Perturbative Expansion of Nonorthogonal Product Approach for Charge Transfer States.

Modeling of the excited states of multichromophoric systems is crucial for the understanding of photosynthesis functioning. The excitonic Hamiltonian method is widely used for such calculations. Excited states of the combined system are constructed from the wave functions of individual chromophores while their interactions are described by excitonic couplings. In the current study, we enhance a previously proposed nonorthogonal product approach to incorporate dynamic correlation effects accounted for by the multireference perturbation theory. We discuss the problems of constructing the excitonic Hamiltonian including charge transfer states for the molecular systems where the overlap contribution to the excitonic couplings is non-negligible. The benchmark calculations were performed for a model system. It was shown that the overlap component of the excitonic coupling is of great importance. The enhanced method provides an accurate description of the excited state energies and other properties.

Calculation of the excited states properties of LH1 complex of Thermochromatium tepidum.

Calculation of the excited states properties of pigment complexes is one of the key problems in the photosynthesis research. The excited states of LH1 complex of Thermochromatium tepidum were studied by means of the high-precision quantum chemistry methods. The influence of different parameters of the calculation procedure was examined. The optimal scheme of calculation was chosen by comparison of calculated results with the experimental data on absorption, electronic and magnetic circular dichroism spectra. The high importance of the account of the second excited states of bacteriochlorophylls and of site heterogeneity was shown. © 2018 Wiley Periodicals, Inc.

Role of hydrogen bond alternation and charge transfer states in photoactivation of the Orange Carotenoid Protein.

Here, we propose a possible photoactivation mechanism of a 35-kDa blue light-triggered photoreceptor, the Orange Carotenoid Protein (OCP), suggesting that the reaction involves the transient formation of a protonated ketocarotenoid (oxocarbenium cation) state. Taking advantage of engineering an OCP variant carrying the Y201W mutation, which shows superior spectroscopic and structural properties, it is shown that the presence of Trp201 augments the impact of one critical H-bond between the ketocarotenoid and the protein. This confers an unprecedented homogeneity of the dark-adapted OCP state and substantially increases the yield of the excited photoproduct S*, which is important for the productive photocycle to proceed. A 1.37 Å crystal structure of OCP Y201W combined with femtosecond time-resolved absorption spectroscopy, kinetic analysis, and deconvolution of the spectral intermediates, as well as extensive quantum chemical calculations incorporating the effect of the local electric field, highlighted the role of charge-transfer states during OCP photoconversion.

Electron-Vibrational Spectra and Dynamics of the Lutein Molecule.

The carotenoid molecules such as lutein play an important role in the absorption of light and the following transfer of energy during photosynthesis. However, the study of these processes by the experimental methods only is quite difficult because some of the transitions between the electronic states of carotenoids are optically forbidden and the effect of vibrational states change also must be taken into account. In the present work, electronic-vibrational states of the lutein molecule in the LHCII complex of higher plants and in the diethyl ether solution were described using the ab initio methods. For lutein of LHCII, the electronic energy transfer processes were modeled. The role of the "hot" S1 states of lutein was shown to be of great importance.

Protein Vibration Effects on Primary Electron Transfer Dynamics in Rhodobacter sphaeroides Photosynthetic Reaction Center.

Primary electron transfer (ET) in the chromophore subsystem in a bacterial reaction center (RC) is a unique process, and is coupled with the protein motion, which, like the ET, is caused by photoexcitation of these chromophores. ET is also coupled with dissipative processes, which are caused by interaction between chromophores and vibrations of its surrounding protein. We propose a new dynamics calculation method that accounts for both these effects of protein vibrations. Within this method, the photoinduced protein motion causes an addition of coherent component to the ET rate. We performed dynamics calculation using this method and parameters, which were determined from the ab initio wave functions of the chromophore subsystem and protein normal vibrational modes. We showed that it is this protein motion that causes oscillations in the time-dependencies of stimulated emission intensities and of absorption at 1020 nm. Moreover, the latter oscillations are related to the coherent component of the ET rate.