Numerical validation of IFT in the analysis of protein-surfactant complexes with SAXS and SANS

The Nanostructured Self-Assembly and Thermoresponsiveness in Water of Amphiphilic Copolymers Carrying Oligoethylene Glycol and Polysiloxane Side Chains

Amphiphilic copolymer self-assembly is a straightforward approach to obtain responsive micelles, nanoparticles, and vesicles that are particularly attractive for biomedicine, i.e., for the delivery of functional molecules. Here, amphiphilic copolymers of hydrophobic polysiloxane methacrylate and hydrophilic oligo (ethylene glycol) methyl ether methacrylate with different lengths of oxyethylenic side chains were synthesized via controlled RAFT radical polymerization and characterized both thermally and in solution. In particular, the thermoresponsive and self-assembling behavior of the water-soluble copolymers in water was investigated via complementary techniques such as light transmittance, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) measurements. All the copolymers synthesized were thermoresponsive, displaying a cloud point temperature (T_cp) strongly dependent on macromolecular parameters such as the length of the oligo(ethylene glycol) side chains and the content of the SiMA counits, as well as the concentration of the copolymer in water, which is consistent with a lower critical solution temperature (LCST)-type behavior. SAXS analysis revealed that the copolymers formed nanostructures in water below T_cp, whose dimension and shape depended on the content of the hydrophobic components in the copolymer. The hydrodynamic diameter (D_h) determined by DLS increased with the amount of SiMA and the associated morphology at higher SiMA contents was found to be pearl-necklace-micelle-like, composed of connected hydrophobic cores. These novel amphiphilic copolymers were able to modulate thermoresponsiveness in water in a wide range of temperatures, including the physiological temperature, as well as the dimension and shape of their nanostructured assemblies, simply by varying their chemical composition and the length of the hydrophilic side chains.

The Amphoteric Surfactant N,N-Dimethyldodecylamine N-Oxide Unfolds β-Lactoglobulin above the Critical Micelle Concentration

Understanding the interactions between surfactants and proteins is important for the formulation of consumer products as surfactant binding can alter protein activity and stability. Additionally, the structure of the protein-surfactant complex can influence surface activity, which is important for emulsion and foam development. N,N-Dimethyldodecylamine N-oxide (DDAO) is an amphoteric surfactant that is nonionic at high pH. It is often used as a foam booster in detergent formulations and for the extraction of membrane proteins. In this study, a variety of biophysical characterization methods was used to investigate the impact of DDAO at pH 8 on the structure of the globular protein β-lactoglobulin (βLG). Pyrene fluorescence and surface tension studies show that βLG had minimal impact on the critical micelle concentration (CMC) of DDAO, while fluorescence and circular dichroism spectroscopy found unfolding of βLG at concentrations of DDAO greater than the CMC. Small-angle X-ray scattering results confirm changes in the structure of βLG at DDAO concentrations above the CMC. Taken together, DDAO behaves like nonionic and zwitterionic surfactants below its CMC with limited interaction with βLG, while it induces protein unfolding at concentrations higher than the CMC, resulting in a protein-surfactant complex structure that resembles a protein-decorated micelle.

Dependence of the Nanoscale Composite Morphology of Fe3O4 Nanoparticle-Infused Lysozyme Amyloid Fibrils on Timing of Infusion: A Combined SAXS and AFM Study

Understanding the formation process and the spatial distribution of nanoparticle (NP) clusters on amyloid fibrils is an essential step for the development of NP-based methods to inhibit aggregation of amyloidal proteins or reverse the assembling trend of the proto-fibrillary complexes that prompts pathogenesis of neuro degeneration. For this, a detailed structural determination of the diverse hybrid assemblies that are forming is needed, which can be achieved by advanced X-ray scattering techniques. Using a combined solution small angle X-ray scattering (SAXS) and atomic force microscopy (AFM) approach, this study investigates the intrinsic trends of the interaction between lysozyme amyloid fibrils (LAFs) and Fe3O4 NPs before and after fibrillization at nanometer resolution. AFM images reveal that the number of NP clusters interacting with the lysozyme fibers does not increase significantly with NP volume concentration, suggesting a saturation in NP aggregation on the fibrillary surface. The data indicate that the number of non-adsorbed Fe3O4 NPs is highly dependent on the timing of NP infusion within the synthesis process. SAXS data yield access to the spatial distribution, aggregation manner and density of NP clusters on the fibrillary surfaces. Employing modern data analysis approaches, the shape and internal structural morphology of the so formed nanocomposites are revealed. The combined experimental approach suggests that while Fe3O4 NPs infusion does not prevent the fibril-formation, the variation of NP concentration and size at different stages of the fibrillization process can impose a pronounced impact on the superficial and internal structural morphologies of these nanocomposites. These findings may be applicable in devising advanced therapeutic treatments for neurodegenerative diseases and designing novel bio-inorganic magnetic devices. Our results further demonstrate that modern X-ray methods give access to the structure of-and insight into the formation process of-biological-inorganic hybrid structures in solution.

