Proteins in membrane mimetic systems. Insertion of myelin basic protein into microemulsion droplets

Modulating shape transition in surfactant stabilized reverse microemulsions

The formation of reverse microemulsions (RMs) of spherical shape in the oil/water/surfactant ternary mixture at high molar ratio of water to surfactant (ω) is well established. Using dynamic light scattering, small-angle X-ray and neutron scattering, we elucidate the formation of non-spherical reverse microemulsions stabilised by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at ω = 10 and volume fractions of the dispersed phase, Φ, ranging from 0.005 to 0.20. In addition, we propose a strategy to tune the aspect ratio of non-spherical droplets and colloidal interactions by (i) varying the volume fraction of the dispersed phase (ii) changing the temperature, and (iii) by substituting the aliphatic oil with a mixture of aliphatic and aromatic hydrocarbons. This tunability of anisotropy along with a precise control of the interactions in the RMs, their ability to form spontaneously and their thermodynamic stability is crucial to provide a handle on reaction kinetics, synthesis of anisotropic nanoparticles as well as for their application as lubricants and viscosity modifiers.

Rate of Molecular Transfer of Allyl Alcohol across an AOT Surfactant Layer Using Muon Spin Spectroscopy

The transfer rate of a probe molecule across the interfacial layer of a water-in-oil (w/o) microemulsion was investigated using a combination of transverse field muon spin rotation (TF-μSR), avoided level crossing muon spin resonance (ALC-μSR), and Monte Carlo simulations. Reverse microemulsions consist of nanometer-sized water droplets dispersed in an apolar solvent separated by a surfactant monolayer. Although the thermodynamic, static model of these systems has been well described, our understanding of their dynamics is currently incomplete. For example, what is the rate of solute transfer between the aqueous and apolar solvents, and how this is influenced by the structure of the interface? With an appropriate choice of system and probe molecule, μSR offers a unique opportunity to directly probe these interfacial transfer dynamics. Here, we have employed a well characterized w/o microemulsion stabilized by bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT), with allyl alcohol (CH2═CH-CH2-OH, AA) as the probe. Resonances due to both muoniated radicals, CMuH2-C*H-CH2-OH and C*H2-CHMu-CH2-OH, were observed with the former being the dominant species. All resonances displayed solvent dependence, with those in the microemulsion observed as a single resonance located at intermediate magnetic fields to those present in either of the pure solvents. Observation of a single resonance is strong evidence for interfacial transfer being in the fast exchange limit. Monte Carlo calculations of the ΔM = 0 ALC resonances are consistent with the experimental data, indicating exchange rates greater than 10(9) s(-1), placing the rate of interfacial transfer at the diffusion limit.

Fluorescence lifetime imaging microscopy and fluorescence resonance energy transfer from cyan to yellow fluorescent protein validates a novel method to cluster proteins on solid surfaces

A novel method to distribute proteins on solid surfaces is proposed. Proteins microencapsulated in the water pool of reverse micelles were used to coat a solid surface with well-individualized round spots of 1 to 3 microm in diameter. The number of spots per unit area can be increased through the concentration of reverse micelles, and networks of spots were obtained at high concentrations of large reverse micelles. Moreover, depending on the pool size of the water reverse micelles, proteins can be deposited far from each other or in close proximity within the range of 50 to 70 A. This proximity obtained with small reverse micelles was proved through fluorescence lifetime imaging microscopy and fluorescence resonance energy transfer (FLIM-FRET) measurements for the most relevant FRET pair in cell biology studies, the cyan and yellow fluorescent proteins. This novel procedure has several advantages and reveals the potential for study of protein-protein interactions on solid surfaces and for developing novel biomaterials and molecular devices based on biorecognition elements.

Investigation of the mobility of amphiphilic polymer—AOT reverse microemulsion systems using electron spin resonance

Amphiphilic polymers can be used as tools to manipulate the behavior of reverse microemulsions. EPR spectroscopy employing the spin probe 5-doxyl stearic acid was used to study the adsorption of a comb type polymer (polymaleic anhydride octyl vinyl ether) and a diblock polymer (polybutadiene-ethylene oxide) onto reverse microemulsion droplets formed from Aerosol-OT/heptane/water. The findings indicate that the comb type polymer was adsorbed by the reverse microemulsion drops at low polymer concentrations causing a structural change of the micelle.

