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Alpha‐retinals as Rhodopsin Chromophores—Preference for the 9‐ Z Configuration and Partial Agonist Activity †
Author(s) -
Wang Yajie,
BoveeGeurts Petra H. M.,
Lugtenburg Johan,
DeGrip Willem J.
Publication year - 2008
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.2008.00321.x
Subject(s) - photoisomerization , polyene , opsin , rhodopsin , chemistry , retinaldehyde , retinal , photochemistry , stereochemistry , chromophore , isomerization , quantum yield , biochemistry , fluorescence , physics , quantum mechanics , catalysis
The visual pigment rhodopsin, the photosensory element of the rod photoreceptor cell in the vertebrate retina, shows in combination with an endogenous ligand, 11‐ Z retinal, an astonishing photochemical performance. It exhibits an unprecedented quantum yield (0.67) in a highly defined and ultrafast photoisomerization process. This triggers the conformational changes leading to the active state Meta(rhodopsin) II. Retinal is covalently bound to Lys‐296 of the protein opsin in a protonated Schiff base. The resulting positive charge delocalization over the terminal part of the polyene chain of retinal creates a conjugation defect that upon photoexcitation moves to the opposite end of the polyene. Shortening the polyene as in 4,5‐dehydro,5,6‐dihydro (alpha), 5,6‐dihydro or 7,8‐dihydro‐analogs might facilitate photoisomerization of a 9‐ Z and a 11‐ Z bond. Here we describe pigment analogs generated with bovine opsin and 11‐ Z or 9‐ Z 4,5‐dehydro,5,6‐dihydro‐retinal that were further characterized by UV–Vis and FTIR spectroscopy. The preference of opsin for native 11‐ Z retinal over the 9‐ Z isomer is reversed in 4,5‐dehydro,5,6‐dihydro‐retinal. 9‐ Z 4,5‐dehydro,5,6‐dihydro‐retinal readily generated a photosensitive pigment. This modification has no effect on the quantum yield, but affects the Batho↔blueshifted intermediate (BSI) equilibrium and leads to a strong decrease in the G‐protein activation rate because of a downshift of the p K a of the Meta I↔Meta II equilibrium.