Evaluation of the Effectiveness of Surfactants and Denaturants to Elute and Denature Adsorbed Protein on Different Surface Chemistries

pH-induced changes in IgE molecules measured by atomic force microscopy

The environment surrounding proteins is tightly linked to its dynamics, which can significantly influence the conformation of proteins. This study focused on the effect of pH conditions on the ultrastructure of Immunoglobulin E (IgE) molecules. Herein, the morphology, height, and area of IgE molecules incubated at different pH were imaged by atomic force microscopy (AFM), and the law of IgE changes induced by pH value was explored. The experiment results indicated that the morphology, height and area of IgE molecules are pH dependent and highly sensitive. In particular, IgE molecules were more likely to present small-sized ellipsoids under acidic conditions, while IgE molecules tend to aggregate into large-sized flower-like structures under alkaline conditions. In addition, it was found that the height of IgE first decreased and then increased with the increase of pH, while the area of IgE increased with the increase of pH. This work provides valuable information for further study of IgE, and the methodological approach used in this study is expected to developed into AFM to investigate the changes of IgE molecules mediated by other physical and chemical factors. RESEARCH HIGHLIGHTS: The ultrastructure of IgE molecules is pH dependent and highly sensitive. IgE molecules were tend to present small-sized ellipsoids under acidic pH. Alkaline pH drives IgE self-assembly into flower-like aggregates.

Protein Adsorption Loss─The Bottleneck of Single-Cell Proteomics

Single-cell proteomics is a promising field to provide direct yet comprehensive molecular insights into cellular functions without averaging effects. Here, we address a grand technical challenge impeding the maturation of single-cell proteomics─protein adsorption loss (PAL). Even though widely known, there is currently no quantitation on how profoundly and selectively PAL has affected single-cell proteomics. Therefore, the mitigations to this challenge have been generic, and their efficacy was only evaluated by the size of the resolved proteome with no specificity on individual proteins. We use the existing knowledge of PAL, protein expression, and the typical surface area used in single-cell proteomics to discuss the severity of protein loss. We also summarize the current solutions to this challenge and briefly review the available methods to characterize the physical and chemical properties of protein surface adsorption. By citing successful strategies in single-cell genomics for measurement errors in individual transcripts, we pinpoint the urgency to benchmark PAL at the proteome scale with individual protein resolution. Finally, orthogonal single-cell proteomic techniques that have the potential to cross validate PAL are proposed. We hope these efforts can promote the fruition of single-cell proteomics in the near future.

Surfactant Protection Efficacy at Surfaces Varies with the Nature of Hydrophobic Materials

OBJECTIVE

Monoclonal antibodies are in contact with many different materials throughout their life cycle from production to patient administration. Plastic surfaces are commonly found in single use bags, syringes, perfusion bags and tubing and their hydrophobic nature makes them particularly prone for adsorption of therapeutic proteins. The addition of surfactants in therapeutic formulations aims at minimizing surface and interface adsorption of the active molecules. However, their protection efficacy related to the nature of the plastic material is still poorly investigated.

METHODS

We use real-time surface-sensitive techniques and immunosorbent assays, to quantify surfactant and monoclonal antibody adsorption on hydrophobic model surfaces and different plastic polymers to analyse the effect of material surface properties on the level of surfactant protection.

RESULTS

We show that Polysorbate 80 protects monoclonal antibodies significantly better from adsorption on a polystyrene surface than on a hexadecane self-assembled monolayer, used as a model surface with similar hydrophobicity. This enhanced protective effect on polystyrene is observed for different antibodies and also other surfactants, and its extent depends on the surfactant concentration for a given antibody concentration. A comparative adsorption study allows ranking different in-use plastics and highlights the dependence of Polysorbate 80 protection efficacy on the nature of the plastic material.

CONCLUSION

This study demonstrates that, beyond hydrophobicity, the nature of plastic polymer surfaces affects surfactant adsorption and thereby impacts their protection efficacy in therapeutic antibody formulations.