Cutinase-AOT interactions in reverse micelles: the effect of 1-hexanol

Cutinase encapsulated in dioctyl sulfosuccinate reverse micelles displays very low stability, undergoing fast denaturation due to an anchoring at the micellar interface. The denaturation process and the structure of the reverse micelle were characterized using biophysical techniques. The kinetics of denaturation observed from fluorescence match the increase of the hydrodynamic radius of reverse micelles. Denaturation in reverse micelles is mainly the unfolding of the three-dimensional structure since the decrease in the circular dichroism ellipticity in the far-UV range is very small. The process is accompanied by an increase in the steady-state anisotropy, as opposed to what happens for denaturation in aqueous solution. Since 1-hexanol used as co-surfactant in dioctyl sulfosuccinate reverse micelles slows or even prevents cutinase denaturation, its effect on cutinase conformation and on the size of reverse micelles was analyzed. When 1-hexanol is present, cutinase is encapsulated in a large reverse micelle, as deduced from dynamic light scattering. The large reverse micelle filled with cutinase was built from the fusion of reverse micelles according to a pseudo-unimolecular process ranging in time from a few minutes to 2h depending on the reverse micellar concentration. This slow equilibrium driven by the encapsulated cutinase has not been reported previously. The encapsulation of cutinase in dioctyl sulfosuccinate reverse micelles establishes a completely new equilibrium characterized by a bimodal population of empty and filled reverse micelles, whose characteristics depend greatly on the interfacial characteristics, that is, on the absence or presence of 1-hexanol.

Enzyme catalysis in reverse micelles

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviour and the corresponding activity of enzymes the application of these media can lead to favourable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes.

Reverse micellar extraction for downstream processing of proteins/enzymes

New developments in the area of downstream processing are, hopefully, to fulfill the promises of modern biotechnology. The traditional separation processes such as chromatography or electrophoresis can become prohibitively expensive unless the product is of high value. Hence, there is a need to develop efficient and cost-effective downstream processing methods. Reverse micellar extraction is one such potential and a promising liquid-liquid extraction technique, which has received immense attention for isolation and purification of proteins/enzymes in the recent times. This technique is easy to scale-up and offers continuous operation. This review, besides briefly considering important physico-chemical and biological aspects, highlights the engineering aspects including mass transfer, mathematical modeling, and technology development. It also discusses recent developments in reverse micellar extraction such as affinity based separations, enzymatic reactions in reverse micelles coupled with membrane processes, reverse micellar extraction in hollow fibers, etc. Special emphasis has been given to some recent applications of this technique.

Dynamic light scattering of cutinase in AOT reverse micelles

The fungal lipolytic enzyme cutinase, incorporated into sodium bis-(2ethylhexyl) sulfosuccinate reversed micelles has been investigated using dynamic light scattering. The reversed micelles form spontaneously when water is added to a solution of sodium bis-(2ethylhexyl) sulfosuccinate in isooctane. When an enzyme is previously dissolved in the water before its addition to the organic phase, the enzyme will be incorporated into the micelles. Enzyme encapsulation in reversed micelles can be advantageous namely to the conversion of water insoluble substrates and to carry out synthesis reactions. However protein unfolding occurs in several systems as for cutinase in sodium bis-(2ethylhexyl) sulfosuccinate reversed micelles. Dynamic light scattering measurements of sodium bis-(2ethylhexyl) sulfosuccinate reversed micelles with and without cutinase were taken at different water to surfactant ratios. The results indicate that cutinase was attached to the micellar wall and that might cause cutinase unfolding. The interactions between cutinase and the bis-(2ethylhexyl) sulfosuccinate interface are probably the driving force for cutinase unfolding at room temperature. Twenty-four hours after encapsulation, when cutinase is unfolded, a bimodal distribution was clearly observed. The radii of reversed micelles with unfolded cutinase were determined and found to be considerable larger than the radii of the empty reversed micelles. The majority of the reversed micelles were empty (90-96% of mass) and the remainder (4-10%) containing unfolded cutinase were larger by 26-89 A.

