On the Structure of Rhodopsin. Ii. A Model of Rhodopsin and Its Changes Following Light Absorption

A model for the structure of the visual pigment rhodopsin is presented. Part of the opsin portion of rhodopsin is assumed as being in the form of an α‐helix. The retinene‐derived portion of rhodopsin spans at least two turns of the α‐helical portion of opsin forming a “retinene opsin” loop. The photoisomerization of the retinene portion causes a change in the loop and produces a strained system. A relief‐of‐strain process is described in which a proton binding group, attached to opsin, is shown to migrate down the retinene chain producing various semi‐carbonium ions via a “semi‐protonation process. This model accounts for most of the presently known photochemical phenomena associated with rhodopsin bleaching and predicts several new, as yet undiscovered, intermediates, and where in the bleaching sequence these intermediates will be found. A possible scheme for rhodopsin regeneration is presented.