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

The mechanism of tl ans-cis isomerization of carotenoid molecules through t he formation of the carotenoid carbocation (CarH(+)) intermediate in the pr esence of acid is presented. Various cis:isomers of carotenoids (p-carotene , canthaxanthin and 8'-apo-caroten-8'-al) are predicted from AM1 calculatio ns 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 calcula ted 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 x 10(5) M-1 cm(-1).