Acoustic Spectroscopy for Characterizing Heptane/H(2)O/AOT Reverse Microemulsions

Acoustic spectroscopy was used to monitor the droplet size distribution in a classic three component system of heptane, water, and aerosol-OT (AOT). The size of the reverse microemulsion drops was varied by changing the molar ratio of water to AOT surfactant. The acoustic results for this transparent microemulsion were found to be in close agreement with literature results obtained with small angle neutron scattering and small angle X-ray scattering. The system was investigated well into the turbid region where microemulsion changes to macroemulsion. The droplet size distribution was found to shift to a bimodal form due to this transition. Copyright 1999 Academic Press.

Enzymes in low water systems

Water is fundamental for enzyme action and for formation of the three-dimensional structure of proteins. Hence, it may be assumed that studies on the interplay between water and enzymes can yield insight into enzyme function and formation. This has proven correct, because the numerous studies that have been made on the behavior of water-soluble and membrane enzymes in systems with a low water content (reverse micelles or enzymes suspended in nonpolar organic solvents) have revealed properties of enzymes that are not easily appreciated in aqueous solutions. In the low water systems, it has been possible to probe the relation between solvent and enzyme kinetics, as well as some of the factors that affect enzyme thermostability and catalysis. Furthermore, the studies show that low water environments can be used to stabilize conformers that exhibit unsuspected catalytic properties, as well as intermediates of enzyme function and formation that in aqueous media have relatively short life-times. The structure of enzymes in these unnatural conditions is actively being explored.

Multiple emulsions for the delivery of proteins

Multiple emulsions are unique in that a true liquid phase is maintained separate from an external aqueous phase. This may be especially important for bioactive molecules that cannot be appropriately stabilized in the solid state. In addition, the separation of aqueous phases enables highly specialized environments, conducive to protein activity, to be prepared. The physical instability of conventional systems remains a major factor limiting their wider application. Attempts to improve the physical stability of the aqueous dispersions through interfacial complexation and the use of micro-emulsions are improving the short-term stability. As an alternative approach, solid-state emulsions attempt to store the multiple emulsion as a solid. Although solid-state emulsions appear to have the potential to be useful protein delivery systems, a substantial experimental data base has yet to be generated.

Kinetic models in reverse micelles

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Interactions of myelin basic protein with palmitoyllysophosphatidylcholine: characterization of the complexes and conformations of the protein

The stoichiometry of palmitoyllysophosphatidylcholine/myelin basic protein (PLPC/MBP) complexes, the location of the protein in the lysolipid micelles, and the conformational changes occurring in the basic protein and peptides derived from it upon interaction with lysolecithin micelles were investigated by circular dichroic spectropolarimetry, ultracentrifugation, electron paramagnetic resonance (EPR) and 31P, 13C, and 1H nuclear magnetic resonance spectroscopy (NMR), and electron magnetic resonance spectroscopy (NMR), and electron microscopy. Ultracentrifugation measurements indicated that well-defined complexes were formed by the association of one protein molecule with approximately 141 lysolipid molecules. Small-angle X-ray scattering data indicated that the PLPC/MBP complexes form particles with a radius of gyration of 3.8 nm. EPR spectral parameters of the spin labels 5-, and 16-doxylstearate incorporated into lysolecithin/basic protein aggregates, and 13C- and 1H-NMR relaxation times of PLPC indicated that the addition of the protein did not affect the environment and location of the labels and the organization of the lysolipid micelles. The data suggested that MBP lies primarily near the surface of the micelles, with segments penetrating beyond the interfacial region into the hydrophobic interior, but without any part of the protein being protected against rapid exchange of its amide groups with the aqueous environment. The basic protein acquired about 20% alpha-helix when bound to lysolipid micelles. Circular dichroic spectra of sequential peptides derived by cleavage of the protein revealed the formation of alpha-helical regions in the association with lysolecithin. Specific residues in myelin basic protein that participated in binding to the micelles were identified from magnetic resonance data on changes in the chemical shifts and intensities of assigned resonances, and line broadening of peaks by fatty acid spin-labels incorporated into the micelles.

