title: Chromophore configuration of iodopsin and its photoproducts formed at low temperatures.
abstract: The photochemical reactions of iodopsin at low temperatures were investigated by a combination of absorption spectroscopy and chromophore extraction to show the formation of isomeric photoproducts other than the all-trans intermediates. We first confirmed that the chromophore in iodopsin is an 11-cis-retinal. Next, iodopsin samples were irradiated with light of different wavelengths at selected temperatures ranging from -190 to 0 degrees C, and their retinylidene chromophores were extracted as oximes after warming the sample to 0 degree C. The isomeric composition of the extracted chromophores was analyzed by high-performance liquid chromatography. It was confirmed that bathoiodopsin produced at -190 degrees C has an all-trans chromophore, but a considerable amount of its chromophore thermally reisomerizes to the 11-cis form upon warming. Photoproducts formerly assigned as lumi- and metaiodopsins [Yoshizawa, T., & Wald, G. (1967) Nature 214, 566-571; Hubbard, R., & Kropf, A. (1959) Nature 183, 448-450] were produced by extensive irradiation of iodopsin at -80 and -40 degrees C, respectively, with red light, but their chromophores were identified to be 7-cis-retinals instead of all-trans-retinals. Thus these photoproducts are artificial byproducts formed as a result of photon absorption by all-trans intermediates. The absorption spectrum of the 7-cis product formed from bovine rhodopsin shows no spectral shift when it is warmed from -80 to 0 degrees C, but the spectrum of 7-cis species formed from iodopsin shifted about 40 nm to the blue at a transition temperature of -60 degrees C. This result indicates a unique chromophore-opsin interaction in iodopsin. Four all-trans intermediates of iodopsin were identified above -80 degrees C under irradiation conditions in which no 7-cis products accumulated. Their absorption maxima were estimated to be approximately 570, approximately 530, approximately 470, and approximately 380 nm. These species should correspond to BL-iodopsin, lumiiodopsin, metaiodopsin I, and metaiodopsin II, respectively, assigned by room temperature laser photolysis [Shichida, Y., Okada, T., Kandori, H., Fukada, Y., & Yoshizawa, T. (1993) Biochemistry 32, 10832-10838].
authors.affiliation: Department of Biophysics, Faculty of Science, Kyoto University, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Imamoto
authors.forename: T
authors.initials: T
authors.lastname: Yoshizawa
authors.forename: Y
authors.initials: Y
authors.lastname: Shichida
mesh_terms: D000818:Animals; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D003080:Cold Temperature; D008956:Models, Chemical; D010777:Photochemistry; D012168:Retinal Pigments / Q000737:chemistry* / Q000528:radiation effects; D012172:Retinaldehyde / Q000737:chemistry*; D017299:Rod Opsins; D013054:Spectrophotometry, Atomic
chemical_list: D012168:Retinal Pigments; D017299:Rod Opsins; C034941:iodopsin; D012172:Retinaldehyde