Rodlike complexes of a polyelectrolyte (hyaluronan) and a protein (lysozyme) observed by SANS

We study by small-angle neutron scattering (SANS) the structure of hyaluronan -lysozyme complexes. Hyaluronan (HA) is a polysaccharide of 9 nm intrinsic persistence length that bears one negative charge per disaccharide monomer (M(mol) = 401.3 g/mol); two molecular weights, M(w) = 6000 and 500,000 Da were used. The pH was adjusted at 4.7 and 7.4 so that lysozyme has a global charge of +10 and +8, respectively. The lysozyme concentration was varied from 3 to 40 g/L at constant HA concentration (10 g/L). At low protein concentration, samples are monophasic, and SANS experiments reveal only fluctuations of concentration, although, at high protein concentration, clusters are observed by SANS in the dense phase of the diphasic samples. In between, close to the onset of the phase separation, a distinct original scattering is observed. It is characteristic of a rod-like shape, which could characterize "single" complexes involving one or a few polymer chains. For the large molecular weight (500,000), the rodlike rigid domains extend to much larger length scale than the persistence length of the HA chain alone in solution and the range of the SANS investigation. They can be described as a necklace of proteins attached along a backbone of diameter of one or a few HA chains. For the short chains (M(w) ≈ 6000), the rod length of the complexes is close to the chain contour length (～ 15 nm).     

Enhancement of viral fusion by nonadsorbing polymers.

Nonadsorbing polymers such as dextran and poly(ethylene glycol) enhance binding as well as extents of fusion of influenza virus with erythrocytes. Kinetics and extent of viral membrane fusion were measured using an assay based on lipid mixing of a fluorescent dye. The effects of nonadsorbing polymers were in the concentration range from 0 to 10 wt%, far below the concentration required to overcome hydration repulsion forces. The enhancing effects were dependent on the molecular weight of nonadsorbing polymer, and only occurred at molecular weight > 1500; this links the phenomena we observe to the so-called "excluded volume effect" of nonadsorbing polymers. The time delay between triggering and the onset of influenza virus fusion was significantly reduced in the presence of nonadsorbing polymers. High molecular weight poly(ethylene glycol) also induced fusion of vesicular stomatitis virus with intact erythrocytes, which do not serve as target of vesicular stomatitis virus fusion in the absence of the polymer. The forces between membranes which determine rate-limiting processes in viral fusion and how they are affected by nonadsorbing polymers are discussed.     

Probing lysozyme conformation with light reveals a new folding intermediate

A means to control lysozyme conformation with light illumination has been developed using the interaction of the protein with a photoresponsive surfactant. Upon exposure to the appropriate wavelength of light, the azobenzene surfactant undergoes a reversible photoisomerization, with the visible-light (trans) form being more hydrophobic than the UV-light (cis) form. As a result, surfactant binding to the protein and, thus, protein unfolding, can be tuned with light. Small-angle neutron scattering (SANS) measurements were used to provide detailed information of the protein conformation in solution. Shape-reconstruction methods applied to the SANS data indicate that under visible light the protein exhibits a native-like form at low surfactant concentrations, a partially swollen form at intermediate concentrations, and a swollen/unfolded form at higher surfactant concentrations. Furthermore, the SANS data combined with FT-IR spectroscopic analysis of the protein secondary structure reveal that unfolding occurs primarily in the alpha domain of lysozyme, while the beta domain remains relatively intact. Thus, the surfactant-unfolded intermediate of lysozyme appears to be a separate structure than the well-known alpha-domain intermediate of lysozyme that contains a folded alpha domain and unfolded beta domain. Because the interactions between the photosurfactant and protein can be tuned with light, illumination with UV light returns the protein to a native-like conformation. Fluorescence emission data of the nonpolar probe Nile red indicate that hydrophobic domains become available for probe partitioning in surfactant-protein solutions under visible light, while the availability of these hydrophobic domains to the probe decrease under UV light. Dynamic light scattering and UV-vis spectroscopic measurements further confirm the shape-reconstruction findings and reveal three discrete conformations of lysozyme. The results clearly demonstrate that visible light causes a greater degree of lysozyme swelling than UV light, thus allowing for the protein conformation to be controlled with light.     

