Energetics Of Electron Transfer Reactions Research Articles

Kinetics and Energetics of Electron Transfer Reactions in a Photosynthetic Bacterial Reaction Center Assembled with Zinc Bacteriochlorophylls

Electron transfer processes were studied in the reaction center (RC) of a Rhodobacter sphaeroides magnesium chelatase (bchD) mutant that assembles with six chemically identical chlorin molecules. A previous study [Jaschke & Beatty, Biochemistry, 2007] and this work show the complete absence of bacteriochlorophyll (containing Mg as the metal) and bacteriopheophytin from the bchD mutant RC. Instead, bacteriochlorophylls containing a Zn atom as the metal (Zn-BChl) occupy the binding sites of the special pair (P), accessory bacteriochlorophyll (B), and primary electron acceptor (H). In spite of significant differences in cofactor composition, electron transfer from excited P through B to H proceeds with high efficiency and with rates nearly identical to the wild type RC. The rate of electron transfer from H to QA is also the same as that observed in the wild type RC. Thus, the protein-cofactor interactions, mainly through electron sharing between the metal of the BChl and the protein, play an important role in adjusting the energies of the cofactors to form an efficient electron transfer system. The study also suggests that the overall electron transfer from P to H is more sensitive to the energy change between P and B than B and H, and can tolerate a large variation in the redox energy of H.

Non-Gaussian statistics of binding/unbinding events and the energetics of electron transfer reactions

We describe a model of electron transfer reactions affected by local binding to the donor or acceptor sites of a particle in equilibrium with the solution. The statistics of fluctuations of the donor–acceptor energy gap caused by binding/unbinding events are non-Gaussian, and the resulting free energy surfaces of electron transfer are non-parabolic. The band-width of the charge-transfer optical transition is predicted to pass through a maximum as a function of the concentration of binding particles in the solution. The breakdown of the link between the Stokes shift and the reorganization energy of electron transfer is also predicted by the model.

Energetics of Electron‐Transfer Reactions in Soft Condensed Media

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Energetics of Electron-Transfer Reactions in Soft Condensed Media

The coupling of electronic transitions within molecules to condensed-phase media involves a complex hierarchy of spatial and dynamical scales. Thermodynamics of activation is related to the length scale of microscopic interactions reflected in the non-Arrhenius reaction kinetics. Solvent dynamics make a particularly strong impact on the activation barrier when the time scale of the reaction is comparable to the relaxation time of the solvent, and the reaction barrier becomes nonergodic. Finally, molecular polarizability is responsible for complex nonparabolic free energy surfaces for electron transfer. We discuss the application of these ideas to soft condensed solvents such as supercooled liquids, liquid crystals, and photosynthetic reaction centers.

Energetics of electron transfer reactions in polar solvents

The free energy change of an electron transfer reaction in a polar solvent is rigorously analyzed within the framework of the dielectric continuum model. An appropriate expression for the electrostatic energy between the two product ions separated by R is derived. The present result does not support a recent claim by Suppan that, if R is close to the contact distance, the electrostatic energy should be much larger in magnitude than estimated from the usual expression −e 2 ϵ s R .

Effect of ion—electrode contact on the energetics of the heterogeneous electron transfer

Using molecular dynamics computer simulation technique we studied the effect of the ion—electrode contact on the energetics of electron transfer reactions. We observed that when the solvation shell of the ion undergoes large changes due to the ion transfer from the bulk to the interface, the energetics of the reaction at the interface differs substantially from the energetics of the reaction in bulk.

Electrostatic influence on energetics of electron transfer reactions.

Electron transfer chains in biological systems must operate efficiently to satisfy metabolic energetic requirements. The component proteins in these chains are expected to exhibit characteristic structural features that facilitate electron transfer to the appropriate donor and acceptor proteins. A survey of soluble one-electron carrier proteins indicates a significant tendency for lower potential proteins to be more negatively charged than higher potential proteins. Consideration of the electrostatic consequences of this pattern of charge asymmetry suggests that the reduction potential difference between the two proteins will be minimized in the precursor complex associated with electron transfer. An equivalent statement is that the change in free energy accompanying electron transfer in the complex will approach zero. This behavior is consistent with theoretical arguments advanced by Albery and Knowles [Albery, W. J. & Knowles, J. R. (1976) Biochemistry 15, 5631-5640], which suggest that for the most efficient enzymes, the free energy difference between enzyme-bound species should approach zero. A more general derivation of this prediction is provided. The observed charge asymmetry in electron transfer proteins provides a structural mechanism for satisfying this requirement, thus accelerating the overall rate of electron transfer.

Possible role of electron donor–acceptor complexes in chemical reactions: kinetic and spectroscopic studies

Kinetics and energetics of electron-transfer reactions of a few electron acceptors with NNN′N′-tetramethyl-p-phenylenediamine and other donors are reported. The kinetics are markedly affected by the solvent polarity and the acceptor strength. Solvent effects on the kinetics of the reaction between NN-dimethylaniline and tetracyanoethylene have been investigated in detail; the rate of formation of the tetracyanoethane derivative (which is the reaction intermediate), as well as of its subsequent reaction to yield NN-dimethyl-4-tricyanovinylaniline, are found to be equally solvent-sensitive. Reactions of a few substituted anilines with chloranil have been studied and the rate data are markedly dependent on the donor strength. Solvent effects on the reaction of aniline with chloranil are found to be appreciable.