COMPUTATIONAL STUDIES OF THE MOLECULAR-STRUCTURE AND ELECTRONIC SPECTROSCOPY OF CAROTENOIDS

Semiempirical INDO/S spectral calculations have been carried out for t he all-trans carotenoids, 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetra hydrospheroidene, and 3,4-dihydrospheroidene, as well as the all-trans and 15, 15'-cis isomers of spheroidene. These biologically important molecules contain seven, eight, nine, and ten conjugated double bonds, respectively. In order to make calculations on these large systems tr actable, their structures have been approximated with the pi-electron conjugated moieties. Geometries used for the INDO calculations were ob tained by computing fully optimized structures for the carotenoids wit h the AM 1 Hamiltonian. Steric interaction involving methyl substituen ts results in some twisting along the carotenoid chain. Single-double bond alternation is clearly evident for each molecule. Agreement betwe en INDO calculated excitation energies and relative oscillator strengt hs for the allowed excited states are in good agreement with the exper imental data. Calculations have also been performed for several distor ted forms of 15,15'-cis-spheroidene to model absorption spectral data that have been obtained for spheroidene bound to reaction centers of R hodobacter sphaeroides wild type strain 2.4.1.