Highly precise characterization of the hydration state upon thermal denaturation of human serum albumin using a 65 GHz dielectric sensor

Biological functions of proteins depend on harmonization with hydration water surrounding them. Indeed, dynamical transition of proteins, such as thermal denaturation, is slaved to changes in mobility of hydration water. However, its commitment during dynamical transition is yet to be fully understood, due to technical limitations in precisely characterizing the amount of hydration water. A state-of-art CMOS dielectric sensor consisting of 65 GHz LC resonators addressed this issue by utilizing the feature that oscillation frequency sensitively shifts in response to the complex dielectric constant at 65 GHz with ultimately high precision. This study aimed to establish an analytical algorithm to derive the hydration number from the measured frequency shift and to demonstrate the transition of hydration number upon thermal denaturation of human serum albumin. The determined hydration number in the native state drew a “global” hydration picture beyond the first solvation shell, with substantially reduced uncertainty of the hydration number (about ± 1 %). This allowed detection of a rapid increase in the hydration number at around 55 °C during the heating process, excellently in phase with the irreversible rupture of the α-helical structure into solvent-exposed extended chains, whereas the hydration number did not trace the forward path in the subsequent cooling process. Our result indicates that weakening of water hydrogen bonds triggers unfolding of the protein structure first, followed by the changes in the number of hydration water as a consequence of thermal denaturation.