Semiempirical and Raman spectroscopic studies of carotenoids

✍ Scribed by R. J. Weesie; J. C. Merlin; J. Lugtenburg; G. Britton; F. J. H. M. Jansen; J. P. Cornard

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 279 KB
• Volume: 5
• Category: Article
• ISSN: 1075-4261
• DOI: 10.1002/(sici)1520-6343(1999)5:1<19::aid-bspy4>3.0.co;2-e

✦ Synopsis

Semiempirical AM1 calculations have been carried out for ␤-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 ␤-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 C i 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 13 C labeled at the positions 12,12Ј; 13,13Ј; 14,14Ј; 15,15Ј; 15, and 20,20Ј 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 [