Orientation of a monoclonal antibody adsorbed at the solid/solution interface: a combined study using atomic force microscopy and neutron reflectivity

Protecting monoclonal antibodies via competitive interfacial adsorption of nonionic surfactants

HYPOTHESIS

Bioengineered monoclonal antibodies (mAbs) have gained significant recognition as medical therapies. However, during processing, storage and use, mAbs are susceptible to interfacial adsorption and desorption, leading to structural deformation and aggregation, and undermining their bioactivity. To suppress antibody surface adsorption, nonionic surfactants are commonly used in formulation. But how surface hydrophobicity affects the adsorption and desorption of mAbs and nonionic surfactants individually and as a mixture remains inconclusive.

EXPERIMENTS

The rapid tuning of the siliconized surface from hydrophobic to hydrophilic was controlled by the UV oxidation time of a self-assembled trimethoxy(7-octen-1-yl)silane (TMOS) monolayer. Spectroscopic ellipsometry and neutron reflection were used to determine the dynamic adsorption and structural changes of the co-adsorbed mAb (COE-3) and the commercial nonionic surfactant PS80, which is composed primarily of polyoxyethylene-sorbitan monooleate with an average molecular weight of about 1310 g/mol.

FINDINGS

COE-3 adsorption on both TMOS or UV-TMOS surface was irreversible. However, nonionic surfactant PS80 could partially remove pre-adsorbed COE-3 from these surfaces, forming a co-adsorption layer. Interestingly, while the hydrophobic TMOS surface prevented mAb adsorption when pre-treated with PS80, the amphiphilic UV-TMOS did not. Furthermore, when COE-3 and PS80 were injected as a mixture, PS80 formed a preventative layer on both surfaces against COE-3 adsorption. These results highlight the significance of surface hydrophobicity in controlling mAb adsorption in the presence of nonionic surfactants.

pH-Dependent Conformational Plasticity of Monoclonal Antibodies at the SiO2/Water Interface: Insights from Neutron Reflectivity and Molecular Dynamics

Investigating the molecular conformations of monoclonal antibodies (mAbs) adsorbed at the solid/liquid interface is crucial for understanding mAb solution stability and advancing the development of mAb-based biosensors. This study examines the pH-dependent conformational plasticity of a human IgG1k mAb, COE-3, at the SiO2/water interface under varying pH conditions (pH 5.5 and 9). By integrating neutron reflectivity (NR) and molecular dynamics (MD) simulations, we reveal that the mAb irreversibly deposits onto the interface at pH 5.5, with surface density saturation reached at 20 ppm bulk concentration. At pH 5.5, the adsorbed mAb adopts a stable "flat-on" orientation, while at pH 9, it assumes a more flexible conformation and a "tilted" orientation. This pH-dependent orientation shift is reversible and influenced by the distinct surface charge properties of the Fab and Fc fragments, with the Fc fragment more prone to desorption at higher pH. The root-mean-square deviation (RMSD) analysis further shows that COE-3 maintains structural stability upon adsorption across both pH levels, showing minimal unfolding or denaturation. These findings highlight how pH-dependent electrostatic interactions between mAb fragments and the SiO2 interface drive conformational adjustments in the intact mAb, offering insights into adsorption-induced aggregation and suggesting pH modulation as a mechanism for controlling biosensor efficiency.

Virtual Screening and Validation of Affinity DNA Functional Ligands for IgG Fc Segment

The effective attachment of antibodies to the immune sensing interface is a crucial factor that determines the detection performance of immunosensors. Therefore, this study aims to investigate a novel antibody immobilization material with low molecular weight, high stability, and excellent directional immobilization effect. In this study, we employed molecular docking technology based on the ZDOCK algorithm to virtually screen DNA functional ligands (DNAFL) for the Fc segment of antibodies. Through a comprehensive analysis of the key binding sites and contact propensities at the interface between DNAFL and IgG antibody, we have gained valuable insights into the affinity relationship, as well as the principles governing amino acid and nucleotide interactions at this interface. Furthermore, molecular affinity experiments and competitive binding experiments were conducted to validate both the binding ability of DNAFL to IgG antibody and its actual binding site. Through affinity experiments using multi-base sequences, we identified bases that significantly influence antibody-DNAFL binding and successfully obtained DNAFL with an enhanced affinity towards the IgG Fc segment. These findings provide a theoretical foundation for the targeted design of higher-affinity DNAFLs while also presenting a new technical approach for immunosensor preparation with potential applications in biodetection.

The Adsorption of P2X2 Receptors Interacting with IgG Antibodies Revealed by Combined AFM Imaging and Mechanical Simulation

The adsorption of proteins onto surfaces significantly impacts biomaterials, medical devices, and biological processes. This study aims to provide insights into the irreversible adsorption process of multiprotein complexes, particularly focusing on the interaction between anti-His6 IgG antibodies and the His6-tagged P2X2 receptor. Traditional approaches to understanding protein adsorption have centered around kinetic and thermodynamic models, often examining individual proteins and surface coverage, typically through Molecular Dynamics (MD) simulations. In this research, we introduce a computational approach employing Autodesk Maya 3D software for the investigation of multiprotein complexes' adsorption behavior. Utilizing Atomic Force Microscopy (AFM) imaging and Maya 3D-based mechanical simulations, our study yields real-time structural and kinetic observations. Our combined experimental and computational findings reveal that the P2X2 receptor-IgG antibody complex likely undergoes absorption in an 'extended' configuration. Whereas the P2X2 receptor is less adsorbed once is complexed to the IgG antibody compared to its individual state, the opposite is observed for the antibody. This insight enhances our understanding of the role of protein-protein interactions in the process of protein adsorption.