Small-Angle Neutron Scattering Characterization of Monoclonal Antibody Conformations and Interactions at High Concentrations

Small-angle neutron scattering (SANS) is used to probe the solution structure of two protein therapeutics (monoclonal antibodies 1 and 2 (MAb1 and MAb2)) and their protein-protein interaction (PPI) at high concentrations. These MAbs differ by small sequence alterations in the complementarity-determining region but show very large differences in solution viscosity. The analyses of SANS patterns as a function of different solution conditions suggest that the average intramolecular structure of both MAbs in solution is not significantly altered over the studied protein concentrations and experimental conditions. Even though a strong repulsive interaction is expected for both MAbs due to their net charges and low solvent ionic strength, analysis of the SANS data shows that the effective PPI for MAb1 is dominated by a very strong attraction at small volume fraction that becomes negligible at large concentrations. The MAb1 PPI cannot be modeled simply by a spherically symmetric central forces model. It is proposed that an anisotropic attraction strongly affects the local interprotein structure and leads to an anomalously large viscosity of concentrated MAb1 solutions. Conversely, MAb2 displays a repulsive interaction potential throughout the concentration series probed and a comparatively small solution viscosity.     

Small-Angle Neutron Scattering Characterization of Monoclonal Antibody Conformations and Interactions at High Concentrations

Small-angle neutron scattering (SANS) is used to probe the solution structure of two protein therapeutics (monoclonal antibodies 1 and 2 (MAb1 and MAb2)) and their protein-protein interaction (PPI) at high concentrations. These MAbs differ by small sequence alterations in the complementarity-determining region but show very large differences in solution viscosity. The analyses of SANS patterns as a function of different solution conditions suggest that the average intramolecular structure of both MAbs in solution is not significantly altered over the studied protein concentrations and experimental conditions. Even though a strong repulsive interaction is expected for both MAbs due to their net charges and low solvent ionic strength, analysis of the SANS data shows that the effective PPI for MAb1 is dominated by a very strong attraction at small volume fraction that becomes negligible at large concentrations. The MAb1 PPI cannot be modeled simply by a spherically symmetric central forces model. It is proposed that an anisotropic attraction strongly affects the local interprotein structure and leads to an anomalously large viscosity of concentrated MAb1 solutions. Conversely, MAb2 displays a repulsive interaction potential throughout the concentration series probed and a comparatively small solution viscosity.     

Inhibition of pulmonary surfactant adsorption by serum and the mechanisms of reversal by hydrophilic polymers: theory

A theory based on the Smolukowski analysis of colloid stability shows that the presence of charged, surface-active serum proteins at the alveolar air-liquid interface can severely reduce or eliminate the adsorption of lung surfactant from the subphase to the interface, consistent with the observations reported in the companion article (pages 1769-1779). Adding nonadsorbing, hydrophilic polymers to the subphase provides a depletion attraction between the surfactant aggregates and the interface, which can overcome the steric and electrostatic resistance to adsorption induced by serum. The depletion force increases with polymer concentration as well as with polymer molecular weight. Increasing the surfactant concentration has a much smaller effect than adding polymer, as is observed. Natural hydrophilic polymers, like the SP-A present in native surfactant, or hyaluronan, normally present in the alveolar fluids, can enhance adsorption in the presence of serum to eliminate inactivation.     

