Enhanced protein adsorption upon bulk phase separation

From adsorption to crystallization of proteins: Evidence for interface-assisted nucleation

Protein crystallization is among the key processes in biomolecular research, but the underlying mechanisms are still elusive. Here, we address the role of inevitable interfaces for the nucleation process. Quartz crystal microbalance with dissipation monitoring (QCM-D) with simultaneously optical microscopy, confocal microscopy, and grazing-incidence small angle X-rays scattering (GISAXS) were employed to investigate the temporal behavior from the initial stage of protein adsorption to crystallization. Here we studied the crystallization of the Human Serum Albumin (HSA), the most abundant blood protein, in the presence of a charged surface and a trivalent salt. We found evidence for interface-assisted nucleation of crystals. The kinetic stages involved are initial adsorption followed by enhanced adsorption after longer times, subsequent nucleation, and finally crystal growth. The results highlight the importance of interfaces for protein phase behavior and in particular for nucleation.

The ion-activated attractive patchy particle model and its application to the liquid-vapor phase transitions

Patchy particles are an intriguing subject of study and indeed a model system in the field of soft matter physics. In recent years, patchy particle models have been applied to describe a wide variety of systems, including colloidal crystals, macromolecular interactions, liquid crystals, and nanoparticle assemblies. Given the importance of the topic, rationalizing and capturing the basic features of these models is crucial to their correct application in specific systems. In this study, we extend the ion-activated attractive patchy particles model previously employed to elucidate the phase behavior of protein solutions in the presence of trivalent salts. Our extension incorporates the effect of repulsion between unoccupied and occupied binding sites, depicted as patches. Furthermore, we examine the influence of model parameters on the liquid-vapor coexistence region within the phase diagram, employing numerical methods. A deeper understanding of this model will facilitate a better comprehension of the effects observed in experiments.

Effects of flexibility in coarse-grained models for bovine serum albumin and immunoglobulin G

We construct a coarse-grained, structure-based, low-resolution, 6-bead flexible model of bovine serum albumin (BSA, PDB: 4F5S), which is a popular example of a globular protein in biophysical research. The model is obtained via direct Boltzmann inversion using all-atom simulations of a single molecule, and its particular form is selected from a large pool of 6-bead coarse-grained models using two suitable metrics that quantify the agreement in the distribution of collective coordinates between all-atom and coarse-grained Brownian dynamics simulations of solutions in the dilute limit. For immunoglobulin G (IgG), a similar structure-based 12-bead model has been introduced in the literature [Chaudhri et al., J. Phys. Chem. B 116, 8045 (2012)] and is employed here to compare findings for the compact BSA molecule and the more anisotropic IgG molecule. We define several modified coarse-grained models of BSA and IgG, which differ in their internal constraints and thus account for a variation of flexibility. We study denser solutions of the coarse-grained models with purely repulsive molecules (achievable by suitable salt conditions) and address the effect of packing and flexibility on dynamic and static behavior. Translational and rotational self-diffusivity is enhanced for more elastic models. Finally, we discuss a number of effective sphere sizes for the BSA molecule, which can be defined from its static and dynamic properties. Here, it is found that the effective sphere diameters lie between 4.9 and 6.1 nm, corresponding to a relative spread of about ±10% around a mean of 5.5 nm.

Evidence for a semisolid phase state of aerosols and droplets relevant to the airborne and surface survival of pathogens

Ambient humidity can influence the survival of pathogens in respiratory aerosols and droplets, although the mechanism and optimum humidity level for public health remain unclear. Here, we present evidence for a humidity-dependent, semisolid state of aerosols and droplets relevant to pathogen survival. These observations indicate that a semisolid state may protect pathogens from inactivation by hindering disinfection reactions at intermediate-to-low humidity levels. The formation of the semisolid state was dependent on the composition of the aerosols, which suggests that the humidity for optimum pathogen destruction will depend on the composition of respiratory particles released from an infected host. These observations can be used to help interpret laboratory studies and inform public health recommendations.

The phase state of respiratory aerosols and droplets has been linked to the humidity-dependent survival of pathogens such as SARS-CoV-2. To inform strategies to mitigate the spread of infectious disease, it is thus necessary to understand the humidity-dependent phase changes associated with the particles in which pathogens are suspended. Here, we study phase changes of levitated aerosols and droplets composed of model respiratory compounds (salt and protein) and growth media (organic–inorganic mixtures commonly used in studies of pathogen survival) with decreasing relative humidity (RH). Efflorescence was suppressed in many particle compositions and thus unlikely to fully account for the humidity-dependent survival of viruses. Rather, we identify organic-based, semisolid phase states that form under equilibrium conditions at intermediate RH (45 to 80%). A higher-protein content causes particles to exist in a semisolid state under a wider range of RH conditions. Diffusion and, thus, disinfection kinetics are expected to be inhibited in these semisolid states. These observations suggest that organic-based, semisolid states are an important consideration to account for the recovery of virus viability at low RH observed in previous studies. We propose a mechanism in which the semisolid phase shields pathogens from inactivation by hindering the diffusion of solutes. This suggests that the exogenous lifetime of pathogens will depend, in part, on the organic composition of the carrier respiratory particle and thus its origin in the respiratory tract. Furthermore, this work highlights the importance of accounting for spatial heterogeneities and time-dependent changes in the properties of aerosols and droplets undergoing evaporation in studies of pathogen viability.

