Geometrical E/Z isomers of (6R)- and (6S)-neoxanthin and biological implications

9'Z-(3S,5R,6R,3'S,5'R,6'S)-Neoxanthin reisolated from spinach (Spinacea oleracea) and characterized by HPLC, VIS, MS, and 2D H-1 NMR, has been submitted to photoinduced stereomutation in the presence of iodine or diphenyl diselenide at conditions not involving isomerization of the allenic bond. The six individual geometrical isomers, all-E, 9Z, 9'Z, 13Z, 13'Z, 15Z and three minor di-Z-isomers, presumably 9,9'-di-Z,9', 13-di-Z and 9',13'-di-Z, present in the equilibrium mixture have been characterized by HPLC, VIS data, H-1 NMR and reversibility tests. Judged by the quantitative composition of the equilibrium mixture the naturally occurring 9'Z-isomer is thermodynamically less stable than the all-E-isomer. The availability of these isomers facilitates future search in natural sources. 9'Z-(6R)-Neoxanthin represented > 90% of total neoxanthin in spinach and broccoli (Brassica oleracea var. italica), consistent with previous findings of its abundance in chloroplasts. all-E-Violaxanthin, the presumed biosynthetic precursor of neoxanthin, represented > 90% of total violaxanthin in the same sources. It is postulated that a neoxanthin Delta 9'-isomerase is present. and involved in the biosynthesis of abscisic acid in higher plants. Allenic S-isomers are of interest as postulated biosynthetic precursors of R-allenes. All-E-(6S)- and 9'Z-(6S)-neoxanthin were available as semi-synthetic model compounds. The allenic (6S)-diastereomers could not be detected in spinach or broccoli. (C) 2000 Elsevier Science Ltd. All rights reserved.