Capacitive immunosensor based on grafted Anodic Aluminum Oxide for the detection of matrix metalloproteinase 9 found in chronic wounds

Chronic wounds impose a significant burden on healthcare resources, society and more specifically on patients. Preliminary research showed that as of today, there is not a system that can do a precise monitoring of these wounds so that healthcare systems can manage them with efficiency. The overall aim of our project is to produce a capacitive sensor able to detect a specific molecule in chronic wounds, thus giving information concerning its inflammation state. In this article, we present a system that uses nanoporous Anodic Aluminum Oxide (AAO) grafted with a commercially available anti-MMP9 antibody able to interact with Matrix Metalloproteinase 9, an enzyme that works as an indicator of inflammation. In order to produce a proof-of-concept we chose to compare two methods of functionalization followed by a thorough analysis with biological, electrical and optical testing. This study produced reproducible results for each functionalization method, chemisorption being the best choice for the immobilization of conventional antibodies on AAO-based sensors for a detection of MMP9 in pure and complex conditions. This proof-of-concept and its analysis allowed a better understanding of the needs of the overall project and will be helpful to produce a prototype of smart dressing in the near future.

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.

A novel anti-tumor/anti-tumor-associated fibroblast/anti-mPEG tri-specific antibody to maximize the efficacy of mPEGylated nanomedicines against fibroblast-rich solid tumor

The therapeutic efficacy of methoxypolyethylene glycol (mPEG)-coated nanomedicines in solid tumor treatment is hindered by tumor-associated fibroblasts (TAFs), which promote tumor progression and form physical barriers. We developed an anti-HER2/anti-FAP/anti-mPEG tri-specific antibody (TsAb) for one-step conversion of mPEG-coated liposomal doxorubicin (Lipo-Dox) to immunoliposomes, which simultaneously target HER2⁺ breast cancer cells and FAP⁺ TAFs. The non-covalent modification did not adversely alter the physical characteristics and stability of Lipo-Dox. The TsAb-Lipo-Dox exhibited specific targeting and enhanced cytotoxicity against mono- and co-cultured HER2⁺ breast cancer cells and FAP⁺ TAFs, compared to bi-specific antibody (BsAb) modified or unmodified Lipo-Dox. An in vivo model of human breast tumor containing TAFs also revealed the improved tumor accumulation and therapeutic efficacy of TsAb-modified mPEGylated liposomes without signs of toxicity. Our data indicate that arming clinical mPEGylated nanomedicines with the TsAb is a feasible and applicable approach for overcoming the difficulties caused by TAFs in solid tumor treatment.

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.

Surface Biomodification of Liposomes and Polymersomes for Efficient Targeted Drug Delivery

Chemotherapy has seen great progress in the development of performant treatment strategies. Nanovesicles such as liposomes and polymersomes demonstrated great potential in cancer therapy. However, these nanocarriers deliver their content passively, which faces a lot of constraints during blood circulation. The main challenge resides in degradation and random delivery to normal tissues. Hence, targeting drug delivery using specific molecules (such as antibodies) grafted over the surface of these nanocarriers came as the answer to overcome many problems faced before. The advantage of using antibodies is their antigen/antibody recognition, which provides a high level of specificity to reach treatment targets. This review discusses the many techniques of nanocarrier functionalization with antibodies. The aim is to recognize the various approaches by describing their advantages and deficiencies to create the most suitable drug delivery platform. Some methods are more suitable for other applications rather than drug delivery, which can explain the low success of some proposed targeted nanocarriers. In here, a critical analysis of how every method could impact the recognition and targeting capacity of some nanocarriers (liposomes and polymersomes) is discussed to make future research more impactful and advance the field of biomedicine further.

Adsorption of non-ionic surfactant and monoclonal antibody on siliconized surface studied by neutron reflectometry

The adsorption of monoclonal antibodies (mAbs) on hydrophobic surfaces is known to cause protein aggregation and degradation. Therefore, surfactants, such as Poloxamer 188, are widely used in therapeutic formulations to stabilize mAbs and protect mAbs from interacting with liquid-solid interfaces. Here, the adsorption of Poloxamer 188, one mAb and their competitive adsorption on a model hydrophobic siliconized surface is investigated with neutron scattering coupled with contrast variation to determine the molecular structure of adsorbed layers for each case. Small angle neutron scattering measurements of the affinity of Poloxamer 188 to this mAb indicate that there is negligible binding at these solution conditions. Neutron reflectometry measurements of the mAb show irreversible adsorption on the siliconized surface, which cannot be washed off with neat buffer. Poloxamer 188 can be adsorbed on the surface already occupied by mAb, which enables partial removal of some adsorbed mAb by washing with buffer. The adsorption of the surfactant introduces significant conformational changes for mAb molecules that remain on the surface. In contrast, if the siliconized surface is first saturated with the surfactant, no adsorption of mAb is observed. Competitive adsorption of mAb and Poloxamer 188 from solution leads to a surface dominantly occupied with surfactant molecules, whereas only a minor amount of mAb absorbs. These findings clearly indicate that Poloxamer 188 can protect against mAb adsorption as well as modify the adsorbed conformation of previously adsorbed mAb.

