Neutron reflectometry from poly (ethylene-glycol) brushes binding anti-PEG antibodies: evidence of ternary adsorption

The Internal Structure of the Velvet Worm Projectile Slime: A Small-Angle Scattering Study

For prey capture and defense, velvet worms eject an adhesive slime which has been established as a model system for recyclable complex liquids. Triggered by mechanical agitation, the liquid bio-adhesive rapidly transitions into solid fibers. In order to understand this mechanoresponsive behavior, here, the nanostructural organization of slime components are studied using small-angle scattering with neutrons and X-rays. The scattering intensities are successfully described with a three-component model accounting for proteins of two dominant molecular weight fractions and nanoscale globules. In contrast to the previous assumption that high molecular weight proteins-the presumed building blocks of the fiber core-are contained in the nanoglobules, it is found that the majority of slime proteins exist freely in solution. Only less than 10% of the slime proteins are contained in the nanoglobules, necessitating a reassessment of their function in fiber formation. Comparing scattering data of slime re-hydrated with light and heavy water reveals that the majority of lipids in slime are contained in the nanoglobules with homogeneous distribution. Vibrating mechanical impact under exclusion of air neither leads to formation of fibers nor alters the bulk structure of slime significantly, suggesting that interfacial phenomena and directional shearing are required for fiber formation.

Blood Protein Exclusion from Polymer Brushes

We report interactions between adsorbed copolymers of poly(ethylene glycol) (PEG) in the presence of two abundant blood proteins, serum albumin and an immunoglobulin G, up to physiological blood concentrations. We directly and nonintrusively measure interactions between PEG triblock copolymers (PEG-PPO-PEG) adsorbed to hydrophobic colloids and surfaces using Total Internal Reflection Microscopy, which provides kT- and nanometer-scale resolution of interaction potentials (energy vs separation). In the absence of protein, adsorbed PEG copolymer repulsion is consistent with dimensions and architectures of PEG brushes on both colloids and surfaces. In the presence of proteins, we observe concentration dependent depletion attraction and no change to brush repulsion, indicating protein exclusion from PEG brushes. Because positive and negative protein adsorption are mutually exclusive, our observations of concentration dependent depletion attraction with no change to brush repulsion unambiguously indicate the absence of protein coronas at physiological protein concentrations. These findings demonstrate a direct sensitive approach to determine interactions between proteins and particle/surface coatings important to diverse biotechnology applications.

Zwitterionic doxorubicin loaded micelles based on polyethyleneimine for enhanced antitumor therapy in vivo

Zwitterionic groups of doxorubicin loaded micelles enhanced their antitumor performance in vivo.

Structure of Self-Initiated Photopolymerized Films: A Comparison of Models

Self-initiated photografting and photopolymerization (SI-PGP) uses UV illumination to graft polymers to surfaces without additional photoinitiators using the monomers as initiators, "inimers". A wider use of this method is obstructed by a lack of understanding of the resulting, presumably heterogeneous, polymer structure and of the parallel degradation under continuous UV illumination. We have used neutron reflectometry to investigate the structure of hydrated SI-PGP-prepared poly(HEMA-co-PEG₁₀MA) (poly(2-hydroxyethyl methacrylate-co-(ethylene glycol)₁₀ methacrylate)) films and compared parabolic, sigmoidal, and Gaussian models for the polymer volume fraction distributions. Results from fitting these models to the data suggest that either model can be used to approximate the volume fraction profile to similar accuracy. In addition, a second layer of deuterated poly(methacrylic acid) (poly(dMAA)) was grafted over the existing poly(HEMA-co-PEG₁₀MA) layer, and the resulting double-grafted films were also studied by neutron reflectometry to shed light on the UV-polymerization process and the inevitable UV-induced degradation which competes with the grafting.