Interaction of a bovine serum albumin (BSA) protein with mixed anionic-cationic surfactants and the resultant structure

The interaction of a bovine serum albumin (BSA) protein with the mixture of anionic sodium dodecyl sulfate (SDS) and cationic dodecyltrimethylammonium bromide (DTAB) has been investigated by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). Both SDS and DTAB as individuals interact electrostatically as well as hydrophobically with BSA and form connected protein-decorated micelle like complexes in the aqueous solution, in which the well-defined surfactant micelles are organized along the randomly distributed unfolded polypeptide chain of the protein. The protein-surfactant interaction has been tuned by adding different molar mixtures of SDS and DTAB in BSA aqueous solution. It is found that a lower molar fraction of either surfactant in the protein-mixed surfactant complexes results in the formation of a connected protein-decorated micelle structure similar to those of pure surfactants. As the molar fraction of one of the surfactants in the mixture approaches the equimolar fraction, the structure formed by the protein-mixed surfactant is very different from the connected protein-decorated micelle like structure. Different microstructures of BSA-mixed surfactant complexes are formed, mostly governed by the structure of mixed surfactants arising from the strong electrostatic interaction of oppositely charged components. In this case, unfolded proteins wrap the structures of mixed surfactants around their surface. Along with the connected protein-decorated micelle like structure, rod-like and bilayer vesicles of protein-surfactant complexes are formed at different molar fractions of mixed surfactants.

An integrative toolbox to unlock the structure and dynamics of protein-surfactant complexes

The interactions between protein and surfactants play an important role in the stability and performance of formulated products. Due to the high complexity of such interactions, multi-technique approaches are required to study these systems. Here, an integrative approach is used to investigate the various interactions in a model system composed of human growth hormone and sodium dodecyl sulfate. Contrast variation small-angle neutron scattering was used to obtain information on the structure of the protein, surfactant aggregates and surfactant-protein complexes. 1H and 1H-13C HSQC nuclear magnetic resonance spectroscopy was employed to probe the local structure and dynamics of specific amino acids upon surfactant addition. Through the combination of these advanced methods with fluorescence spectroscopy, circular dichroism and isothermal titration calorimetry, it was possible to identify the interaction mechanisms between the surfactant and the protein in the pre- and post-micellar regimes, and interconnect the results from different techniques. As such, the protein was revealed to evolve from a partially unfolded conformation at low SDS concentration to a molten globule at intermediate concentrations, where the protein conformation and local dynamics of hydrophobic amino acids are partially affected compared to the native state. At higher surfactant concentrations the local structure of the protein appears disrupted, and a decorated micelle structure is observed, where the protein is wrapped around a surfactant assembly. Importantly, this integrative approach allows for the identification of the characteristic fingerprints of complex transitions as seen by each technique, and establishes a methodology for an in-detail study of surfactant-protein systems.

Unfolding and Refolding of Protein by a Combination of Ionic and Nonionic Surfactants

The interaction of protein and surfactant yields protein-surfactant complexes which have a wide range of applications in the cosmetics, foods, and pharmaceutical industries among others. Ionic and nonionic surfactants are known to interact differently with the protein. The interplay of electrostatic and hydrophobic interactions governs the resultant structure of protein-surfactant complexes. The present study enlightens the paramount role of the hydrophobic interaction, tuned by the hydrophobic tail length of ionic surfactants, in the unfolding of anionic bovine serum albumin (BSA) protein. The unfolding of BSA in the presence of four different tail-length cationic surfactants, that is, C10TAB, C12TAB, C14TAB, and C16TAB, has been investigated by small-angle neutron scattering and dynamic light scattering. All cationic surfactants unfold the protein at a certain concentration range. The propensity of protein unfolding increases with increasing the hydrophobic tail length. The denatured structure of BSA upon addition of cationic surfactants is characterized by the random flight model representing a beads-on-a-string chain-like complex. The unfolded protein binds the surfactant micelles in the protein-surfactant cluster. The micelles get elongated with the increasing concentration of cationic surfactants, whereas the number of micelles per cluster is decreased. In the final stage, the protein-surfactant cluster merges to one large micelle with unfolded protein wrapping the micelle surface. The pathway of protein unfolding is described in terms of the changes in the micellar size, the number of micelles formed per cluster, the separation between the micelles in the cluster, the aggregation number of micelles, and the number of proteins per cluster. The protein-surfactant interaction is further examined in the presence of a nonionic surfactant, that is, C12E10. The nonionic surfactant significantly suppresses the interaction of BSA protein with ionic surfactants by forming mixed micelles. As a result of the mixed micelles formation by ionic-nonionic surfactants, the ionic surfactant moves out from the unfolded BSA protein, and this enables the protein to refold back to its native structure. The propensity of mixed micelle-driven refolding of proteins is significantly changed with changing the tail length of the ionic surfactant.

