STUDIES ON PLANT BILE PIGMENTS—9. PHOTOOXIDATION OF A-DIHYDROBILINDIONE AS PHYTOCHROME MODEL IN ACID MEDIUM

Abstract— The photooxidation of octaethyl-2, 3-dihydrobilindione ( 2 ) as a model for the P_r-chromophore has been studied in acidic methanol. When 2 is titrated with HCl to the cation and irradiated under N₂ with an excess of I₂, violin and rhodin type pigments are formed bearing methoxy-substituents at either C-4,5 or C-15,16, respectively. The reaction is slowed down as compared to neutral conditions, it is no longer regioselective at the C-5 methine bridge, and the formation of dimers is suppressed. From the data of this and earlier chemical model studies on the photooxidation of 2 , and the known properties of phytochrome, a reaction for the P_r→ P_fr phototransformation is suggested, in which the chromophore is attacked by the protein moiety in a sequence of oxidation and nucleophilic substitution, which may lead to a primary redox signal for the physiological response.