Model Studies of Phytochrome Photochromism: Protein-Mediated Photoisomerization of a Linear Tetrapyrrole in the Absence of Covalent Bonding

The phototrigger function of phytochrome, the major photosensory pigment in plants, can be performed by a noncovalently bound linear tetrapyrrole (see picture). This tetrapyrrole was embedded into the chromophore pocket of the phytochrome apoprotein without covalent attachment (A), rather than being bound to cysteine Cys(321) as in the native chromoprotein (B).