Time-Dependent DFT on Phytochrome Chromophores: AWay to the Right Conformer

ABSTRACT A theoretical approach based on time-dependent density functional theory (TDDFT) togetherwith a polarizable continuummodel for incorporating the bulk effect of the surrounding environment is used to estimate the excitation energies for the phytochromobilin chromophore of plant phytochrome. The TDDFT results reproduce well the experimental values of the absorption maxima and those of the ratio of the spectroscopic bands (Qbandover the Soret band orQ/S index). Our results suggest that the phytochromobilin within the Pr form of the phytochrome holoprotein should adopt a semicyclic ZZZssa structure as it does in bacteria. SECTION Biophysical Chemistry P hytochromes are photoreceptors in plants and are alsofound in nonoxygenic bacteria, cyanobacteria, andfungi. In plants, phytochromes regulate photomorpho-genic responses as germination, de-elotation, shade avoid-ance, phototropism, chloroplast movement, stomatal open-ing, and flowering. The photocycle of the receptor involves a Z/E photoisomerization around a double bond of its phyto