Formation of large unilamellar vesicles using alkyl maltoside detergents

Towards understanding the crystallization of photosystem II: influence of poly(ethylene glycol) of various molecular sizes on the micelle formation of alkyl maltosides

The influence of poly(ethylene glycol) (PEG) polymers H-(O-CH2-CH2)p-OH with different average molecular sizes p on the micelle formation of n-alkyl-β-D-maltoside detergents with the number of carbon atoms in the alkyl chain ranging from 10 to 12 is investigated with the aim to learn more about the detergent behavior under conditions suitable for the crystallization of the photosynthetic pigment-protein complex photosystem II. PEG is shown to increase the critical micelle concentration (CMC) of all three detergents in the crystallization buffer in a way that the free energy of micelle formation increases linearly with the concentration of oxyethylene units (O-CH2-CH2) irrespective of the actual molecular weight of the polymer. The CMC shift is modeled by assuming for simplicity that it is dominated by the interaction between PEG and detergent monomers and is interpreted in terms of an increase of the transfer free energy of a methylene group of the alkyl chain by 0.2 kJ mol-1 per 1 mol L-1 increase of the concentration of oxyethylene units at 298 K. Implications of this effect for the solubilization and crystallization of protein-detergent complexes as well as detergent extraction from crystals are discussed.

Microfluidic technologies for lipid vesicle generation

Encapsulating biological and non-biological materials in lipid vesicles presents significant potential in both industrial and academic settings. When smaller than 100 nm, lipid vesicles and lipid nanoparticles are ideal vehicles for drug delivery, facilitating the delivery of payloads, improving pharmacokinetics, and reducing the off-target effects of therapeutics. When larger than 1 μm, vesicles are useful as model membranes for biophysical studies, as synthetic cell chassis, as bio-inspired supramolecular devices, and as the basis of protocells to explore the origin of life. As applications of lipid vesicles gain prominence in the fields of nanomedicine, biotechnology, and synthetic biology, there is a demand for advanced technologies for their controlled construction, with microfluidic methods at the forefront of these developments. Compared to conventional bulk methods, emerging microfluidic methods offer advantages such as precise size control, increased production throughput, high encapsulation efficiency, user-defined membrane properties (i.e., lipid composition, vesicular architecture, compartmentalisation, membrane asymmetry, etc.), and potential integration with lab-on-chip manipulation and analysis modules. We provide a review of microfluidic lipid vesicle generation technologies, focusing on recent advances and state-of-the-art techniques. Principal technologies are described, and key research milestones are highlighted. The advantages and limitations of each approach are evaluated, and challenges and opportunities for microfluidic engineering of lipid vesicles to underpin a new generation of therapeutics, vaccines, sensors, and bio-inspired technologies are presented.

A novel strategy for partial purification of alkane hydroxylase from P. chrysogenum SNP5 through reconstituting its native membrane into liposome

Integral proteins or enzymes are still challenging to purify into their native state because of their need for an amphipathic environment and cofactors. Alkane hydroxylase (AlkB) is a membrane-bound enzyme that catalyzes the hydroxylation of a range of alkanes that have a broad spectrum of applications. In the current study, a novel approach has been explored for partial purification of alkane hydroxylase (AlkB) in its native state through restructuring the lipid bilayer of Penicillium chrysogenum SNP5 into a liposome to extend the native and protective environment to AlkB enzyme. Three different methods i.e., reverse-phase evaporation method (RPEM), detergent-based method (DBM), and ethanol injection method (EIM) have been used for reconstituting its native membrane into liposome. On characterizing liposomes through fluorescence imaging, AFM, and particle size analysis, the reverse-phase evaporation method gave the best results based on the size distribution (i.e., 100-300 nm), the morphology of liposomes, and maximum AlkB specific activity (i.e., 140.68 U/mg). The maximum reconstitution efficiency of 29.48% was observed in RPEM followed by 17.3% in DBM and 12.3% in EIM. On the characterization of the purified AlkB, the molecular weight was measured of 44.6 KDa and the thermostability of liposomes synthesized with the RPEM method was obtained maximum at 55 °C. This approach may open a new strategy for the purification of integral enzymes/proteins in their native state in the field of protein purification and its applications in diversified industries.