Switchable β-lactoglobulin (BLG) adsorption on protein resistant oligo (ethylene glycol) (OEG) self-assembled monolayers (SAMs)

HYPOTHESIS

Although protein adsorption at an interface is very common and important in biology and biotechnology, it is still not fully understood - mainly due to the intricate balance of forces that ultimately control it. In food processing (and medicine), controlling and manipulating protein adsorption, as well as avoiding protein adsorption (biofilm formation or membrane fouling) by the production of protein-resistant surfaces is of substantial interest. A major factor conferring resistance towards protein adsorption to a surface is the presence of tightly bound water molecules, as is the case in oligo ethylene glycol (OEG)-terminated self-assembled monolayers (SAMs). Due to strong attractive protein-protein and protein-surface interactions observed in systems containing trivalent salt ions, we hypothesize that these conditions may lead to a breakdown of protein resistance in OEG SAMs.

EXPERIMENTS

We studied the adsorption behavior of BLG in the presence of a lanthanum(III) chloride (LaCl₃) at concentrations of 0, 0.1, 0.8 and 5.0 mM on normally protein resistant triethylene glycol-termianted (EG3) SAMs on a gold surface. We used quartz-crystal microbalance with dissipation (QCM-D) and neutron reflectivity (NR) to characterize the morphology of the interfacial region of the SAM.

FINDINGS

We demonstrate that the protein resistance of the EG3 SAM breaks down beyond a threshold salt concentration c^∗ and mirrors the bulk behaviour of this system, showing reduced adsorption beyond a second critical salt concentration c^∗∗. These results demonstrate for the first time the controlled switching of the protein-resistant properties of this type of SAM by the addition of trivalent salt.

Metabolic channeling: predictions, deductions, and evidence

With the elucidation of myriad anabolic and catabolic enzyme-catalyzed cellular pathways crisscrossing each other, an obvious question arose: how could these networks operate with maximal catalytic efficiency and minimal interference? A logical answer was the postulate of metabolic channeling, which in its simplest embodiment assumes that the product generated by one enzyme passes directly to a second without diffusion into the surrounding medium. This tight coupling of activities might increase a pathway's metabolic flux and/or serve to sequester unstable/toxic/reactive intermediates as well as prevent their access to other networks. Here, we present evidence for this concept, commencing with enzymes that feature a physical molecular tunnel, to multi-enzyme complexes that retain pathway substrates through electrostatics or enclosures, and finally to metabolons that feature collections of enzymes assembled into clusters with variable stoichiometric composition. Lastly, we discuss the advantages of reversibly assembled metabolons in the context of the purinosome, the purine biosynthesis metabolon.

Bulk phase behaviour vs interface adsorption: Effects of anions and isotopes on β-lactoglobulin (BLG) interactions

HYPOTHESIS

Protein adsorption is highly relevant in numerous applications ranging from food processing to medical implants. In this context, it is important to gain a deeper understanding of protein-protein and protein-surface interactions. Thus, the focus of this investigation is on the interplay of bulk properties and surface properties on protein adsorption. It was hypothesised that the type of solvent and ions in solution should significantly influence the protein's bulk and interface behaviour, which has been observed in literature and previous work for other net negatively charged, globular proteins such as bovine serum albumin (BSA).

EXPERIMENTS

The phase behaviour of β-lactoglobulin (BLG) with lanthanum chloride (LaCl₃) and iodide (LaI₃) in normal water H₂O(l) and heavy water (D₂O(l)) was established via optical microscopy and ultraviolet-visible spectroscopy. The formation of an adsorption layer and its properties such as thickness, density, structure, and hydration was investigated via neutron reflectivity, quartz-crystal microbalance with dissipation, and infra-red measurements.

FINDINGS

β-lactoglobulin does not show significant anion-induced or isotope-induced effects - neither in bulk nor at the solid-liquid interface, which deviates strongly from the behaviour of bovine serum albumin. We also provide a comprehensive discussion and comparison of protein-specific bulk and interface behaviour between bovine serum albumin and β-lactoglobulin dependent on anion, cation, solvent, and substrate properties. These findings pave the way for understanding the transition from adsorption to crystallisation.

Bulk Phase Behavior vs Interface Adsorption: Specific Multivalent Cation and Anion Effects on BSA Interactions

Proteins are ubiquitous and play a critical role in many areas from living organisms to protein microchips. In humans, serum albumin has a prominent role in the foreign body response since it is the first protein which will interact with, e.g., an implant or stent. In this study, we focused on the influence of salts (i.e., different cations (Y³⁺, La³⁺) and anions (Cl^-, I^-) on bovine serum albumin (BSA) in terms of its bulk behavior as well as the role of charges for protein adsorption at the solid-liquid interface in order to understand and control the underlying molecular mechanisms and interactions. This is part of our group's effort to gain a deeper understanding of protein-protein and protein-surface interactions in the presence of multivalent ions. In the bulk, we established two new phase diagrams and found not only multivalent cation-triggered phase transitions, but also a dependence of the protein behavior on the type of anion. The attractive interactions between proteins were observed to increase from Cl^- < NO₃^- < I^-, resulting in iodide preventing re-entrant condensation and promoting liquid-liquid phase separation in bulk. Using ellipsometry and a quartz-crystal microbalance with dissipation (QCM-D), we obtained insight into the growth of the protein adsorption layer. Importantly, we found that phase transitions at the substrate can be triggered by certain interface properties, whether they exist in the bulk solution or not. Through the use of a hydrophilic, negatively charged surface (native silica), the direct binding of anions to the interface was prevented. Interestingly, this led to re-entrant adsorption even in the absence of re-entrant condensation in bulk. However, the overall amount of adsorbed protein was enhanced through stronger attractive protein-protein interactions in the presence of iodide salts. These findings illustrate how carefully chosen surface properties and salts can directly steer the binding of anions and cations, which guide protein behavior, thus paving the way for specific/triggered protein-protein, protein-salt, and protein-surface interactions.