Photoresponsive Viscosity and Host−Guest Association in Aqueous Mixtures of Poly-Cyclodextrin with Azobenzene-Modified Poly(acrylic)acid

In aqueous solutions, beta-cyclodextrin (CD) and cyclodextrin-containing polymers (PolCD) associate with azobenzene-modified polyacrylate (AMP). Inclusion complexes in solution of CD (or PolCD) and AMP, and the viscosity of these mixtures, have been studied as a function of the composition of AMP and concentrations of samples. AMPs are random copolymers containing a low fraction of a light-responsive hydrophobic moieties (<10 mol % of 6-[4-alkylamido]phenylazobenzene acrylamide), and a charged hydrophilic unit, sodium acrylate. PolCDs are beta-cyclodextrin randomly conjugated with epichlorohydrin and fractionated to yield copolymers of average number of CD per chain equal to 50. In dilute solutions, the composition of complexes has been investigated by capillary electrophoresis and UV-vis spectrometry. Association between PolCD and AMP appears more complex than the conventional Benesi-Hildebrand scheme. We identified a tight (quantitative) binding regime followed by a gradual increase of the density of AMP-bound PolCD upon increasing the concentration of PolCD. At higher concentrations, the formation of large clusters has been characterized by the increase of viscosity by several decades. Light-triggered trans-conformation of the azobenzene moieties of AMPs leads to a marked photoswitch of viscosity. Reversible viscosity swings by up to 6-fold were achieved by alternative exposure to UV and visible lights. In contrast, the composition of PolCD/AMP complexes in dilute regime does not respond to light, though subtle modifications of the structures of complexes are reflected by variation of electrophoretic mobilities and UV spectra. The properties of interpolymer clusters and photoviscosity are accordingly the result of modification of the dynamics of association. In practice, the low concentration of photochrome makes it possible to obtain rapid responses in samples having a thickness of the order of cm. The data reported provide guidelines for the formulations of CD/polymer systems, specifically, viscosity enhancers, which should show promising developments in pharmaceuticals or cosmetics.

Enthalpy of interaction and binding isotherms of non-ionic surfactants onto micellar amphiphilic polymers (amphipols)

The interactions in water between short amphiphilic macromomolecules, known as amphipols, and three neutral surfactants (detergents), dodecylmaltoside (DM), n-octylthioglucoside (OTG), and n-octyltetraethyleneoxide (C8E4), have been assessed by static and dynamic light-scattering (SLS and DLS), capillary electrophoresis (CE), and isothermal titration calorimetry (ITC). The amphipols selected are random copolymers of the hydrophobic n-octylacrylamide (25-30 mol %), a charged hydrophilic monomer, either acrylic acid ( approximately 35 mol %) or a phosphorylcholine-modified acrylamide (40-70 mol %), and, optionally, N-isopropylacrylamide (30-40 mol %). In water, the copolymers form micelles of small size (hydrodynamic radius: approximately 5 nm). Neutral surfactants, below their critical micellar concentration (cmc), form mixed micelles with the amphipols irrespective of the chemical structure of the detergent or the polymer. The fraction of detergent in the surfactant/polymer complexes increases significantly (cooperatively) as the surfactant concentration nears the cmc. The ITC data, together with data gathered by CE, were fitted via a regular mixing model, which allowed us to predict the detergent concentration in equilibrium with complexes and the heat evolved upon transfer of detergent from water into a mixed surfactant/polymer complex. The enthalpy of transfer was found to be almost equal to the enthalpy of micellization, and the regular mixing model points to a near-ideal mixing behavior for all systems. Amphipols are promising tools in biochemistry where they are used, together with neutral surfactants, for the stabilization and handling of proteins. This study provides guidelines for the optimization of current protein purification protocols and for the formulations of surfactant/polymer systems used in pharmaceutics, cosmetics, and foodstuffs.

