Glass transition and dynamics in BSA–water mixtures over wide ranges of composition studied by thermal and dielectric techniques

• Published in: Biochimica et biophysica acta Vol. 1814; no. 12; pp. 1984 - 1996
• Main Authors: Panagopoulou, A., Kyritsis, A., Sabater i Serra, R., Gómez Ribelles, J.L., Shinyashiki, N., Pissis, P.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.12.2011
• Subjects: Animals; bovine serum albumin; Calorimetry, Differential Scanning; Cattle; cooling; Crystallization; crystals; Dielectric relaxation; Dielectric Spectroscopy; differential scanning calorimetry; enthalpy; Glass - chemistry; Glass transition; heat; Hydrated protein; ice; Kinetics; melting; mixtures; Models, Biological; Molecular mobility; pellets; Phase Transition; Plasticization; room temperature; Serum Albumin, Bovine - chemistry; Serum Albumin, Bovine - metabolism; solutions; sorption; sorption isotherms; Spectrometry, Mass, Electrospray Ionization; spectroscopy; Temperature; Thermodynamics; Uncrystallized water; Water - chemistry; Water - metabolism; water content; Glass transition; Uncrystallized water; Hydrated protein; Molecular mobility; Plasticization; Dielectric relaxation
• ISSN: 1570-9639; 0006-3002; 1878-1454
• DOI: 10.1016/j.bbapap.2011.07.014

Protein–water dynamics in mixtures of water and a globular protein, bovine serum albumin (BSA), was studied over wide ranges of composition, in the form of solutions or hydrated solid pellets, by differential scanning calorimetry (DSC), thermally stimulated depolarization current technique (TSDC) and dielectric relaxation spectroscopy (DRS). Additionally, water equilibrium sorption isotherm (ESI) measurements were performed at room temperature. The crystallization and melting events were studied by DSC and the amount of uncrystallized water was calculated by the enthalpy of melting during heating. The glass transition of the system was detected by DSC for water contents higher than the critical water content corresponding to the formation of the first sorption layer of water molecules directly bound to primary hydration sites, namely 0.073 (grams of water per grams of dry protein), estimated by ESI. A strong plasticization of the Tg was observed by DSC for hydration levels lower than those necessary for crystallization of water during cooling, i.e. lower than about 0.3 (grams of water per grams of hydrated protein) followed by a stabilization of Tg at about −80°C for higher water contents. The α relaxation associated with the glass transition was also observed in dielectric measurements. In TSDC a microphase separation could be detected resulting in double Tg for some hydration levels. A dielectric relaxation of small polar groups of the protein plasticized by water, overlapped by relaxations of uncrystallized water molecules, and a separate relaxation of water in the crystallized water phase (bulk ice crystals) were also recorded. [Display omitted] ► Thermal and dielectric studies of BSA–water mixtures. ► Studies of water and protein dynamics over extremely wide ranges of water content. ► Evolution of dynamics with hydration level. ► Determination of critical water contents. ► Assignment of relaxations to particular origins.