Interfacial Assembly Inspired by Marine Mussels and Antifouling Effects of Polypeptoids: A Neutron Reflection Study

Polypeptoid-coated surfaces and many surface-grafted hydrophilic polymer brushes have been proven efficient in antifouling-the prevention of nonspecific biomolecular adsorption and cell attachment. Protein adsorption, in particular, is known to mediate subsequent cell-surface interactions. However, the detailed antifouling mechanism of polypeptoid and other polymer brush coatings at the molecular level is not well understood. Moreover, most adsorption studies focus only on measuring a single adsorbed mass value, and few techniques are capable of characterizing the hydrated in situ layer structure of either the antifouling coating or adsorbed proteins. In this study, interfacial assembly of polypeptoid brushes with different chain lengths has been investigated in situ using neutron reflection (NR). Consistent with past simulation results, NR revealed a common two-step structure for grafted polypeptoids consisting of a dense inner region that included a mussel adhesive-inspired oligopeptide for grafting polypeptoid chains and a highly hydrated upper region with very low polymer density (molecular brush). Protein adsorption was studied with human serum albumin (HSA) and fibrinogen (FIB), two common serum proteins of different sizes but similar isoelectric points (IEPs). In contrast to controls, we observed higher resistance by grafted polypeptoid against adsorption of the larger FIB, especially for longer chain lengths. Changing the pH to close to the IEPs of the proteins, which generally promotes adsorption, also did not significantly affect the antifouling effect against FIB, which was corroborated by atomic force microscopy imaging. Moreover, NR enabled characterization of the in situ hydrated layer structures of the polypeptoids together with proteins adsorbed under selected conditions. While adsorption on bare SiO2 controls resulted in surface-induced protein denaturation, this was not observed on polypeptoids. Our current results therefore highlight the detailed in situ view that NR may provide for characterizing protein adsorption on polymer brushes as well as the excellent antifouling behavior of polypeptoids.

Membrane stiffness and myelin basic protein binding strength as molecular origin of multiple sclerosis

Myelin basic protein (MBP) and its interaction with lipids of the myelin sheath plays an important part in the pathology of multiple sclerosis (MS). Previous studies observed that changes in the myelin lipid composition lead to instabilities and enhanced local curvature of MBP-lipid multilayer structures. We investigated the molecular origin of the instability and found that the diseased lipid membrane has a 25% lower bending rigidity, thus destabilizing smooth \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$>1\,$$\end{document}>1µm curvature radius structures such as in giant unilamellar vesicles. MBP-mediated assembling of lipid bilayers proceeds in two steps, with a slow second step occurring over many days where native lipid membranes assemble into well-defined multilayer structures, whereas diseased lipid membranes form folded assemblies with high local curvature. For both native and diseased lipid mixtures we find that MBP forms dense liquid phases on top of the lipid membranes mediating attractive membrane interactions. Furthermore, we observe MBP to insert into its bilayer leaflet side in case of the diseased lipid mixture, whereas there is no insertion for the native mixture. Insertion increases the local membrane curvature, and could be caused by a decrease of the sphingomyelin content of the diseased lipid mixture. These findings can help to open a pathway to remyelination strategies.

Recent Advances in Studying Interfacial Adsorption of Bioengineered Monoclonal Antibodies

Monoclonal antibodies (mAbs) are an important class of biotherapeutics; as of 2020, dozens are commercialized medicines, over a hundred are in clinical trials, and many more are in preclinical developmental stages. Therapeutic mAbs are sequence modified from the wild type IgG isoforms to varying extents and can have different intrinsic structural stability. For chronic treatments in particular, high concentration (≥ 100 mg/mL) aqueous formulations are often preferred for at-home administration with a syringe-based device. MAbs, like any globular protein, are amphiphilic and readily adsorb to interfaces, potentially causing structural deformation and even unfolding. Desorption of structurally perturbed mAbs is often hypothesized to promote aggregation, potentially leading to the formation of subvisible particles and visible precipitates. Since mAbs are exposed to numerous interfaces during biomanufacturing, storage and administration, many studies have examined mAb adsorption to different interfaces under various mitigation strategies. This review examines recent published literature focusing on adsorption of bioengineered mAbs under well-defined solution and surface conditions. The focus of this review is on understanding adsorption features driven by distinct antibody domains and on recent advances in establishing model interfaces suitable for high resolution surface measurements. Our summary highlights the need to further understand the relationship between mAb interfacial adsorption and desorption, solution aggregation, and product instability during fill-finish, transport, storage and administration.

Surfaces Affect Screening Reliability in Formulation Development of Biologics

PURPOSE

The ability to predict an antibody's propensity for aggregation is particularly important during product development to ensure the quality and safety of therapeutic antibodies. We demonstrate the role of container surfaces on the aggregation process of three mAbs under elevated temperature and long-term storage conditions in the absence of mechanical stress.

METHODS

A systematic study of aggregation is performed for different proteins, vial material, storage temperature, and presence of surfactant. We use size exclusion chromatography and micro-flow imaging to determine the bulk concentration of aggregates, which we combine with optical and atomic force microscopy of vial surfaces to determine the effect of solid-liquid interfaces on the bulk aggregate concentration under different conditions.

RESULTS

We show that protein particles under elevated temperature conditions adhere to the vial surfaces, causing a substantial underestimation of aggregation propensity as determined by common methods used in development of biologics. Under actual long-term storage conditions at 5°C, aggregate particles do not adhere to the surface, causing an increase in bulk concentration of particles, which cannot be predicted from elevated temperature screening tests by common methods alone. We also identify specific protein - surface interactions which promote oligomer formation in the nanometre range.

CONCLUSIONS

Special care should be taken when interpreting size exclusion and particle count data from stability studies if different temperatures and vial types are involved. We propose a novel combination of methods to characterise vial surfaces and bulk solution for a full understanding of protein aggregation processes in a sample.

Detection of Brucella abortus by a platform functionalized with protein A and specific antibodies IgG

Oriented immobilization of antibodies on a sensor surface is critical for enhancing both the antigen-binding capacity and the sensitivity of immunosensors. In this study, we describe a strategy to adsorb immunoglobulin G (IgG) anti-Brucella antibodies onto a silicon surface, oriented by protein A obtained from Staphylococcus aureus (SpA). X-ray photoelectron spectroscopy and atomic force microscopy were used to characterize topographically, morphologically, and chemical changes of the sensor functionalization. The activity of the biosensor was assessed by confocal microscopy, scanning electronic microscopy, and bacteria capture assays (BCA). According to the BCA, the efficiency of Brucella abortus detection with the SpA-IgG anti Brucella biosensor was three-fold higher than that of the random orientated IgG anti Brucella biosensor. The limit of detection was 1 × 10⁶ CFU/ml. These data show that the orientation of antibodies immobilization is crucial to developing immunosensors for bacterial antigen detection as Brucella spp and improve its sensibility level. Functionalization with protein A increases Brucella detection by an antibody-coated surface. Functionalized silicon surface for Brucella detection was characterized by atomic force microscopy, X-ray photoelectron spectroscopy and confocal microscopy.

