Computational study on spectral properties of the selected pigments from various photosystems; structure – transition energy relationship

Zuzana Vokáčová and Jaroslav V. Burda

Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech Republic

In this study, the most important kinds of pigments (chlorophylls, bacteriochrophylls, phycobilins, and carotenoids) from various photosystems were explored. For the most stable conformations, electronic transitions were determined at the TDDFT/6-31+G(d) level with B3PW91 functional and compared with measured spectra. The group of carotenoids was also investigated at the TDA/TDDFT level with the functional BLYP. The energies of Q_y transitions are systematically blue shifted by about 50-100 nm in the case of (bacterio)chlorophyll and pheophytin molecules. Nevertheless, the correct relative order of the Q lines among various chlorophyll types was obtained in comparison with experimental results. Much better agreement was obtained for the Soret band where the differences between calculated and measured transitions are at most 35 nm. In the case of phycobillins the first transition line is estimated to be at lower frequencies (around 500 nm) with a very similar blue shift of by about 100 nm from experimental values. The influence of anchoring cysteine side chain(s) was found marginal. Dominant role of linear polyene chain on determined spectral lines in the case of carotenoids was found. Nevertheless, the impact of b-cycles, epoxy- or keto-groups is clearly visible, too. The high intensity of the first allowed transition matches different character of HOMO and LUMO. On the contrary to the Q_y line of chlorophyll molecules, in the case of carotenoids, calculated transitions are red shifted in average by about 70 nm from the measured spectra.