Light-responsive hydrophobic association of azobenzene-modified poly(acrylic acid) with neutral surfactants

Photoresponsive association between azobenzene-modified poly(acrylic acid)s (AMPs) and the nonionic surfactants tetraethylene glycol monododecyl ether and octadecyl ether (C12E4 and C18E4) has been achieved in dilute aqueous solution. The binding was investigated by (i) spectrophotometry that probes the polarity close to the azobenzene chromophore, (ii) capillary electrophoresis to obtain the amount of C12E4 bound per polymer chain, and (iii) pressure-area curves of Langmuir films to obtain information on the adsorption of AMP at the water-C18E4 interface. Increasing hydrophobicity of AMP (with increasing degree of modification with azobenzene side-groups) tightened the association with C12E4 in the dark. Exposure to UV light rapidly converted the azobenzene to their more polar cis isomer, which in turn weakened the association with surfactant. Almost complete photorelease of bound C12E4 was obtained with the optimal structure of AMP. Adsorption on large interfaces is much less sensitive to light. The possible origin of the photoresponse is analyzed in terms of AMP affinity for surfactant assemblies and azobenzene penetration in the hydrophobic core of micelles. We propose that the photoswing of polarity is amplified by the binding to small micelles because of the small number of anchors involved. A few azobenzene anchors afford tight binding in the dark, but also detach more easily than the whole AMP chain upon photoisomerization.

Long-living channels of well defined radius opened in lipid bilayers by polydisperse, hydrophobically-modified polyacrylic acids

In the presence of random amphiphilic polymers devoid of secondary structure, lipid membranes become leaky to albumin and dextran (fluorescence of giant unilamellar vesicles) and their conductance (black lipid films) shows the transient opening of channels of typical radius 1-5 nm.

Size, charge, and interactions with giant lipid vesicles of quantum dots coated with an amphiphilic macromolecule

Semiconductor colloidal quantum dots (QDs) are promising fluorescent probes for biology. Initially synthesized in organic solvents, they can be dispersed in aqueous solution by noncovalent coating with amphiphilic macromolecules, which renders the particles hydrophilic and modifies their interactions with other biological compounds. Here, after coating QDs with an alkyl-modified polyacrilic acid, we investigated their colloidal properties in aqueous buffers by electrophoresis, electron microscopy, light scattering, and rate zonal centrifugation. Despite polymer dispersity and variation of the size of the inorganic nanoparticles, the polymer-dot complexes appeared relatively well-defined in terms of hydrodynamic radius and surface charge. Our data show that these complexes contain isolated QD surrounded by a polymer layer with thickness 8-10 nm. We then analyzed their interaction with giant unilamellar vesicles, either neutral or cationic, by optical microscopy. At neutral pH, we found the absence of binding of the coated particles to lipid membrane, irrespective of their lipid composition. An abrupt surface aggregation of the nanoparticles on the lipid membrane was observed in a narrow pH range (6.0-6.2), indicative of critical binding triggered by polymer properties. The overall features of QDs coated with amphiphilic polymers open the route to using these nanoparticles in vivo as optically stable probes with switchable properties.

Well-defined nanoparticles formed by hydrophobic assembly of a short and polydisperse random terpolymer, amphipol A8-35

Amphipols are short amphilic polymers designed for applications in membrane biochemistry and biophysics and used, in particular, to stabilize membrane proteins in aqueous solutions. Amphipol A8-35 was obtained by modification of a short-chain parent polymer (poly(acrylic acid); PAA) with octyl- and isopropylamine, to yield an amphiphilic product with an average molar mass of 9-10 kg x mol(-1) (sodium salt form) and a polydispersity index of 2.0 to 3.1, depending on the source of PAA. The behavior of A8-35 in aqueous buffers was studied by size exclusion chromatography, static and dynamic light scattering, equilibrium and sedimentation velocity analytical ultracentrifugation, and small angle neutron scattering. Despite the variable length of the chains and the random distribution of hydrophobic groups along them, A8-35 self-organizes into well-defined assemblies. The data are best compatible with most of the polymer forming compact assemblies (particles) with a molar mass of approximately 40 kg x mol(-1), a radius of gyration of approximately 2.4 nm, and a Stokes radius of approximately 3.15 nm. Each particle contains, on average, four A8-35 macromolecules and 75-80 octyl chains. Neutron scattering reveals a sharp interface between the particles and water. A minor (approximately 0.1%) mass fraction of the material forms much larger aggregates, whose proportion may increase under certain conditions of preparation or handling, such as low pH. They can be removed by gel filtration.