Long-circulation zwitterionic dendrimer nanodrugs for phototherapy of tumors

The development of stealth and effective antitumor nanodrugs has been drawing great attention. Herein, generation five poly(amide amine) dendrimer (G5 PAMAM) was modified by zwitterionic material carboxybetaine methacrylamide (CBMAA) on its surface to prepare zwitterionic dendrimer (G5-CBMAA_n). The results showed that G5-CBMAA₃₀ had the longest blood circulation time due to its thickest zwitterionic layer, and its residual rate after injection into mice at 2 and 12 h was as high as 47.22 % and 14.37 %, respectively. Nanodrug G5-CBMAA₃₀-ICG was prepared by containing indocyanine green (ICG) in the cavity of G5-CBMAA₃₀. G5-CBMAA₃₀-ICG had better tumor targeting ability and antitumor effect than free ICG in mice after laser irradiation, and the tumor inhibition rate was 96.6 % after 14 days' treatment. The prepared G5-CBMAA₃₀-ICG has great potential applications in the field of antitumor by phototherapy.

Accurate Correction of the "Bulk Response" in Surface Plasmon Resonance Sensing Provides New Insights on Interactions Involving Lysozyme and Poly(ethylene glycol)

Surface plasmon resonance is a very well-established surface sensitive technique for label-free analysis of biomolecular interactions, generating thousands of publications each year. An inconvenient effect that complicates interpretation of SPR results is the “bulk response” from molecules in solution, which generate signals without really binding to the surface. Here we present a physical model for determining the bulk response contribution and verify its accuracy. Our method does not require a reference channel or a separate surface region. We show that proper subtraction of the bulk response reveals an interaction between poly(ethylene glycol) brushes and the protein lysozyme at physiological conditions. Importantly, we also show that the bulk response correction method implemented in commercial instruments is not generally accurate. Using our method, the equilibrium affinity between polymer and protein is determined to be K_D = 200 μM. One reason for the weak affinity is that the interaction is relatively short-lived (1/k_off < 30 s). Furthermore, we show that the bulk response correction also reveals the dynamics of self-interactions between lysozyme molecules on surfaces. Besides providing new insights on important biomolecular interactions, our method can be widely applied to improve the accuracy of SPR data generated by instruments worldwide.

Investigating the Conformation of Surface-Adsorbed Proteins with Standing-Wave X-ray Fluorescence

Protein adsorption to surfaces is at the heart of numerous technological and bioanalytical applications, but sometimes, it is also associated with medical risks. To deepen our insights into processes involving layers of surface-adsorbed proteins, high-resolution structural information is essential. Here, we use standing-wave X-ray fluorescence (SWXF) in combination with an optimized liquid-cell setup to investigate the underwater conformation of the random-coiled phosphoprotein β-casein adsorbed to hydrophilic and hydrophobized solid surfaces. The orientation of the protein, as determined through the distributions of sulfur and phosphorus, is found to be sensitive to the chemical nature of the substrate. While no preferred orientations are observed on hydrophobized surfaces, on hydrophilic Al oxide, β-casein is adsorbed as a diblock copolymer with the phosphorylated domain I attached to the surface. Our results demonstrate that targeting biologically relevant chemical elements with SWXF enables a detailed investigation of biomolecular layers under near-physiological conditions.

Control of Polymer Brush Morphology, Rheology, and Protein Repulsion by Hydrogen Bond Complexation

Polymer brushes are widely used to alter the properties of interfaces. In particular, poly(ethylene glycol) (PEG) and similar polymers can make surfaces inert toward biomolecular adsorption. Neutral hydrophilic brushes are normally considered to have static properties at a given temperature. As an example, PEG is not responsive to pH or ionic strength. Here we show that, by simply introducing a polymeric acid such as poly(methacrylic acid) (PMAA), the highly hydrated brush barrier can change its properties entirely. This is caused by multivalent hydrogen bonds in an extremely pH-sensitive process. Remarkably, it is sufficient to reduce the pH to 5 for complexation to occur at the interface, which is two units higher than in the corresponding bulk systems. Below this critical pH, PMAA starts to bind to PEG in large amounts (comparable to the PEG amount), causing the brush to gradually compact and dehydrate. The brush also undergoes major rheology changes, from viscoelastic to rigid. Furthermore, the protein repelling ability of PEG is lost after reaching a threshold in the amount of PMAA bound. The changes in brush properties are tunable and become more pronounced when more PMAA is bound. The initial brush state is fully recovered when releasing PMAA by returning to physiological pH. Our findings are relevant for many applications involving functional interfaces, such as capture-release of biomolecules.

