The Conformation of Bilin Chromophores in Biliproteins: Ramachandran-Type Calculations

Bilin Conformation, Phycocyanin, Phycoerythrin, Phytochrome Ramachandran-type calculations are performed for conformations of bilin chromophores pres­ ent in the biliproteins phycocyanin, phycoerythrin and phytochrome. The atomic coordinates are taken from x-ray data of crystalline model compounds, namely biliverdin for pyrrole rings B, C, D and substituted succinimides for the hydrogenated ring A including a thioether containing ^-sub­ stituent. Maxima and minima for steric hindrance are calculated for rotation of the thioether side chain, the rotation of pyrrole rings at single bonds (syn-anti-forms) and at double bonds (Z-E-isomers) of the methine bridges. Whereas quasi-planar structures are possible for all syn, Z-forms, only twisted structures are possible if anti, E-forms are considered. The relevance for the bilin con­ formations of native biliproteins and of the Pr ^ Pfr phototransformation is discussed.