Towards redox active liquid crystalline phases of lipids: a monoolein/water system with entrapped derivatives of ferrocene

Development of Redox-Active Lyotropic Lipid Cubic Phases for Biosensing Platforms

Enzyme-based electrochemical biosensors play an important role in point-of-care diagnostics for personalized medicine. For such devices, lipid cubic phases (LCP) represent an attractive method to immobilize enzymes onto conductive surfaces with no need for chemical linking. However, research has been held back by the lack of effective strategies to stably co-immobilize enzymes with a redox shuttle that enhances the electrical connection between the enzyme redox center and the electrode. In this study, we show that a monoolein (MO) LCP system doped with an amphiphilic redox mediator (ferrocenylmethyl)dodecyldimethylammonium bromide (Fc12) can be used for enzyme immobilization to generate an effective biosensing platform. Small-angle X-ray scattering (SAXS) showed that MO LCP can incorporate Fc12 while maintaining the Pn3m symmetry morphology. Cyclic voltammograms of Fc12/MO showed quasi-reversible behavior, which implied that Fc12 was able to freely diffuse in the lipid membrane of LCP with a diffusion coefficient of 1.9 ± 0.2 × 10-8 cm2 s-1 at room temperature. Glucose oxidase (GOx) was then chosen as a model enzyme and incorporated into 0.2%Fc12/MO to evaluate the activity of the platform. GOx hosted in 0.2%Fc12/MO followed Michaelis-Menten kinetics toward glucose with a KM and Imax of 8.9 ± 0.5 mM and 1.4 ± 0.2 μA, respectively, and a linearity range of 2-17 mM glucose. Our results therefore demonstrate that GOx immobilized onto 0.2% Fc12/MO is a suitable platform for the electrochemical detection of glucose.

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.

Lyotropic liquid crystal engineering moving beyond binary compositional space - ordered nanostructured amphiphile self-assembly materials by design

Ordered amphiphile self-assembly materials with a tunable three-dimensional (3D) nanostructure are of fundamental interest, and crucial for progressing several biological and biomedical applications, including in meso membrane protein crystallization, as drug and medical contrast agent delivery vehicles, and as biosensors and biofuel cells. In binary systems consisting of an amphiphile and a solvent, the ability to tune the 3D cubic phase nanostructure, lipid bilayer properties and the lipid mesophase is limited. A move beyond the binary compositional space is therefore required for efficient engineering of the required material properties. In this critical review, the phase transitions upon encapsulation of more than 130 amphiphilic and soluble additives into the bicontinuous lipidic cubic phase under excess hydration are summarized. The data are interpreted using geometric considerations, interfacial curvature, electrostatic interactions, partition coefficients and miscibility of the alkyl chains. The obtained lyotropic liquid crystal engineering design rules can be used to enhance the formulation of self-assembly materials and provides a large library of these materials for use in biomedical applications (242 references).

Gel-Phase-like Ordered Membrane Properties Observed in Dispersed Oleic Acid/1-Oleoylglycerol Self-Assemblies: Systematic Characterization Using Raman Spectroscopy and a Laurdan Fluorescent Probe

Aqueous dispersions of oleic acid (OA) and those modified with 1-oleoylglycerol (monoolein, MO) form various kinds of self-assembled structures: micelles, vesicles, oil-in-water (O/W) emulsions, hexagonal phases, and dispersed cubic phases. Conventionally, these self-assembled structures have been characterized using cryogenic transmission electron microscopy or X-ray diffraction spectroscopy. However, these methodologies require specialized treatment before they can be used, which may lead to the self-assemblies not adopting their true equilibrium state. Herein, we systematically characterized the self-assemblies composed of OA and MO in aqueous solution using Raman spectroscopy and fluorescent probe 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan). The OA/MO dispersions at pH 5.0 showed increased chain packing in comparison to the OA micelle at pH 11 or OA vesicle at pH 9.0, which were characterized by the intensity ratio of the Raman peaks at 2850 and 2890 cm^-1, R = I₂₈₉₀/I₂₈₅₀. In the Laurdan fluorescence measurements, the obtained spectra were deconvoluted to two peak fractions (A₁: λ_em= 490 nm; A₂: λ_em = 440 nm), and the peak area ratio, A₁/(A₁ + A₂), was defined as the membrane hydrophilicity Ø_m. The Ø_m value of the OA/MO dispersion at pH 5.0 was similar to that of the OA O/W emulsion, indicating that the membrane surfaces of these self-assemblies were relatively dehydrated compared to the OA micelle or OA vesicle. To categorize the type of self-assembly dispersion, a Cartesian diagram plot was systematically drawn: R on the x axis and Ø_m on the y axis, with the cross point at x = 1, y = 0.5. By comparing the membrane properties of the OA-based micelles, O/W emulsions, and dispersed cubic phases, we determined that the OA/MO dispersion at pH 5.0 possessed higher chain packing (R > 1) and a dehydrated membrane surface (Ø_m < 0.5), which is similar to that of the ordered membranes in gel phases. This characterization method can be useful in evaluating the ordered membrane properties in dispersed self-assemblies in aqueous media.

