Structure and hydration of purple membranes in different conditions.

The unit cell dimension of the bacteriorhodopsin lattice in purple membranes decreases by the same amount (2%) upon drying the membranes at room temperature as when they are cooled to liquid nitrogen temperatures. Neutron diffraction experiments with H2O:2H2O exchange, however, show that whereas in the dry membranes the lipid headgroups are dehydrated and the decrease in dimension is due to a smaller area occupied by the lipid molecules, the water of hydration remains in place in the cooled membranes, and the decrease in dimension is due to thermal contraction only. These data suggest a hypothesis that functional bacteriorhodopsin, in the wet state at room temperature, has a relatively soft environment that would allow large amplitude motions of the protein; in the dry membranes at room temperature (which are inactive), the amplitudes of protein motions would be inhibited by a more close-packed environment as they are reduced, due to thermal contraction, in the cold membranes.