Phase Behavior and Interactions of the Membrane-Protein Bacteriorhodopsin

We present a synchrotron x-ray diffraction study of melting in stacks of two-dimensional crystalline arrays of the membrane protein bacteriorhodopsin. Two distinct regimes have been found as a function of the intermembrane distance d. In the ``coupled'' regime for d<250 the temperature (Tₘ) of the melting transition decreases with increasing d, demonstrating the effect of the repulsive membrane interactions on the intramembrane protein ordering. For d>250 a ``decoupled'' regime is found with higher Tₘ* independent of d. Below Tₘ* a solid-liquid-solid reentrant behavior is observed as d is increased.