Applications of neutron reflectometry in biology

Over the last 10 years, neutron reflectometry (NR) has emerged as a powerful technique for the investigation of biologically relevant thin films. The great advantage of NR with respect to many other surface-sensitive techniques is its sub-nanometer resolution that enables structural characterizations at the molecular level. In the case of bio-relevant samples, NR is non-destructive and can be used to probe thin films at buried interfaces or enclosed in bulky sample environment equipment. Moreover, recent advances in biomolecular deutera-tion enabled new labeling strategies to highlight certain structural features and to resolve with better accuracy the location of chemically similar molecules within a thin film.

In this chapter I will describe some applications of NR to bio-relevant samples and discuss some of the data analysis approaches available for biological thin films. In particular, examples on the structural characterization of biomembranes, protein films and protein-lipid interactions will be described.

Molecular Simulations of PEGylated Biomolecules, Liposomes, and Nanoparticles for Drug Delivery Applications

Since the first polyethylene glycol (PEG)ylated protein was approved by the FDA in 1990, PEGylation has been successfully applied to develop drug delivery systems through experiments, but these experimental results are not always easy to interpret at the atomic level because of the limited resolution of experimental techniques. To determine the optimal size, structure, and density of PEG for drug delivery, the structure and dynamics of PEGylated drug carriers need to be understood close to the atomic scale, as can be done using molecular dynamics simulations, assuming that these simulations can be validated by successful comparisons to experiments. Starting with the development of all-atom and coarse-grained PEG models in 1990s, PEGylated drug carriers have been widely simulated. In particular, recent advances in computer performance and simulation methodologies have allowed for molecular simulations of large complexes of PEGylated drug carriers interacting with other molecules such as anticancer drugs, plasma proteins, membranes, and receptors, which makes it possible to interpret experimental observations at a nearly atomistic resolution, as well as help in the rational design of drug delivery systems for applications in nanomedicine. Here, simulation studies on the following PEGylated drug topics will be reviewed: proteins and peptides, liposomes, and nanoparticles such as dendrimers and carbon nanotubes.

Conformation of Single and Interacting Lipopolysaccharide Surfaces Bearing O-Side Chains

The outer surfaces of Gram-negative bacteria are composed of lipopolysaccharide (LPS) molecules exposing oligo- and polysaccharides to the aqueous environment. This unique, structurally complex biological interface is of great scientific interest as it mediates the interaction of bacteria with antimicrobial agents as well as with neighboring bacteria in colonies and biofilms. Structural studies on LPS surfaces, however, have so far dealt almost exclusively with rough mutant LPS of reduced molecular complexity and limited biological relevance. Here, by using neutron reflectometry, we structurally characterize planar monolayers of wild-type LPS from Escherichia coli O55:B5 featuring strain-specific O-side chains in the presence and absence of divalent cations and under controlled interaction conditions. The model used for the reflectivity analysis is self-consistent and based on the volume fraction profiles of all chemical components. The saccharide profiles are found to be bimodal, with dense inner oligosaccharides and more dilute, extended O-side chains. For interacting LPS monolayers, we establish the pressure-distance curve and determine the distance-dependent saccharide conformation.

Structure and Conformation of Wild-Type Bacterial Lipopolysaccharide Layers at Air-Water Interfaces

The outer membrane of Gram-negative bacteria is of great scientific interest because it mediates the action of antimicrobial agents. The membrane surface is composed of lipopolysaccharide (LPS) molecules with negatively charged oligosaccharide headgroups. To a certain fraction, LPSs additionally display linear polysaccharides termed O-side chains (OSCs). Structural studies on bacterial outer surfaces models, based on LPS monolayers at air-water interfaces, have so far dealt only with rough mutant LPSs lacking these OSCs. Here, we characterize monolayers of wild-type LPS from Escherichia coli O55:B5 featuring strain-specific OSCs in the presence of defined concentrations of monovalent and divalent ions. Pressure-area isotherms yield insight into in-plane molecular interactions and monolayer elastic moduli. Structural investigations by x-ray and neutron reflectometry reveal the saccharide conformation and allow quantifying the area per molecule and the fraction of LPS molecules carrying OSCs. The OSC conformation is satisfactorily described by the self-consistent field theory for end-grafted polymer brushes. The monolayers exhibit a significant structural response to divalent cations, which goes beyond generic electrostatic screening.