Coadsorption of a Monoclonal Antibody and Nonionic Surfactant at the SiO2/Water Interface

During the formulation of therapeutic monoclonal antibodies (mAbs), nonionic surfactants are commonly added to attenuate structural rearrangement caused by adsorption/desorption at interfaces during processing, shipping, and storage. We examined the adsorption of a mAb (COE-3) at the SiO₂/water interface in the presence of pentaethylene glycol monododecyl ether (C₁₂E₅), polysorbate 80 (PS80-20EO), and a polysorbate 80 analogue with seven ethoxylates (PS80-7EO). Spectroscopic ellipsometry was used to follow COE-3 dynamic adsorption, and neutron reflection was used to determine interfacial structure and composition. Neither PS80-20EO nor C₁₂E₅ had a notable affinity for COE-3 or the interface under the conditions studied and thus did not prevent COE-3 adsorption. In contrast, PS80-7EO did coadsorb but did not influence the dynamic process or the equilibrated amount of absorbed COE-3. Near equilibration, COE-3 underwent structural rearrangement and PS80-7EO started to bind the COE-3 interfacial layer and subsequently formed a well-defined surfactant bilayer via self-assembly. The resultant interfacial layer comprised an inner mAb layer of about 70 Å thickness and an outer surfactant layer of a further 70 Å, with distinct transitional regions across the mAb-surfactant and surfactant-bulk water boundaries. Once formed, such interfacial layers were very robust and worked to prevent further mAb adsorption, desorption, and structural rearrangement. Such robust interfacial layers could be anticipated to exist for formulated mAbs stored in type II glass vials; further research is required to understand the behavior of these layers for siliconized glass syringes.

The influence of covalent immobilization conditions on antibody accessibility on nanoparticles

The accessibility of particle-coupled antibodies is important for many analytical applications, but comprehensive data on parameters controlling the accessibility are scarce. Here we report on the site-specific accessibility of monoclonal antibodies, immobilized on magnetic nanoparticles (500 nm) by the widely used covalent EDC coupling method, with the variation of four key coupling parameters (surface activation and immobilization pH, crosslinker and antibody concentration ratios). By developing quantitative radio-labelled assays, the number of immobilized antibodies, the Fab domain accessibility (in a sandwich immunoassay), and the Fc domain accessibility (in a Protein G assay) were determined. For sub-monolayer surface coverage, the observed numbers of accessible Fab and Fc domains are equal and scale linearly with the antibody density. For above monolayer coverage, the fractions of accessible Fab and Fc domains decrease, in an unequal manner. The results show that the antibody accessibility is primarily determined by the antibody surface density, rather than by chemical reactivity or the charge state, and that crowded conditions affect Fab and Fc accessibility in an unequal manner.

Current Conjugation Methods for Immunosensors

Recent advances in the development of immunosensors using polymeric nanomaterials and nanoparticles have enabled a wide range of new functions and applications in diagnostic and prognostic research. One fundamental challenge that all immunosensors must overcome is to provide the specificity of target molecular recognition by immobilizing antibodies, antibody fragments, and/or other peptides or oligonucleotide molecules that are capable of antigen recognition on a compact device surface. This review presents progress in the application of immobilization strategies including the classical adsorption process, affinity attachment, random cross-linking and specific covalent linking. The choice of immobilization methods and its impact on biosensor performance in terms of capture molecule loading, orientation, stability and capture efficiency are also discussed in this review.

Synthesis, Radiolabelling and In Vitro Imaging of Multifunctional Nanoceramics

Molecular imaging has become a powerful technique in preclinical and clinical research aiming towards the diagnosis of many diseases. In this work, we address the synthetic challenges in achieving lab-scale, batch-to-batch reproducible copper-64- and gallium-68-radiolabelled metal nanoparticles (MNPs) for cellular imaging purposes. Composite NPs incorporating magnetic iron oxide cores with luminescent quantum dots were simultaneously encapsulated within a thin silica shell, yielding water-dispersible, biocompatible and luminescent NPs. Scalable surface modification protocols to attach the radioisotopes 64Cu (t1/2=12.7 h) and 68Ga (t1/2=68 min) in high yields are reported, and are compatible with the time frame of radiolabelling. Confocal and fluorescence lifetime imaging studies confirm the uptake of the encapsulated imaging agents and their cytoplasmic localisation in prostate cancer (PC-3) cells. Cellular viability assays show that the biocompatibility of the system is improved when the fluorophores are encapsulated within a silica shell. The functional and biocompatible SiO2 matrix represents an ideal platform for the incorporation of 64Cu and 68Ga radioisotopes with high radiolabelling incorporation.

