Isolation of the Phytochrome Chromophore. The Cleavage Reaction with Hydrogen Bromide

The cleavage of the bilin chromophore from C-phycocyanin with hydrogen bromide yields 3E-configurated phycocyanobilin (4) as the major and 3 Z-configurated phycocyanobilin (5) as the minor reaction product. The reaction of synthetic 3E-configurated phytochromobilin (2) with hydrogen bromide and methanol leads only to a methanol adduct at the C-18 side chain (7) whereas the same reaction with the 3Z-configurated phytochromobilin (3) leads to 7 and 2. The bilin chromophore was cleaved also from phytochrome after preparation of phytochromobilin peptides. The detection of 2 and 7 suggested that 3Z-and 3E-configurated phytochromobilin were the primary products of cleavage from phytochrome. A reaction scheme is given which can explain the results of the reaction with hydrogen bromide and methanol