A broad glass transition in hydrated proteins

We performed Raman and Brillouin scattering measurements to estimate glass transition temperature, T_g, of hydrated protein. The measurements reveal very broad glass transition in hydrated lysozyme with approximate T_g ∼ 180 ± 15 K. This result agrees with a broad range of T_g ∼ 160–200 K reported in literature for hydrated globular proteins and stresses the difference between behavior of hydrated biomolecules and simple glass-forming systems. Moreover, the main structural relaxation of the hydrated protein system that freezes at T_g ∼ 180 K remains unknown. We emphasize the difference between the “dynamic transition”, known as a sharp rise in mean-squared atomic displacement <r²> at temperatures around T_D ∼ 200–230 K, and the glass transition. They have different physical origin and should not be confused.