W0336

Measurement of the Elastic Constant Tensor of Protein Crystals by Brillouin Spectroscopy. S. Speziale^a, F. Jiang^a,b, C.L. Caylor^b, S. Kriminski^b, C-S. Zha^c, R.E. Thorne^b, and T.S. Duffy^a. ^aPrinceton Univ., Princeton, NJ 08544, ^bCornell Univ., Ithaca, NY 14853, ^cCornell High-Energy Synchrotron Source, Ithaca, NY 14853.

Brillouin scattering is a promising non-contact method to investigate the elastic tensor of protein crystals, providing a new probe of inter- and intramolecular interactions [1]. Quasi-longitudinal sound velocities and the second-order elastic moduli of tetragonal hen egg-white lysozyme crystals were determined as a function of relative humidity. 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 [] direction. Dehydration from 98% to79% relative humidity increases the sound velocities and decreases the velocity anisotropy in (110) from 8.2% to 2.8%. 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% relative humidity. At equilibrium hydration (98% relative humidity) 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 quasi-longitudinal results with axial compressibilities determined from high-pressure x-ray diffraction. We constrain the adiabatic bulk modulus to the range 2.8 - 5.1 GPa. Comparison with measurements performed in other frequency ranges shows the response of lysozyme crystals to be viscoelastic.

[1] S. Speziale et al. (2003). Biophys. J. (in press).