Relationship between structure, dynamics and function of hydrated purple membrane investigated by neutron scattering and dielectric spectroscopy.

We investigated the influence of hydration water on the relationship between structure, dynamics and function in a biological membrane system. For the example of the purple membrane (PM) with its protein bacteriorhodopsin (BR), a light-driven proton pump, complementary information from neutron diffraction, quasi-elastic neutron scattering (QENS) and dielectric spectroscopy will form a comprehensive picture of the structural and dynamic behavior of the PM in the temperature range between 150 and 290 K. Temperature- and humidity-dependent changes in the membrane system influence the accessibility of the different photocycle intermediates of BR. The melting of the 'freezing bound water' between 220 and 250 K could be related to the transition from the M1 to the M2 intermediate, which represents the key step in the photocycle. The dynamic transition in the vicinity of 180 K was shown to be necessary to ensure that the M1 intermediate can be populated and that the melting of crystallized bulk water above 255 K enables the completion of the photocycle.