Partial specific volume and solvent interactions of amphipol A8-35

Amphipols are small amphiphilic polymers that can stabilize and keep soluble membrane proteins in aqueous solutions in the absence of detergent. A prerequisite to solution studies of membrane protein/amphipol complexes is the determination of the partial specific volume v2 and effective charge z of the polymer. The ratio (R) of the buoyant molar masses of particles in D2O and H2O solutions, obtained from sedimentation velocity (sH/sD method) and sedimentation equilibrium experiments, and their contrast match point (CMP), determined in small-angle neutron scattering experiments, depend on v2 and z. When z is known, v2 can be estimated from R with a good accuracy as long as v2 is close to 1. The effects of labile H/D exchange and of polyelectrolyte counter-ion dissociation in general cannot be neglected. The accuracy, advantages, and limits of the sH/sD method have been studied in details using model macromolecules (DNA, protein, and polysaccharide). The sH/sD method appears particularly advantageous for the study of heterogeneous samples. Measurements of density, sD/sH buoyant molar masses in H2O, D2O, and D2(18)O, and CMP of hydrogenated and partially deuterated A8-35, a polyacrylate-based amphipol containing 35 underivatized carboxylates per 100 monomers, led to a consistent description of its buoyancy and charge properties.

Effects of Protein−Polyelectrolyte Affinity and Polyelectrolyte Molecular Weight on Dynamic Properties of Bovine Serum Albumin−Poly(diallyldimethylammonium chloride) Coacervates

Bovine serum albumin (BSA) and poly(diallyldimethylammonium chloride) (PDADMAC) spontaneously form, over a range of ionic strength I and pH, dense fluids rich in both macroions. To study their nanostructure, these coacervates were prepared at low I and high pH (strong interaction) or at high I and lower pH (weaker interaction), with polymer MWs ranging from 90K to 700K, and then examined by dynamic light scattering (DLS) and rheology. DLS shows a dominant and surprisingly fast protein diffusional mode independent of polymer MW; accompanied by robust slow modes, slower by 1-2 orders of magnitude, which are also insensitive to MW and are present regardless of I, pH, and sample aging. High MW sensitivity was observed by rheology for the terminal time (order of milliseconds), which increased as well with the strength of polyelectrolyte-protein interaction. Viscoelastic behavior also indicated a tenuous network, solidlike at low strain but re-forming after breakage by shear. Two models, both of which have strengths and defects, are put forward: (I) macroion-rich domains dispersed in a continuum of macroion-poor domains near the percolation limit and (II) a semidilute solution of PDADMAC chains with interchain friction modulated by transient BSA-PDADMAC association.

Association of octyl-modified poly(acrylic acid) onto unilamellar vesicles of lipids and kinetics of vesicle disruption

