MEASUREMENT OF COHERENT DEBYE-WALLER FACTOR IN IN-VIVO DEUTERATED C-PHYCOCYANIN BY INELASTIC NEUTRON-SCATTERING

Quasielastic neutron scattering measurements of dry and 35% D2O hydrat ed amorphous protein powder of C-phycocyanin were made as a function o f temperature ranging from 313K down to 100K. The protein is grown fro m blue-green algae cultured in D2O and is deuterated up to 99%. The sc attering is thus dominated by coherent scattering. Within the best ene rgy resolution of the time-of-flight instrument, which is 28 mu eV FWH M, the scattering appears entirely elastic. For this reason we are abl e to extract a coherent Debye-Waller factor by making an independent m easurement of the static structure factor. We observe a considerable d ifference in the q dependence of the Debye-Waller factor between the d ry and hydrated proteins; furthermore, there is an interesting tempera ture dependence of the Debye-Waller factor that is quite different fro m that predicted for dense hard-sphere liquids.