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 the formation of the carotenoid carbocation (CarH(+)) intermediate in the presence of acid is presented. Various cis:isomers of carotenoids (p-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 x 10(5) M-1 cm(-1).