Conformational Changes and Cooperation of Bacteriorhodopsin
The mechanism of coupling between the photocycle and the vectorial proton transfer process of bacteriorhodopsin (BR) of the purple membrane of Halobacterium halobium implies an oriented and tight interaction between the chromophore and the protein conformation of BR within the structure of the purple membrane. Recently, the orientation of the transition dipole moment of the retinal chromophore in its dark state was analyzed by linear dichroism at 568 nm in thin layers of purple membrane of less than 1 μm to 4 μm thickness. We found an angle between the transition moment and the plane of the membrane of ≤ 27° (1), which is schematically drawn in the computed sequence model recently published by Ovchinnikov et al. (2) (see Fig. 1). It should be mentioned here that the direction of the chromophore (to the left or to the right) is not yet clarified. However, the picture illustrates the orientation of the chromophore at its approximate angle within a distribution of polar and nonpolar groups along the folds of the amino acid sequence thus far resolved.
KeywordsElectric Field Pulse Transition Dipole Moment Linear Dichroism Purple Membrane Relative Occupancy
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