Computational study on spectral properties of the selected pigments from various photosystems: structure-transition energy relationship.

In this study, the most important kinds of pigments (chlorophylls, bacteriochlorophylls, 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 the B3PW91 functional and compared to measured spectra. The group of carotenoids was also investigated at the TDA/TDDFT level with the BLYP functional. The energies of Qy 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 through comparison with experimental results. Much better agreement was obtained for the Soret band, for which the differences between calculated and measured transitions were at most 35 nm. In the case of phycobilins, the first transition line was estimated to be at lower frequencies (around 500 nm) with a very similar blue shift of about 100 nm from experimental values. The influence of anchoring cysteine side chain(s) was found to be marginal. A dominant effect of the linear polyene chain on the determined spectral lines was found in the case of carotenoids. Nevertheless, the impact of beta-cycles and epoxy and keto groups is clearly visible as well. The high intensity of the first allowed transition matches different characters of the HOMO and LUMO. In the case of fucoxanthin, the TDA method also predicts the Bu- state to lie below the 1Bu+ state. Because the shift of electron transitions is approximately proportional to the size of the pi-conjugated system, the shift of the calculated transitions compared to experimental values is practically constant for the same excitations of (bacterio)chlorophyll and phycobilin molecules. However, this is not true for carotenoids, for which both the transition energy and the shift of the transition vary with the number of conjugated double bonds.