Interactions Between Chromophore and Protein in Phytochrome Identified by Novel Oxa‐, Thia‐ and Carba‐Chromophores

Six new bilin chromophores of the plant photoreceptor phytochrome have been synthesized, carrying at the photoisomerizing ring D an oxygen or a sulfur atom or a methylene group instead of the pyrrole nitrogen atom. These furanone‐, thiophenone‐ or cyclopentenone‐containing compounds bound covalently to the recombinant apophytochrome phyA of Avena sativa . The novel chromoproteins showed hypsochromically shifted absorption spectra with respect to native phytochrome and a strongly diminished photochemical activity, but a three‐ to four‐fold higher fluorescence quantum yield. These results demonstrate that, on the one hand, also ring D‐modified chromophores can be forced into a partially extended structure, required for incorporation into the apoprotein binding pocket and covalent binding. On the other hand, the modifications introduced into ring D of the chromophores strongly impede the formation of stable far red‐absorbing forms of plant photoreceptor phytochrome (P fr ‐form) of the chromoproteins, highlighting especially the role of the pyrrole nitrogen atom and hydrogen bonding for the precise interactions between that part of the chromophore and the protein for the P fr ‐formation.