Refined definition of the critical micelle concentration and application to alkyl maltosides used in membrane protein research

The critical micelle concentration (CMC) of nonionic detergents is defined as the breaking point in the monomer concentration as a function of the total detergent concentration, identified by setting the third derivate of this function to zero. Combined with a mass action model for micelle formation, this definition yields analytic formulae for the concentration ratio of monomers to total detergent at the CMC and the relationship between the CMC and the free energy of micellization g mic. The theoretical breaking point is shown to coincide with the breaking point of the experimental titration curve, if the fluorescence enhancement of 8-anilino-1-naphthalene-sulfonic acid (ANS) or a similar probe dye is used to monitor micelle formation. Application to a series of n-alkyl-β-d-maltosides with the number of carbon atoms in the alkyl chain ranging from 8 to 12 demonstrates the good performance of a molecular thermodynamic model, in which the free energy of micellization is given by g mic = σΦ + g pack + g st. In this model, σ is a fit parameter with the dimension of surface tension, Φ represents the change in area of hydrophobic molecular surfaces in contact with the aqueous phase, and g pack and g st are contributions, respectively, from alkyl chain packing in the micelle interior and steric repulsion of detergent head groups. The analysis of experimental data from different sources shows that varying experimental conditions such as co-solutes in the aqueous phase can be accounted for by adapting only σ, if the co-solutes do not bind to the detergent to an appreciable extent. The model is considered a good compromise between theory and practicability to be applied in the context of in vitro investigations of membrane proteins.

Recent Progress of DNA Nanostructures on Amphiphilic Membranes

Employing DNA nanostructures mimicking membrane proteins on artificial amphiphilic membranes have been widely developed to understand the structures and functions of the natural membrane systems. In this review, the recent developments in artificial systems constructed by amphiphilic membranes and DNA nanostructures are summarized. First, the preparations and properties of the amphipathic bilayer models are introduced. Second, the interactions are discussed between the membrane and the DNA nanostructures, as well as their coassembly behaviors. Next, the alternative systems related to membrane protein-mediated signal transmission, selective distribution, transmembrane channels, and membrane fusion are also introduced. Moreover, the constructions of membrane skeleton protein-mimicking DNA nanostructures are also highlighted.

Manufacturing drug co-loaded liposomal formulations targeting breast cancer: Influence of preparative method on liposomes characteristics and in vitro toxicity

Developing more efficient manufacturing methods for nano therapeutic systems is becoming important, not only to better control their physico-chemical characteristics and therapeutic efficacy but also to ensure scale-up is cost-effective. The principle of cross-flow chemistry allows precise control over manufacturing parameters for the fabrication of uniform liposomal formulations, as well as providing reproducible manufacturing scale-up compared to conventional methods. We have herein investigated the use of microfluidics to produce PEGylated DSPC liposomes loaded with doxorubicin and compared their performance against identical formulations prepared by the thin-film method. The isoprenylated coumarin umbelliprenin was selected as a co-therapeutic. Umbelliprenin-loaded and doxorubicin:umbelliprenin co-loaded liposomes were fabricated using the optimised microfluidic set-up. The role of umbelliprenin as lipid bilayer fluidity modulation was characterized, and we investigated its role on liposomes size, size distribution, shape and stability compared to doxorubicin-loaded liposomes. Finally, the toxicity of all liposomal formulations was tested on a panel of human breast cancer cells (MCF-7, MDA-MB 231, BT-474) to identify the most potent formulation by liposomal fabrication method and loaded compound(s). We herein show that the microfluidic system is an alternative method to produce doxorubicin:umbelliprenin co-loaded liposomes, allowing fine control over liposome size (100-250 nm), shape, uniformity and doxorubicin drug loading (>80%). Umbelliprenin was shown to confer fluidity to model lipid biomembranes, which helps to explain the more homogeneous size and shape of co-loaded liposomes compared to liposomes without umbelliprenin. The toxicity of doxorubicin:umbelliprenin co-loaded liposomes was lower than that of free doxorubicin, due to the delayed release of doxorubicin from liposomes. An alternative, rapid and easy manufacturing method for the production of liposomes has been established using microfluidics to effectively produce uniform doxorubicin:umbelliprenin co-loaded liposomal formulations with proven cytotoxicity in human breast cancer cell lines in vitro.