Zwitterlation mitigates protein bioactivity loss in vitro over PEGylation

Conjugation with poly(ethylene glycol) (PEG) or PEGylation is a widely used tool to overcome the shortcomings of native proteins, such as poor stability, inadequate pharmacokinetic (PK) profiles, and immunogenicity. However, PEGylation is often accompanied by an unwanted detrimental effect on bioactivity, particularly, resulting from the amphiphilic nature of PEG. This is especially true for PEGylated proteins with large binding targets. Pegasys, a PEGylated interferon alpha-2a (IFN-α2a) bearing a 40 kDa branched PEG, is a typical example that displays only 7% in vitro activity of the unmodified IFN-α2a. In this work, by employing IFN-α2a as a model protein, we demonstrated that a protein conjugated with zwitterionic polymers (or zwitterlation) could significantly mitigate the antiproliferative bioactivity loss in vitro after polymer conjugation. The retained antiproliferative activity of zwitterlated IFN-α2a is 4.4-fold higher than that of the PEGylated IFN-α2a with the same polymer molecular weight, or 3-fold higher than that of the PEGylated IFN-α2a with a similar hydrodynamic size. It is hypothesized that nonspecific interactions between zwitterionic polymers and IFN-α2a/IFN-α2a receptors can be mitigated due to the super-hydrophilic nature of zwitterionic polymers. This, in turn, reduces the 'nonspecific blocking' between IFN-α2a and IFN-α2a receptors. In addition, we demonstrated that zwitterlated IFN-α2a showed a prolonged circulation time and a mitigated accelerated blood clearance after repeated injections in rats.

End Point Versus Backbone Specificity Governs Characteristics of Antibody Binding to Poly(ethylene glycol) Brushes

End-grafted poly(ethylene glycol) (PEG) brushes are widely used in order to suppress undesired protein adsorption to surfaces exposed to blood or other biological fluids. The specific adsorption of antibodies (Abs) to PEG brushes associated with PEG's antigenicity is drawing increasing attention because it can affect clinical applications. Here, the adsorption to PEG brushes of two Ab types, specifically binding the polymer backbone and the polymer endpoints, is structurally characterized by neutron reflectometry. The measurements yield volume fraction profiles of PEG and of the adsorbed Abs with sub-nanometer resolution perpendicular to the surface. For all brush parameters in terms of grafting density and polymerization degree, the Ab profiles clearly differ between backbone binders and endpoint binders. The adsorbed Ab amount per unit area is substantial for both Ab types and for all brush parameters investigated, even for dense brushes, which impose a considerable osmotic barrier to Ab insertion. The results therefore indicate that variation of brush parameters alone is insufficient to prevent undesired Ab adsorption. Instead, our work motivates further efforts in the search for nonantigenic brush chemistry.

Gating Protein Transport in Solid State Nanopores by Single Molecule Recognition

Control of molecular translocation through nanoscale apertures is of great interest for DNA sequencing, biomolecular filters, and new platforms for single molecule analysis. However, methods for controlling the permeability of nanopores are very limited. Here, we show how nanopores functionalized with poly(ethylene glycol) brushes, which fully prevent protein translocation, can be reversibly gated to an "open" state by binding of single IgG antibodies that disrupt the macromolecular barrier. On the basis of surface plasmon resonance data we propose a two-state model describing the antibody-polymer interaction kinetics. Reversibly (weakly) bound antibodies decrease the protein exclusion height while irreversibly (strongly) bound antibodies do not. Our results are further supported by fluorescence readout from pore arrays and high-speed atomic force microscopy on single pores. This type of dynamic barrier control on the nanoscale provides new possibilities for biomolecular separation and analysis.