Comparison of the dynamic structure of alpha-chymotrypsin in aqueous solution and in reversed micelles by fluorescent active-site probing

A highly fluorescent anthraniloyl (Ant) group was covalently attached to the active site of alpha-chymotrypsin (CT), probably at Ser195. Ant-CT is stable at neutral pH for months, allowing a detailed fluorescence study of Ant-CT as a model protein to investigate its physical properties in 0.1 M Tris/HCl, pH 8.2, and in reversed micelles of n-octane, 0.1 M Tris/HCl, pH 8.2, and sodium bis(2-ethylhexyl)sulfosuccinate (AOT). Steady-state fluorescence measurements of the progressive red-shift of the center of gravity of the emission band as function of degree of hydration, wo, defined as [H2O]/[AOT], indicate that the average polarity in the vicinity of the probe is approaching that of bulk water at wo > 12. Time-resolved fluorescence measurements of Ant-CT in water and in reversed micelles showed that the active site has different properties in reversed micelles compared to those in water. Some specific changes at very low water content (0.6 < wo < 5) can be observed, which correlate with enzyme activity measurements in the same wo region (unpublished results). These effects are, for instance, significant changes in the average fluorescence lifetime and the internal flexibility of the probe. The overall rotational-correlation time of the enzyme in AOT reversed micelles seems to be independent on wo (5 < wo < 29), which suggests that the enzyme creates its own micelle.

Ligand binding at membrane mimetic interfaces. Human serum albumin in reverse micelles

The behaviour of human serum albumin in the presence of three chemically distinct ligands: oxyphenylbutazone, dansylsarcosine and hemin, has been compared in buffer and in reverse micelles of isooctane, water, and either sodium bis(2-ethylhexyl)sulfosuccinate or hexadecyl trimethylammonium bromide, systems selected to mimic the membrane-water interface. Upon micellar incorporation, the dansylsarcosine-albumin complex dissociated, as evidenced by fluorescence emission spectroscopy (red shift from 485 nm to 570 nm) and by fluorescence polarization measurements. In contrast, the hemin-albumin complex remained stable in reverse micelles, as judged from the Soret absorption band at 408 nm and the molar absorption coefficient of 8.4 x 10(4) M-1 cm-1. The oxyphenylbutazone to albumin binding curves reveal that while the association constant remained unchanged (Ka approximately 1.0 x 10(5) M-1), only a fraction of the albumin molecules present reacted with the ligand. The results were unaffected by the nature and the concentration of the surfactant. These findings can be interpreted in the light of conformational changes induced in human serum albumin by the large micellar inner surface area. The blue shift of the fluorescence emission maximum from 344 nm in buffer to 327 nm in sodium bis(2-ethylhexyl)sulfosuccinate micelles and the lesser reactivity/accessibility of the fluorophore to oxidation by N-bromosuccinimide, indicate perturbations of the sole tryptophan-214 microenvironment. However, the distance between the indole residue and tyrosine-411 does not seem substantially modified by the 15% decrease affecting the alpha helices of the albumin molecule. It is proposed that the results reported herein reflect the interactions of albumin with a membrane-like interface which generates two protein subpopulations differing in their membrane-surface and ligand affinities. Overall and local conformational changes, originating from this surface-induced effect, may thus constitute a ligand-release facilitating mechanism acting at cellular membrane levels.