Effect of the Anomeric Configuration on the Micellization of Hexadecylmaltoside Surfactants

The self-assembly of the two anomeric forms of n-hexadecyl-d-maltopyranoside (denoted α-C₁₆G₂ and β-C₁₆G₂) has been studied in dilute aqueous solution by means of surface tension measurements, scattering methods (dynamic light scattering, static light scattering, and small-angle X-ray and neutron scattering), and cryo-transmission electron microscopy at different surfactant concentrations and temperatures. Surface tension measurements demonstrate differences in the surfactant adsorption at the air-water interface, where α-C₁₆G₂ shows a lower CMC than β-C₁₆G₂. Similarly, micelle morphology was found to profoundly depend on anomerism. β-C₁₆G₂ preferentially forms very elongated micelles with large persistence lengths, whereas α-C₁₆G₂ assembles into smaller micelles for which the structure varies with concentration and temperature. The differences between the two surfactant anomers in terms of self-assembly can be attributed to the interaction between neighboring headgroups. Specifically, β-C₁₆G₂ allows for a closer packing in the palisade layer, hence reducing the micelle curvature and promoting the formation of more elongated micelles. Strong intermolecular headgroup interactions may also account for the observed rigidity of the micelles.

Lipid-based nanovesicles for nanomedicine

Multifunctional lipid-based nanovesicles (L-NVs) prepared by molecular self-assembly of membrane components together with (bio)-active molecules, by means of compressed CO₂-media or other non-conventional methods lead to highly homogeneous, tailor-made nanovesicles that are used for advanced nanomedicine. Confocal microscopy image of siRNA transfection using L-NVs, reprinted with permission from de Jonge,et al.,Gene Therapy, 2006,13, 400–411.

One step preparation of quantum dot-embedded lipid nanovesicles by a microfluidic device

Synthetic carriers that mimic “natural lipid-based vesicles” (micro/nanovesicles, exosomes) have found broad application in biomedicine for the delivery of biomolecules and drugs. Here, an innovative approach for their production is presented.

Liposome: classification, preparation, and applications

Liposomes, sphere-shaped vesicles consisting of one or more phospholipid bilayers, were first described in the mid-60s. Today, they are a very useful reproduction, reagent, and tool in various scientific disciplines, including mathematics and theoretical physics, biophysics, chemistry, colloid science, biochemistry, and biology. Since then, liposomes have made their way to the market. Among several talented new drug delivery systems, liposomes characterize an advanced technology to deliver active molecules to the site of action, and at present, several formulations are in clinical use. Research on liposome technology has progressed from conventional vesicles to 'second-generation liposomes', in which long-circulating liposomes are obtained by modulating the lipid composition, size, and charge of the vesicle. Liposomes with modified surfaces have also been developed using several molecules, such as glycolipids or sialic acid. This paper summarizes exclusively scalable techniques and focuses on strengths, respectively, limitations in respect to industrial applicability and regulatory requirements concerning liposomal drug formulations based on FDA and EMEA documents.