Aggregate Formation of Surface-Modified Nanoparticles in Solvents and Polymer Nanocomposites

A new method based on the combination of small-angle scattering, reverse Monte Carlo simulations, and an aggregate recognition algorithm is proposed to characterize the structure of nanoparticle suspensions in solvents and polymer nanocomposites, allowing detailed studies of the impact of different nanoparticle surface modifications. Experimental small-angle scattering is reproduced using simulated annealing of configurations of polydisperse particles in a simulation box compatible with the lowest experimental q-vector. Then, properties of interest like aggregation states are extracted from these configurations and averaged. This approach has been applied to silane surface-modified silica nanoparticles with different grafting groups, in solvents and after casting into polymer matrices. It is shown that the chemistry of the silane function, in particular mono- or trifunctionality possibly related to patch formation, affects the dispersion state in a given medium, in spite of an unchanged alkyl-chain length. Our approach may be applied to study any dispersion or aggregation state of nanoparticles. Concerning nanocomposites, the method has potential impact on the design of new formulations allowing controlled tuning of nanoparticle dispersion.

Myoglobin and α-Lactalbumin Form Smaller Complexes with the Biosurfactant Rhamnolipid Than with SDS

Biosurfactants (BSs) attract increasing attention as sustainable alternatives to petroleum-derived surfactants. This necessitates structural insight into how BSs interact with proteins encountered by current chemical surfactants. Thus, small-angle x-ray scattering (SAXS) has been used for studying the structures of complexes made of the proteins α-Lactalbumin (αLA) and myoglobin (Mb) with the biosurfactant rhamnolipid (RL). For comparison, complexes between αLA and the chemical surfactant sodium dodecyl sulfate (SDS) were also investigated. The SAXS data for pure RL micelles can be described by prolate core-shell structures with a core radius of 7.7 Å and a shell thickness of 12 Å, giving an aggregation number of 11. The small core radius is attributed to RL's complex hydrophobic tail. Data for the αLA-RL complex agree with a 12-molecule micelle with a single protein molecule in the shell. For Mb-RL, the analysis gives complexes of two connected micelles, each containing 10 RL and one protein in the shells. αLA-RL and Mb-RL form surfactant-saturated complexes above 5.6 and 4.7 mM RL, respectively, leaving the remaining RL in free micelles. The SAXS data for SDS agree with oblate-shaped micelles with a core of 20 Å, core eccentricity 0.7, and shell thickness of 5.45 Å, with an aggregation number of 74. The αLA-SDS complexes contain a prolate micelle with a core radius of 11-14 Å and a shell of 8-12 Å with up to 3 αLA per particle and up to 43 SDS per αLA, both considerably larger than for RL. Unlike the RL-protein complexes, the number of surfactant molecules in αLA-SDS complexes increases with surfactant concentration, and saturate at higher surfactant concentrations than αLA-RL complexes. The results highlight how RL and SDS follow similar overall rules of self-assembly and interactions with proteins, but that differences in the strength of protein-surfactant interactions affect the formed structures.

Structural and magnetic properties of multi-core nanoparticles analysed using a generalised numerical inversion method

The structural and magnetic properties of magnetic multi-core particles were determined by numerical inversion of small angle scattering and isothermal magnetisation data. The investigated particles consist of iron oxide nanoparticle cores (9 nm) embedded in poly(styrene) spheres (160 nm). A thorough physical characterisation of the particles included transmission electron microscopy, X-ray diffraction and asymmetrical flow field-flow fractionation. Their structure was ultimately disclosed by an indirect Fourier transform of static light scattering, small angle X-ray scattering and small angle neutron scattering data of the colloidal dispersion. The extracted pair distance distribution functions clearly indicated that the cores were mostly accumulated in the outer surface layers of the poly(styrene) spheres. To investigate the magnetic properties, the isothermal magnetisation curves of the multi-core particles (immobilised and dispersed in water) were analysed. The study stands out by applying the same numerical approach to extract the apparent moment distributions of the particles as for the indirect Fourier transform. It could be shown that the main peak of the apparent moment distributions correlated to the expected intrinsic moment distribution of the cores. Additional peaks were observed which signaled deviations of the isothermal magnetisation behavior from the non-interacting case, indicating weak dipolar interactions.