Grafting Density Impacts Local Nanoscale Hydrophobicity in Poly(ethylene glycol) Brushes

Accumulated single-molecule observations of a fluorescent solvatochromic probe molecule were found to provide detailed local information about nanoscale hydrophobicity in polymer brushes. Using this approach, we showed that local hydrophobicity in poly(ethylene glycol) (PEG) brushes was spatially heterogeneous and increased with the surface grafting density of the polymer chains. These findings may provide an explanation for prior observations of the denaturation of surface-adsorbed proteins on PEG brushes with high grafting densities, which is believed to influence protein-mediated cell-surface interactions. Moreover, by employing the broad range of existing environmentally sensitive fluorophores, this approach may potentially be used to characterize nanoscale changes in a variety of physicochemical properties within polymeric materials.

A Versatile Method for the Distance-Dependent Structural Characterization of Interacting Soft Interfaces by Neutron Reflectometry

Interactions between soft interfaces govern the behavior of emulsions and foams and crucially influence the functions of biological entities like membranes. To understand the character of these interactions, detailed insight into the interfaces' structural response in terms of molecular arrangements and conformations is often essential. This requires the realization of controlled interaction conditions and surface-sensitive techniques capable of resolving the structure of buried interfaces. Here, we present a new approach to determine the distance-dependent structure of interacting soft interfaces by neutron reflectometry. A solid/water interface and a water/oil interface are functionalized independently and initially macroscopically separated. They are then brought into contact and structurally characterized under interacting conditions. The nanometric distance between the two interfaces can be varied via the exertion of osmotic pressures. Our first experiments on lipid-anchored polymer brushes interacting across water with solid-grafted polyelectrolyte brushes and with bare silicon surfaces reveal qualitatively different interaction scenarios depending on the chemical composition of the two involved interfaces.

Neutron Reflectometry Elucidates Protein Adsorption from Human Blood Serum onto PEG Brushes

Poly(ethylene glycol) (PEG) brushes are reputed for their ability to prevent undesired protein adsorption to material surfaces exposed to biological fluids. Here, protein adsorption out of human blood serum onto PEG brushes anchored to solid-supported lipid monolayers was characterized by neutron reflectometry, yielding volume fraction profiles of lipid headgroups, PEG, and adsorbed proteins at subnanometer resolution. For both PEGylated and non-PEGylated lipid surfaces, serum proteins adsorb as a thin layer of approximately 10 Å, overlapping with the headgroup region. This layer corresponds to primary adsorption at the grafting surface and resists rinsing. A second diffuse protein layer overlaps with the periphery of the PEG brush and is attributed to ternary adsorption due to protein-PEG attraction. This second layer disappears upon rinsing, thus providing a first observation of the structural effect of rinsing on protein adsorption to PEG brushes.

Neutron reflectometry yields distance-dependent structures of nanometric polymer brushes interacting across water

The interaction between surfaces displaying end-grafted hydrophilic polymer brushes plays important roles in biology and in many wet-technological applications. In this context, the conformation of the brushes upon their mutual approach is crucial, because it affects interaction forces and the brushes' shear-tribological properties. While this aspect has been addressed by theory, experimental data on polymer conformations under confinement are difficult to obtain. Here, we study interacting planar brushes of hydrophilic polymers with defined length and grafting density. Via ellipsometry and neutron reflectometry we obtain pressure-distance curves and determine distance-dependent polymer conformations in terms of brush compression and reciprocative interpenetration. While the pressure-distance curves are satisfactorily described by the Alexander-de-Gennes model, the pronounced brush interpenetration as seen by neutron reflectometry motivates detailed simulation-based studies capable of treating brush interpenetration on a quantitative level.

Synergistic Effects of Bound Micelles and Temperature on the Flexibility of Thermoresponsive Polymer Brushes

