| Abstract [eng] |
The Photosystem II (PSII) is one of the main protein supercomplexes participating in photosynthetic processes. It constitutes the light–harvesting complex II (LHCII), which is known not only as a very efficient light collector but also as one of the main pieces in plants’ defensive apparatus. Specifically, LHCII is known to take part in the process called non-photochemical quenching (NPQ) which manifests itself by reducing a lifetime of fluorescing spectral components. It is well known that charge transfer (CT) states play big role either as intermediates or simply as markers of the quenching process. One of the marks, which certainly signals the quenching processes is a know red-wing (maximum ∼ 700-710 nm) in the fluorescence spectrum, rising alongside the main 680 nm peak. Concensus between the different mechanisms of NPQ is not seen in the literature. Some argue, that the ’red state’ is just the mark, which signals the NPQ, while the quenching states are different in nature, while others say, that the red-shoulder in the fluorescence spectrum also corresponds to some quenching intermediate. The present computational study was conducted in order to investigate the effects of the LHCII protein environment on the CT states of close–lying and strongly interacting chlorophyll (Chl) dimers. The effects of the environment were taken into account by implicit solvation of the Chl dimers in the dielectric medium via the PCM method. To a higher order, the environmental effects were considered by explicit introduction and protonation of amino acids in the vicinity of the Chl dimers. Excited state energies and multiple other physical parameters, such as static and transition from the ground state dipole moments, were probed at TDDFT/CAM-B3LYP/cc-pVDZ level of theory. It was found that implicit solvation of the Chl dimers did not affect their CT state energies greatly, nor did it considerably change the distribution of energy levels. The effect on the CT state energies of chlorophyll dimers was only observed upon explicit introduction and protonation of the amino acid structures. In particular, the inclusion and protonation of GLN131 amino acid was found to reduce the first CT state of the Chla604–Chlb606 dimer by ~ 0.9 eV. Even more, the first excited state of Chla604–Chlb606–GLN131 + structure is also the first CT state. Thus, it is highly probable that the S 1 state of the Chla604–Chlb606–GLN131+ structure could correspond to the ’red’ fluorescence, which was observed in the fluorescence experiments of LHCII oligomers and crystals. Additional data on the effects of protonation of 2 distinct amino acids explains a wide band visible in the red region of the fluorescence spectra of LHCII oligomers and crystals. Other indirect structural data also suggests that ’red’ fluorescent states have a higher probability to form close to Chla604–Chlb606 dimer as it is in the vicinity of the stromal boundary of the LHCII complex. |
