Add time:08/14/2019         Source:sciencedirect.com

Eleven cis-trans isomers of okenone were isolated by means of HPLC using a silica-gel column from an isomeric mixture which was obtained by iodine-sensitized photo-isomerization of the all-trans isomer. The configurations of eight isomers among them were determined by NMR spectroscopy using the isomerization shifts of the olefinic 1Hs and the 1H–1H NOE correlations to be all-trans, 7-cis, 7-cis,8-s-cis, 9-cis, 9′-cis, 13-cis, 13′-cis and 9,9′-di-cis, and their electronic-absorption and resonance-Raman spectra were recorded. Based on the results: (1) the chemical shifts of the olefinic 1Hs in NMR; (2) the wavelength of the Ag−→Bu+ transition; and (3) the relative intensity of the Ag−→Ag+ versus the Ag−→Bu+ transition in electronic absorption; (4) the CC stretching frequency; and (5) the relative intensity of the C10–C11 (C10′–C11′) versus the C14–C15 (C14′–C15′) stretching vibration in resonance Raman were compared among the all-trans, 7-cis, 9-cis (9′-cis) and 13-cis (13′-cis) isomers of β-carotene, canthaxanthin, β-apo-8′-carotenal, neurosporene, spheroidene and okenone. Relevance of the systematic changes in the above five different parameters originally found in β-carotene was examined in the rest of the carotenoids, and the effects of the peripheral groups on them were explained in terms of the length and asymmetry of the conjugated system consisting of the CC and CO bonds.

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