A molecular-thermodynamic model for the interactions between globular proteins in aqueous solutions: applications to bovine serum albumin (BSA), lysozyme, alpha-chymotrypsin, and immuno-gamma-globulins (IgG) solutions

Molecular Serum Albumin Unmask Nanobio Properties of Molecular Graphenes in Shungite Carbon Nanoparticles

Serum albumin is a popular macromolecule for studying the effect of proteins on the colloidal stability of nanoparticle (NP) dispersions, as well as the protein-nanoparticle interaction and protein corona formation. In this work, we analyze the specific conformation-dependent phase, redox, and fatty acid delivery properties of bovine albumin in the presence of shungite carbon (ShC) molecular graphenes stabilized in aqueous dispersions in the form of NPs in order to reveal the features of NP bioactivity. The formation of NP complexes with proteins (protein corona around NP) affects the transport properties of albumin for the delivery of fatty acids. Being acceptors of electrons and ligands, ShC NPs are capable of exhibiting both their own biological activity and significantly affecting conformational and phase transformations in protein systems.

Colloid osmotic parameterization and measurement of subcellular crowding

Crowding of the subcellular environment by macromolecules is thought to promote protein aggregation and phase separation. A challenge is how to parameterize the degree of crowding of the cell interior or artificial solutions that is relevant to these reactions. Here I review colloid osmotic pressure as a crowding metric. This pressure is generated by solutions of macromolecules in contact with pores that are permeable to water and ions but not macromolecules. It generates depletion forces that push macromolecules together in crowded solutions and thus promotes aggregation and phase separation. I discuss measurements of colloid osmotic pressure inside cells using the nucleus, the cytoplasmic gel, and fluorescence resonant energy transfer (FRET) biosensors as osmometers, which return a range of values from 1 to 20 kPa. I argue for a low value, 1-2 kPa, in frog eggs and perhaps more generally. This value is close to the linear range on concentration-pressure curves and is thus not crowded from an osmotic perspective. I discuss the implications of a low crowding pressure inside cells for phase separation biology, buffer design, and proteome evolution. I also discuss a pressure-tension model for nuclear shape, where colloid osmotic pressure generated by nuclear protein import inflates the nucleus.

Self-Interaction of Human Serum Albumin: A Formulation Perspective

In the present study, small-angle X-ray scattering (SAXS) and static light scattering (SLS) have been used to study the solution properties and self-interaction of recombinant human serum albumin (rHSA) molecules in three pharmaceutically relevant buffer systems. Measurements are carried out up to high protein concentrations and as a function of ionic strength by adding sodium chloride to probe the role of electrostatic interactions. The effective structure factors (S _eff) as a function of the scattering vector magnitude q have been extracted from the scattering profiles and fit to the solution of the Ornstein-Zernike equation using a screened Yukawa potential to describe the double-layer force. Although only a limited q range is used, accurate fits required including an electrostatic repulsion element in the model at low ionic strength, while only a hard sphere model with a tunable diameter is necessary for fitting to high-ionic-strength data. The fit values of net charge agree with available data from potentiometric titrations. Osmotic compressibility data obtained by extrapolating the SAXS profiles or directly from SLS measurements has been fit to a 10-term virial expansion for hard spheres and an equation of state for hard biaxial ellipsoids. We show that modeling rHSA as an ellipsoid, rather than a sphere, provides a much more accurate fit for the thermodynamic data over the entire concentration range. Osmotic virial coefficient data, derived at low protein concentration, can be used to parameterize the model for predicting the behavior up to concentrations as high as 450 g/L. The findings are especially important for the biopharmaceutical sector, which require approaches for predicting concentrated protein solution behavior using minimal sample consumption.

Binding analysis of carbon nanoparticles to human immunoglobulin G: Elucidation of the cytotoxicity of CNPs and perturbation of immunoglobulin conformations

The chemical compositions, sizes and fluorescent properties of synthesized carbon nanoparticles (CNPs) were characterized. Escherichia coli (E. coli) cells were used as a model to study the cytotoxicity of CNPs, and the results of the cellular uptake of CNPs yielded excellent results: the CNPs demonstrated good biocompatibility and were non-toxic to the growth of the E. coli cells. Moreover, to assess the potential toxicity of CNPs to human health, the binding behavior of CNPs with human immunoglobulin G (HIgG) was examined by fluorescence quenching spectroscopy, synchronous fluorescence spectroscopy and circular dichroism spectroscopy under physiological conditions. The fluorescence quenching constants and parameters for the interaction at different temperatures had been calculated according to Scatchard. The thermodynamic parameters, such as enthalpy change (ΔH), entropy change (ΔS) and free energy change (ΔG), were calculated, and the results indicated strong static quenching and showed that van der Waals forces, hydrogen bonds and hydrophobic interactions were the predominant intermolecular forces stabilizing the CNP-HIgG complex. Synchronous fluorescence and circular dichroism spectra provided information regarding the conformational alteration of HIgG in the presence of CNPs. These findings help to characterize the interactions between CNPs and HIgG, which may clarify the potential risks and undesirable health effects of CNPs, as well as the related cellular trafficking and systemic translocation.

