Raman spectra of carotenoids in natural products

Abstract

Resonance Raman spectra of naturally occurring carotenoids have been obtained from nautilus, periwinkle ( Littorina littorea) and clam shells under 514.5 nm excitation and these spectra are compared with the resonance Raman spectra obtained in situ from tomatoes, carrots, red peppers and saffron. The tomatoes, carrots and red peppers gave rise to resonance Raman spectra exhibiting a ν 1 band at ca. 1520 cm −1, in keeping with its assignment to carotenoids with ca. nine conjugated carboncarbon double bonds in their main chains, whereas the resonance Raman spectrum of saffron showed a ν 1 band at 1537 cm −1 which can be assigned to crocetin, having seven conjugated carboncarbon double bonds. A correlation between ν 1 wavenumber location and effective conjugated chain length has been used to interpret the data obtained from the shells, and the wavenumber position (1522 cm −1) of the ν 1 band of the carotenoid in the orange clam shell suggests that it contains nine conjugated double bonds in the main chain. However, the black periwinkle and nautilus shells exhibit ν 1 bands at 1504 and 1496 cm −1, respectively. On the basis of the correlation between ν 1 wavenumber location and effective conjugated chain length, this indicates that they contain carotenoids with longer conjugated chains, the former having ca. 11 double bonds and the latter ca. 13 or even more. Raman spectra of the nautilus, periwinkle and clam shells also exhibited a strong band at 1085 cm −1 and a doublet with components at 701 and 705 cm −1, which can be assigned to biogenic calcium carbonate in the aragonite crystallographic form.