Semiempirical and Raman spectroscopic studies of carotenoids.

Semiempirical AM1 calculations have been carried out for beta-carotene and the three xanthophylls (zeaxanthin, canthaxanthin, and astaxanthin) containing oxygen functions (hydroxy/keto groups) found in the majority of natural pigment. The fully optimized geometries correspond well with the X-ray structures of beta-carotene and canthaxanthin and indicate that substitutions on the terminal rings have a minimal effect on the conformation of the chromophore. Twisting along the polyenic chain results from steric interaction involving methyl substituents, and a Ci point group can be proposed for the four investigated carotenoids. AM1 calculated excitation energies for the strongly allowed excited states can be compared to with the experimental absorption band in the visible region, considering solvent effect. Resonance Raman (RR) and Fourier transform (FT) Raman spectra of natural astaxanthin as well as astaxanthins specifically 13C labeled at the positions 12,12'; 13,13'; 14,14'; 15,15'; 15, and 20,20' were recorded. Furthermore the RR and FT Raman spectra of the asymmetric carotenoid 20-norastaxanthin are presented. The data reveal a substantial amount of information about the coupling between the different vibrations, and enabled an extensive experimental verification of the theoretical normal-coordinate analysis previously performed on polyenic molecules [J Raman Spectrosc 1983, 14, 310-321; Advances in Infrared and Raman Spectroscopy, Vol. 12, 1985, pp. 115-178; Spectrochim Acta 1996, 53, 381-392; Biochim Biophys Acta 1994, 1185, 188-196]. The results make up a very interesting dataset which allowed the interpretation and/or observation of several, hitherto never observed or not well understood, effects in the Raman spectra of the differently labeled astaxanthins.