Elastic interactions of photosynthetic reaction center proteins affecting phase transitions and protein distributions

Reaction-center proteins of Rhodopseudomonas Sphaeroides reconstituted into phosphatidylcholine vesicles shift and broaden the fluid-gel transition of the lipid bilayer. The amount of broadening and temperature shift of the transition depend both on protein concentration and on lipid chain length. In particular, the direction of the transition temperature shift is very sensitive to lipid chain length. Electron micrographs show homogeneous protein distribution on the fluid surface whereas the solid phase contains protein aggregates the type depending on chain length. The results can qualitatively be understood in the framework of a mattress model of lipid/protein interactions in membranes.