Between the ground- and M-state of bacteriorhodopsin the retinal transition dipole moment tilts out of the plane of the membrane by only 3 degrees.

The orientation of the transition dipole moments in the ground state and the M-intermediate of bacteriorhodopsin were determined by time-resolved and steady-state polarized absorption spectroscopy on samples of oriented immobilized purple membranes. The angle between the transition dipole moment and the membrane normal decreases from 66.8 +/- 0.5 degrees in the all-trans ground state to 64.1 +/- 0.8 degrees in the 13-cis M-state. The light-induced isomerization of the chromophore is thus accompanied by an orientational change of only about 3 degrees out of the plane of the membrane. The absorption anisotropy at 410 nm remains constant over more than 4 decades of time covering both the rise and decay of M. Conformational changes accompanying a sequential M1----M2 transition thus do not affect the chromophore orientation.