Studies on Plant Bile Pigments, II. Chemical and Photochemical Oxygenation of a Phytochrome PrChromophore Model Pigment to Purpurins

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Abstract
The chemical and photochemical oxygenation of 2,3-dihydrooctaethyl-1,19(21,24H)-bilindione (1), as a model for the chromophores of both phytochrome Pr and phycocyanin, was studied in neutral and alkaline solution, and in the presence of Zn2+. By comparison with octaethyl-1,19(21,24H)-bilindione (5), the influence of ring A hydrogenation on the reactivity of bilins was assessed. In the dark, 1 is attacked selectively and rapidly at C-5 yielding purpurins, while 5 reacts slowly, and is attacked predominantly at C-10. Photooxidation of 5 yields the tripyrrinic purpurin 7 [2,3,7,8,12,13-hexaethyl-1-formyl-17(15H)-tripyrrinone] only. Photoreactivity of 1 is considerably enhanced, yielding purpurins and violins rapidly. In spite of UV-visible and mass spectroscopic similarities, the purpurin 7 differs from the purpurins 6a,b [octaethyl-2,3-dihydro-4-hydroxy-1,5,19(21,24H)-bilintrione and its C-4 epimer] by the loss of ring A. The facile cleavage at the C-5 methine bridge and the spectroscopic properties of purpurins are discussed.