Photoisomerization of Sixteen Isomers of Retinal. Initial Product Distribution in Direct and Sensitized Irradiation. Photochemistry of Polyenes 31*

Abstract

Abstract— Initial product distributions of all 16 geometric isomers of retinal in hexane solutions have been determined. With direct irradiation, the product ratios are characterized by a preference for isomerization at the 13,14‐bond. In particular, all isomers containing the 13‐cis geometry give the corresponding 13‐trans isomer as the major product. Preference for one‐photon‐one‐bond isomerization was also noted, although a substantial amount of the all‐trans isomer was detected for all poly‐cis, 13‐trans isomers. In sensitized irradiation, the initial mixture shows extensive one‐photon‐two(or multiple)‐bond isomerization to the corresponding unhindered isomers. In cases of hindered isomers, multiple‐bond isomerized products are dominant. The different results are accountable by the different shapes of the excited state potential curves for singlet and triplet states.

The case of medium-dependent dual mechanisms for photoisomerization: One-bond-flip and Hula-Twist

“…And, from a variety of arylalkenes, where fluorescence is more common or efficient than in the simple dienes and trienes, different excited-state properties of the isomeric excited singlets have been demonstrated (11)(12)(13). For longer polyenes, the effort has been largely devoted to unravelling the complex chemistry associated with the large number of geometric isomers (14,15). However, selective excitations of conformers were implicated (16).…”

Hula‐Twist. A Stereoselective and Regioselective Photoisomerization Reaction Mechanism

“…We have also detected a high regioselectivity (HT-1 or HT-2) in photoisomerization of isomers of the parent diphenylbutadiene (DPB). 21 Thus, for Z,Z-DPB (19), we observed first formation of the stable Z,E-DPB, which is only consistent with HT-1 (see Fig. 3).…”

“…18 We have also observed that 11-cis-retinal (the key chromophore in rhodopsin) was reactive when irradiated in an organic glass at 77 K. There are two possible modes of HT isomerization, HT-11 and HT-12. The exact nature of the low temperature photoproduct and a comparison with its solution photochemistry 19 remain to be examined.…”

Isorhodopsin: An Undervalued Visual Pigment Analog

“…This was also of great help when the recombinant DNA era arrived in the 1980s and proficient heterologous hosts for the production of the apoprotein (opsin) in cell culture were developed [249][250][251][252][253]. In order to generate the holoproteins, 11-cis retinal had to be supplied to the cell culture, but this compound has several disadvantages, including low thermal stability in biological environments [17,142], not being commercially available, and elaborate purification after chemical synthesis or after photoproduction from all-trans retinal [17,254,255]. Conversely, 9-cis retinal has better thermal stability [6] and is commercially available; hence, it seems to be a more suitable alternative.…”