| Abstract [eng] |
Photosystem I (PSI) is the most efficient light-to-energy conversion apparatus with quantum yield almost equal to 1. One of the prerequisites for high efficiency is very fast energy transfer between the molecules in light harvesting complex. Light-harvesting complex of PSI absorbs and emits light at the longest wavelengths compared to other pigment–protein complexes found in land plants. In plants, light harvesting antenna of PSI is composed of four species of LHCI complexes. They all have very similar structure; however, their spectral properties are different. The red-shifted peak in the fluorescence spectra of these light harvesting sub-complexes is observed at different wavelengths. The excitation dynamics in LHCI is highly affected by the charge-transfer (CT) states that occur between two or more pigments. Some sites in which CT states occur in LHCI are known, however, they do not completely explain the red-shifted peak in fluorescence spectrum. The energy of the excited states of pigments (including the CT states) are highly affected by the surrounding environment, consisting of other pigments and the protein chain. Therefore, it is necessary to account for the environment in order to model light-harvesting complexes properly. The aim of this work is to describe the electrostatic impact that the protein environment has on the excited electronic states of single pigments and intermolecular CT states, forming between two chlorophylls. The main tasks of this work were to identify CT states, forming between two chlorophyll molecules in the Lhca1–Lhca4 subcomplexes, model the electrostatic environment surrounding the chlorophylls while considering possible protein protonation patterns, and calculate the resulting energy shifts for the chlorophyll monomer Q y states and chlorophyll dimer CT states. We chose to analyze the structure of LHCI that was obtained as the 1st–4th chains of PSI supercomplex structure at 2.6 Å resolution, freely accessible on Protein Data Bank (PDB ID: 5L8R). The structure was compared to the structure of LHCI at 2.8 Å resolution (PDB ID: 4XK8) in order to later set recent results side by side to the previous studies. Since short distance between the chlorophylls is a prerequisite for a CT state to form, we calculated the distances between magnesium atoms of all chlorophylls (for all four subcomplexes) and chose 44 pairs in LHCI, that satisfied this condition (relatively short distance was chosen to be ≤ 12.5 Å ). We performed quantum chemical calculations to obtain energies of chlorophyll dimer CT states in vacuo. The values of the sum of Mulliken partial charges, static and transition dipole moments for excited states were analyzed and thus 69 CT states forming in LHCI were found. The environment (chlorophylls, carotenoids and the protein chain) was included in our calculations by obtaining atomic partial charges of both environmental blocks and dimers of interest and evaluating the electrostatic interaction between these charges. In case of the protein chains, we estimated the most probable protonation pattern for every subcomplex in neutral solution and thus found that seven amino acids are in non-standard protonation states in LHCI (ASP40, GLU98 and HIS223 in Lhca1, GLU156 in Lhca2, GLU120 and GLU161 in Lhca3 and GLU145 in Lhca4 protein chain). The energy shifts caused by the environment were calculated using the CDC method for both chlorophyll monomer Qy states and dimer CT states. There are five cases in LHCI antenna, where the energy of all monomer Qy states exceeds the energy of the CT states. These CT states are a505+a510– and a509–a515+ in Lhca1, b513–a514+in Lhca2, b314–a315+ in Lhca3 and a304+b314– in Lhca4 subcomplex. The energies of these states correspond to the spectral position of the red shifted peak in the fluorescence spectra of Lhca1/Lhca4 and Lhca2/Lhca3 dimers if the effective dielectric constant in CDC method is chosen appropriately. |