The persistence length is a key parameter for the quantitative interpretation of the flexibility of polymers. We have studied complexes composed of a spherical poly(N-isopropylacrylamide) (PNIPAM) brush and a sodium dodecyl sulfate micelle in an effort to characterize the flexibility of tethered PNIPAM below the lower critical solution temperature T_LCST. An analytical mean-field model is used to describe the persistence length L_p in a broad range of ψ, the number of bound micelles per chain. The persistence length of micelle-constrained PNIPAM is quantitatively correlated with the thermal energy k_BT, electrostatic repulsion f_C, and effective excluded-volume parameter ν_eff. The persistence length per ψ, which depends on T and f_C, is found to scale with a synergistic effect f_C/(ψk_BT). The results reveal that the bound-micelle charges affecting the persistence length are analogous to the fixed charges of polyelectrolytes, though the bound micelles are separated by a large number of neutral monomers. The extension ⟨L⟩ of micelle-constrained PNIPAM decreases as ⟨L⟩ ∼ f_C^-β_F with f_C, where β_F ≈ 0.58-0.8 depending on ψ, but as the universal power law ⟨L⟩ ∼ (f_C/k_BT)^-0.6 with the synergistic effect f_C/(k_BT), irrespective of ψ. In spite of the intricate interplay among the multiple components in the system, the extension scales as a function of ν_eff as ⟨L⟩ ∼ (ν_eff/ψL_p)^-β_V, where β_V ≈ 0.35 for the significant monomer interaction and β_V ≈ 0.2 for the weak or negligible monomer interaction.

Atom-scale depth localization of biologically important chemical elements in molecular layers

In nature, biomolecules are often organized as functional thin layers in interfacial architectures, the most prominent examples being biological membranes. Biomolecular layers play also important roles in context with biotechnological surfaces, for instance, when they are the result of adsorption processes. For the understanding of many biological or biotechnologically relevant phenomena, detailed structural insight into the involved biomolecular layers is required. Here, we use standing-wave X-ray fluorescence (SWXF) to localize chemical elements in solid-supported lipid and protein layers with near-Ångstrom precision. The technique complements traditional specular reflectometry experiments that merely yield the layers' global density profiles. While earlier work mostly focused on relatively heavy elements, typically metal ions, we show that it is also possible to determine the position of the comparatively light elements S and P, which are found in the most abundant classes of biomolecules and are therefore particularly important. With that, we overcome the need of artificial heavy atom labels, the main obstacle to a broader application of high-resolution SWXF in the fields of biology and soft matter. This work may thus constitute the basis for the label-free, element-specific structural investigation of complex biomolecular layers and biological surfaces.

Dense Poly(ethylene glycol) Brushes Reduce Adsorption and Stabilize the Unfolded Conformation of Fibronectin

Polymer brushes, in which polymers are end-tethered densely to a grafting surface, are commonly proposed for use as stealth coatings for various biomaterials. However, although their use has received considerable attention, a mechanistic understanding of the impact of brush properties on protein adsorption and unfolding remains elusive. We investigated the effect of the grafting density of poly(ethylene glycol) (PEG) brushes on the interactions of the brush with fibronectin (FN) using high-throughput single-molecule tracking methods, which directly measure protein adsorption and unfolding within the brush. We observed that, as grafting density increased, the rate of FN adsorption decreased; however, surface-adsorbed FN unfolded more readily, and unfolded molecules were retained on the surface for longer residence times relative to those of folded molecules. These results, which are critical for the rational design of PEG brushes, suggest that there is a critical balance between protein adsorption and conformation that underlies the utility of such brushes in physiological environments.

Self-assembly behavior of gels composed of dibenzylidene sorbitol derivatives and poly(ethylene glycol)

The strongest intermolecular interactions between DBS molecules in the DBS/PEG systems led to the more regular structure compared with the DMDBS/PEG and TBPMN/PEG systems.

Zwitterionic gel encapsulation promotes protein stability, enhances pharmacokinetics, and reduces immunogenicity

Advances in protein therapy are hindered by the poor stability, inadequate pharmacokinetic (PK) profiles, and immunogenicity of many therapeutic proteins. Polyethylene glycol conjugation (PEGylation) is the most successful strategy to date to overcome these shortcomings, and more than 10 PEGylated proteins have been brought to market. However, anti-PEG antibodies induced by treatment raise serious concerns about the future of PEGylated therapeutics. Here, we demonstrate a zwitterionic polymer network encapsulation technology that effectively enhances protein stability and PK while mitigating the immune response. Uricase modified with a comprehensive zwitterionic polycarboxybetaine (PCB) network exhibited exceptional stability and a greatly prolonged circulation half-life. More importantly, the PK behavior was unchanged, and neither anti-uricase nor anti-PCB antibodies were detected after three weekly injections in a rat model. This technology is applicable to a variety of proteins and unlocks the possibility of adopting highly immunogenic proteins for therapeutic or protective applications.