Protein rotation and chromophore orientation in reconstituted bacteriorhodopsin vesicles

Bacteriorhodopsin has been reconstituted into lipid vesicles with dipalmitoyl and dimyristoyl phosphatidylcholine. Circular dichroism (CD) measurements show that the proteins are in a monomeric state above the main lipid phase transition temperature (Tc), 41 and 23 degrees C for dipalmitoyl and dimyristoyl phosphatidylcholine, respectively. Below Tc, the CD spectrum is the same as that found for the purple membrane. The latter result implies that the orientation of the chromophore at these temperatures is most likely the same as in the purple membrane (70 degrees +/- 5 degrees from the normal to the membrane plane). Transient dichroism measurements show that below Tc the proteins are immobile, while above this temperature protein rotation around an axis normal to the plane of the membrane is occurring. In addition, from the data the angle of the chromophore for the rotating proteins with respect to the rotational diffusion axis can be calculated. This angle is found to be 30 degrees +/- 3 degrees and 29 degrees +/- 4 degrees in dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine, respectively. This is considerably smaller than the value of 70 degrees +/- 5 degrees for the natural biomembrane. A reversible reorientation of the chromophore above and below the respective main Tc transition temperature could explain the change of angle observed provided that all the molecules rotate above Tc.