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An efficient synthetic access to phytochromobilin (1), the chromophore of the plant photoreceptor phytochrome, and to its constitutional isomer “iso”-phytochromobilin (2) is reported. Both tetrapyrroles are assembled spontaneously with a recombinant apoprotein to form photochemically functional phytochromes. Interestingly, in the case of 2 the absorption maximum of the physiologically active phytochrome state selectively undergoes a hypsochromic shift (Pfr; fr=far red).
Lindner et al. (1998) studied this question.