Orientation of retinal in purple membrane determined by polarized Raman spectroscopy.

The orientation of the retinal molecule in the purple membrane was determined by polarized Raman spectroscopy for stacked purple membranes. The depolarization ratios of C = C stretching vibration mode were measured for three scattering geometries of purple membrane films. From the depolarization ratios we estimated the tilt angle of the transition dipole moment of retinal to the membrane normal and the rotational angle of the molecular plane along the transition dipole moment of retinal. The molecular plane of M intermediate was found to be almost perpendicular to the membrane plane. We confirmed that the tilt angle was 65 +/- 2 degrees for both bR and M intermediates.