Between the ground‐ and M‐state of bacteriorhodopsin the retinal transition dipole moment tilts out of the plane of the membrane by only 3°

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Abstract
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° in the all-trans ground state to 64.1±0.8° in the 13-cis M-state. The light-induced isomerization of the chromophore is thus accompanied by an orientational change of only about 3° 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

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