Orientation of Intermediates in the Bleaching of Shear-Oriented Rhodopsin

Cattle rhodopsin can be highly oriented by shearing a wet paste of digitonin micelles of this visual pigment between two quartz slides. This orients the rhodopsin micelles so that their chromophores lie mainly parallel to the direction of shear. In such preparations the orientation of rhodopsin and intermediates of its bleaching by light have been measured with plane-polarized light from -195°C to room temperature. The chromophore maintains essentially the same orientation as in rhodopsin in all the intermediates of bleaching: bathorhodopsin (prelumirhodopsin), lumirhodopsin, and metarhodopsins I and II. When, however, the retinaldehyde chromophore is hydrolyzed from opsin in the presence of hydroxylamine, the retinaldehyde oxime that results rotates so as to lie mainly across the direction of shear. That is, the retinal oxime, though free, orients itself upon the oriented matrix of the opsin-digitonin micelles. These experiments show the rhodopsin-digitonin micelle to be markedly asymmetric, with the chromophore lying parallel to its long axis. The asymmetry could originate in the formation of the micelle, in rhodopsin itself, or by its linear polymerization under the conditions of the experiment. If rhodopsin itself is markedly asymmetric, for which there is some evidence, then, since in the rod outer segments its chromophores lie parallel to the disk membranes, the molecules themselves must lie with their long axes parallel to the membranes.