Dynamics of proteins in hydrated state and in solution as studied by dielectric relaxation spectroscopy

An investigation was carried out of the interactions between water and three globular proteins: Candida antarctica Lipase B (CaLB), Bovine Serum Albumin (BSA), and Lysozyme (Lys). Measurements were performed on hydrated proteins and protein solutions using dielectric relaxation spectroscopy over a wide range of frequency (10 ^-2-10 ⁹ Hz) and temperature (-100 to +80 °C). Three dielectric dispersions were observed in hydrated proteins, and an explanation of their molecular origin was offered in terms of the three regions in globular proteins where the absorbed water is located. Those three processes evolve gradually into an overlapping broad dispersion with increasing water content and temperature; this finding was common to hydrated proteins and aqueous solutions alike. Dielectric modulus formalism was employed to compare the dynamics of proteins in aqueous and nonaqueous solutions in the temperature range from 20 to 80 °C. Dimethyl sulfoxide (DMSO) was the nonaqueous solvent of choice because of its ability to interfere with hydrogen bonding. General trends in aqueous and nonaqueous solutions are similar but with notably different time scales. For a given concentration and temperature, the relaxation process is slower in DMSO than in the aqueous solution. Low-temperature (below 0 °C) dynamics in aqueous solutions were characterized by the presence of two processes: their origin and their temperature and concentration dependence are discussed.