Interfacial Adsorption of Monoclonal Antibody COE-3 at the Solid/Water Interface

Spectroscopic ellipsometry (SE) and neutron reflection (NR) data for the adsorption of a monoclonal antibody (mAb, termed COE-3, pI 8.44) at the bare SiO₂/water interface are compared here to the simulations based on Derjaguin-Landau-Verwey-Overbeek theory. COE-3 adsorption was characterized by an initial rapid increase in the surface-adsorbed amount (Γ) followed by a plateau. Only the initial rate of the increase in Γ was strongly correlated with the bulk concentration (0.002-0.2 mg/mL), with Γ at the plateau being about 2.2 mg/m² (pH 5.5). Simulations captured COE-3 adsorption at equilibrium most accurately, the point at which the outgoing flux of molecules within the adsorbed plane matched the adsorption flux. Increasing the buffer pH from 5.5 to 9 increased Γ at equilibrium to ∼3 mg/m² (0.02 mg/mL COE-3), revealing a dominant role for lateral repulsion between adsorbed mAb molecules. In contrast, increasing the buffer ionic strength (pH 6) reduced Γ, which was captured by simulations accounting for electrostatic screening by ions, in addition to mAb/SiO₂ attractive forces and lateral repulsion. NR data at the same bulk concentrations corroborated the SE data, albeit with slightly higher Γ due to longer adsorption times for data acquisition; for example, at pH 9, Γ was 3.6 mg/m² (0.02 mg/mL COE-3), equivalent to a relatively high volume fraction of 0.5. An adsorbed monolayer with a thickness of 50-52 Å was consistently determined by NR, corresponding to the short axial lengths of fragment antigen-binding and fragment crystallization and implying minimal structural perturbation. Thus, the simulations enabled a mechanistic interpretation of the experimental data of mAb adsorption at the SiO₂/water interface.

Variability in Microplate Surface Properties and Its Impact on ELISA

BACKGROUND

Microplate-based immunoassays are widely used in clinical and research settings to measure a broad range of biomarkers present in complex matrices. Assay variability within and between microplates can give rise to false-negative and false-positive results leading to incorrect conclusions. To date, the contribution of microplates to this variability remains poorly characterized and described. This study provides new insights into variability in immunoassays attributable to surface characteristics of commercial microplates.

METHODS

Well-to-well assay variation in γ-treated and nontreated 96-well opaque microplates suitable for chemiluminescence assays was determined by use of a validated sandwich ELISA. Microplate surface characteristics were assessed by sessile drop contact angle measurements, scanning electron microscopy, energy dispersive x-ray spectroscopy, and atomic force microscopy.

RESULTS

All microplate types tested exhibited vendor-specific assay response profiles; and "rogue" plates with very high intraassay variation and deviant mean assay responses were found. Within-plate, location-dependent bias in assay responses and variability in well contact angle were also observed. We demonstrate substantial differences in well-surface properties with putative effects on protein-coating reproducibility and hence consistency in immunoassay responses. A surface "cleaning" effect on manufacturing residues was attributed to γ-irradiation, and treated microplates manifest increased polar functionalities, surface roughness, and assay responses.

CONCLUSIONS

Our data suggest that tighter control of variability in surface roughness, wettability, chemistry, and level of residual contaminants during microplate preparation is warranted to improve consistency of ELISA assay read out.

Full-length antibodies versus single-chain antibody fragments for a selective impedimetric lectin-based glycoprofiling of prostate specific antigen

The main aim of the research was to design a functional impedimetric biosensor able to glycoprofile prostate specific antigen (PSA), a biomarker for prostate cancer (PCa), with high specificity using lectins as glycan recognising proteins. Traditionally, full-length antibody is immobilised on the biosensor interface for specific capture of PSA with subsequent glycoprofiling of PSA by addition of lectins. Since full-length antibodies contain glycans in the Fc domain, particular attention has to be paid to suppress direct binding of lectins to immobilised full-length antibodies, which would compromise accurate glycoprofiling. This issue is addressed here using a recombinant single-chain antibody fragments (scAb), which do not contain any carbohydrate moiety. Surface plasmon resonance was applied to prove negligible interaction of lectins with immobilised scAb fragments, while substantial binding of lectins to full length antibodies was observed. Eight different biosensor designs were tested for their ability to detect PSA. The biosensor device based on scAb fragments covalently immobilised on the gold electrode surface, patterned by a mixed SAM using standard amine coupling chemistry, proved to be the most sensitive. The scAb fragment-based biosensor exhibited sensitivity of 15.9 ± 0.8% decade^-1 (R² = 0.991 with an average RSD of 4.9%), while the full antibody-based biosensor offered sensitivity towards PSA of 4.2 ± 0.1% decade^-1 (R² = 0.999 with an average RSD of 4.8%). Moreover, the selectivity of the scAb-based biosensor was tested using a kallikrein 2 protein, a protein structurally similar to PSA, and the results indicated high selectivity for PSA detection.

Surface engineering of poly(methylmethacrylate): Effects on fluorescence immunoassay

The authors present surface engineering modifications through chemistry of poly(methylmethacrylate) (PMMA) that have dramatic effects on the result of surface-bound fluorescence immunoassays, both for specific and nonspecific signals. The authors deduce the most important effect to be clustering of antibodies on the surface leading to significant self-quenching. Secondary effects are attributable to the formation of sparse multilayers of antibody. The authors compare PMMA as an antibody support surface with ultraviolet-ozone oxidized PMMA and also to substrates that were, after the oxidation, surface modified by a four-unit poly(ethyleneglycol) carboxylic acid (PEG₄), a branched tricarboxylic acid, and a series of carboxylic acid-terminated dendrimers, from generation 1.5 to 5.5. Fluorescence immunoassay and neutron reflectometry were used to compare the apparent antibody surface loading, antigen binding and nonspecific binding on these various surfaces using anti-human IgG as a model antibody, chemically coupled to the surface by amide formation. Simple physical adsorption of the antibody on PMMA resulted in a thick antibody multilayer with small antigen binding capacity. On the carboxylated surfaces, with chemical coupling, a simple monolayer was formed. The authors deduce that antibody clustering was driven by conformational inflexibility and high carboxylate density. The PEG₄-modified surface was the most conformationally flexible. The dendrimer-modified interfaces showed a collapse and densification. In fluorescence immunoassay, the optimal combination of high specific and low nonspecific fluorescence signal was found for the G3.5 dendrimer.