Molecular modeling and multi-spectroscopic approaches to study the interaction between antibacterial drug and human immunoglobulin G

Mechanistic and conformational studies on the interaction of sulfamethoxazole (SMX) with human immunoglobulin G (HIgG) were performed by molecular modeling and multi-spectroscopic methods. The interaction mechanism was firstly predicted through molecular modeling that confirmed the interaction between SMX and HIgG. The binding parameters and thermodynamic parameters at different temperatures had been calculated according to the Stern-Volmer, Scatchard, Sips and Van 't Hoff equations, respectively. Experimental results showed that the fluorescence intensity of HIgG was quenched by the gradual addition of SMX. The binding constants of SMX with HIgG decreased with the increase of temperature, which meant that the quenching mechanism was a static quenching. Meanwhile, the results also confirmed that there was one independent class of binding site on HIgG for SMX during their interaction. The thermodynamic parameters of the reaction, namely standard enthalpy ΔH(0) and entropy ΔS(0), had been calculated to be -14.69 kJ·mol(-1) and 22.99 J·mol(-1) ·K(-1), respectively, which suggested that the electrostatic and hydrophobic interactions were the predominant intermolecular forces in stabilizing the SMX-HIgG complex. Furthermore, experimental results obtained from three-dimensional fluorescence spectroscopy, UV-vis absorption spectroscopy and circular dichroism (CD) spectroscopy confirmed that the conformational structure of HIgG was altered in the presence of SMX.

Identification and Partial Characterization of Two Populations of Prostasomes by a Combination of Dynamic Light Scattering and Proteomic Analysis

Prostasomes are vesicles secreted by prostate epithelial cells and are found in abundance in the semen. Here we characterized two different prostasome populations isolated from human seminal fluid. Prostasomes were isolated using differential centrifugation, while dynamic light scattering (DLS) was used to characterize their size and size distribution. Their protein content was analyzed using two-dimensional electrophoresis and mass spectrometry. DLS showed two distinct prostasome subpopulations in centrifuged seminal plasma, with an average hydrodynamic radius of 80 and 300 nm. The larger population was isolated after centrifugation at 20,000 × g (P20), while the smaller one was recovered at 100,000 × g (P100). The two fractions had a similar lipid composition, showing an elevated content of sphingomyelin and cholesterol. The P100 vesicles showed a significant over-expression of proteins involved in the endosomal sorting complexes required for transport (ESCRT) machinery such as Alix, TSG101, and syntenin-1. Some proteins possibly involved in prostate cancer were present only in one specific population (TMPRSS2 in P100 and VCP in P20). The different size and protein profile in the two subpopulations of prostasomes might support differential roles of the semen vesicles toward the target cells, and/or different secretion pathways from the organ of origin.

Two-Yukawa fluid at a hard wall: Field theory treatment

We apply a field-theoretical approach to study the structure and thermodynamics of a two-Yukawa fluid confined by a hard wall. We derive mean field equations allowing for numerical evaluation of the density profile which is compared to analytical estimations. Beyond the mean field approximation, analytical expressions for the free energy, the pressure, and the correlation function are derived. Subsequently, contributions to the density profile and the adsorption coefficient due to Gaussian fluctuations are found. Both the mean field and the fluctuation terms of the density profile are shown to satisfy the contact theorem. We further use the contact theorem to improve the Gaussian approximation for the density profile based on a better approximation for the bulk pressure. The results obtained are compared to computer simulation data.

