Finding of an unexpected thermal anomaly at very low temperatures due to water confined within a globular protein, bovine serum albumin

Heat capacities and enthalpy relaxation rates of completely anhydrous bovine serum albumin (BSA) and hydrous BSA with 1.85% (w/w) water were measured by using an adiabatic calorimeter to examine the thermal behavior at low temperatures of a small amount of water left within the globular protein molecule. The heat capacities of the hydrous BSA were larger a little on account of the presence of water molecules. In addition, two new phenomena appeared for the hydrous BSA while not for the anhydrous one: spontaneous exothermic and endothermic effects depending on the pre-cooling rates were observed and interpreted as due to a glass transition. Anomalous behavior of heat capacities was found in 60-140 K and recognized as potentially originating from a kind of phase transition. It is suggested that the water left a little within the BSA is responsible for both the phenomena occurring at low temperatures.