ИСТИНА

Peripheral antennae LHCII, CP24, CP26 and CP29 play an important role in energy absorption and transfer in PSII light-harvesting supercomplex. Contributions of single chromophores to absorption and circular dichroism (CD) spectra reveal exciton structure and energy transfer mechanisms in the supercomplex. CD spectra are particularly useful in this case because they allow to distinguish individual excitonic states. In this work, we modeled the line-broadened spectra for the four antennae using molecular dynamics and excitonic Hamiltonian approach. The structures of LHCII and CP29 complexes were determined from X-ray data by various authors, but no structural information is available for CP24 and CP26. For the latter two, we deduced the structure using amino acid sequences encoded by lhcb5 and lhcb6 genes which were aligned with a LHCII subpart as a template. Only the 500-700 nm region (the chlorophyll Qy band) was studied, so excitation of carotenoids was neglected. Spectra were modeled using excitonic Hamiltonian approach including atom-atom Coulomb couplings. Excitation energies of chlorophylls in each binding site type were calculated using CASSCF[4,4] at cc-pVDZ level (three states averaging) with perturbation theory correction (XMCQDPT2). Pigment-pigment and pigment-protein interactions were calculated using ESP and TrESP [1] charges obtained by fitting matrix elements of electrostatic potential with the CASSCF wavefunctions. To account for spectral broadening, molecular dynamics was simulated with 2.5 ns duration giving 2500 nuclear geometries for spectra calculations. Averaging of linear spectra gave the resulting broadened spectra. The obtained spectra fit well the experimental data. This work shows that the described approach can be used for non-empirical modeling of line broadening in the absorption and CD spectra. Based on the well-established TrESP method, this approach allows non-empirical modeling with the accuracy about 3-5 nm in the Qy absorption region.