Tuning of protein-surfactant interaction to modify the resultant structure

Small-angle neutron scattering and dynamic light scattering studies have been carried out to examine the interaction of bovine serum albumin (BSA) protein with different surfactants under varying solution conditions. We show that the interaction of anionic BSA protein (pH7) with surfactant and the resultant structure are strongly modified by the charge head group of the surfactant, ionic strength of the solution, and mixed surfactants. The protein-surfactant interaction is maximum when two components are oppositely charged, followed by components being similarly charged through the site-specific binding, and no interaction in the case of a nonionic surfactant. This interaction of protein with ionic surfactants is characterized by the fractal structure representing a bead-necklace structure of micellelike clusters adsorbed along the unfolded protein chain. The interaction is enhanced with ionic strength only in the case of site-specific binding of an anionic surfactant with an anionic protein, whereas it is almost unchanged for other complexes of cationic and nonionic surfactants with anionic proteins. Interestingly, the interaction of BSA protein with ionic surfactants is significantly suppressed in the presence of nonionic surfactant. These results with mixed surfactants thus can be used to fold back the unfolded protein as well as to prevent surfactant-induced protein unfolding. For different solution conditions, the results are interpreted in terms of a change in fractal dimension, the overall size of the protein-surfactant complex, and the number of micelles attached to the protein. The interplay of electrostatic and hydrophobic interactions is found to govern the resultant structure of complexes.

λ-Orthogonal pericyclic macromolecular photoligation

A photochemical strategy enabling λ-orthogonal reactions is introduced to construct macromolecular architectures and to encode variable functional groups with site-selective precision into a single molecule by the choice of wavelength. λ-Orthogonal pericyclic reactions proceed independently of one another by the selection of functional groups that absorb light of specific wavelengths. The power of the new concept is shown by a one-pot reaction of equimolar quantities of maleimide with two polymers carrying different maleimide-reactive endgroups, that is, a photoactive diene (photoenol) and a nitrile imine (tetrazole). Under selective irradiation at λ=310-350 nm, any maleimide (or activated ene) end-capped compound reacts exclusively with the photoenol functional polymer. After complete conversion of the photoenol, subsequent irradiation at λ=270-310 nm activates the reaction of the tetrazole group with functional enes. The versatility of the approach is shown by λ-orthogonal click reactions of complex maleimides, functional enes, and polymers to the central polymer scaffold.

Fine structures of self-assembled beta-cyclodextrin/Pluronic in dilute and dense systems: a small angle X-ray scattering study

The evolution of the fine structures of self-assembled polypseudorotaxane (PPR) in Pluronic (PL F108) solutions containing dilute to dense beta-cyclodextrin (β-CD) was illustrated for the first time by small angle X-ray scattering (SAXS). Dense β-CD (∼19 w/v%) was found feasible to be dispersed in 24% citric acid solution. 5% of PL F108 formed cylindrical micelles of 1 nm in radius and 8 nm in length in the presence of 24% citric acid through the dehydration of citric acid and citrate. PPR was formed through host-guest interaction between PL F108 and β-CD. In dilute β-CD system (1%), the single chains of PPR with separated β-CD stacks on PL F108 were formed. The numbers of β-CD in each stack increased from 1 to 4 on increasing β-CD concentration to 9%. In a dense β-CD system, PPR condensed to correlated structures majorly composed of two unit blocks through the hydrogen bonds between PPRs. Two distinguishable correlated domains with correlation lengths of 50 nm (marked α-phase) and 46 nm (marked β-phase) along the chains, but without fine periodic structure within each individual domain, were identified in the 10% β-CD solution. Periodic stacking of β-CD in the domains developed in the 12% solution. As β-CD concentration increased from 12 to 19%, the correlated heights of α and β phases reduced from 41 and 32 nm to 30 and 10 nm, respectively. There were 48 β-CDs that stabilized on each PL F108 chain in the 19% β-CD system, which is in good agreement with stoichiometry.

Anionic branched surfactants as alternative denaturing agents for protein separations

Denaturation of a group of model proteins of diverse size and composition with three branched alkyl surfactants-sodium 2-ethylhexyl sulfate (2-EHS), sodium 3,7-dimethyloctyl sulfate (3,7-DMOS), and sodium 2-butyloctyl sulfate (2-BOS)-has been investigated using circular dichroism (CD), small-angle X-ray scattering, and polyacrylamide gel electrophoresis (PAGE). Circular dichroism reveals that 2-BOS disrupts to a higher extent the secondary structure for most of the proteins. Also, it is found that upon adsorption the shape of the protein-surfactant complexes varies from "pearl necklace" to ellipsoidal depending on the surfactant that is used. Macroscopic separations also reveal that branching sodium alkyl sulfates with n-butyl (2-BOS) and n-methyl (3,7-DMOS) groups significantly affects their performance in PAGE. 3,7-DMOS and 2-BOS result in anomalous migrations that deviate from the expected electrophoretic mobility. A combined interpretation of spectroscopy, scattering, and polyacrylamide gel electrophoresis suggests that 2-BOS promotes stronger modification of proteins during denaturation. The findings in this work aim to improve protein electrophoretic separations and the design of novel surfactants.