7-Cis isomers of retinal via 7-cis- and 7,9-dicis-β-C18-tetraene ketones

An Organic Capsule as a Matrix to Capture and Store Reactive Molecules at Room Temperature in Aqueous Solution: 7-cis-β-Ionone†

Low-temperature m atrix isolation method is the most popular one to generate and store reactive molecules and characterize them by in situ IR spectroscopy. Recognizing the need for a simpler method to trap and store such molecules and characterize by NMR spectroscopy at room temperature in solution, we have performed experiments exploring the value of water-soluble octa acid (OA) capsule as a storage vessel. The molecule we have chosen to illustrate the feasibility is the highly hindered 7-cis-β-ionone, which has been established to exist in equilibrium with its cyclic form with the later favored at room temperature. In this study, we have shown that confined space can be an alternative to temperature to tilt an equilibrium toward higher energy isomer. During the course of the study, we were surprised to note that 7-trans-β-ionone aggregates in water and has distinct ¹ H NMR spectra. Ability to assemble characterizable organic aggregates in water reveals the value of water as a reaction medium that is yet to be fully explored by photochemists. Finally, we have clarified the likely mechanism of secondary photoreaction of α-pyran to the final photoproduct that involves 1,5-hydrogen migration.

Regeneration of rhodopsin and bacteriorhodopsin. The role of retinal analogues as inhibitors

The rate of regeneration of rhodopsin, from 11-cis-retinal and opsin, and bacteriorhodopsin from all-trans-retinal and bacterio-opsin, in the presence or absence of compounds whose structures partially resemble retinal were measured. Some of these compounds severely slowed down the regeneration process, but did not influence the extent of regeneration. In the case of compounds with a carbonyl functional group they were not joined to the active site of the apo-protein via a Schiff's base linkage since after treatment with NaBH4 an active apo-protein remained. The most effective inhibitors of rhodopsin regeneration were molecules whose structure could be superimposed on 9-cis or 11-cis retinal up to carbon atom 11. These C13 and C15 molecules were not distinguished between aldehyde, ketone or alcohol functional groups. The regeneration of bacteriorhodopsin was not inhibited by retinal analogues with short side chains. The most effective inhibitors were the all-trans C17-aldehyde (beta-ionylideneacetaldehyde) or C18-ketone (beta-ionylidenepent-3-ene-2-one) which, compared to retinal, lack two or three carbon atoms from the end of the poylene chain. The inhibition was very dependent upon the presence of the all-trans isomer and required aldehyde or ketone as functional group nitriles and alcohols were less effective. However, similarly to retinol, the all-trans C17 and C18 alcohols underwent a bathochromic shift and showed fine-structured spectra when mixed with bacterio-opsin.

Photochemical studies of 7-cis-rhodopsin at low temperatures. Nature and properties of the bathointermediate

The photoreaction of 7-cis-rhodopsin derived from 7-cis-retinal and cattle opsin was studied by low-temperature spectrophotometry. Upon irradiation of 7-cis-rhodopsin at liquid nitrogen temperature (-190 degrees C) with blue light, its spectrum shifted to the longer wavelengths, indicating the formation of a bathoproduct. The bathoproduct thus formed was found to be identical with bathorhodopsin formed from rhodopsin in their spectroscopic, photochemical, and thermal properties. Therefore, we believe that the bathoproduct is, in fact, bathorhodopsin. The fact that 7-cis-rhodopsin can be readily converted to rhodopsin and to 9-cis-rhodopsin shows that the identical retinal binding site of opsin is involved in the three isomeric rhodopsins. These results appear to be consistent with the notion that the chromophore of bathorhodopsin is a twisted all-trans isomer, which is readily obtainable from the 7-cis, 9-cis, and 11-cis isomers.