AM1, INDO/S and optical studies of carbocations of carotenoid molecules. Acid induced isomerization

The mechanism of trans–cis isomerization of carotenoid molecules through the formation of the carotenoid carbocation (CarH+) intermediate in the presence of acid is presented. Various cis-isomers of carotenoids (β-carotene, canthaxanthin and 8′-apo-caroten-8′-al) are predicted from AM1 calculations of rotation barriers for CarH+, as well as from the stabilities of CarH+. AM1 dipole moments D and INDO/S optical transitions of CarH+ were calculated for all protonation sites. Optical spectra of CarH+ solutions exhibit broad lines in the region of 700–1000 nm with extinction coefficients of 1–3 × 105 M–1 cm–1.