β-Carotene was extracted from spinach Photosystem I reaction centers (one consisting of the Psa A, B, C, D and E subunits and the other consisting of the Psa A and B subunits alone), and the extract was subjected to high-pressure liquid chromatography using an apparatus equipped with a two-dimensional diode-array detector; all the procedures were performed at ≈ 4 °C in complete darkness. Both 15-cis and all-trans-β-carotene were identified in the extract by means of electronic absorption spectroscopy. Thus, universal presence of 15-cis carotenoid in the reaction centers of purple photosynthetic bacteria and of spinach Photosystem I and Photosystem II has been shown.

Modified ABTS cation radical decolorization assay and EPR technique were applied to screen the antioxidant activity of three flavonoids with different polarity: 7-O-β-[2-O-feruloyl-β-glucuronopyranosyl (1→2) glucuronopyranoside] (tricine), 4'-methoxy-5,7-dihydroxyflavone 6-C-β-glucopyranoside (isocytisoside) and I 3' II 8 biapigenine (amentoflavone), with nonpolar all-trans β-carotene used as standard carotenoid molecule. The ABTS [2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid] cation radical decolorization assay was modified as follows: (1) measurements extended up to 8 days after preparation, (2) method adapted for flavonoids with different polarity and β-carotene, (3) concentrations in the 0.01-10 μM range of both trolox and antioxidants in order to use the same experimental conditions for both this technique and EPR measurement.

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