MOLECULAR MOTIONS AND HYDRATION OF PURPLE MEMBRANES AND DISK MEMBRANES STUDIED BY NEUTRON-SCATTERING

Fast stochastic equilibrium fluctuations (time scale: 10(-10)-10(-13) seconds) In purple membranes (PM) and in disk membranes (DM) have been measured with quasielastic incoherent neutron scattering. The compari son of predominantly stochastic motions occurring in purple membranes and in disk membranes revealed qualitatively similar dynamical behavio ur. Models of internal motions within restricted volumes have been sho wn to be useful to fit the spectra from both samples. From fits using these models we found ''amplitudes'' 15 to 20% larger for motions in D M samples compared to PM samples. This indicates a higher internal fle xibility of the DM. Because the dynamical behaviour is very sensitive to the hydration of the protein-lipid complex, we also performed neutr on diffraction experiments to determine lamellar spacings as a measure of level of hydration and as a function of temperature. From these st udies the interaction of solvent molecules with the surface of the pro tein-lipid complex appears to be qualitatively similar for both types of membranes.