Steric hindrance between chromophore substituents as the driving force of rhodopsin photoisomerization: 10-methyl-13-demethyl retinal containing rhodopsin.
A visual chromophore analogue, 10-methyl-13-demethyl (dm) retinal, was synthesized and reconstituted with bleached bovine rhodopsin to form a visual pigment derivative with absorbance maximum at 505 nm. The investigations with this new compound were stimulated from recent results using 13-dm retinal as a chromophore that revealed a remarkable loss in quantum efficiency (phi of 13-dm retinal-containing rhodopsin: 0.30, Ternieden and Gärtner, J. Photochem. Photobiol. B Biol, 33, 83-86, 1996). The quantum efficiency of the new pigment was determined as 0.59 by quantitative bleaching using reconstituted rhodopsin as a reference. The very similar quantum efficiencies of rhodopsin and the new pigment give experimental support for the recently presented hypothesis that a steric hindrance between the substituents at positions 10 and 13 in 11-cis-retinal is elevated during the photoisomerization and thus facilitates the rapid photoisomerization of the visual chromophore (Peteanu et al., Proc. Natl. Acad. Sci. USA 90, 11762-11766, 1993). Such steric hindrance is removed from the molecule by the elimination of the methyl group from position 13 and can be re-established via a rearrangement of the substitution pattern by introducing a methyl group at position 10 of 13-dm retinal.
Koch
D
D
Max-Planck-Institut für Strahlenchemie, Mülheim an der Ruhr, Germany.
Gärtner
W
W
D000818:Animals; D002417:Cattle; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D010777:Photochemistry; D012172:Retinaldehyde / Q000031:analogs & derivatives* / Q000737:chemistry; D012243:Rhodopsin; D013054:Spectrophotometry, Atomic
C106450:1O-methyl-13-demethylretinal; D012243:Rhodopsin; D012172:Retinaldehyde