ИСТИНА

Non-photochemical quenching (NPQ) of chlorophyll fluorescence plays an important role in protection of photosynthetic apparatus against overexcitation and photodamage. While in higher plants the pH-sensitive NPQ trigger (PsbS protein) and the quenching site (LHCII protein) are spatially separated in the photosystem II, less complex organisms such as green algae have both functions combined in one stress-related protein LHCSR belonging to the same family as LHCII. The recently engineered fusion protein LHCII-SR based on LHCII structure with C-terminus replaced by pH-sensitive one of LHCSR exhibits NPQ activation upon acidification of solution, thus serving as a promising object for structure-based modeling of NPQ activation. In this work, we propose a multiscale model of this process combining molecular dynamics simulation of pH-induced conformational changes in the protein with a modeling of excitation energy transfer between the pigments in the protein for selected structures. Molecular dynamics simulation at neutral and lowered pH shows that acidification leads to aggregation of the C-terminus and the main domain of the protein. First, it was analyzed whether such protein movement can influence excitation energy transfer from the Qy state of chlorophyll to the 2Ag- state of lutein which is believed to be the rate limiting step of NPQ in the complex. Excitation energy transfer rate was calculated using Förster theory with empirical spectral densities for pigments. Exciton coupling between the states was calculated on MD structures using TrESP-charges based on RASSCF wavefunctions of the pigments [1]. The obtained results show that the C-terminus aggregation with the main domain increases exciton coupling and, therefore, excitation energy transfer rate. Second, a more precise model was built on the base of protein structures corresponding to high and low transfer rates. Redfield-type electron-nuclear dynamics was simulated for the whole complex on the base of exciton Hamiltonian built in the basis of Qy states of chlorophylls and 2Ag- states of luteins. The protein environment was represented by spectral densities calculated as an interaction between the TrESP-charges of the pigments and the normal modes of protein. Radiationless decay of 2Ag- state was introduced into the model in empirical form with a fixed rate constant.