Neutron reflectivity measurement of protein A-antibody complex at the solid-liquid interface

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The orientation of IgG4 adsorbed at the solid-liquid interface was probed.

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A chromatography resin was mimicked by attaching protein A to a silica surface.

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Neutron reflectivity was used to measure protein A and adsorbed IgG structures.

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Protein A-modified silica was blocked with either BSA or PEG before IgG adsorption.

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Adsorbed IgG extended up to 230 Å from the surface, depending on blocking strategy.

Chromatography is a ubiquitous unit operation in the purification of biopharmaceuticals yet few studies have addressed the biophysical characterisation of proteins at the solution-resin interface. Chromatography and other adsorption and desorption processes have been shown to induce protein aggregation which is undesirable in biopharmaceutical products. In order to advance understanding of how adsorption processes might impact protein stability, neutron reflectivity was used to characterise the structure of adsorbed immunoglobulin G (IgG) on model surfaces. In the first model system, IgG was adsorbed directly to silica and demonstrated a side-on orientation with high surface contact. A maximum dimension of 60 Å in the surface normal direction and high density surface coverage were observed under pH 4.1 conditions. In chromatography buffers, pH was found to influence IgG packing density and orientation at the solid-liquid interface. In the second model system, which was designed to mimic an affinity chromatography surface, protein A was attached to a silica surface to produce a configuration representative of a porous glass chromatography resin. Interfacial structure was probed during sequential stages from ligand attachment, through to IgG binding and elution. Adsorbed IgG structures extended up to 250 Å away from the surface and showed dependence on surface blocking strategies. The data was suggestive of two IgG molecules bound to protein A with a somewhat skewed orientation and close proximity to the silica surface. The findings provide insight into the orientation of adsorbed antibody structures under conditions encountered during chromatographic separations.

Antibody adsorption on the surface of water studied by neutron reflection

Surface and interfacial adsorption of antibody molecules could cause structural unfolding and desorbed molecules could trigger solution aggregation, resulting in the compromise of physical stability. Although antibody adsorption is important and its relevance to many mechanistic processes has been proposed, few techniques can offer direct structural information about antibody adsorption under different conditions. The main aim of this study was to demonstrate the power of neutron reflection to unravel the amount and structural conformation of the adsorbed antibody layers at the air/water interface with and without surfactant, using a monoclonal antibody 'COE-3' as the model. By selecting isotopic contrasts from different ratios of H₂O and D₂O, the adsorbed amount, thickness and extent of the immersion of the antibody layer could be determined unambiguously. Upon mixing with the commonly-used non-ionic surfactant Polysorbate 80 (Tween 80), the surfactant in the mixed layer could be distinguished from antibody by using both hydrogenated and deuterated surfactants. Neutron reflection measurements from the co-adsorbed layers in null reflecting water revealed that, although the surfactant started to remove antibody from the surface at 1/100 critical micelle concentration (CMC) of the surfactant, complete removal was not achieved until above 1/10 CMC. The neutron study also revealed that antibody molecules retained their globular structure when either adsorbed by themselves or co-adsorbed with the surfactant under the conditions studied.

Modelling random antibody adsorption and immunoassay activity

One of the primary considerations in immunoassay design is optimizing the concentration of capture antibody in order to achieve maximal antigen binding and, subsequently, improved sensitivity and limit of detection. Many immunoassay technologies involve immobilization of the antibody to solid surfaces. Antibodies are large molecules in which the position and accessibility of the antigen-binding site depend on their orientation and packing density. In this paper we propose a simple mathematical model, based on the theory known as random sequential adsorption (RSA), in order to calculate how the concentration of correctly oriented antibodies (active site exposed for subsequent reactions) evolves during the deposition process. It has been suggested by experimental studies that high concentrations will decrease assay performance, due to molecule denaturation and obstruction of active binding sites. However, crowding of antibodies can also have the opposite effect by favouring upright orientations. A specific aim of our model is to predict which of these competing effects prevails under different experimental conditions and study the existence of an optimal coverage, which yields the maximum expected concentration of active particles (and hence the highest signal).

Design of lipidic platforms anchored within nanometric cavities by peptide hooks

A stable confinement of liposomes within arrays of hybrid polymer/Au nanocavities was achieved using peptide hooks covalently linked to the Au floor.

Different Organization of Type I Collagen Immobilized on Silanized and Nonsilanized Titanium Surfaces Affects Fibroblast Adhesion and Fibronectin Secretion

Silanization has emerged in recent years as a way to obtain a stronger and more stable attachment of biomolecules to metallic substrates. However, its impact on protein conformation, a key aspect that influences cell response, has hardly been studied. In this work, we analyzed by atomic force microscopy (AFM) the distribution and conformation of type I collagen on plasma-treated surfaces before and after silanization. Subsequently, we investigated the effect of the different collagen conformations on fibroblasts adhesion and fibronectin secretion by immunofluorescence analyses. Two different organosilanes were used on plasma-treated titanium surfaces, either 3-chloropropyl-triethoxy-silane (CPTES) or 3-glycidyloxypropyl-triethoxy-silane (GPTES). The properties and amount of the adsorbed collagen were assessed by contact angle, X-ray photoelectron spectroscopy, optical waveguide lightmode spectroscopy, and AFM. AFM studies revealed different conformations of type I collagen depending on the silane employed. Collagen was organized in fibrillar networks over very hydrophilic (plasma treated titanium) or hydrophobic (silanized with CPTES) surfaces, the latter forming little globules with a beads-on-a-string appearance, whereas over surfaces presenting an intermediate hydrophobic character (silanized with GPTES), collagen was organized into clusters with a size increasing at higher protein concentration in solution. Cell response was strongly affected by collagen conformation, especially at low collagen density. The samples exhibiting collagen organized in globular clusters (GPTES-functionalized samples) favored a faster and better fibroblast adhesion as well as better cell spreading, focal adhesions formation, and more pronounced fibronectin fibrillogenesis. In contrast, when a certain protein concentration was reached at the material surface, the effect of collagen conformation was masked, and similar fibroblast response was observed in all samples.

