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Biophysical Journal 85:3202-3213 (2003)
© 2003 The Biophysical Society

Sound Velocity and Elasticity of Tetragonal Lysozyme Crystals by Brillouin Spectroscopy

S. Speziale *, F. Jiang * {dagger}, C. L. Caylor {dagger}, S. Kriminski {dagger}, C.-S. Zha {ddagger}, R. E. Thorne {dagger} and T. S. Duffy *

* Department of Geosciences, Princeton University, Princeton, New Jersey; {dagger} Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York; and {ddagger} Cornell High-Energy Synchrotron Source, Ithaca, New York

Correspondence: Address reprint requests to S. Speziale, Dept. of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 08544. Tel.: 609-258-3261; Fax: 609-258-1274; speziale{at}princeton.edu.

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 C11 + C12 + 2C66 = 12.81 ± 0.08 GPa, C11 = 5.49 ± 0.03 GPa, and C33 = 5.48 ± 0.05 GPa were directly determined. Inversion of the measured sound velocities in the [110] plane constrains the combination C44 + 1/2C13 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.




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