ИСТИНА

Stress pigment-protein complex LHCSR3 expressed in high light conditions is responsible for regulation of the non-photochemical quenching (NPQ) in green algae using pH-sensitive C-terminus as switch button for NPQ process after acidification of lumen. In the recent experiment, artificial protein LHCII-SR was synthesized from LHCII body and LHCSR3 C-terminus and demonstrated pH-depencence of fluorescence decay (FD) rate. We have studied pH-induced conformational changes in this protein and their impact to structure of the exciton states and their properties. Nuclear structure of the complex was reconstructed using primary sequence of the LHCII-SR and tertiary structure of the LHCII. Constant-pH molecular dynamics was used to reveal stable conformations in the pH interval from 4.4 to 7.5 responsible for altering of the FD rate. Energy levels of the complex in stable conformations were modeled using exciton Hamiltonian including excited states pigments lying in visible spectrum. Diagonal elements of the Hamiltonian were represented by excitation energies of individual pigments, exciton couplings were evaluated from transition densities of them. These properties were calculated using QM/MM approach with time-dependent DFT for QM part, and polarizable force field AMBER02 for MM one. Adsorption, linear, and circular dichroism spectra were modeled based on eigenvectors of the Hamiltonian; homegeneous broadening is incorporated using Redfield theory, inhomogeneous one – using random shift method. These spectra were compared with experimental ones in order to check correctness of the computations. Chlorophyll-xanthophyll exciton couplings responsible for FD were compared for conformations obtained earlier demonstrating significant increase. These results can be combined in future with open system dynamics and lead to quantitative description of chrophyll-xanthophyll energy transfer. The reported study was funded by RFBR according to the research project № 18-34-00700.