Interrogating protonated/deuterated fibronectin fragment layers adsorbed to titania by neutron reflectivity and their concomitant control over cell adhesion

The fibronectin fragment, 9th-10th-type III domains (FIII9-10), mediates cell attachment and spreading and is commonly investigated as a bioadhesive interface for implant materials such as titania (TiO2). How the extent of the cell attachment-spreading response is related to the nature of the adsorbed protein layer is largely unknown. Here, the layer thickness and surface fraction of two FIII9-10 mutants (both protonated and deuterated) adsorbed to TiO2 were determined over concentrations used in cell adhesion assays. Unexpectedly, the isotopic forms had different adsorption behaviours. At solution concentrations of 10 mg l(-1), the surface fraction of the less conformationally stable mutant (FIII9'10) was 42% for the deuterated form and 19% for the protonated form (fitted to the same monolayer thickness). Similarly, the surface fraction of the more stable mutant (FIII9'10-H2P) was 34% and 18% for the deuterated and protonated forms, respectively. All proteins showed a transition from monolayer to bilayer between 30 and 100 mg l(-1), with the protein longitudinal orientation moving away from the plane of the TiO2 surface at high concentrations. Baby hamster kidney cells adherent to TiO2 surfaces coated with the proteins (100 mg l(-1)) showed a strong spreading response, irrespective of protein conformational stability. After surface washing, FIII9'10 and FIII9'10-H2P bilayer surface fractions were 30/25% and 42/39% for the lower/upper layers, respectively, implying that the cell spreading response requires only a partial protein surface fraction. Thus, we can use neutron reflectivity to inform the coating process for generating bioadhesive TiO2 surfaces.

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

Neutron reflectometry provides evidence of ternary protein adsorption within polyethylene glycol (PEG) brushes. Anti-PEG Immunoglobulin G antibodies (Abs) binding the methoxy terminated PEG chain segment specifically adsorb onto PEG brushes grafted to lipid monolayers on a solid support. The Abs adsorb at the outer edge of the brush. The thickness and density of the adsorbed Ab layer, as well as its distance from the grafting surface grow with increasing brush density. At high densities most of the protein is excluded from the brush. The results are consistent with an inverted "Y" configuration with the two FAB segments facing the brush. They suggest that increasing the grafting density favors narrowing of the angle between the FAB segments as well as overall orientation of the bound Abs perpendicular to the surface.

Engineered self-assembling monolayers for label free detection of influenza nucleoprotein

Integrating nanotechnology into useable devices requires a combination of bottom up and top down methodology. Often the techniques to measure and control these different components are entirely different, so methods that can analyse the nanoscale component in situ are of increasing importance. Here we describe a strategy that employs a self-assembling monolayer of engineered protein chimeras to display an array of oriented antibodies (IgG) on a microelectronic device for the label free detection of influenza nucleoprotein. The structural and functional properties of the bio-interface were characterised by a range of physical techniques including surface plasmon resonance, quartz-crystal microbalance and neutron reflectometry. This combination of methods reveals a 13.5 nm thick engineered-monolayer that (i) self-assembles on gold surfaces, (ii) captures IgG with high affinity in a defined orientation and (iii) specifically recognises the influenza A nucleoprotein. Furthermore we also show that this non-covalent self-assembled structure can render the dissociation of bound IgG irreversible by chemical crosslinking in situ without affecting the IgG function. The methods can thus describe in detail the transition from soluble engineered molecules with nanometre dimensions to an array that demonstrates the principles of a working influenza sensor.

Surface orientation control of site-specifically immobilized nitro-reductase (NfsB)

We demonstrate the control of enzyme orientation for enzymes chemically immobilized on surfaces. Nitro-reductase (NfsB) has the ability to reduce a broad range of nitro-containing compounds and has potential applications in a broad range of areas including the detection and decomposition of explosives. The enzyme was tethered through unique surface cysteine residues to a self-assembled monolayer (SAM) terminated with maleimide groups. One cysteine was introduced close to the active site (V424C), and the other, at a remote site (H360C). The surface-tethered NfsB variants were interrogated by a combination of surface-sensitive sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) to determine how the mode of attachment altered the enzyme's orientation. The activities of the two immobilized NfsB variants were measured and can be well correlated to the deduced orientations. The relationships among enzyme engineering, surface immobilization, enzyme orientation, and enzyme activity were revealed.

Interfacial structure of immobilized antibodies and perdeuterated HSA in model pregnancy tests measured with neutron reflectivity

Experimental studies of antibody adsorption and antigen binding that mimicked pregnancy test immunoassays have been performed using neutron reflectivity studies of a model antibody/antigen system immobilized on the silica/water interface. The study revealed the nature of the antibody/antigen interaction and also the importance of a blocking protein, in this case human serum albumin (HSA), that enhances the immunoassay's specificity and efficiency. Of central importance to this study has been the use of a perdeuterated human serum albumin (d-HSA), providing contrast that highlights the orientation and position of the blocking agent within the adsorbed layer. It was found that the adsorbed HSA filled the gaps between the preadsorbed antibodies on the substrate, with decreased adsorption occurring as a function of increased antibody surface coverage. In addition, the antigen binding capacity of the adsorbed antibodies was investigated as a function of antibody surface coverage. The amount of specifically bound antigen was found to saturate at approximately 0.17 mg/m(2) and became independent of the antibody surface coverage. The ratio of bound antigen to immobilized antibody decreased with increased antibody surface coverage. These results are of importance for a full understanding of immunoassay systems that are widely used in clinical tests and in the detection of environmental contaminants.

