CIRCULAR DICHROISM OF SQUID RHODOPSIN

The linkage between 11-cis retinal and opsin in squid rhodopsin was studied by means of the measurement of circular dichroism (CD). The CD revealed by 11-cis retinal when bound with opsin is probably derived from the stereospecific interaction between them. The magnitude and the shape of the CD spectrum of squid rhodopsin in the visible range are different from those of cattle rhodopsin. This fact may be due to the difference in protein conformation, especially around the attachment site of retinal, and this is probably related to the stability of the linkage of retinal and opsin in those visual pigments. On the other hand, squid rhodopsin shows a complicated negative Cotton effect in the ultraviolet range, as is generally found in other proteins or polypeptides containing an α-helix. On illumination, squid rhodopsin loses its optical activity in the visible range at the metarhodopsin stage. The ultraviolet CD spectrum does not change, though the absorbance near 233 nm is diminished. This is ascribed to a small conformational change in the protein part. From the above results it is concluded that the conformation of 11-cis retinal is influenced by the asymmetric conformation of the protein near the retinal and the optical activity is induced. Retinal becomes optically inactive at the metarhodopsin stage because the spatial arrangement between retinal and opsin has been changed by photoisomerization. © 1968.