Structural parameters of the myelin transmembrane proteolipid in reverse micelles

The Folch-Pi proteolipid is the most abundant structural protein from the central nervous system myelin. This protein-lipid complex, normally insoluble in water, requires only a small amount of water for solubilization in reverse micelles of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) in isooctane. The characterization of the proteolipid-free and proteolipid-containing micelles was undertaken by light scattering and fluorescence recovery after fringe pattern photobleaching (FRAPP) experiments. Quasi elastic light scattering (QELS) was carried out at a high (200 mM) AOT concentration, at low water-to-surfactant mole ratio (Wo = 7) and at increasing protein occupancy. Two apparent hydrodynamic radii, differing tenfold in size, were obtained from correlation functions. The smaller one (RaH = 5.2 nm) remains constant and corresponds to that measured for protein-free micelles. The larger one increases linearly with protein concentration. In contrast, FRAPP measurements of self-diffusion coefficients were found unaffected by the proteolipid concentration. Accordingly, they have been performed at constant protein/surfactant mole ratios. The equivalent RH, extrapolated to zero AOT concentration for protein-free reverse micelles (2.9 nm) and in the presence of the proteolipid (4.6 nm), do not reveal the mode of organization previously suggested by QELS measurements. The complex picture emerging from this work represents a first step in the characterization of an integral membrane protein in reverse micelles.

Myelin proteins in reverse micelles: tight lipid association required for insertion of the Folch-Pi proteolipid into a membrane-mimetic system

The solubility and reactivity of the Folch-Pi proteolipid from bovine CNS have been studied in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate, isooctane, and water. Such a membrane-mimetic system resembles the aqueous spaces of the native myelin sheath in terms of its physicochemical properties. Although the proteolipid is completely insoluble in water, it can be inserted into the water-containing micellar system. In contrast, the lipid-depleted protein failed to be incorporated into these organized assemblies. The lipid requirements for insertion of the proteolipid were studied, therefore, after delipidation by several precipitations with isooctane, a nondenaturing solvent. Novel extraction procedures and quantitative analyses by HPLC of the protein-bound lipids revealed the persistence of a lipid-protein complex (6 +/- 1 mol of lipid/mol of protein) displaying optimal micellar solubilization. Competition experiments carried out with brain lipids provide evidence for a preference of the myelin protein for sulfatide, phosphatidylinositol, and phosphatidylserine, in that order. The resulting proteolipid, although differing in relative composition, showed good solubility in the membrane-mimetic system. In contrast, reconstitution experiments carried out with the lipid-depleted protein resulted in weak lipid binding and poor micellar incorporation. These results suggest that the tightly bound acidic lipids may stabilize a protein conformation required for insertion into the micellar system.

Conformational aspects and rotational dynamics of synthetic adrenocorticotropin-(1-24) and glucagon in reverse micelles

The tryptophan (Trp) rotational dynamics and the secondary structure of the peptide hormones adrenocorticotropin-(1-24) [ACTH(1-24)]--the fully active N-terminal fragment of adrenocorticotropin-(1-39)--and glucagon were studied in aqueous solutions and in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/water/isooctane, a system selected to mimic the membrane-water interface. In aqueous solutions, the total fluorescence intensity decays of their single Trp residue [Trp-9 and Trp-25 for ACTH(1-24) and glucagon, respectively] are multiexponential. This is also the case for ACTH(5-10), a fragment of the adrenocorticotropin "message" region. Time-resolved fluorescence anisotropy data evidence a high degree of rotational freedom of the single Trp residue. Transfer of these peptides from water to the aqueous core of reverse micelles induces severe restrictions of the Trp internal motion and of its local environment. The results indicate that the Trp-9 residue in ACTH(1-24 is maintained in the close neighborhood of the water-AOT molecular interface where the water molecules are strongly immobilized. By contrast, the Trp residues in ACTH(5-10) and glucagon are likely to be located closer to the center of the micellar aqueous core where the water molecules are in a more mobile state. Furthermore, the above location of Trp can be extended to the peptide chains themselves as evidenced by the overall correlation time values of the peptide-containing micelles. Nevertheless, in all peptides, the indole ring remains susceptible to oxidation by N-bromosuccinimide. Circular dichroism measurements evidence the induction in glucagon of alpha-helices remaining unaffected by the micellar water content. Conversely, beta-sheet structures are favored in ACTH(1-24) at low water-to-surfactant molar ratios (w0) but are disrupted by subsequent additions of water. These results are discussed in terms of the possible role of the micellar interfaces in selecting the preferred peptide dynamical conformation(s)