Solvent dependence of dynamic transitions in protein solutions

A transition as a function of increasing temperature from harmonic to anhar monic: dynamics has been observed in globular proteins by using spectroscop ic, scattering, and computer simulation techniques. We present here results of a dynamic: neutron scattering analysis of the solvent dependence of the picosecond-time scale dynamic transition behavior of solutions of a simple single-subunit enzyme, xylanase. The protein is examined in powder form, i n D2O, and in four two-component perdeuterated single-phase cryosolvents in which it is active and stable. The scattering profiles of the mixed solven t systems in the absence of protein are also determined. The general featur es of the dynamic transition behavior of the protein solutions follow those of the solvents. The dynamic transition in all of the mixed cryosolvent-pr otein systems is much more gradual than in pure D2O, consistent with a dist ribution of energy barriers. The differences between the dynamic behaviors of the various cryosolvent protein solutions themselves are remarkably smal l. The results are consistent with a picture in which the picosecond-time s cale atomic dynamics respond strongly to melting of pure water solvent but are relatively invariant in cryosolvents of differing compositions and melt ing points.