Low-Temperature Glass Transitions of Quenched and Annealed Bovine Serum Albumin Aqueous Solutions

Abstract

To investigate the glass transition behaviors of a 20% (w/w) aqueous solution of bovine serum albumin, heat capacities and enthalpy relaxation rates were measured by adiabatic calorimetry at temperatures ranging from 80 to 300K. One series of measurements was carried out after quenching from 300 down to 80K and another after annealing in 200–240K. The quenched sample showed a heat capacity jump indicating a glass transition temperature Tg=170K, and the annealed sample showed a smaller jump with the Tg shifted toward the higher temperature side. The temperature dependence of the enthalpy relaxation rates for the quenched sample indicated the presence of two enthalpy relaxation effects: one at around 110K and the other over a wide temperature range (120–190K). The annealed sample showed three separate relaxation effects giving 1) Tg=110K, 2) 135K, and 3) temperature higher than 180K, whereas nothing around 170K. These effects were thought to originate, respectively, from the rearrangement motions of 1) primary hydrate water forming a direct hydrogen bond with the protein, 2) part of the internal water localized in the opening of a protein structure, and 3) the disordered region in the protein.