Enhanced enzymatic activity through photoreversible conformational changes

The interaction of a light-responsive surfactant with lysozyme at pH 5.0 has been investigated as a means to control protein structure and enzymatic activity with light illumination. The cationic azobenzene surfactant undergoes a reversible photoisomerization upon exposure to the appropriate wavelength of light, with the visible-light (trans) form being more hydrophobic and, thus, inducing a greater degree of protein unfolding than the UV-light (cis) form. Conformational changes as a function of photoresponsive surfactant concentration and light illumination were measured through shape-reconstruction analysis of small-angle neutron scattering (SANS) data. The SANS-based in vitro structures indicate that lysozyme transitions from a nativelike structure at low surfactant concentration to a partially unfolded conformation at higher surfactant concentrations under visible light illumination, while UV-light illumination causes the protein to refold to a near-native structure. Protein swelling occurs principally away from the active site near the hinge region connecting the alpha and beta domains, leading to an increase in the observed separation distance of the alpha and beta domains in the ensemble SANS measurements, a likely result of enhanced domain motions and increased flexibility within the protein. This swelling of the hinge region is accompanied by an 8-fold increase in enzymatic activity relative to the native state. Both enzyme swelling and superactivity observed under visible light can be reversed to nativelike conditions upon exposure to UV light, leading to complete photoreversible control of the structure and function of lysozyme.     

Enhanced surfactant adsorption via polymer depletion forces: a simple model for reversing surfactant inhibition in acute respiratory distress syndrome

Lung surfactant adsorption to an air-water interface is strongly inhibited by an energy barrier imposed by the competitive adsorption of albumin and other surface-active serum proteins that are present in the lung during acute respiratory distress syndrome. This reduction in surfactant adsorption results in an increased surface tension in the lung and an increase in the work of breathing. The reduction in surfactant adsorption is quantitatively described using a variation of the classical Smolukowski analysis of colloid stability. Albumin adsorbed to the interface induces an energy barrier to surfactant diffusion of order 5 k(B)T, leading to a reduction in adsorption equivalent to reducing the surfactant concentration by a factor of 100. Adding hydrophilic, nonadsorbing polymers such as polyethylene glycol to the subphase provides a depletion attraction between the surfactant aggregates and the interface that eliminates the energy barrier. Surfactant adsorption increases exponentially with polymer concentration as predicted by the simple Asakura and Oosawa model of depletion attraction. Depletion forces can likely be used to overcome barriers to adsorption at a variety of liquid-vapor and solid-liquid interfaces.     

Polymer-cushioned lipid bilayers in porous alumina

Using small-angle neutron scattering (SANS) and cyclic voltammetry (CV), we show that model biological membranes can be deposited on a polymer cushion confined in highly regular porous alumina. The thicknesses of the dilute polymer cushion chemically bound to the alumina and of the supported bilayer are obtained for two polyethylene glycol cushions (PEG₅₀₀₀ and PEG₂₀₀₀₀) and for a cushion made of chains bearing a lipid anchor at their free end (DSPE-PEG₃₄₀₀). The bilayers are studied well below and well above the chain melting temperature of the lipid mixture (DMPC/DMPE: 80/20), using a coenzyme (Ubiquinone, UQ₁₀) as a redox probe for the voltammetry experiments. Analysis of the SANS form factor of the bilayers shows that the bilayer thickness can be extracted in this particular geometry. Using PEG chains grafted at a low surface density (D<2R_g), the thickness of the complete molecular construction is obtained by CV, which shows (after subtracting the bilayer thickness) that the polymer cushion thickness can be varied from 50 to 150 Å. The values obtained with three different chain lengths, are in perfect agreement with the radius derived from the Flory theory.     

Protein partitioning at the isoelectric point: influence of polymer molecular weight and concentration and protein size

We report the partition coefficient, K(p') at the isoelectric point of lysozyme, chymotrypsinogen A, albumin, transferrin, and catalase in 64 different polyethylene(PEG)/ dextran(Dx)/water systems. We study the trends of the partition coefficient with protein type, polymer concentration, and polymer molecular weight. We find that the partition coefficient decreases with increasing tie line length for lysozyme, albumin, transferrin, and catalase for which K(p) is less than 1, but increases for chymotrysinogen for which K(p) is larger than 1. The effect of the tie line length on the partition coefficient is larger for the large proteins than for the small proteins. The partition coefficient decreases with increasing protein molecular weight except for lysozyme suggesting that lysozyme is present as a dimer or a trimer. The partition coefficient decreases with increasing PEG molecular weight, but the magnitude of the increase is larger for the smaller PEG molecular eights and tends to level of at high PEG molecular weight. The partition coefficient increases with increasing dextran (Dx) molecular weight for chymotrypsinogen but decreases for catalase. The partition coefficients of lysozyme, albumin, and transferrin increase with increasing Dx molecular weight from Dx 10(4) to Dx 1.1 x 10(5) and then slightly decrease from Dx 1.1 x 10(5) to Dx 5 x 10(5). The experimental results are analyzed using a statistical thermodynamics model. The experimental results are analyzed using a statistical thermodynamics model. The experiments suggest that protein partitioning at the isoelectric point in aqueous two-phase systems is strongly related to the size of the proteins and polymers. Finally, the impossibility of obtaining data completely independent of polymer concentration is emphasized.     

