Detailed description of Raman spectra of carotenoids: experiment and DFT calculations of all-trans-isomers

For trans-isomers of ten carotenoids (α-carotene, β-carotene, γ-carotene, ζ-carotene, lycopene, neurosporene, spheroidene, spirilloxanthin, lutein, astaxanthin), we carried out experimental and density functional theory (DFT) studies of their Raman spectra. We analyzed the relationship of the intensities and positions of all Raman bands in the range of 800-1700 cm^-1 with the conjugation length and constitution of the end groups of carotenoid molecules. We found out that the Raman bands at about 1285, 1632, and 1660 cm^-1 are markers of presence of acyclic end groups in carotenoid molecules. Using as the example of dietary supplements and carotenoid-enriched seed oils, we demonstrate that, although most carotenoid Raman bands in the spectral region of 800-1700 cm^-1 have sufficiently low intensities, nevertheless, they can be found out in the Raman spectra of complex systems. These less intense Raman bands can be used to more accurately identify carotenoids in complex systems, in addition to analyzing the stretching CC and CC vibrational modes.