Micro-Brillouin scattering study of low temperature elastic properties of protein crystals

Recently the crystal structures of small protein crystals have been well studied, whereas elastic properties of these crystals are not yet fully understood. The elastic properties of a tetragonal hen egg white lysozyme (HEWL) crystal is studied over a wide temperature range by using micro-Brillouin scattering (MBS), which is a powerful tool to measure the elastic properties of a very small crystal. A HEWL crystal is immersed in the cryoprotective liquid of glycerol which undergoes a glass transition around 185 K without any crystallization. The sound velocity of hypersonic acoustic phonons shows continuous increase with decreasing temperature and no abrupt jump due to crystallization. Typical relaxation behavior is observed in the gigahertz frequency range, and the relaxation time obeys the Arrhenius law. Its relaxation parameters are much different from those of bulk glycerol. The relaxation times of a HEWL crystal immersed in glycerol suggests that the observed dynamical process is probably related to the local motion of protein and solvent molecules near the surface of protein molecule.