Effects of the terminal-methyl position and of the length of the conjugated chain on the cis isomers produced by photo-isomerization: Analysis by HPLC and configurational determination by H-1-NMR spectroscopy of isomeric analogues of retinal and beta-apo-12'-carotenal

An analogue of retinal (beta-apo-12'-carotenal), its terminal-methyl group being shifted from the 13 to 14 (from 13' to 14') position, was subjected to direct (iodine-sensitized) photo-isomerization. The configurations of five (seven) isomers of the retinal (beta-apo-12'-carotenal) analogue were determined by H-1-NMR spectroscopy to be all-trans, 7-, 9- and 11-mono-cis and 9,11-di-cis (all-trans, 9-, 13- and 13'-mono-cis and 9,13-, 9,13'- and 13,13'-di-cis). Comparative direct photo-isomerization in acetonitrile of the above parent and analogue aldehydes showed that the kinds and the amounts of cis isomer produced in the stationary-state mixtures are drastically affected by the terminal-methyl position and by the length of the conjugated chain. The results are discussed in terms of enhanced polarization upon excitation in the terminal C=O and the neighbouring C=C bonds in the conjugated chain. (C) 1997 Published by Elsevier Science B.V.