1H NMR, electronic-absorption and resonance-Raman spectra of isomeric okenone as compared with those of isomeric β-carotene, canthaxanthin, β-apo-8′-carotenal and spheroidene

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 (1)Hs and the H-1-H-1 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 (1)Hs in NMR; (2) the wavelength of the A(g)(-)-->B-u(+) transition; and (3) the relative intensity of the A(g)(-)-->A(g)(+) versus the A(g)(-)-->B-u(+) 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-trails, 7-cis, 9-cis (9'-cis) and 13-cis (13'-cis) isomers of beta-carotene, canthaxanthin, beta-apo-8'-carotenal, neurosporene, spheroidene and okenone. Relevance of the systematic changes in the above five different parameters originally found in beta-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. (C) 1998 Elsevier Science B.V. All rights reserved.