Highly Efficient and Selective Syntheses of Phycobilin Derivatives toward Analysis of structure and Function of Phytochrome

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Phytochrome, one of the major photosensory chromo proteins in plants, mediates a variety of light responsive developmental processes in a red/far-red photo reversible manner. The chromophore named phytochromobilin (PФB) is a linear tetrapyrrole similar in structure to phycocyanobilin (PCB), which is a chromophore of the light-harvesting pigment phycocyanin and used as a substitute for PФB to reconstitute with apoproteins. In order to analyze the structural requirements of the chromophore in phytochrome, we studied on the syntheses of phycobilin derivatives and succeeded to synthesize PФB, PCB, and PCB derivatives modified at the A-, B-, C-, and D-rings in free acid forms.Total syntheses of PФB and PCB were achieved by employing our original synthetic reactions, I.e., 1) rearrangement of the tosyl group of 2-tosylpyrroles to the 5-position under acidic conditions, 2) efficient transformation of 2-bromo-5-tosylpyrroles to the corresponding 5-tosylpyrrolinones under acidic conditions, 3) … More Wittig-type coupling reaction between 5-tosylpyrrolinones and 2-formylpyrrole, 4) palladium catalyzed deprotection of allyl esters of propanoic acid side-chains.To analyze the structural requirements of the chromophore for the spectral properties of phytochrome B (PhyB), we have designed and chemically synthesized 20 analogs of PCB and reconstituted them with PhyB apoprotein (PHYB). It was found that the A-ring -acts mainly as the anchor for legation to PHYB, because the modification of the side chains at the C2 and C3 positions did not significantly influence the formation or difference spectra of adducts. In contrast, the side chains of the B- and C-rings are crucial to position the chromophore properly in the chromophore pocket of PHYB and for photo reversible spectral changes. The side-chain structure of the D-ring is required for the photo reversible spectral change of the adducts. From these studies, we concluded that each pyrrole ring of the linear tetrapyrrole chromophore plays a different role in chromophore assembly and the photo chromic properties of PhyB. Less