Water-soluble polymers containing a few hydrophobic anchors are known to bind onto lipid vesicles and are used as stabilizers of liposome-based formulas. In contrast, polymers with high hydrophobicity destabilize the lipid bilayers. With macromolecules of intermediate hydrophobic/hydrophilic balance, a gradual sweep of the stabilization-destabilization capacity can be achieved and is considered as promising triggered systems for drug release, although the mechanism of permeabilization and membrane breakage using polymers is essentially conjectural to date. As a model system, we used short octyl-modified poly(acrylic acid)s (MW 8000 g/mol) sensitive to pH, temperature, and ionic strength in conjunction with small unilamellar vesicles mainly comprised of DPPC or egg-PC. Kinetics of vesicle fragmentation was followed using static and dynamic light scattering. Polymer adsorption was studied by nonradiative energy transfer between pyrene-labeled lipids and a naphthalene-modified polymer. The permeability of the vesicles was characterized by calcein leakage experiments. The key findings were (i) the lack of coupling between the density of bound polymer and the rate of disruption and (ii) the qualitative difference depending on whether the polymer contains or not isopropyl side groups. Point i relates to the increase of the rate of polymer adsorption with increasing bulk polymer concentration, while the breakage is essentially unaffected. Point ii relates to the stabilization of large membrane fragments (Stokes radius ca. 40 nm) in the presence of a polymer with no isopropyl side groups, while micelle-like assemblies (Stokes radius 8 nm) containing the lipids are obtained with an isopropyl-containing polymer of similar hydrophobicity. Both polymers display similar efficiency at disrupting small vesicles. The mechanism of polymer-induced disruption appears to differ markedly from the disruption steps now recognized for conventional (molecular) surfactant and is discussed on the basis of data obtained with different membrane fluidity, polymer structure, concentration, and hydrophilicity.

Amphipols: polymeric surfactants for membrane biology research

Membrane proteins classically are handled in aqueous solutions as complexes with detergents. The dissociating character of detergents, combined with the need to maintain an excess of them, frequently results in more or less rapid inactivation of the protein under study. Over the past few years, we have endeavored to develop a novel family of surfactants, dubbed amphipols (APs). APs are amphiphilic polymers that bind to the transmembrane surface of the protein in a noncovalent but, in the absence of a competing surfactant, quasi-irreversible manner. Membrane proteins complexed by APs are in their native state, stable, and they remain water-soluble in the absence of detergent or free APs. An update is presented of the current knowledge about these compounds and their demonstrated or putative uses in membrane biology.

Ionic strength dependence of protein-polyelectrolyte interactions

The effect of univalent electrolyte concentration on protein-polyelectrolyte complex formation has been measured by frontal analysis continuous capillary electrophoresis (FACCE) and turbidimetry for the interaction of bovine serum albumin (BSA) with a synthetic hydrophobically modified polyacid, for BSA with (porcine mucosal) heparin (Hp), a highly charged polyanion, and for Hp and insulin. All three highly diverse systems display maxima or plateaus in complex formation in the range of ionic strength 5 < I < 30 mM, confirmed in the case of BSA-Hp by multiple techniques. Similar maxima are reported in the literature, but with little discussion, for BSA-poly(dimethyldiallylammonium chloride), lysozyme-hyaluronic acid, and lysozyme-chondroitin sulfate, always in the I range 5-30 mM. While inversion of salt effect has been discussed specifically for the interaction of gelatin and sodium polystyrenesulfonate with gelatin(28) and with beta-lactoglobulin,(10) the general nature of this phenomenon, regardless of polyelectrolyte origin, molecular weight, and charge sign has not been recognized. The position of the maxima and their occurrence when protein and polyelectrolyte have the same net charge imply that they arise when Debye lengths extend, at low I, beyond half the protein diameter so that addition of salt screens repulsions, as well as attractions. This appears to be a general effect caused by electrostatic repulsions that can coexist simultaneously with hydrophobic interactions. Modeling of protein electrostatics via Delphi is used to visualize this effect for BSA, lysozyme, insulin, and beta-lactoglobulin.

Allosteric transitions of Torpedo acetylcholine receptor in lipids, detergent and amphipols: molecular interactions vs. physical constraints

The binding of a fluorescent agonist to the acetycholine receptor from Torpedo electric organ has been studied by time-resolved spectroscopy in three different environments: in native membrane fragments, in the detergent CHAPS, and after complexation by amphipathic polymers ('amphipols'). Binding kinetics was similar in the membrane and in amphipols, demonstrating that the receptor can display unaltered allosteric transitions outside its natural lipid environment. In contrast, allosteric equilibria were strongly shifted towards the desensitized state in CHAPS. Therefore, the effect of CHAPS likely results from molecular interactions rather than from the loss of bulk physical properties of the membrane environment.