Adsorbed protein film on pump surfaces leads to particle formation during fill-finish manufacturing

During fill-finish manufacturing, therapeutic proteins may aggregate or form subvisible particles in response to the physical stresses encountered within filling pumps. Understanding and quantitating this risk is important since filling may be the last unit operation before the patient receives their dose. We studied particle formation from lab-scale to manufacturing-scale using sensitive and robust protein formulations. Filling experiments with a ceramic rotary piston pump were integrated with a rinse-stripping method to investigate the relationship between protein adsorption and particle formation. For a sensitive protein, multilayer film formation on the piston surface correlated with high levels of subvisible particles in solution. For a robust protein formulation, adsorption and subvisible particle formation were minimal. These results support an aggregation mechanism that is initiated by adsorption to pump surfaces and propagated by mechanical and/or hydrodynamic disruption of the film. The elemental analysis confirmed that ceramic wear debris remained at trace levels and did not contribute appreciably to protein aggregation.

On the relationship between structure and catalytic effectiveness in solid surface-immobilized enzymes: Advances in methodology and the quest for a single-molecule perspective

The integration of enzymes with solid materials is important in many biotechnological applications, including the use of immobilized enzymes for biocatalytic synthesis. The development of functional enzyme-material composites is restrained by the lack of molecular-level insight into the behavior of enzymes in confined, surface-near environments. Here, we review recent advances in surface-sensitive spectroscopic techniques that push boundaries for the determination of enzyme structure and orientation at the solid-liquid interface. We discuss recent evidence from single-molecule studies showing that analyses sensitive to the temporal and spatial heterogeneities in immobilized enzymes can succeed in disentangling the effects of conformational stability and active-site accessibility on activity. Different immobilization methods involve distinct trade-off between these effects, thus emphasizing the need for a holistic (systems) view of immobilized enzymes for the rational development of practical biocatalysts.

Characterization of Sample Loss Caused by Competitive Adsorption of Proteins in Vials Using Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis

Sample loss caused by competitive protein adsorption on solid surfaces from complex samples remains to be a major hurdle in sensitive analyses of proteins. No label-free techniques can easily quantify individual proteins adsorbed on irregular surfaces of Eppendorf vials or Falcon tubes, which are commonly used to contain complex biological samples. Multiplexed characterization of such adsorption by different proteins is technically challenging. Herein, we developed a direct protein analysis based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis for the characterization of sample loss occurred on the curved surface with limited area. Using this simple and easily accessible method, we discovered the effect of ethylenediaminetetraacetic acid on surface adsorption of different milk proteins, specifically an augmented loss of milk proteins in low-binding sample vials. In this study, we also identified severe biases of silver staining and established proteomics-based mapping of protein distribution in biological samples for absolute quantification of competitive protein adsorption on irregular surfaces.

Correlating Structural and Functional Heterogeneity of Immobilized Enzymes

Many nanobiotechnology applications rely on stable and efficient integration of functional biomacromolecules with synthetic nanomaterials. Unfortunately, the reasons for the ubiquitous loss of activity of immobilized enzymes remain poorly understood due to the difficulty in distinguishing between distinct molecular-level mechanisms. Here, we employ complementary single-molecule fluorescence methods that independently measure the impact of immobilization on the structure and function ( i. e., substrate binding kinetics) of nitroreductase (NfsB). Stochastic statistical modeling methods were used to unambiguously quantify the effects of immobilized NfsB structural dynamics on function, allowing us to explicitly separate effects due to conformation and accessibility. Interestingly, we found that nonspecifically tethered NfsB exhibited enhanced stability compared to site-specifically tethered NfsB; however, the folded state of site-specifically tethered NfsB had faster substrate binding rates, suggesting improved active site accessibility. This demonstrated an unexpected intrinsic trade-off associated with competing bioconjugation methods, suggesting that it may be necessary to balance conformational stability versus active site accessibility. This nuanced view of the impact of immobilization will facilitate a rational approach to the integration of enzymes with synthetic nanomaterials.