Active Gating, Molecular Pumping, and Turnover Determination in Biomimetic Lipidic Cubic Mesophases with Reconstituted Membrane Proteins

Understanding the mechanisms controlling molecular transport in bioinspired materials is a central topic in many branches of nanotechnology. In this work, we show that biomolecules of fundamental importance in biological processes, such as glucose, can be transported in an active, controlled, and selective manner across macroscopic lipidic cubic mesophases, by correctly reconstituting within them their corresponding membrane protein transporters, such as Staphylococcus epidermidis (GlcP_Se). Importantly, by duly exploiting the symporter properties of GlcP_Se of coupled glucose/H⁺ transport, the diffusion of glucose can further be tuned by independent physiological stimuli, such as parallel or antiparallel pH gradients, offering an important model to study molecular exchange processes in cellular machinery. We finally show that by measuring the transport properties of the lipidic mesophases with and without the GlcP_Se membrane protein reconstituted within, it becomes possible to determine its intrinsic conductance. We generalize these findings to other membrane proteins from the antiporters family, such as the bacterial ClC exchanger from Escherichia coli (EcClC), providing a robust method for evaluating the turnover rate of the membrane proteins in general.

Cubosomes: remarkable drug delivery potential

Cubosomes are nanostructured liquid crystalline particles, made of certain amphiphilic lipids in definite proportions, known as biocompatible carriers in drug delivery. Cubosomes comprise curved bicontinuous lipid bilayers that are organized in three dimensions as honeycombed structures and divided into two internal aqueous channels that can be exploited by various bioactive ingredients, such as chemical drugs, peptides and proteins. Owing to unique properties such as thermodynamic stability, bioadhesion, the ability of encapsulating hydrophilic, hydrophobic and amphiphilic substances, and the potential for controlled release through functionalization, cubosomes are regarded as promising vehicles for different routes of administration. Based on the most recent reports, this review introduces cubosomes focusing on their structure, preparation methods, mechanism of release and potential routes of administration.

Competitive displacement of sodium caseinate by low-molecular-weight emulsifiers and the effects on emulsion texture and rheology

Low-molecular-weight (LMW) emulsifiers are used to promote controlled destabilization in many dairy-type emulsions in order to obtain stable foams in whippable products. The relation between fat globule aggregation induced by three LMW emulsifiers, lactic acid ester of monoglyceride (LACTEM), saturated monoglyceride (GMS), and unsaturated monoglyceride (GMU) and their effect on interfacial protein displacement was investigated. It was found that protein displacement by LMW emulsifiers was not necessary for fat globule aggregation in emulsions, and conversely fat globule aggregation was not necessarily accompanied by protein displacement. The three LMW emulsifiers had very different effects on emulsions. LACTEM induced shear instability of emulsions, which was accompanied by protein displacement. High stability was characteristic for emulsions with GMS where protein was displaced from the interface. Emulsions containing GMU were semisolid, but only low concentrations of protein were detected in the separated serum phase. The effects of LACTEM, GMS, and GMU may be explained by three different mechanisms involving formation of interfacial α-gel, pickering stabilization and increased exposure of bound casein to the water phase. The latter may facilitate partial coalescence. Stabilizing hydrocolloids did not have any effect on the LMW emulsifiers' ability to induce protein displacement.

