RHODOPSIN RECONSTITUTION IN VESICLES FORMED FROM SIMPLE, FULLY SYNTHETIC AMPHIPHILES

The visual pigment bovine rhodopsin has been incorporated into closed bilayer vesicles formed from fully synthetic double-chained amphiphiles 1 and 2 carrying N-methylpyridinium head groups. The reconstituted rhodopsin exhibits the same absorption spectrum as in the intact biological membrane. This indicates that the chromophoric center of the pigment is not affected by reconstitution into the artificial, positively charged vesicles. Illumination of rhodopsin bound to vesicles of 1 a t 25 or 16 "C results in spectrochemical changes indicative of the formation of metarhodopsin I1 and, subsequently, of metarhodopsin I11 and all-trans-retinal. These findings reveal that rhodopsin molecules are located in a membrane-like environment. Metarhodopsin I is the first detectable photoproduct when rhodopsin bound to vesicles formed from 1 is illuminated a t 1.5 "C. This different behavior a t lower temperature is primarily a kinetic effect and is not caused by an increased rigidity of the bilayer below the phase-transition temperature. Illumination of rhodopsin reconstituted in the presence of amphiphile 2 at 25 "C leads to less than 30% of metarhodopsin I. Electron micrographs obtained by both negative staining and freeze-fracture techniques provide strong additional evidence for the incorporation of rhodopsin into the hydrocarbon part of the vesicle membrane. It is concluded that this microenvironment allows the essential internal flexibility for the chromoprotein to retain its functional activity. Thus, vesicles derived from simple synthetic amphiphiles may well become powerful tools in mimicking membrane