Protein interactions in solution characterized by light and neutron scattering: comparison of lysozyme and chymotrypsinogen.

The effects of pH and electrolyte concentration on protein-protein interactions in lysozyme and chymotrypsinogen solutions were investigated by static light scattering (SLS) and small-angle neutron scattering (SANS). Very good agreement between the values of the virial coefficients measured by SLS and SANS was obtained without use of adjustable parameters. At low electrolyte concentration, the virial coefficients depend strongly on pH and change from positive to negative as the pH increases. All coefficients at high salt concentration are slightly negative and depend weakly on pH. For lysozyme, the coefficients always decrease with increasing electrolyte concentration. However, for chymotrypsinogen there is a cross-over point around pH 5.2, above which the virial coefficients decrease with increasing ionic strength, indicating the presence of attractive electrostatic interactions. The data are in agreement with Derjaguin-Landau-Verwey-Overbeek (DLVO)-type modeling, accounting for the repulsive and attractive electrostatic, van der Waals, and excluded volume interactions of equivalent colloid spheres. This model, however, is unable to resolve the complex short-ranged orientational interactions. The results of protein precipitation and crystallization experiments are in qualitative correlation with the patterns of the virial coefficients and demonstrate that interaction mapping could help outline new crystallization regions.     

Small angle neutron scattering from lysozyme solutions in unsaturated and supersaturated states (SANS from lysozyme solutions)

Small angle neutron scattering (SANS) method was used to study lysozyme solutions, with particular interest in an understanding of the crystallization process at the initial stage. It is found that (1) in the unsaturated solution, the protein molecules aggregate with a continuous increase in size when NaCl concentration is increased, and (2) in the supersaturated solution, an irreversible change, superimposed on the former process, occurs when the supersaturation is realized. These facts indicate the usefulness of SANS in detecting changes of protein molecules in solution on the nanometer scale. The reliability of the SANS results are indicated by (1) comparing them with those of small angle X-ray scattering (SAXS), and (2) comparing the effect of D(2)O and H(2)O as solvent. Since the interparticle interaction is essential in the crystallization process and a simple Guinier plot analysis is not allowed, a more rigorous framework of analyzing data with interference function is developed, through which both average interparticle distance and particle size are estimated.     

The conformation of the poly(ethylene glycol) chain in mono-PEGylated lysozyme and mono-PEGylated human growth hormone

Covalent conjugation of poly(ethylene glycol) or "PEGylation" has proven an effective strategy to improve pharmaceutical protein efficacy by hindering recognition by proteases, inhibitors, and antibodies and by retarding renal clearance. Because it determines the strength and range of intermolecular steric forces and the hydrodynamic properties of the conjugates, the configuration of protein-conjugated PEG chains is the key factor determining how PEGylation alters protein in vivo circulation time. Mono-PEGylated proteins are typically described as having a protective PEG shroud wrapped around the protein, but recent dynamic light scattering studies suggested that conjugates adopt a dumbbell configuration, with a relatively unperturbed PEG random coil adjacent to the globular protein. We used small-angle neutron scattering (SANS) to distinguish between the dumbbell model and the shroud model for chicken-egg lysozyme and human growth hormone covalently conjugated to a single 20 kDa PEG chain. The SANS contrast variation technique was used to isolate the PEG portion of the conjugate. Scattering intensity profiles were well described by the dumbbell model and inconsistent with the shroud model.     