Aqueous self-assembly of phytantriol in ternary systems: Effect of monoolein, distearoylphosphatidylglycerol and three water-miscible solvents

The aqueous phase behavior of phytantriol (PT) in mixtures of monoolein (MO), distearoylphosphatidylglycerol (DSPG), propylene glycol (PG), polyethylene glycol 400 (PEG 400) and 2-methyl-2,4-pentanediol (MPD) was investigated by visual inspection, polarized light microscopy and small angle X-ray diffraction at room temperature. The phase diagrams of PT and MO in water are qualitatively very similar and PT/MO mixtures in excess water form one cubic phase of space group Pn3m irrespective of mixing ratio. The addition of the charged membrane lipid DSPG to the PT system gives rise to a considerable water swelling of the cubic phases as well as the occurrence of a cubic phase of space group Im3m. Whereas all three solvents studied give rise to a sponge (L3) phase in the MO-water system, this phase was only found when MPD was added to the PT-water system. The results are discussed with respect to the chemical differences between PT and MO.

Diamond cubic phase of monoolein and water as an amphiphilic matrix for electrophoresis of oligonucleotides

We used a cubic liquid crystal formed by the nonionic monoglyceride monoolein and water as a porous matrix for the electrophoresis of oligonucleotides. The diamond cubic phase is thermodynamically stable when in contact with a water-rich phase, which we exploit to run the electrophoresis in the useful submarine mode. Oligonucleotides are separated according to size and secondary structure by migration through the space-filling aqueous nanometer pores of the regular liquid crystal, but the comparatively slow migration means the cubic phase will not be a replacement for the conventional DNA gels. However, our demonstration that the cubic phase can be used in submarine electrophoresis opens up the possibility for a new matrix for electrophoresis of amphiphilic molecules. From this perspective, the results on the oligonucleotides show that water-soluble particles of nanometer size, typical for the hydrophilic parts of membrane-bound proteins, may be a useful separation motif. A charged contamination in the commercial sample of monoolein, most likely oleic acid that arises from its hydrolysis, restricts useful buffer conditions to a pH below 5.6.

Liquid crystalline phases and their dispersions in aqueous mixtures of glycerol monooleate and glyceryl monooleyl ether

The aqueous phase behavior of mixtures of 1-glycerol monooleate (GMO) and its ether analogue, 1-glyceryl monooleyl ether (GME) has been investigated by a combination of polarized microscopy, X-ray diffraction, and NMR techniques. Three phase diagrams of the ternary GMO/GME/water system have been constructed at 25, 40, and 55 degrees C. The results demonstrate that the increasing amount of GME favors the formation of the reversed phases, evidenced by the transformation of the lamellar and bicontinuous cubic liquid crystalline phases of the binary GMO/water system into reversed micellar or reversed hexagonal phases. For a particular liquid crystalline phase, increasing the GME content has no effect on the structural characteristics and hydration properties, thus suggesting ideal mixing with GMO. Investigations of dispersed nanoparticle samples using shear and a polymeric stabilizer, Pluronic F127, show the possibility of forming two different kinds of bicontinuous cubic phase nanoparticles by simply changing the GMO/GME ratio. Also NMR self-diffusion measurements confirm that the block copolymer, Pluronic F127, used to facilitate dispersion formation, is associated with nanoparticles and provides steric stabilization.

Mechanical and Chemical Protection of a Wired Enzyme Oxygen Cathode by a Cubic Phase Lyotropic Liquid Crystal