Nonionic amphiphilic polymers derived from Tris(hydroxymethyl)-acrylamidomethane keep membrane proteins soluble and native in the absence of detergent

A new family of amphipols-amphiphilic polymers designed to form water-soluble complexes with membrane proteins-was synthesized by free-radical telomerization of Tris(hydroxymethyl)-acrylamidomethane (THAM) and derivatized THAM. Some of these polymers were found to prevent aggregation and denaturation of two model membrane proteins, bacteriorhodopsin and cytochrome b(6) f, in the absence of detergent micelles.

Slow Reorganization of Small Phosphatidylcholine Vesicles upon Adsorption of Amphiphilic Polymers

Static or dynamic light scattering measurements were performed in parallel, on dilute mixtures of DPPC/DPPA vesicles (typical radius 60 nm) and hydrophobically modified polymers. This technique gave evidence of the slow kinetics involved in both the reorganization of an adsorbed polymer layer and the membrane breakage. Hours, or sometimes days, were required in order to follow the variation of both the hydrodynamic radius and the scattering intensity at intermediate stages. Images of the intermediate species were collected using freeze-fracture electron microscopy (FFEM). Comparison of different polymers (of varying molecular weight or structure) revealed the prime importance of hydrophobicity on the disruption of membranes. Although the presence of a few percent of pendant alkyl chains along the polymer backbone induced adsorption to membranes, only the association with the more hydrophobic ones (>25 mol% of pendant octyl groups) resulted in small mixed objects of micellar size (radius about 10 nm). The drop of the mean radius of intermediate structures formed upon the vesicle breakage was also sensitive to temperature. A tentative mechanism was proposed on the basis of kinetics and FFEM studies. Copyright 2001 Academic Press.

Interaction of amphipols with sarcoplasmic reticulum Ca2+-ATPase

Amphipols are short-chain amphipathic polymers designed to keep membrane proteins soluble in aqueous solutions. We have evaluated the effects of the interaction of amphipols with sarcoplasmic reticulum Ca(2+)-ATPase either in a membrane-bound or a soluble form. If the addition of amphipols to detergent-solubilized ATPase was followed by removal of detergent, soluble complexes formed, but these complexes retained poor ATPase activity, were not very stable upon long incubation periods, and at high concentrations they experienced aggregation. Nevertheless, adding excess detergent to diluted detergent-free ATPase-amphipol complexes incubated for short periods immediately restored full activity to these complexes, showing that amphipols had protected solubilized ATPase from the rapid and irreversible inactivation that otherwise follows detergent removal. Amphipols also protected solubilized ATPase from the rapid and irreversible inactivation observed in detergent solutions if the ATPase Ca(2+) binding sites remain vacant. Moreover, in the presence of Ca(2+), amphipol/detergent mixtures stabilized concentrated ATPase against inactivation and aggregation, whether in the presence or absence of lipids, for much longer periods of time (days) than detergent alone. Our observations suggest that mixtures of amphipols and detergents are promising media for handling solubilized Ca(2+)-ATPase under conditions that would otherwise lead to its irreversible denaturation and/or aggregation.

Hydrophobically driven attachments of synthetic polymers onto surfaces of biological interest: lipid bilayers and globular proteins

This paper gives a brief overview of the consequences of associations between amphiphilic water-soluble polymers and small colloidal particles of biological interest: proteins and vesicles. Typical structures of water-soluble synthetic polymers containing hydrophobic groups are presented. The segregation between polar and apolar units in these polymers induces self-organisation in micro-domains despite the lack of specific primary structure. In the presence of other amphiphilic particles like proteins and vesicles, mixed assemblies are formed. Examples of polymer associations with vesicles or globular proteins, mainly focused on the acrylic derivatives, bring out common features in these mixtures. When the size of the polymer is of the same order of magnitude as that of the particle, adsorption of polymer chains creates a protective layer around each individual particle. Depending on the hydrophobicity of the partners, the association can stabilise the dispersion of unmodified particles or induce structural changes (membrane disruption, leakage). When small particles are added to solutions of long polymers, multimolecular complexation occurs. In this case, the size of the resulting aggregates depends on the concentrations. It goes from the size of one polymer molecule up to formally infinity as revealed by gelation. The identification of non-specific association modes between biological nanoparticles and macromolecules might be revealed by the general behaviour of these synthetic mixed systems.

