocarriers in nanomedicine technologies. We hope that this work provides new guidelines for the rational design of LLC NP systems with lipopeptides for biomedical applications.

Supramolecular chirality and crystallization from biocatalytic self-assembly in lipidic cubic mesophases

Biocatalytic self-assembly in a nanoconfined environment is widely used in nature to construct complex structures that endow special characteristics to life. There is tremendous interest in mimicking such bottom-up processes to fabricate functional materials. In this study, we have investigated a novel biomimetic scaffold based on lipidic cubic mesophases (LCMs), which provide a special nanoconfined environment for biocatalytic self-assembly and subsequent formation of organic crystals. (R)-Benzoin generated in situ from benzaldehyde in a reaction catalyzed by the enzyme benzaldehyde lyase (BAL) exhibits - when confined within LCMs - enhanced chirality compared to (R)-benzoin in solution or (R)-benzoin-doped LCMs. We infer that a metastable state is formed under kinetic control that displays enhanced supramolecular chirality. As they age, these metastable structures can further grow into thermodynamically stable crystals. The biomimetic, nanoconfined environment provided by the LCMs plays a key role in the development of supramolecular chirality and subsequent crystallization.

Cubosomes: The Next Generation of Smart Lipid Nanoparticles?

Cubosomes are highly stable nanoparticles formed from the lipid cubic phase and stabilized by a polymer based outer corona. Bicontinuous lipid cubic phases consist of a single lipid bilayer that forms a continuous periodic membrane lattice structure with pores formed by two interwoven water channels. Cubosome composition can be tuned to engineer pore sizes or include bioactive lipids, the polymer outer corona can be used for targeting and they are highly stable under physiological conditions. Compared to liposomes, the structure provides a significantly higher membrane surface area for loading of membrane proteins and small drug molecules. Owing to recent advances, they can be engineered in vitro in both bulk and nanoparticle formats with applications including drug delivery, membrane bioreactors, artificial cells, and biosensors. This review outlines recent advances in cubosome technology enabling their application and provides guidelines for the rational design of new systems for biomedical applications.

Lipidic Mesophase-Embedded Palladium Nanoparticles: Synthesis and Tunable Catalysts in Suzuki-Miyaura Cross-Coupling Reactions

Lipidic cubic phases (LCPs) can reduce Pd²⁺ salts to palladium nanoparticles (PdNPs) of ∼5 nm size in their confined water channels under mild conditions. The resulting PdNP-containing LCPs were used as nanoreactor scaffolds to catalyze Suzuki-Miyaura cross-coupling reactions in the aqueous channels of the mesophase. To turn on catalysis, PdNP-containing LCPs were activated by swelling the aqueous channels of the lipidic framework, thereby enabling diffusion of the water-soluble substrates to the catalysts. The mesophases play a threefold role: they act as reducing agents for Pd²⁺, as limiting templates for their growth, and as support. The system was characterized and investigated by small-angle X-ray scattering (SAXS), cryo-transmission electron microscopy, dynamic light scattering, and nuclear magnetic resonance. Bulk LCPs and three dispersed palladium/lipid hybrid nanoparticle types were applied in the catalysis. The latter-liposomes, hexosomes, and cubosomes-can be obtained by design through combination of lipids and additives. The Suzuki-Miyaura cross-coupling of 5-iodo-2'-deoxyuridine and phenylboronic acid was used as a model reaction to study these systems. Bulk Pd-LCPs deliver the Suzuki-Miyaura product in 24 h in conversions up to 98% at room temperature, whereas with palladium/lipid dispersions at 40 °C, 68% of the starting material was transformed to the product after 72 h.

Overview of Biochemical Assays in Lipidic Cubic Phase

The development of novel biochemical methods to efficiently characterize membrane protein (MP) properties in lipidic cubic phase (LCP) is important for studying complicated MPs and their multimeric complexes. Here, we summarize recent LCP-related assays and provide an outlook on their applications in structure and function studies of MPs.

Lipidic Mesophases as Novel Nanoreactor Scaffolds for Organocatalysts: Heterogeneously Catalyzed Asymmetric Aldol Reactions in Confined Water

The unique molecular architecture of lipidic cubic phases (LCPs) and their cubosome dispersions comprise a well-defined, curved bilayer that spans the entire three-dimensional (3-D) material space, encompassing a network of two periodic, curved, and nonintersecting 3-D aqueous channels. The ensuing large lipid/water interfacial area makes these biomaterials an interesting matrix for the lateral immobilization of organocatalysts to catalyze organic reactions in confined water. Herein, we report for the first time the design, synthesis, assembly, and characterization of catalytically active LCPs and cubosomes and demonstrate their applicability as self-assembled, biomimetic, and recyclable nanoreactor scaffolds. Small-angle X-ray scattering, cryo-transmission electron microscopy, and dynamic light scattering were applied for the characterization of the mesophases. These mesophases can be recycled and enable efficient catalytic activity as well as modulation of the diastereo- and enantioselectivity for the aldol reaction of several benzaldehyde derivatives and cyclohexanone in water.