Effect of aqueous solution and load on the formation of DLC transfer layer against Co-Cr-Mo for joint prosthesis

Diamond-like carbon (DLC) coating exhibits excellent mechanical properties such as high hardness, low friction and wear, which offer a promising solution for the metal-on-metal hip joint implants. In the study, the hydrogen-free DLC coating with the element Cr as the interlay addition was deposited on the surface of the Co-Cr-Mo alloy by a unbalanced magnetron sputtering method. The coating thickness was controlled as 2 µm. Nano-indentation test indicated the hardness was about 13 GPa. DLC coated Co-Cr-Mo alloy disc against un-coated Co-Cr-Mo alloy pin (spherical end SR9.5) comprised the friction pairs in the pin-on-disc tribotest under bovine serum albumin solution (BSA) and physiological saline(PS).The tribological behavior under different BSA concetrations(2-20 mg/ml), and applied load (2-15N) was investigated.DLC transfer layer did not form under BSA solution, even though different BSA concetration and applied load changed. The coefficient of friction(COF) under 6 mg/ml BSA at 10 N was the lowest as 0.10. A higher COF of 0.13 was obtained under 20 mg/ml BSA. The boundary absorption layer of protein is the main factor for the counterparts. However, the continous DLC transfer layer was observed under PS solution, which make a lower COF of 0.08.

Quantitative assessment of the accuracy of the Poisson-Boltzmann cell model for salty suspensions

The cell model is a ubiquitous, fast, and relatively easily implemented model used to estimate the osmotic pressure of a colloidal dispersion. It has been shown to yield accurate approximations of the pressure in dispersions with a low salt content. It is generally accepted that it performs well when long-ranged interactions are involved and the structure of the dispersion is solidlike. The aim of the present work is to determine quantitatively the error committed by assuming the pressure computed with the cell model is the real osmotic pressure of a dispersion. To this end, cell model pressures are compared to a correct estimation of the actual pressures obtained from Poisson-Boltzmann Brownian dynamics simulations including many-body electrostatics and the thermal motion of the colloids. The comparison is performed for various colloidal sizes and charges, salt contents, and volume fractions. It is demonstrated that the accuracy of the cell model predictions is a function of only the average intercolloid distance scaled by Debye's length κd̅ and the normalized colloidal charge. The cell model is accurate for κd̅ < 1 and not reliable for κd̅ > 5 independently of the colloidal charge. In the 1 < κd̅ < 5 range, covering a wide set of experimental conditions, the colloidal surface charge has a large influence on the error associated with the cell approximation. The results presented in this article should provide a useful reference to determine a priori if the cell model can be expected to predict accurately an equation of state for a given set of physicochemical parameters.

Protein-protein interactions in dilute to concentrated solutions: α-chymotrypsinogen in acidic conditions

Protein-protein interactions were investigated for α-chymotrypsinogen by static and dynamic light scattering (SLS and DLS, respectively), as well as small-angle neutron scattering (SANS), as a function of protein and salt concentration at acidic conditions. Net protein-protein interactions were probed via the Kirkwood-Buff integral G22 and the static structure factor S(q) from SLS and SANS data. G22 was obtained by regressing the Rayleigh ratio versus protein concentration with a local Taylor series approach, which does not require one to assume the underlying form or nature of intermolecular interactions. In addition, G22 and S(q) were further analyzed by traditional methods involving fits to effective interaction potentials. Although the fitted model parameters were not always physically realistic, the numerical values for G22 and S(q → 0) were in good agreement from SLS and SANS as a function of protein concentration. In the dilute regime, fitted G22 values agreed with those obtained via the osmotic second virial coefficient B22 and showed that electrostatic interactions are the dominant contribution for colloidal interactions in α-chymotrypsinogen solutions. However, as protein concentration increases, the strength of protein-protein interactions decreases, with a more pronounced decrease at low salt concentrations. The results are consistent with an effective "crowding" or excluded volume contribution to G22 due to the long-ranged electrostatic repulsions that are prominent even at the moderate range of protein concentrations used here (<40 g/L). These apparent crowding effects were confirmed and quantified by assessing the hydrodynamic factor H(q → 0), which is obtained by combining measurements of the collective diffusion coefficient from DLS data with measurements of S(q → 0). H(q → 0) was significantly less than that for a corresponding hard-sphere system and showed that hydrodynamic nonidealities can lead to qualitatively incorrect conclusions regarding B22, G22, and static protein-protein interactions if one uses only DLS to assess protein interactions.