Adsorption behavior of a human monoclonal antibody at hydrophilic and hydrophobic surfaces

One aspiration for the formulation of human monoclonal antibodies (mAb) is to reach high solution concentrations without compromising stability. Protein surface activity leading to instability is well known, but our understanding of mAb adsorption to the solid-liquid interface in relevant pH and surfactant conditions is incomplete. To investigate these conditions, we used total internal reflection fluorescence (TIRF) and neutron reflectometry (NR). The mAb tested ("mAb-1") showed highest surface loading to silica at pH 7.4 (~12 mg/m(2)), with lower surface loading at pH 5.5 (~5.5 mg/m(2), further from its pI of 8.99) and to hydrophobized silica (~2 mg/m(2)). The extent of desorption of mAb-1 from silica or hydrophobized silica was related to the relative affinity of polysorbate 20 or 80 for the same surface. mAb-1 adsorbed to silica on co-injection with polysorbate (above its critical micelle concentration) and also to silica pre-coated with polysorbate. A bilayer model was developed from NR data for mAb-1 at concentrations of 50-5000 mg/L, pH 5.5, and 50-2000 mg/L, pH 7.4. The inner mAb-1 layer was adsorbed to the SiO₂ surface at near saturation with an end-on" orientation, while the outer mAb-1 layer was sparse and molecules had a "side-on" orientation. A non-uniform triple layer was observed at 5000 mg/L, pH 7.4, suggesting mAb-1 adsorbed to the SiO₂ surface as oligomers at this concentration and pH. mAb-1 adsorbed as a sparse monolayer to hydrophobized silica, with a layer thickness increasing with bulk concentration - suggesting a near end-on orientation without observable relaxation-unfolding.

Interfacial recognition of human prostate-specific antigen by immobilized monoclonal antibody: effects of solution conditions and surface chemistry

The specific recognition between monoclonal antibody (anti-human prostate-specific antigen, anti-hPSA) and its antigen (human prostate-specific antigen, hPSA) has promising applications in prostate cancer diagnostics and other biosensor applications. However, because of steric constraints associated with interfacial packing and molecular orientations, the binding efficiency is often very low. In this study, spectroscopic ellipsometry and neutron reflection have been used to investigate how solution pH, salt concentration and surface chemistry affect antibody adsorption and subsequent antigen binding. The adsorbed amount of antibody was found to vary with pH and the maximum adsorption occurred between pH 5 and 6, close to the isoelectric point of the antibody. By contrast, the highest antigen binding efficiency occurred close to the neutral pH. Increasing the ionic strength reduced antibody adsorbed amount at the silica-water interface but had little effect on antigen binding. Further studies of antibody adsorption on hydrophobic C8 (octyltrimethoxysilane) surface and chemical attachment of antibody on (3-mercaptopropyl)trimethoxysilane/4-maleimidobutyric acid N-hydroxysuccinimide ester-modified surface have also been undertaken. It was found that on all surfaces studied, the antibody predominantly adopted the 'flat on' orientation, and antigen-binding capabilities were comparable. The results indicate that antibody immobilization via appropriate physical adsorption can replace elaborate interfacial molecular engineering involving complex covalent attachments.

Antibody adsorption and orientation on hydrophobic surfaces

The orientation of a monoclonal, anti-streptavidin human IgG1 antibody on a model hydrophobic, CH(3)-terminated surface (1-dodecanethiol self-assembled monolayer on gold) was studied by monitoring the mechanical coupling between the adsorbed layer and the surface as well as the binding of molecular probes to the antibodies. In this study, the streptavidin antigen was used as a probe for the Fab portions of the antibody, while bacteria-derived Protein G' was used as a probe for the Fc region. Bovine serum albumin (BSA) acted as a blocking protein. Monolayer coverage occurred around 468 ng/cm(2). Below 100 ng/cm(2), antibodies were found to adsorb flat-on, tightly coupled to the surface and unable to capture their antigen, whereas the Fc region was able to bind Protein G'. At half-monolayer coverage, there was a transition in the mechanism of adsorption to allow for vertically oriented antibodies, as evidenced by the binding of both Protein G' and streptavidin as well as looser mechanical coupling with the surface. Monolayer coverage was characterized by a reduced level in probe binding per antibody and an even less rigid coupling to the surface.

Comparative study of random and oriented antibody immobilization as measured by dual polarization interferometry and surface plasmon resonance spectroscopy

Dual polarization interferometry (DPI) is used for a detailed study of antibody immobilization with and without orientation control, using prostate specific antigen (PSA) and its antibody as model. Thiol modified DPI chips were activated by a heterobifunctional cross-linker (sulfo-GMBS). PSA antibody was either directly immobilized via covalent binding or coupled via the Fc-fragment to protein G covalently attached to the activated chip. The direct covalent binding leads to a random antibody orientation and the coupling through protein G leads to an end-on orientation. Ethanolamine (ETH) was used to block remaining active sites following the direct antibody immobilization and protein G immobilization. A homobifunctional cross-linker (BS3) was used to stabilize the antibody layer coupled on protein G. DPI provides a real-time measurement of the stepwise molecular binding processes and gives detailed geometrical and structural values of each layer, i.e., thickness, mass, and density. These values evidence the end-on orientation of closely packed antibody on protein G layer and reveal structural effects of ETH blocking/deactivation and BS3 stabilization. With the end-on immobilized antibody, PSA at 10 pg/mL can be detected by DPI through a sandwich complex that satisfies the clinical requirement (assuming <30 pg/mL as clinically safe). However, the randomly immobilized antibody failed to detect PSA at 1 ng/mL. In a parallel study using surface plasmon resonance (SPR) spectroscopy, random and end-on antibody immobilization on streptavidin-modified gold surface was evaluated to further validate the importance of antibody orientation control. With the closely packed antibody layer on protein G surface, SPR can also detect PSA at 10 pg/mL.