Nanostructured Lipid-based Films for Substrate Mediated Applications in Biotechnology

Amphiphilic in nature, lipids spontaneously self-assemble into a range of nanostructures in the presence of water. Among lipid self-assembled structures, liposomes and supported lipid bilayers have long held scientific interest for their main applications in drug delivery and plasma membrane models, respectively. In contrast, lipid-based multi-layered membranes on solid supports only recently begun drawing scientists' attention. New studies on lipid films show that the stacking of multiple bilayers on a solid support yields interestingly complex features to these systems. Namely, multiple layers exhibit cooperative structural and dynamic behavior. In addition, the materials enable compartmentalization, templating, and enhanced release of several molecules of interest. Importantly, supported lipid phases exhibit long-range periodic nano-scale order and orientation that is tunable in response to a changing environment. Herein, we summarize current and pertinent understanding of lipid-based film research focusing on how unique structural characteristics enable the emergence of new applications in biotechnology including label-free biosensors, macroscale drug delivery, and substrate-mediated gene delivery. Our very recent contributions to lipid-based films, focusing on the structural characterization at the meso, nano, and molecular-scale, using Small-Angle X-ray Scattering, Atomic Force Microscopy, Photothermal Induced Resonance, and Solid-State NMR will be also highlighted.

Formation of highly ordered liquid crystalline coatings – an in situ GISAXS study

In situ GISAXS and AFM reveal the formation of highly geometrically organized glycerol monooleate based liquid crystalline films on silicon wafers.

Enzyme-Triggered Dissociation of a FRET-Based Protein Biosensor Monitored by Synchrotron SAXS

Protein biosensors are widely used for the monitoring of metabolite concentration and enzymatic activities inside living cells and in in vitro applications. Neutrophil elastase (NE) is a serine protease of relevance in inflammatory diseases whose activity can lead to pathological conditions if unregulated. This study focuses on the structural characterization of a biosensor for NE activity based on Förster resonance energy transfer (FRET). The cleavage by NE results in dissociation of the FRET fluorescent protein pair and alteration of the fluorescent emission spectrum. We have used small angle x-ray scattering at a high intensity synchrotron source, combined with model-free analysis of the scattering data, to demonstrate the structure of the biosensor and the effect of its exposure to NE on size and shape. These investigations, together with biochemical studies, established the nanostructure-activity relationship that may contribute to the detailed understanding of the FRET-based biosensor and guide the rational design of new biosensor constructs.

Direct demonstration of lipid phosphorylation in the lipid bilayer of the biomimetic bicontinuous cubic phase using the confined enzyme lipid A phosphoethanolamine transferase

Retention of amphiphilic protein activity within the lipid bilayer membrane of the nanostructured biomimetic bicontinuous cubic phase is crucial for applications utilizing these hybrid protein-lipid self-assembly materials, such as in meso membrane protein crystallization and drug delivery. Previous work, mainly on soluble and membrane-associated enzymes, has shown that enzyme activity may be modified when immobilized, including membrane bound enzymes. The effect on activity may be even greater for amphiphilic enzymes with a large hydrophilic domain, such as the Neisserial enzyme lipid A phosphoethanolamine transferase (EptA). Encapsulation within the biomimetic but non-endogenous lipid bilayer membrane environment may modify the enzyme conformation, while confinement of the large hydrophilic domain with the nanoscale water channels of a continuous lipid bilayer structure may prevent full function of this enzyme. Herein we show that NmEptA remains active despite encapsulation within a nanostructured bicontinuous cubic phase. Full transfer of the phosphoethanolamine (PEA) group from a 1,2-dioleoyl-glycero-phosphoethanolamine (DOPE) doped lipid to monoolein (MO), which makes up the bicontinuous cubic phase, is shown. The reaction was found to be non-specific to the alkyl chain identity. The observed rate of enzyme activity is similar to other membrane bound enzymes, with complete transfer of the PEA group occurring in vitro, under the conditions studied, over a 24 hour timescale.