Dynamics in a 20 wt% Bovine Serum Albumin Aqueous Solution Studied by Thermal and Dielectric Relaxation Measurements: Discovery of an Ice Ih–XI Phase Transition

Abstract

This review reorganizes our recent study on a 20 wt% bovine serum albumin (BSA) aqueous solution, focusing on the role of thermal history in water dynamics. Adiabatic calorimetry and dielectric relaxation measurements under comparable thermal protocols revealed an endothermic peak at ~72 K only in quenched samples, attributed to a transition from proton-ordered ice XI to proton-disordered ice Ih, indicating partial proton ordering prior to heating. The low-temperature anomaly around 110 K is reassigned as a superposition of the glass transition of primary hydration water and proton reorientation in ice Ih. While these processes can be distinguished in dielectric measurements, they overlap in calorimetry. A faster relaxation observed only in quenched samples is discussed in relation to ice XI formation, although its mechanism remains unclear. These results highlight the importance of thermal history and the coexistence of multiple water states in protein aqueous systems.