Quantitation of nonspecific protein adsorption at solid–liquid interfaces for single-cell proteomics

Protein nonspecific adsorption that occurred at the solid–liquid interface has been subjected to intense physical and chemical characterizations due to its crucial role in a wide range of applications, including food and pharmaceutical industries, medical implants, biosensing, and so on. Protein-adsorption caused sample loss has largely hindered the studies of single-cell proteomics; the prevention of such loss requires the understanding of protein–surface adsorption at the proteome level, in which the competitive adsorption of thousands and millions of proteins with vast dynamic range occurs. To this end, we feel the necessity to review current methodologies on their potentials to characterize — more specifically to quantify — the proteome-wide adsorption. We hope this effort can help advancing single-cell proteomics and trace proteomics.

Denaturation of proteins by surfactants studied by the Taylor dispersion analysis

We showed that the Taylor Dispersion Analysis (TDA) is a fast and easy to use method for the study of denaturation proteins. We applied TDA to study denaturation of β-lactoglobulin, transferrin, and human insulin by anionic surfactant sodium dodecyl sulfate (SDS). A series of measurements at constant protein concentration (for transferrin was 1.9 x 10⁻⁵ M, for β- lactoglobulin was 7.6 x 10⁻⁵ M, and for insulin was 1.2 x 10⁻⁴ M) and varying SDS concentrations were carried out in the phosphate-buffered saline (PBS). The structural changes were analyzed based on the diffusion coefficients of the complexes formed at various surfactant concentrations. The concentration of surfactant was varied in the range from 1.2 x 10⁻⁴ M to 8.7 x 10⁻² M. We determined the minimum concentration of the surfactant necessary to change the native conformation of the proteins. The minimal concentration of SDS for β-lactoglobulin and transferrin was 4.3 x 10⁻⁴ M and for insulin 2.3 x 10⁻⁴ M. To evaluate the TDA as a novel method for studying denaturation of proteins we also applied other methods i.e. electronic circular dichroism (ECD) and dynamic light scattering (DLS) to study the same phenomenon. The results obtained using these methods were in agreement with the results from TDA.

pH-Dependent Ordered Fibrinogen Adsorption on Polyethylene Single Crystals

Nanostructured surfaces have the potential to influence the assembly as well as the orientation of adsorbed proteins and may, thus, strongly influence the biomaterials' performance. For the class of polymeric (bio)materials a reproducible and well-characterized nanostructure is the ordered chain folded surface of a polyethylene single crystal (PE-SC). We tested the hypothesis that the trinodal-rod-shaped protein human plasma fibrinogen (HPF) adsorbs on the (001) surface of PE-SCs along specific crystallographic directions. The PE-SC samples were prepared by isothermal crystallization in dilute solution and characterized by atomic force microscopy (AFM) before as well as after HPF adsorption at different concentrations and pH values. At a physiological pH of 7.4, connected HPF molecules, or e.g., fibrils, fibril networks, or sponge-like structures, were observed on PE-SC surfaces that featured no preferential orientation. The observation of these nonoriented multiprotein assemblies was explained by predominant protein-protein interactions and limited surface diffusion. However, at an increased pH of 9.2, single HPF molecules, e.g., spherical-shaped and trinodal-rod-shaped HPF molecules as well as agglomerates, were observed on the PE-SC surface. The presence of single HPF molecules at increased pH was explained by decreased protein-protein interactions. These single trinodal-rod-shaped HPF molecules oriented preferentially along crystallographic [100] and [010] directions on the PE-SC surface which was explained by an increased amount of intermolecular bonds along these crystallographic directions with increased surface atom density. The study established that HPF molecules can align on chemically homogeneous surface topographies one order of magnitude smaller than the dimension of the protein. This advances the understanding of how to control the assembly and orientation of proteins on nanostructured polymer surfaces. Controlled protein adsorption is a crucial key to improve the surface functionality of future implants and biosensors.