ISOMERIZATION OF THE RETINYLIDENE CHROMOPHORE OF BACTERIORHODOPSIN INLIGHT ADAPTATION - INTRINSIC ISOMERIZATION OF THE CHROMOPHORE AND ITSCONTROL BY THE APOPROTEIN

The dependence of the isomeric configuration of the retinylidene chrom ophore of bacteriorhodopsin on the pH value and on the wavelength of i rradiation (in a photostationary state) were examined by high Performa nce liquid chromatographic analyses of extracted retinal. The process of isomerization of the chromophore during tight adaptation was also t raced. More than 93% of all-trans and less than 5% of 13-cis retinal w ere extracted in the photostationary state for irradiation at 560 nm i n the pH region of 5-9 as well as for irradiation in the wavelength re gion of 400-650 nm at pH 7. Comparison of the above photostationary st ate composition with that of protonated n-butylamine Schiff base of re tinal indicates that strong constraint is applied to the chromophore b y the apo-protein. The constraint can be changed at low or high pH by a partial denaturation or transition of the apo-protein, which results in the generation of 11-cis and 9-cis retinal in the extract. At high er photon density, the isomerization process of the chromophore during light adaptation at pH 7 was characterized, as extracted isomeric ret inal, by (1) the initial decrease in 13-cis and increase in all-trans, (2) a subsequent, transient decrease in all-trans and increase in 11- cis, 9-cis and 13-cis and (3) the final decrease in these cis and incr ease in all-trans toward the above photostationary state composition. The results are discussed in terms of both the photoisomerization patt ern inherent in the retinylidene chromophore and the control by the ap o-protein.