When implanted in animals, enzyme-containing battery electrodes, biofuel cell electrodes, and biosensors are often damaged by components of the biological environment. An O2 cathode, superior to the classical platinum cathode, which would be implanted, as part of a caseless physiological pH miniature Zn-O2 battery or as part of a caseless and membraneless miniature glucose-O2 biofuel cell, is rapidly damaged by serum urate at its operating potential. The cathode is made by electrically connecting, or wiring, reaction centers of bilirubin oxidase to carbon with an electron-conducting redox hydrogel. In the physiological pH 7.3 electrolyte battery or biofuel cell, the O2 cathode should operate at, or positive of, 0.3 V (Ag/AgCl), where the urate anion, a common serum component, is electrooxidized. Because an unidentified urate electrooxidation intermediate, formed in the presence of O2, damages the wired bilirubin oxidase electrocatalyst, urate must be excluded from the cathode. Unlike O2, which permeates through both the lipid and the aqueous interconnected networks of cubic-phase lyotropic liquid crystals, urate permeates only through their continuous three-dimensional aqueous channel networks. The aqueous channels have well-defined diameters of approximately 5 nm in the monoolein/water cubic-phase liquid crystal. Through tailoring the wall charge of the aqueous channels, the anion/cation permeability ratio can be modulated. Thus, doping the monoolein of the monoolein/water liquid crystal with 1,2-dioleoyl-sn-glycero-3-phosphate makes the aqueous channel walls anionic and reduces the urate permeation in the liquid crystal. As a result, the ratio of the urate electrooxidation current to the O2 electroreduction current is reduced from 1:3 to 1:100 for 5-mm O2 cathodes rotating at 1000 rpm. Doping with 1,2-dioleoyl-sn-glycero-3-phosphate also increases the shear strength of the cubic-phase monoolein/water lyotropic liquid crystal. While the undoped liquid crystal is promptly damaged at the 0.1 N m-2 average shear stress generated by rotating the 5-mm-diameter disk cathode at 1000 rpm in a physiological aqueous solution, the 10 mol % 1,2-dioleoyl-sn-glycero-3-phosphate-doped film remains intact. The mechanical strengthening of the lyotropic liquid crystal by the two-tailed 1,2-dioleoyl-sn-glycero-3-phosphate is attributed to cross-linking hydrophobic bonds (i.e., bonds resulting of the increase in entropy upon the freeing of the translation and rotation of multiple water molecules), which is analogous to the strengthening of polymer-based plastic materials by cross-linking through covalent bonds.

Cubic phase nanoparticles (Cubosome): principles for controlling size, structure, and stability

Methods and compositions for producing lipid-based cubic phase nanoparticles were first discovered in the 1990s. Since then a number of studies have been presented, but little is known about how to control key properties such as particle size, morphology, and stability of cubic phase dispersions. In the present work we give examples of how these properties can be tuned by composition and processing conditions. Importantly we show that stable particle dispersions with consistent size and structure can be produced by a simple processing scheme comprising a homogenization and heat treatment step.

Redox Activity and Diffusion of Hydrophilic, Hydrophobic, and Amphiphilic Redox Active Molecules in a Bicontinuous Cubic Phase

The objective was to examine how a bicontinuous cubic phase influences the diffusion and electrochemical activity of dissolved molecules. The cubic phase is a structure with three-dimensional continuous channels of water separated by an apolar membrane. A redox active molecule can dissolve in three different environments. A hydrophobic molecule will prefer the interior of the membrane, a hydrophilic molecule will prefer the water channels, and an amphiphilic molecule will be situated with its headgroup at the surface of the membrane and its tail in the interior. The electrochemical activity was measured with cyclic voltammetry and the transport behavior with chronocoulometry. All the molecules were redox active in the cubic phase; that is, all the molecules could reach the surface of the electrode and react. The cubic phase made the kinetics of the charge transfer slower, showing a quasi-reversible behavior. The reason may be that a layer of the membrane adheres to the hydrophobic electrode surface. The diffusion experiment showed that the diffusion was slower than in solution. The molecules that were restricted to diffuse within the membrane gave particularly low mass transport rates.

Cubic Phases and Cubic Phase Dispersions in a Phospholipid-Based System

A cubic liquid crystalline phase forming system based on the phospholipid dioleoylphosphatidylethanolamine (DOPE) which is fortified with small amounts of PEGylated (poly(ethylene) glycol) glycerol monooleate (PEG(660)-GMO) is characterized. The cubic phase formed by the DOPE/PEG(660)-GMO/water system coexists with water in the dilute part of the phase diagram and can be fragmented into colloidal size particles with retained cubic phase structure.

Electrodes modified with monoolein cubic phases hosting laccases for the catalytic reduction of dioxygen

An enzyme-catalyzed process has been used for dioxygen monitoring. The enzymes were two different laccases (p-diphenol:dioxygen oxidoreductases), chosen as catalysts for dioxygen reduction. The laccases were immobilized in a liquid crystalline cubic phase formed with monoolein. The structures of the cubic phases, both with and without enzymes, were established using small-angle X-ray scattering. The catalytic reduction of dioxygen was performed using a glassy carbon electrode modified with cubic phases containing the enzymes. The modified electrode was used as a dioxygen sensing system, based on the increasing reduction current of a suitable electrochemical probe in the presence of dioxygen.