Complex thermal behavior of 11- cis -retinal, the ligand of the visual pigments

Abstract

Upon heating to 80 °C, 11-cis-retinal yields a mixture of all-trans-retinal and 13-cis-retinal. This isomerization has been studied by means of density functional theory methods, and the computational results suggest a close competition between two mechanisms of very different nature. A classical internal rotation around the C11-C12 cis double bond, via a diradical transition state, accounts for the formation of the all-trans isomer. An intricate sequence of pericyclic reactions, namely a reversible [1,7]-H sigmatropic shift and a reversible 6-π-oxa-electrocyclic reaction, is responsible for the formation of 13- cis-retinal. Experiments using 11-cis-retinal labeled with deuterium at C19 confirmed the mechanistic proposal and also revealed an unprecedented outcome on the product composition of isotopologues.