Phytochrome, an important photoreceptor in green plants, contains a cofactor, phytochromobilin, which shows photo-isomerization to respond as a sensor of light. Phytochrome exists in one of two photo-interconvertible forms: physiologically inactive Pr and active Pfr forms which absorb light in the red (lmax = 668 nm, 1.86 eV) and in the far-red (lmax = 730 nm, 1.70 eV) regions, respectively. However, the crystal structure of the phytochrome have not been resolved. Several resonance raman studies also discussed the structure of the Pr and Pfr forms, but the decisive conclusion has not been obtained.
The SAC-CI method was applied to the absorption spectra of several structural isomers of phytochromobilin and successfully identifed the structure of the key isomers, Pr and Pfr forms which have long been discussed. The Pr and Pfr forms of the phytochromobilin are identied 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.
[1] SAC-CI Theoretical Study on the Structure of Phytochromobilin in the Pr and Pfr Forms, J. Hasegawa, M. Isshiki, K. Fujimoto, and H. Nakatsuji, Chem. Phys. Lett., 410, 90-93 (2005).