Preferential hydration fully controls the renaturation dynamics of collagen in water-glycerol solvents

Protein stability in a natural deep eutectic solvent: Preferential hydration or solvent slaving?

Deep eutectic solvents (DESs) emerged as potential alternative solvent media in multiple areas, including biomolecular (cryo)preservation. Herein, we studied the stability of a small protein (ubiquitin) in water and a betaine-glycerol-water (B:G:W) (1:2:ζ; ζ = 0, 1, 2, 5, 10) DES, through molecular dynamics. An AMBER-based model that accurately describes the density and shear viscosity of the DES is proposed. We find that water molecules are largely trapped in the solvent, precluding the formation of a full hydration layer, seemingly opposite to osmolytes' preferential exclusion/preferential hydration mechanism. Although the protein is stable in the DES, structural fluctuations are largely suppressed and only recovered upon sufficient hydration. This is explained by a solvent-slaving mechanism where β-fluctuations are key, with the non-monotonic hydration of some amino acids with the water content providing an explanation to the non-monotonic folding of some proteins in aqueous DESs. A major thermal stability enhancement in the DES is also observed, caused by a similar slowdown of the backbone torsional dynamics. Our results support a kinetic stabilization of the protein in the DES, whereas a possible thermodynamic stabilization does not follow a preferential hydration or water entrapment mechanism.

Nanocrystallites Modulate Intermolecular Interactions in Cryoprotected Protein Solutions

Studying protein interactions at low temperatures has important implications for optimizing cryostorage processes of biological tissue, food, and protein-based drugs. One of the major issues is related to the formation of ice nanocrystals, which can occur even in the presence of cryoprotectants and can lead to protein denaturation. The presence of ice nanocrystals in protein solutions poses several challenges since, contrary to microscopic ice crystals, they can be difficult to resolve and can complicate the interpretation of experimental data. Here, using a combination of small- and wide-angle X-ray scattering (SAXS and WAXS), we investigate the structural evolution of concentrated lysozyme solutions in a cryoprotected glycerol-water mixture from room temperature (T = 300 K) down to cryogenic temperatures (T = 195 K). Upon cooling, we observe a transition near the melting temperature of the solution (T ≈ 245 K), which manifests both in the temperature dependence of the scattering intensity peak position reflecting protein-protein length scales (SAXS) and the interatomic distances within the solvent (WAXS). Upon thermal cycling, a hysteresis is observed in the scattering intensity, which is attributed to the formation of nanocrystallites in the order of 10 nm. The experimental data are well described by the two-Yukawa model, which indicates temperature-dependent changes in the short-range attraction of the protein-protein interaction potential. Our results demonstrate that the nanocrystal growth yields effectively stronger protein-protein attraction and influences the protein pair distribution function beyond the first coordination shell.

The Impact of Glycerol on an Affibody Conformation and Its Correlation to Chemical Degradation

The addition of glycerol to protein solutions is often used to hinder the aggregation and denaturation of proteins. However, it is not a generalised practice against chemical degradation reactions. The chemical degradation of proteins, such as deamidation and isomerisation, is an important deteriorative mechanism that leads to a loss of functionality of pharmaceutical proteins. Here, the influence of glycerol on the chemical degradation of a protein and its correlation to glycerol-induced conformational changes is presented. The time-dependent chemical degradation of a pharmaceutical protein, GA-Z, in the absence and presence of glycerol was investigated in a stability study. The effect of glycerol on protein conformation and oligomerisation was characterised using asymmetric field-flow fractionation and small-angle neutron scattering in a wide glycerol concentration range of 0–90% v/v. The results from the stability study were connected to the observed glycerol-induced conformational changes in the protein. A correlation between protein conformation and the protective effect of glycerol against the degradation reactions deamidation, isomerisation, and hydrolysis was found. The study reveals that glycerol induces conformational changes of the protein, which favour a more compact and chemically stable state. It is also shown that the conformation can be changed by other system properties, e.g., protein concentration, leading to increased chemical stability.

Direct Evidence for the Effect of Glycerol on Protein Hydration and Thermal Structural Transition

The mechanisms of protein stabilization by uncharged solutes, such as polyols and sugars, have been intensively studied with respect to the chemical thermodynamics of molecular crowding. In particular, many experimental and theoretical studies have been conducted to explain the mechanism of the protective action on protein structures by glycerol through the relationship between hydration and glycerol solvation on protein surfaces. We used wide-angle x-ray scattering (WAXS), small-angle neutron scattering, and theoretical scattering function simulation to quantitatively characterize the hydration and/or solvation shell of myoglobin in aqueous solutions of up to 75% v/v glycerol. At glycerol concentrations below ∼40% v/v, the preservation of the hydration shell was dominant, which was reasonably explained by the preferential exclusion of glycerol from the protein surface (preferential hydration). In contrast, at concentrations above 50% v/v, the partial penetration or replacement of glycerol into or with hydration-shell water (neutral solvation by glycerol) was gradually promoted. WAXS results quantitatively demonstrated the neutral solvation, in which the replacement of hydrated water by glycerol was proportional to the volume fraction of glycerol in the solvent multiplied by an exchange rate (β ≤ 1). These phenomena were confirmed by small-angle neutron scattering measurements. The observed WAXS data covered the entire hierarchical structure of myoglobin, ranging from tertiary to secondary structures. We separately analyzed the effect of glycerol on the thermal stability of myoglobin at each hierarchical structural level. The thermal transition midpoint temperature at each hierarchical structural level was raised depending on the glycerol concentration, with enhanced transition cooperativeness between different hierarchical structural levels. The onset temperature of the helix-to-cross β-sheet transition (the initial process of amyloid formation) was evidently elevated. However, oligomerization connected to fibril formation was suppressed, even at a low glycerol concentration.