Energetics of Bacterial Photosynthesis

Abstract

We report the results of extensive numerical simulations and theoretical calculations of electronic transitions in the reaction center of Rhodobacter sphaeroides photosynthetic bacterium. The energetics and kinetics of five electronic transitions related to the kinetic scheme of primary charge separation have been analyzed and compared to experimental observations. Nonergodic formulation of the reaction kinetics is required for the calculation of the rates due to a severe breakdown of the system ergodicity on the time scale of primary charge separation, with the consequent inapplicability of the standard canonical prescription to calculate the activation barrier. Common to all reactions studied is a significant excess of the charge-transfer reorganization energy from the width of the energy gap fluctuations over that from the Stokes shift of the transition. This property of the hydrated proteins, breaking the linear response of the thermal bath, allows the reaction center to significantly reduce the reaction free energy of near-activationless electron hops and thus raise the overall energetic efficiency of the biological charge-transfer chain. The increase of the rate of primary charge separation with cooling is explained in terms of the temperature variation of induction solvation, which dominates the average donor-acceptor energy gap for all electronic transitions in the reaction center. It is also suggested that the experimentally observed break in the Arrhenius slope of the primary recombination rate, occurring near the temperature of the dynamical transition in proteins, can be traced back to a significant drop of the solvent reorganization energy close to that temperature.

Design and photochemical study of supramolecular donor–acceptor systems assembled via metal–ligand axial coordination

“…The photosynthetic reaction center consists of arrays of porphyrinoids and carotenoids in a self-assembled protein scaffolds to harvest light efficiently. Better efficiencies arise because of the precise nano-architecture which facilitate the intermolecular interactions between the entities [49][50][51][52][53][54][55].…”

Protein electron transfer: Dynamics and statistics

“…11 Similarly, λ St of primary charge separation in bacterial photosynthesis, calculated from long simulations trajectories (i.e., referring to k → 0), is about 0.76 eV, and is split into 0.41 eV from protein and 0.35 eV from water. 98 The reorganization energy referring to the fast reaction rate of photosynthetic charge separation, τ obs 0.3 ps −1 , is reduced to 0.36 eV according to the nonergodic cutoff described by Eqs. (14) and (17).…”

Reorganization Energy for Internal Electron Transfer in Multicopper Oxidases

“…It has been used in several studies. 16,17,18,19,20,21,22,23,24,25 Several other groups have developed extensions to this theory, as well as other methods to calculate the RE at various levels of theory, including MM, QM/MM, and QM. 20,21,23,26 In particular, it has been shown that the RE in a protein can be estimated from QM or QM/MM calculations on snapshots from classical MD simulations.…”