Excitation Wavelength Dependence of Primary Charge Separation in Reaction Centers from Rhodobacter sphaeroides

Abstract

The excitation wavelength dependence of the initial electron transfer rate in both wild type and mutant reaction centers from Rhodobacter sphaeroides has been studied between 840 and 920 nm as a function of temperature (10-295 K). The dynamics of primary charge separation show no resolvable excitation wavelength dependence at room temperature over this spectral range. A small variation in rate with excitation wavelength is observed at cryogenic temperatures. The low temperature results cannot be explained in terms either of a nonequilibrium model that assumes that the primary charge separation starts from a vibrationally hot state or a model that assumes a static inhomogeneous distribution of electron transfer driving forces. Instead these results are consistent with the concept that primary charge separation kinetics are controlled by the dynamics of protein conformational diffusion.

Early Bacteriopheophytin Reduction in Charge Separation in Reaction Centers of Rhodobacter sphaeroides

“…In this model, the initial charge separation is limited by protein dynamics rather than by a static electron transfer barrier, and conformational diffusion occurs until a configuration with a low activation energy for electron transfer is achieved. Subsequent studies on the wavelength dependence (39) and temperature dependence (40) of electron transfer support this model.…”

“…At present, it is difficult to distinguish between these two situations. In view of the reported important role of conformational changes that optimize the electron transfer pathway (38)(39)(40), the fast rate could be interpreted as the intrinsic electron transfer rate in an optimized configuration, with the slower rate delayed by the typical time it takes for the protein to reach a conformation optimal for electron transfer. Heterogeneity in BRC electron transfer rates has been reported before.…”

Role of Protein Dynamics in Guiding Electron-Transfer Pathways in Reaction Centers from Rhodobacter sphaeroides

“…However, one might think that such static heterogeneity would also manifest itself in other measurements, such as excitation wavelength dependence of forward electron transfer, yet this is not observed. 31 Further, the systematic variation in the relative contribution of the two components between mutants suggests this kinetic feature is fundamental to the homogeneous function of reaction centers rather than an effect of multiple populations.…”

The origin of unidirectional charge separation in photosynthetic reaction centers: nonadiabatic quantum dynamics of exciton and charge in pigment–protein complexes

“…[5][6][7] Time-resolved spectroscopic measurements indicated that the excited electron in (P L P M )* is transferred to H L via B L along the L-branch on a time scale of a few ps. [39][40][41][42][43][44][45][46][47][48][49][50] Despite the pseudo-C 2 symmetric cofactor arrangement, the difference in the amino acid sequences between the L-and M-branches leads to the difference in the redox potentials of the pigments via electrostatic interactions and polarization. [51][52][53] A previous study using time-dependent density functional theory (TDDFT) with the quantum mechanics/molecular mechanics/polarizable continuum model (QM/MM/PCM) method indicated that the intermediate states of charge separation along the L-and M-branches, i.e., [P L P M ]c + B L c À and [P L P M ]c + B M c À , are lower and higher in energy than that of (P L P M )*, respectively.…”