A NOVEL BACTERIORHODOPSIN ANALOGUE WITH CONFORMATIONALLY 6-s-cis FIXED RETINALS

Abstract— The artificial pigments of bacteriorhodopsin (bR) were synthesized with retinal analogues which have 6-s-cis fixed conformation. In order to study the shape of binding pocket around β-ionone ring of retinal, we used three different 6-s-cis fixed retinals (1–3), which have a gem-dimethyl group at a different position of the ring structure. These three 6-s-cis fixed retinals produced bR analogue pigments with absorption maxima at 574, 594 and 584 nm, respectively. These analogue pigments showed proton pumping activities upon incorporation into vesicles, indicating that 6-s-trans structure of retinal is not necessary to pump protons. Although these retinal analogues possess the same conjugated π-electron systems, the wavelengths of maximum absorbance of the analogue pigments were different from each other, indicating a distinctly steric interaction between retinal ring portion and apoprotein. The analogue pigments were also different from each other in regeneration rate and the number of the final products. These differences resulted from a difference in the position of the dimethyl substituent at the ring structure. Based on the results the molecular structure around β-ionone ring of retinal-binding pocket was discussed.