Temperature dependence of molecular motions in the membrane protein bacteriorhodopsin from QINS

The internal molecular motions of the membrane protein bacteriorhodopsin (BR) have been investigated by means of quasielastic incoherent neutron scattering (QINS). Measurements using a time-of-flight spectrometer were performed in order to study the quasielastic scattering component in the temperature range 100-297 K. Within an energy range of +/- 1.5 meV, it is possible to fit the data by a single Lorentzian with a line width of 100 mu eV (HWHM). The line width of the quasi-elastic component is roughly T- and Q-independent. The intensity of the quasielastic scattering is negligible below 200 K and shows a pronounced increase above this temperature, which is caused by an appearance of thermal fluctuations leading to transitions between different conformational substates of BR.