Quasielastic light scattering,electrophoresis, and fluorescence studies of lysozyme-poly(2-acrylamido-methylpropylsulfate) complexes

Complexation between lysozyme and sodium poly(2-acrylamido-2-methylpropanesulfonate) (PAMPS) was studied by quasielastic light scattering, electrophoretic light scattering, fluorescence, and turbidimetry in electrolyte solution. These techniques show that complexation occurs at pH 9.6 in an ionic strength buffer of 0.25M NaCl + 25 mM Na₂B₄O₇. At constant lysozyme concentration (C_pro). The structure of the complex depends on the polymer concentration. At low polymer concentration (relative to C_pro), an intrapolymer complex is formed. This intrapolymer complex aggregates to an interpolymer species upon increase in polymer concentration. Complex formation was also studied by fluorescence using pyrene-labeled PAMPS (Py-PAMPS). Energy transfer from singlet-excited tryptophan residues in lysozyme to the pyrene label occurs when the complex is formed. Fluorescence and turbidity data indicate that lysozyme interacts with Py-PAMPS preferentially at pyrene sites, which leads to static quenching of tryptophan fluorescence via energy transfer to the pyrene label. © 1995 John Wiley & Sons, Inc.     

A novel aldehyde dextran sulfonate matrix for affinity biosensors

Aldehyde dextran sulfonate (ADS), a modified oligosaccharide polymer, was used to prepare a new matrix structure for affinity biosensors. The principal difference between the ADS matrix and similar structures developed previously results from presence of two active functional groups in the matrix, namely, aldehyde and sulfonate. These groups perform two different functions in the matrix. The aldehyde group is responsible for covalent bonding in the biomaterials, and the negatively charged sulfonate group provides electrostatic attraction of the positively charged biomolecules. By varying the ratio between the aldehyde and sulfonate groups in the matrix, one can control contributions from the two binding modes (covalent and electrostatic). A number of oligosaccharides, such as simple dextran, aldehyde dextran (AD), aldehyde dextran sulfonate (ADS) and aldehyde ethylcellulose (AEC), were used for preparation of matrix structures. The properties of the obtained matrices were analysed and compared. Surface plasmon resonance (SPR) was used as the main technique to characterize the matrix structures.     

Effect of alcohols on aqueous lysozyme-lysozyme interactions from static light-scattering measurements

Alcohols have been widely used as protein denaturants, precipitants and crystallization reagents. We have studied the effect of alcohols on aqueous hen-egg lysozyme self-interactions by measuring the osmotic second virial coefficient (B22) using static light scattering. Addition of alcohols increases B22, indicating stronger protein-protein repulsion or weaker attraction. For the monohydric alcohols used in this study (methanol, ethanol, 1-propanol, n-butanol, iso-butanol and trifluoroethanol), B22 for lysozyme reaches a common plateau at approximately 5% (v/v) alcohol, while glycerol increases B22 more than monohydric alcohols. For a 0.05 M NaCl hen-egg lysozyme solution at pH 7, B22 increases from 2.4 x 10(-4) to 4.7 x 10(-4) ml mol/g2 upon addition of monohydric alcohols and to 5.8 x 10(-4) ml mol/g2 upon addition of glycerol. We describe the alcohol effect using a simple model that supplements the DLVO theory with an additional alcohol-dependent term representing orientation-averaged hydrophobic interactions. In this model, the increased lysozyme repulsive forces in the presence of monohydric alcohols are interpreted in terms of adsorption of alcohol molecules on hydrophobic sites on the protein surface. This adsorption reduces attractive hydrophobic protein-protein interactions. A thicker lysozyme hydration layer in aqueous glycerol solution can explain the glycerol-increased lysozyme-lysozyme repulsion.     