Interactions between charged polypeptides and nonionic surfactants

The influence of molecular characteristics on the mutual interaction between peptides and nonionic surfactants has been investigated by studying the effects of surfactants on amphiphilic, random copolymers of alpha-L-amino acids containing lysine residues as the hydrophilic parts. The hydrophobic residues were either phenylalanine or tyrosine. The peptide-surfactant interactions were studied by means of circular dichroism spectroscopy and binding isotherms, as well as by 1D and 2D NMR. The binding of surfactant to the peptides was found to be a cooperative process, appearing at surfactant concentrations just below the critical micellar concentration. However, a certain degree of peptide hydrophobicity is necessary to obtain an interaction with nonionic surfactant. When this prerequisite is fulfilled, the peptide mainly interacts with self-assembled, micelle-like surfactant aggregates formed onto the peptide chain. Therefore, the peptide-surfactant complex is best described in terms of a necklace model, with the peptide interacting primarily with the palisade region of the micelles via its hydrophobic side chains. The interaction yields an increased amount of alpha-helix conformation in the peptide. Surfactants that combine small headgroups with a propensity to form small, nearly spherical micelles were shown to give the largest increase in alpha-helix content.

Chiral recognition of thiaheterohelicenes by alkyl beta-D-pyranoside micelles. Influence of extension of helix

Chiral recognition of alkyl beta-D-pyranoside micelles toward [7] and [5] heterohelicenes possessing helical structures was investigated by 1H-NMR and CD (circular dichroism) spectroscopy. In dodecyl maltopyranoside micellar solution, P and M enantiomers of tetrathia[7]heterohelicene (7TH), which have rigid and stable helixes, manifested different chemical shifts in their 1H-NMR spectra due to differences in the diastereomeric interactions, implying that the micelles undergo stronger recognition toward the M enantiomer than the P enantiomer. On the other hand, in octyl glucopyranoside micellar solution, trithia[5]heterohelicene (5TH) and two kinds of its derivatives which are rapidly equilibrated between the enantiomers in solution, gave no distinctly resolved 1H-NMR peaks for either enantiomer even at a lowered temperature. However, these racemic [5]heterohelicenes in the micelles did develop induced CD absorptions owing to a displacement of the equilibrium, suggesting from the signs of their Cotton effects that the micelles prefer the M enantiomer to its antipode in conformity with the 1H-NMR results of 7TH.

Generation of proteoliposomes from subcellular fractions

Intracellular membranes are highly dynamic, yet they retain their identity and functional characteristics. Integral membrane proteins, which must confer this specific membrane identity, remain poorly characterized at the biochemical level, largely because detergent-mediated solubilization is required for purification and analysis, and several properties of integral membrane proteins can only be investigated when the molecule is properly embedded in a lipid bilayer. We present a method for the efficient reconstitution into proteoliposomes of integral membrane proteins from subcellular fractions. Integral membrane proteins were identified on high-resolution two-dimensional gels after selective extraction of soluble and peripheral membrane proteins; they accounted for 8% of the number of resolved polypeptides. A reconstitution procedure based on membrane solubilization with dodecyl-octaoxyethylene (C12E8) and subsequent detergent removal with BioBeads SM-2 resulted in the efficient reconstitution of several membrane proteins into proteoliposomes of uniform density. The generated proteoliposomes strongly resemble the starting membrane fraction in protein composition. This reconstitution allows the functional characterization of integral membrane proteins after enrichment and/or specific (immuno)depletion.

Measurement of intravesicular volumes by salt entrapment

Internal volume is a very sensitive parameter of vesicle morphology. Measurement of captured volumes by solute entrapment is legitimate for most types of vesicles (Perkin, W.R. et al. (1993) Chem. Phys. Lipids 64, 197-217). In this study chloride was selected as the most convenient marker ion because the ubiquity of Cl- in physiological buffers eliminates prelabeling with exogenous markers and because minute concentrations of trapped chloride are well detectable in the presence of large extravesicular nitrate concentrations. Perfect exchange of external chloride for nitrate was shown to be accomplished by gel filtration, dialysis, or sucrose gradient flotation-but only after significant technical improvements and/or elimination of experimental pitfalls. Reliability was cross-checked by simultaneous entrapment of Cl- and K+. Diafiltration and ion exchange chromatography appeared inapplicable for exchange of extravesicular salt. When a representative variety of vesicle preparations was analyzed for internal volume (as well as for external surface and size) unexpected features of vesicle morphology were discovered. This emphasizes the genuine role of macroscopic vesicle characterization in complementing information from electron microscopy.

