V. V. Eremin

The Influence of Dissipation on Vibrational Dynamics in a System of Two Interacting Electronic States

The vibrational dynamics of a nonadiabatic transition between two interacting electronic states in a molecular system in a thermal environment was considered. Two models were used. In one of these, both states, and in the other, only one state interacted with the environment. The electronic states were described by one-dimensional harmonic oscillators on the assumption that the interaction amplitude with the environment (bath) linearly depended on the coordinates of the bath and system. Model parameters typical of electron transfer in photosynthesis reaction centers were selected. The numerical solutions to the Redfield equations for the reduced density matrix were used to calculate the time characteristics of the system, including the mean vibrational energy, product population, and the degree of vibrational motion coherence. The influence of temperature and intensity of interactions with the bath on the time dependence of these values was studied. The character of vibrational dynamics had features common to both models, namely, (1) the vibrational energy monotonically decreased with the time, and this dependence was close to one-exponential in the majority of cases and (2) the time dependence of reaction yield, i.e., product population, was a step function, and the probability of the electron transition decreased as the temperature increased. It was found that there was a fundamental difference between the models under consideration: if only the reaction product interacted with the bath, vibrational coherence was retained for a long time (up to 2000 fs).

Evidence for the purely electronic character of primary electron transfer in purple bacteria Rh. Sphaeroides

A quantum-chemical calculation of the excited electronic states of a Rh. Sphaeroides reaction centre was performed. We discovered a new excited electronic state which can participate in electron transfer (ET). The energy gradient calculations showed that photoexcitation activates only high-frequency vibrational modes. This contradicts the widely accepted picture of ET resulting from vibrational wave packet motion. An alternative model is suggested where ET has a purely dissipative character and occurs only due to pigment--protein interaction. With this model, we demonstrate that oscillations in the femtosecond spectra can be caused by the new electronic state and non-Markovian character of dissipative dynamics.

A model of dissipative energy transfer in natural and artificial photosystems

Within the Redfield theory, a model is proposed that describes dissipative dynamics of electronic excitation energy migration in natural and artificial photosystems. The model explicitly takes into account physicochemical properties of the protein environment, which allows the fitting parameters to be completely excluded from calculations. The model is used to calculate the dynamics of energy transfer in a photosystem of purple bacteria and eight model photosystems, which helped to reveal the main factors that determine the rate and efficiency of energy transfer.

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.

Exciton states and optical properties of the CP26 photosynthetic protein

The photosynthetic complex CP26, one of the minor antennae of the photosystem II, plays an important role in regulation of the excitation energy transfer in the PSII. Due to instability during isolation and purification, it remained poorly studied from the viewpoint of theoretical chemistry because of the absence of X-ray crystallography data. In this work, using the recently determined three-dimensional structure of the complex we apply the quantum chemical approach to study the properties of exciton states in it. Spectral properties, structure of exciton states and roles of the pigments in the complex and photosystem II are discussed.

Quantum dynamic description of dissipative energy transfer in light-harvesting complexes 1. The interaction model between chromophores and protein within the context of Redfield theory

Within the context of Redfield theory, a model that describes the dissipative dynamics of excitation energy transfer within natural and artificial light-harvesting complexes is proposed. On the basis of the dipole-dipole approximation of interaction between the chromophores and the protein, an analytical expression for the elements of the Redfield tensor is obtained, which contains no fitting parameters. This expression takes account of the spatial arrangement of the molecules, their relative orientation, and their excitation energy; it also presents a dependence of the dissipation rate on the temperature of the protein and its physical properties.

Influence of the architecture of light-harvesting antennae on the energy transfer efficiency and rate: Probability analysis

The high efficiency of natural light-harvesting systems is based on the optimal organization of various parts of photosynthetic antennae, carotenoids and porphyrins. The rate and efficiency of energy transfer inside an antenna and between the antenna and the reaction center were studied using probability analysis. The transfer rate and efficiency were found to depend on the antenna architecture. The most efficient antennae are those in which a maximal number of photosensitive elements are in direct contact with the reaction center, whereas the interaction with neighboring elements is minimal. The following types of antennae, in order of decreasing efficiency, were studied: parallel, ring, spherical, cluster, and sequential. Explicit expressions relating the average transfer route length and the fraction of energy received by the reaction center to the number of photosensitive elements and the efficiency of the elementary transfer event were derived. The spatial arrangement of photosensitive elements and the resistance of the antenna to damage of individual elements were taken into account.

The femtosecond dynamics of electron transfer in a modified photosynthesis reaction center: Quantum, classical, and kinetic analyses

The dynamics of electron transfer in a modified photosynthesis reaction center in which electron transfer from the bridge to the acceptor is blocked is considered. A microscopic model of the process is suggested. Within this model, the diabatic electronic states of the donor and bridge are described by one-dimensional displaced harmonic oscillators. The dynamics of the population of electronic states is calculated by the quantum method of wave packets and classical and kinetic modeling. The suggested model is used to study the qualitative dependence of the dynamics of electron transfer on the nonadiabatic interaction potential. The parameters of the model are determined by comparing the experimental and calculated absorption spectra of the product of electron transfer. It is shown that kinetic models can be used to approximately describe the dynamics of electron transfer in reaction centers. The boundaries of the applicability of the kinetic method are considered.