Scanning transmission electron microscopy study of the molecular mass of amphipol/cytochrome b6f complexes

The composition and mass of complexes between Chlamydomonas reinhardtii cytochrome b6f and low molecular mass amphipathic polymers ('amphipols') have been studied using biochemical analysis and scanning transmission electron microscopy at liquid helium temperature (cryo-STEM). Cytochrome b6f was trapped by amphipols either under its native 14-meric state or as a delipidated, lighter form. A good consistency was observed between the masses of either form calculated from their biochemical composition and those determined by cryo-STEM. These data show that association with amphipols preserved the original original state of the protein in detergent solution. Complexation with amphipols appears to facilitate preparation of the samples and mass determination by cryo-STEM as compared to conventional solubilization with detergents.

Dimer to monomer conversion of the cytochrome b6 f complex. Causes and consequences

The molecular weight of the cytochrome b6 f complex purified from Chlamydomonas reinhardtii thylakoid membranes has been determined by combining velocity sedimentation measurements, molecular sieving analyses, and determination of its lipid and detergent content. The complex in its enzymatically active form is a dimer. Upon incubation in detergent solution, it converts irreversibly into an inactive, monomeric form that has lost the Rieske iron-sulfur protein, the b6 f-associated chlorophyll, and, under certain conditions, the small 32-residue subunit PetL. The results are consistent with the view that the dimer is the predominant form of the b6f in situ while the monomer observed in detergent solution is a breakdown product. Indirect observations suggest that subunit PetL plays a role in stabilizing the dimeric state. Delipidation is shown to be a critical factor in detergent-induced monomerization.

Amphipols: polymers that keep membrane proteins soluble in aqueous solutions

Amphipols are a new class of surfactants that make it possible to handle membrane proteins in detergent-free aqueous solution as though they were soluble proteins. The strongly hydrophilic backbone of these polymers is grafted with hydrophobic chains, making them amphiphilic. Amphipols are able to stabilize in aqueous solution under their native state four well-characterized integral membrane proteins: (i) bacteriorhodopsin, (ii) a bacterial photosynthetic reaction center, (iii) cytochrome b6f, and (iv) matrix porin.

Use of neutral surfactants for the capillary electrophoretic separation of hydrophobically modified poly(acrylic acids)

Hydrophobically modified poly(acrylic acids) (HMPAs) are random copolymers of sodium acrylate and dodecyl acrylamide, containing 0-10% mol/mol of dodecyl grafts. The hydrophobic character of different HMPAs of average molecular weight 150,000 was studied by capillary electrophoresis (CE), using neutral surfactants as buffer additives. The differentiation of the electrophoretic mobilities of HMPAs with their hydrophobicity was achieved through the use of nonionic Brij 35 and zwitterionic DAPS surfactants. A nearly baseline separation of the precursor and three HMPAs derivatives was obtained in a poly(ethylene glycol)-coated capillary with a background electrolyte containing 10 mM N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DAPS) and 10 mM borax (pH 9.2). In addition to CE experiments, the polymer-surfactant interactions were also investigated by means of quasi-elastic light scattering (QELS) and viscosimetric measurements. According to the latter results, the separation mechanism was interpreted as an expansion of the polymer coil in the presence of micelles and subsequent change of its frictional properties. A true micellar electrokinetic capillary chromatography (MEKC) partitioning model was discarded on the basis of the relative sizes of the macromolecule and the micelles.