External surface and lamellarity of lipid vesicles: a practice-oriented set of assay methods

Three methods are presented for the determination of external surface of large lipid vesicles of different lamellarity with 2% absolute accuracy. These methods (referred to as EPR, NBD and TNBS assays) use different marker lipids which provide signals (electron paramagnetic resonance, fluorescence of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) residues, and UV absorption increase of 2,4,6-trinitrobenzenesulfonic acid after reaction with aminolipids, respectively). The signals change upon addition of different membrane-impermeant reagents due to reaction with marker lipids at the external vesicle surface. They were applied to the same vesicle samples, including unilamellar and multilamellar vesicles, both at two different lipid compositions. External surface data matched for the three assays within 2%, but only after appropriate redesign or adaptation of so far published procedures. Main improvements related to slow influx of reagents (TNBS and NBD assays) or to redistribution of marker lipids (EPR assay), obscuring determination of outer vesicle surface from fast reaction between reagent and readily accessible marker lipids. Furthermore, suitable strategies were found to obtain accurate 100% values (reaction of all marker lipids present), required to relate external vesicle surface to total surface. This included corrections for light scattering (NBD assay) and for turbidity (TNBS assay). These three methods appear to close a gap in the methodology to determine external surface of vesicles for typical practical needs. In particular, the reliability range of the NBD assay could be extended to marker lipid densities as low as 1 marker lipid per 3000 lipids.

Sequential use of detergents for solubilization and reconstitution of a membrane ion transporter

Solubilization and reconstitution of the cardiac sarcolemmal Na+/Ca2+ exchanger by use of the anionic detergent cholate and its application for reconstitution of the exchanger following solubilization with zwitterionic or nonionic detergents is described. Solubilization and reconstitution with cholate provided a 32.6-fold enrichment of Na+/Ca2+ exchange activity over sarcolemmal vesicles (5.2 to 170 nmol/mg/s) with 202% recovery of total activity. In combination with asolectin, the cholate dilution technique (H. Miyamoto and E. Racker, J. Biol. Chem. 255, 2656, 1980) offers a rapid and simple means for reconstitution and provides good recovery of total and specific Na+/Ca2+ exchange activity. However, the use of anionic detergents for solubilization precludes the use of certain chromatographic procedures for protein purification. Conversely, nonionic and zwitterionic detergents permit effective use of available chromatographic techniques, but can be troublesome during reconstitution. We have combined the advantages of solubilization with nonionic and zwitterionic detergents with the advantages of reconstitution by cholate dilution. Reconstitution of the exchanger, after solubilization with 3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate (Chaps) or n-octyl-beta-D-glucoside, was accomplished by the addition of a cholate/asolectin medium followed by dilution. Na+/Ca2+ exchange activity was enriched 30.7-fold with 196% recovery with Chaps and 34.1-fold with 204% recovery with n-octyl-beta-D-glucoside. The presence of Chaps was found to shift the optimal asolectin concentration for reconstitution from 15 mg/ml (cholate alone) to 25 mg/ml. In addition, pelleting of proteoliposomes subsequent to reconstitution resulted in greatest recovery of total activity when volumes were kept below 1.0 ml.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrogenic transport by the Enterococcus hirae ATPase

A transport ATPase from Enterococcus hirae was reconstituted in lipid vesicles and its electrogenic action investigated with the fluorescent dye oxonol VI as membrane potential probe. Reconstitution in bacterial and in soybean phospholipid mixtures led to transport-active vesicle preparations. Inside-out oriented ATPase molecules were activated by the addition of ATP to the extravesicular medium, generating in all experiments an intravesicularly positive potential. The extravesicular pH strongly influenced the initial pumping rate and the duration of the pumping activity. At neutral pH, transient pumping activity was observed, lasting for 1-2 min, while at pH 5.6, pumping was continuous. The transport activity was not dependent on the ionic composition of the buffer on either side of the membrane. These findings can be interpreted as the action of a proton ATPase, regulated by the cytoplasmic proton concentration and electrogenically translocating protons from the cytoplasm to the extracellular space.

