Sound velocity and elasticity of tetragonal lysozyme crystals by Brillouin spectroscopy.
Quasilongitudinal sound velocities and the second-order elastic moduli of tetragonal hen egg-white lysozyme crystals were determined as a function of relative humidity (RH) by Brillouin scattering. In hydrated crystals the measured sound velocities in the [110] plane vary between 2.12 +/- 0.03 km/s along the [001] direction and 2.31 +/- 0.08 km/s along the [110] direction. Dehydration from 98% to 67% RH increases the sound velocities and decreases the velocity anisotropy in (110) from 8.2% to 2.0%. A discontinuity in velocity and an inversion of the anisotropy is observed with increasing dehydration providing support for the existence of a structural transition below 88% RH. Brillouin linewidths can be described by a mechanical model in which the phonon is coupled to a relaxation mode of hydration water with a single relaxation time of 55 +/- 5 ps. At equilibrium hydration (98% RH) the longitudinal moduli C(11) + C(12) + 2C(66) = 12.81 +/- 0.08 GPa, C(11) = 5.49 +/- 0.03 GPa, and C(33) = 5.48 +/- 0.05 GPa were directly determined. Inversion of the measured sound velocities in the [110] plane constrains the combination C(44) + (1/2)C(13) to 2.99 +/- 0.05 GPa. Further constraints on the elastic tensor are obtained by combining the Brillouin quasilongitudinal results with axial compressibilities determined from high-pressure x-ray diffraction. We constrain the adiabatic bulk modulus to the range 2.7-5.3 GPa.
Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA. speziale@princeton.edu
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D000162:Acoustics; D016880:Anisotropy; D003461:Crystallography / Q000379:methods*; D004527:Egg Proteins / Q000737:chemistry; D004548:Elasticity; D006813:Humidity; D007368:Interferometry / Q000379:methods*; D046911:Macromolecular Substances; D009113:Muramidase / Q000737:chemistry*; D011487:Protein Conformation; D013057:Spectrum Analysis / Q000379:methods*; D013314:Stress, Mechanical; D014867:Water / Q000737:chemistry*
D004527:Egg Proteins; D046911:Macromolecular Substances; D014867:Water; D009113:Muramidase