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The study of the interaction between lipid membranes and amyloidogenic peptides is a turning point for understanding the processes involving the cytotoxicity of peptides involved in neurodegenerative diseases. In this work, we perform an experimental study of model membrane–lysozyme interaction to understand how the formation of amyloid fibrils can be affected by the presence of polar and zwitterionic phospholipid molecules (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine [POPC] and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol [POPG]). The study was conducted above and below the critical micellar concentration (CMC) using dynamic light scattering (DLS), atomic force microscopy (AFM), UV–Vis spectrophotometry, and the quartz crystal microbalance (QCM). Our results show that the presence of phospholipids appears to be a factor favoring the formation of amyloid aggregates. Spectrophotometric and DLS data revealed that the quantity of β{\rm{\beta }}-structure increases in the presence of POPG and POPC at different concentrations. The presence of POPG and POPC increases the speed of the nucleation process, without altering the overall structures of the fibrillar final products.

We provide a quantitative description of the solvation properties of lysozyme in water/ethanol mixtures, which has been obtained by a simultaneous analysis of small-angle neutron scattering and differential scanning calorimetry experiments. All data sets were analyzed by an original method, which integrates the exchange equilibrium model between water and ethanol molecules at the protein surface and activity coefficients data of water/ethanol binary mixtures. As a result, the preferential binding of ethanol molecules at the protein surface was obtained for both native and thermal unfolded protein states. Excess solvation numbers reveal a critical point at ethanol molar fraction ≈0.06, corresponding to the triggering of the hydrophobic clustering of alcohol molecules detected in water/ethanol binary mixtures. [ABSTRACT FROM AUTHOR]

Folded protein stabilization or destabilization induced by cosolvent in mixed aqueous solutions has been studied by differential scanning microcalorimetry and related to difference in preferential solvation of native and denatured states. In particular, the thermal denaturation of a model system formed by lysozyme dissolved in water in the presence of the stabilizing cosolvent glycerol has been considered. Transition temperatures and enthalpies, heat capacity, and standard free energy changes have been determined when applying a two-state denaturation model to microcalorimetric data. Thermodynamic parameters show an unexpected, not linear, trend as a function of solvent composition; in particular, the lysozyme thermodynamic stability shows a maximum centered at water molar fraction of about 0.6. Using a thermodynamic hydration model based on the exchange equilibrium between glycerol and water molecules from the protein solvation layer to the bulk, the contribution of protein-solvent interactions to the unfolding free energy and the changes of this contribution with solvent composition have been derived. The preferential solvation data indicate that lysozyme unfolding involves an increase in the solvation surface, with a small reduction of the protein-preferential hydration. Moreover, the derived changes in the excess solvation numbers at denaturation show that only few solvent molecules are responsible for the variation of lysozyme stability in relation to the solvent composition. [ABSTRACT FROM AUTHOR]

In solution small-angle neutron scattering has been used to study the solvation properties of lysozyme dissolved in water/glycerol mixtures. To detect the characteristics of the protein-solvent interface, 35 different experimental conditions (i.e., protein concentration, water/glycerol fraction in the solvent, content of deuterated compounds) have been considered and a suitable software has been developed to fit simultaneously the whole set of scattering data. The average composition of the solvent in the close vicinity of the protein surface at each experimental condition has been derived. In all the investigated conditions, glycerol resulted especially excluded from the protein surface, confirming that lysozyme is preferentially hydrated. By considering a thermodynamic hydration model based on an equilibrium exchange between water and glycerol from the solvation layer to the bulk, the preferential binding coefficient and the excess solvation number have been estimated. Results were compared with data previously derived for ribonuclease A in the same mixed solvent: even if the investigated solvent compositions were very different, the agreement between data is noticeable, suggesting that a unique mechanism presides over the preferential hydration process. Moreover, the curve describing the excess solvation number as a function of the solvent composition shows the occurrence of a region of maximal hydration, which probably accounts for the changes in protein stability detected in the presence of cosolvents. [ABSTRACT FROM AUTHOR]