Cancer chemotherapy administered by activated carbon particles and liposomes

In cancer chemotherapy, a specific drug delivery system is expected since many anticancer drugs show toxicity against not only cancer cells but also against normal tissues. The dosage form comprising anticancer drugs adsorbed on activated carbon particles or encapsulated in liposomes is developed as a drug-delivery system which enhances the therapeutic efficacy and reduces the adverse effects. The dosage forms are versatile in size and electric charge, so that large amounts of the drugs are distributed to the "targeted" organs or tissues and lesser amounts are distributed to the whole body. The dosage forms are designed to release the drugs slowly for a long time at local sites. Through this process, practical use of the dosage forms as an anticancer drug carrier results in an enhancement of anticancer efficacy on the local lesion and a decrease of systemic toxicity.

Reconstitution of Na+/K+-ATPase into phosphatidylcholine vesicles by dialysis of nonionic alkyl maltoside detergents

The reconstitution of Na+/K+-ATPase from outer medulla of rabbit kidney into large unilamellar liposomes was achieved through detergent removal by dialysis of mixed micellar solutions of synthetic dioleoyl phosphatidylcholine/octyl glucoside and Na+/K+-ATPase/decyl maltoside or decenyl maltoside. Tight, transport-active liposomes were formed when the lipid and the enzyme were solubilized separately in the nonionic detergents and mixed immediately before starting the dialysis. The two maltoside detergents with different structures of the hydrophobic part of the molecule proved to be well suited for the solubilization of Na+/K+-ATPase with high retention of enzyme activity; the inactivation of enzyme being evidently slower with the unsaturated decenyl maltoside. The diameters of the proteoliposomes, 110 and 170 nm, respectively, were also dependent on the structure of the maltoside detergent, the saturated decyl maltoside producing the bigger liposomes. After freeze-fracture, both preparations exhibited intramembranous particles as structural indicators of successful reconstitution. The electrogenic activity of the reconstituted enzyme was determined by fluorescence measurements with Oxonol VI and by tracer-flux measurements with 22Na+.

Proton concentration jumps and generation of transmembrane pH-gradients by photolysis of 4-formyl-6-methoxy-3-nitrophenoxyacetic acid

Proton concentration gradients across membranes are important for many biological energy transducing processes. The kinetics of proton dependent processes can be studied by pH-jump methods in which protons are photochemically released. In the following we describe the synthesis and the properties of photolabile 4-formyl-6-methoxy-3-nitrophenoxyacetic acid, a 'caged proton'. The synthesis is based on vanillin, which is alkylated with chloroacetic acid to give a carboxylic acid (pK = 2.72). In a second step a nitro group ortho to the formyl group is introduced. Photochemical proton release occurs by a reaction mechanism analogous to the well known photochemical formation of 2-nitrosobenzoic acid from 2-nitrobenzaldehyde. The pK values of the photoproduct are 0.75 and 2.76, respectively, thus allowing the use of the compound in a wide pH-range. The quantum yield is 0.18, lower than in the case of the 2-nitrobenzaldehyde/2-nitrosobenzoic acid system (phi = 0.5). The release of the proton in a flash photolysis experiment occurs within less than 1 microseconds. The spectrum of photolabile compound has absorption maxima at 263 nm and 345 nm, respectively. Its permeability across a lipid bilayer membrane is very low (permeability coefficient Pd approximately equal to 10(-9) cm.s-1 at pH 8) so that transmembrane proton concentration gradients can be generated.