Thermodynamic and structural characterization of an antibody gel

Although extensively studied, protein-protein interactions remain highly elusive and are of increasing interest in drug development. We show the assembly of a monoclonal antibody, using multivalent carboxylate ions, into highly-ordered structures. While the presence and function of similar structures in vivo are not known, the results may present a possible unexplored area of antibody structure-function relationships. Using a variety of tools (e.g., mechanical rheology, electron microscopy, isothermal calorimetry, Fourier transform infrared spectroscopy), we characterized the physical, biochemical, and thermodynamic properties of these structures and found that citrate may interact directly with the amino acid residue histidine, after which the individual protein units assemble into a filamentous network gel exhibiting high elasticity and interfilament interactions. Citrate interacts exothermically with the monoclonal antibody with an association constant that is highly dependent on solution pH and temperature. Secondary structure analysis also reveals involvement of hydrophobic and aromatic residues.

Predicting solution aggregation rates for therapeutic proteins: approaches and challenges

Non-native aggregation is a common concern during therapeutic protein product development and manufacturing, particularly for liquid dosage forms. Because aggregates are often net irreversible under the conditions that they form, controlling aggregate levels requires control of aggregation rates across a range of solution conditions. Rational design of product formulation(s) would therefore benefit greatly from methods to accurately predict aggregation rates. This article focuses on the principles underlying current rate-prediction approaches for non-native aggregation, the limitations and strengths of different approaches, and illustrative examples from the authors' laboratories. The analysis highlights a number of reasons why accurate prediction of aggregation rates remains an outstanding challenge, and suggests some of the important areas for research to ultimately enable improved predictive capabilities in the future.

Comparative effects of pH and ionic strength on protein-protein interactions, unfolding, and aggregation for IgG1 antibodies

Changes in protein-protein interactions, protein unfolding, and nonnative aggregation were assessed for a series of human IgG1 antibodies as a function of pH and solution ionic strength (I). Unfolding transitions were characterized with differential scanning calorimetry. Protein-protein interactions were characterized with the apparent second virial coefficient (A(2)) from light scattering. Aggregation pathways were assessed using size-exclusion chromatography and multi-angle laser light scattering, aggregation kinetics, and structural changes monitored by circular dichroism spectroscopy and thioflavine T (ThT) binding. Ionic strength had relatively minor qualitative effects on unfolding, while pH had large effects for all four antibodies. A(2) was sensitive to both pH and I, and indicated that electrostatic interactions and nonuniform surface-charge distributions were important near neutral pH. Depending on solution pH and I, distinct aggregation pathways were found for each antibody, and these shared similar patterns versus pH, I, and A(2). Main differences observed across different antibodies included thermal unfolding transitions in DSC and the effects of pH and I on aggregation kinetics and pathways. These correlated strongly with whether aggregates of a given antibody bound ThT, suggesting possible differences with respect to conformational changes and/or regions of the proteins that are structurally involved in stabilizing the aggregates.

Intermolecular interactions of IgG1 monoclonal antibodies at high concentrations characterized by light scattering

Light scattering intensity measurements of solutions of two purified monoclonal antibodies were performed over a wide range of concentrations (0.5-275 mg/mL) and ionic strengths (0.02 to 0.6 M). Despite extensive sequence homology between these mAbs, alteration of ∼20 amino acids in the complementarity determining regions resulted in different net intermolecular interactions and responses to solution ionic strength. The concentration dependence of scattering was analyzed by comparison with the predictions of three models, allowing for intermolecular interaction of various types. In order of increasing complexity, the three models account for: (1) steric repulsions (simple hard-sphere model), (2) steric repulsion with short-ranged attractive interactions of varying magnitude (adhesive hard-sphere model), and (3) steric and nonsteric repulsive interactions between several species whose relative concentrations may change as a function of total protein concentration as dictated by equilibrium self-association (effective hard-sphere mixture model). Simple scattering models of noninteracting and adhesive hard-sphere species permitted qualitative interpretation of contributions from excluded volume, electrostatic, and van der Waals interactions on net mAb interactions at high concentration as a function of ionic strength. mAb2 electrostatic interactions were repulsive, whereas mAb1 interactions were net attractive at low ionic strengths, attributed to an anisotropic distribution of molecular charge. The effective hard-sphere mixture model can account quantitatively for the dependence of scattering for both antibodies over the entire concentration range and at salt concentrations exceeding 40 mM. This analysis showed that at high ionic strength both mAbs self-associate weakly to form dimer with an affinity that varies little with salt concentration at concentrations exceeding 75 mM. In addition, mAb1 appears to self-associate further to form oligomers with stoichiometry of 4-6 and an affinity that declines substantially with increasing ionic strength. All three models lead to the conclusion that at high concentrations repulsive interactions are predominantly due to excluded volume, whereas additional features are salt-dependent and reflect a substantial electrostatic contribution to intermolecular interactions of both mAbs.