Orientation of Aromatic Residues in Rhodopsin. Rotation of One Tryptophan Upon the Meta I→meta Ii Transition After Illumination

Linear dichroism measurements have been performed in the visible and the UV on suspensions of intact retinal rod outer segments, isolated from frog and from cattle retinas and oriented by a magnetic field. In the UV a sharp double peaked signal is observed around 290 nm. It is characteristic of a tryptophan residue having its 1 L b transition oriented preferentially perpendicular to the membrane plane. Only one tryptophan residue per rhodopsin molecule seems to be involved, and this is insufficient to account for the diamagnetic anisotropy of the rods which causes their orientation. Upon bleaching one observes a rotation of this tryptophan coupled to the Meta I →Meta II transition, and the rotation is reversed in the next step Meta II → Meta III. The correlation with other spectral changes suggests that this tryptophan is in close relation with the retinal chromophore.