Role of the Propionic Acid Side-Chain of C-Phycocyanin Chromophores in the Excited States for the Photosynthesis Process

This paper focuses on a theoretical investigation of the peculiar properties of the chromophore in the C-phycocyanin (C-PC), phycocyanobilin (PCB). The scope is to unravel their key features upon light absorption and transmission occurring in natural photosynthesis. To this aim, by resorting to the time-dependent density functional theory (TDDFT) and natural bond orbital (NBO) methods, we compute the photoabsorption spectra and electronic properties of PCB, showing that three different orientations of the PCB in C-PC contribute to the nonhomogeneous broadening of the entire photoabsorption spectrum of C-PC. Furthermore, the photoabsorption peaks of PCB can undergo a shift up to 40 nm because of solvation effects. Further investigations on the competitive influence of the nearby aspartate residue and two propionic acids on the absorption spectra show that the latter play a significant role in realizing the different photo-response among the three isomers of PCB. In the low-lying electronic excited states, the π conjugated C-C bonds and the twisting angle of the pyrrole rings turn out to be affected. The NBO geometrical analyses of the bond lengths, interatomic angles, and dihedral angles evidenced that the intermolecular interactions of the propionic acid side chains play a crucial role in the determination of the excited state molecular conformations. These results indicate that the absorption spectra and the excited state structures of PCB are efficiently tuned during natural photosynthesis processes.