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Abstract— Carotenoids extracted from the reaction center (RC), the light‐harvesting complex (LH), and the chromatophore membrane of Rhodospirillum rubrum SI were analyzed by high‐performance liquid chromatography. The chemical structures and the configurations of major components were determined by means of mass, Raman, electronic absorption and 1H‐NMR spectroscopy. The results indicated: (1) 15‐cis‐spirilloxanthin is bound to RC; (2) both all‐frans‐spirilloxanthin and aII‐(ran.s‐3,4‐dihydrospirilloxanthin are bound to LH and (3) 13‐cK‐spirilloxanthin is additionally present in the chromatophore membrane. The natural selection of the carotenoid configurations, i.e. 15‐ris by RC and aW‐trans by LH, is discussed in relation to the physiological functions and the photophysical properties of isomeric carotenoids.

All‐transβ‐carotene‐5,6‐epoxide has been found in the thylakoid membranes of spinach and of the cyanobacterium Synechococcus vulcanus Copeland. The epoxide was extracted from the thylakoid membranes with acetone, and was isolated by high‐performance liquid chromatography (HPLC). The structure of the epoxide was identified by means of mass, Raman, and electronic absorption spectroscopy. Changes in the amount of the epoxide, as a result of epoxidation and (apparent) de‐epoxidation reactions in the membranes, were traced by analysis of extracts on HPLC. In isolated thylakoid membranes, only the epoxidation reaction took place. The reaction was caused by irradiation or by the addition of ferricyanide, suggesting that electron transport reactions in the membranes are involved in the epoxidation. In intact spinach leaves, however, both epoxidation and de‐epoxidation took place; the extent of epoxidation correlated with the intensity of light incident on the leaves. The epoxidation and de‐epoxidation of all‐transβ‐carotene are contrasted with those of xanthophylls (in the violaxanthin cycle).