Diamagnetic anisotropy and orientation of alpha helix in frog rhodopsin and meta II intermediate.

The diamagnetic anisotropy of retinal rod outer segments, and its variation upon bleaching, have been measured with a rotating field device. A large molar diamagnetic asymmetry is found for rhodopsin. This cannot be explained by an anisotropy of the aromatic side chains of the protein, nor by the orientation of the retinal chromophore. However, it can be accounted for by an orientation perpendicular to the disc membrane of a major proportion of the alpha-helical segments of the protein. Upon bleaching a decrease of 9 +/- 2% of the diamagnetic asymmetry is observed when going to the meta II intermediate. This change is not mainly due to a reorientation of the retinal, since it is practically insensitive to detachment of the chromophore by addition of NH2OH. Comparison with recent UV linear dichroism results indicate that it may be due to the rotation of a trytophan residue in the bleaching sequence.