Immobilization of a (dextran-adamantane-COOH) polymer onto beta-cyclodextrin-modified silica

Adamantane-modified compounds are known to form stable complexes with beta-cyclodextrins (beta-CD) by host-guest interactions. In this study, the inclusion complex formed between beta-CD cavities and the adamantane group was evaluated for the elaboration of a cation-exchange support. The synthesis of the chromatographic supports involved three steps: (i) a polymer of beta-CD was grafted to diol-modified silica, (ii) a dextran polymer was modified by both adamantane groups and ionizable COOH functions, (iii) the dextran derivative (Ad-Dex-COOH) was bound to the chromatographic support by complexation between the adamantane groups of the dextran and beta-CD cavities of the support. The polymer immobilization on the beta-CD support was successful as the resulting support exhibited weak cation-exchange properties. The stationary phase was easy to prepare under mild conditions (aqueous media, room temperature) and was quite stable when using aqueous mobile phases. The chromatographic behaviour of model proteins was studied in isocratic elution by examining the effect of salt concentration in the buffer on retention. A mixed retention mode was found for lysozyme, revealing both electrostatic and hydrophobic interactions with the stationary phase.     

Understanding the mechanism of action of poly(amidoamine)s as endosomolytic polymers: correlation of physicochemical and biological properties

Bioresponsive poly(amidoamine)s (PAA)s are currently under development as endosomolytic polymers for intracellular delivery of proteins and genes. Here for the first time, small-angle neutron scattering (SANS) is used to systematically investigate the pH-dependent conformational change of an endosomolytic polymer, the PAA ISA 23. The radius of gyration of the ISA23 was determined as a function of pH and counterion, the aim being to correlate changes in polymer conformation with membrane activity assessed using a rat red blood cell haemolysis assay. With decreasing pH, the ISA23 radius of gyration increased to a maximum (R(g) approximately 80 A) around pH = 3, before subsequently decreasing once more. At high pH and therefore high ionic strengths, the polymer is negatively charged and adopts a rather compact structure (R(g) approximately 20 A), presumably with the dissociated carboxylic groups on the exterior of the polymer coil. At low pH, the coil again collapses (R(g) < 20 A), presumably due to the effects of the high ionic strength. It is concluded that the nature of the salt form has no direct bearing on the size of the polymer coil, but it does indirectly determine the prevailing pH and, hence, polymer conformation. Pulsed-gradient spin-echo NMR measurements were in good agreement with the SANS estimates of the radius of gyration, although ISA23 polydispersity does complicate the data interpretation/comparison. These results support the proposed mode of action of PAAs, namely a coil expansion on passing from a neutral pH (extracellular) to an acidic pH (endosomal and lysosomal) environments. The results do, however, suggest that the charge on the polymer shows a closer correlation with the haemolysis activity rather than the polymer conformation.     

Probing the Conformation of FhaC with Small-Angle Neutron Scattering and Molecular Modeling

Probing the solution structure of membrane proteins represents a formidable challenge, particularly when using small-angle scattering. Detergent molecules often present residual scattering contributions even at their match point in small-angle neutron scattering (SANS) measurements. Here, we studied the conformation of FhaC, the outer-membrane, β-barrel transporter of the Bordetella pertussis filamentous hemagglutinin adhesin. SANS measurements were performed on homogeneous solutions of FhaC solubilized in n-octyl-d17-βD-glucoside and on a variant devoid of the α helix H1, which critically obstructs the FhaC pore, in two solvent conditions corresponding to the match points of the protein and the detergent, respectively. Protein-bound detergent amounted to 142 ± 10 mol/mol as determined by analytical ultracentrifugation. By using molecular modeling and starting from three distinct conformations of FhaC and its variant embedded in lipid bilayers, we generated ensembles of protein-detergent arrangement models with 120–160 detergent molecules. The scattered curves were back-calculated for each model and compared with experimental data. Good fits were obtained for relatively compact, connected detergent belts, which occasionally displayed small detergent-free patches on the outer surface of the β barrel. The combination of SANS and modeling clearly enabled us to infer the solution structure of FhaC, with H1 inside the pore as in the crystal structure. We believe that our strategy of combining explicit atomic detergent modeling with SANS measurements has significant potential for structural studies of other detergent-solubilized membrane proteins.