title: Microcalorimetric study of thermal unfolding of lysozyme in water/glycerol mixtures: an analysis by solvent exchange model.
abstract: 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.
authors.affiliation: Dipartimento di Scienze Applicate ai Sistemi Complessi, Universita Politecnica delle Marche and CNISM, Ancona, Italy. f.spinozzi@univpm.it
authors.forename: Francesco
authors.initials: F
authors.lastname: Spinozzi
authors.forename: Maria Grazia
authors.initials: MG
authors.lastname: Ortore
authors.forename: Raffaele
authors.initials: R
authors.lastname: Sinibaldi
authors.forename: Paolo
authors.initials: P
authors.lastname: Mariani
authors.forename: Alessandro
authors.initials: A
authors.lastname: Esposito
authors.forename: Stefania
authors.initials: S
authors.lastname: Cinelli
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Onori
mesh_terms: D000818:Animals; D002151:Calorimetry / Q000379:methods*; D002645:Chickens; D045424:Complex Mixtures / Q000737:chemistry / Q000494:pharmacology*; D005260:Female; D005990:Glycerol / Q000737:chemistry*; D008958:Models, Molecular; D009113:Muramidase / Q000737:chemistry* / Q000378:metabolism; D011489:Protein Denaturation / Q000187:drug effects; D012997:Solvents / Q000737:chemistry / Q000494:pharmacology*; D013696:Temperature; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D045424:Complex Mixtures; D012997:Solvents; D014867:Water; D009113:Muramidase; D005990:Glycerol