Thermodynamic properties of purple membrane.

We measured the density, expansivity, specific heat at constant pressure, and sound velocity of suspensions of purple membrane from Halobacterium halobium and their constituent buffers. From these quantities we calculated the apparent values for the density, expansivity, adiabatic compressibility, isothermal compressibility, specific heat at constant pressure, and specific heat at constant volume for the purple membrane. These results are discussed with respect to previously reported measurements on globular proteins and lipids. Our data suggest a simple additive model in which the protein and lipid molecules expand and compress independently of each other. However, this simple model seems to fail to describe the specific heat data. Our compressibility data suggest that bacteriorhodopsin in native purple membrane binds less water than many globular proteins in neutral aqueous solution, a finding consistent with the lipid surround of bacteriorhodopsin in purple membrane.