Structure of phytochromobilin in the Pr and Pfr forms: SAC-CI theoretical study

Phytochrome, an important photoreceptor in green plants, contains a cofactor, phytochromobilin, which shows photo-isomerization to respond as a sensor of light. The SAC-CI method was applied to the absorption spectra of several structural isomers of phytochromobilin and successfully identified the structure of the key isomers, Pr and Pfr forms which have long been discussed. The Pr and Pfr forms of the phytochromobilin are identified to be ZZZasa and ZZEass isomers, respectively, in their protonated forms. The results also indicated that the structures of the lumi-R and meta-Ra forms, the intermediates during the photochemical cycle, would be ZZEasa isomers.