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 300 K. One series of measurements was carried out after quenching from 300 down to 80 K and another after annealing in 200-240 K. The quenched sample showed a heat capacity jump indicating a glass transition temperature T(g) = 170 K, and the annealed sample showed a smaller jump with the T(g) 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 110 K and the other over a wide temperature range (120-190 K). The annealed sample showed three separate relaxation effects giving 1) T(g) = 110 K, 2) 135 K, and 3) temperature higher than 180 K, whereas nothing around 170 K. 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.

FIGURE 1

General relation found among the relaxation time (τ) (a), enthalpy (H) (b), and enthalpy relaxation rate (−dH/dt) (c) in the case of a glass transition due to freezing of a single relaxation process.

FIGURE 2

Temperature sequence in C_p and −dH/dt measurements for quenched sample (a) and annealed sample (b).

FIGURE 3

Heat capacities of the quenched (circle) and annealed (square) samples. The inset shows the heat capacities on an enlarged vertical scale for the region T = 80–250 K. For clarity, the C_p data of the annealed sample have been shifted upward by 10 kJK⁻¹BSA-mol⁻¹ in the main figure and 5 kJK⁻¹BSA-mol⁻¹ in the inset. Arrow indicates the T_g.

FIGURE 4

Temperature dependence of −dH/dt for the quenched sample. Horizontal bar and arrow indicate the glass transition region and the T_g, respectively. Inset shows enlargement of T = 80–200 K region.

FIGURE 5

Temperature dependence of −dH/dt for the annealed sample. Horizontal bar and arrow indicate the glass transition region and the T_g, respectively.