Basic Aspects of Electron and Proton Transfer Reactions with Applications to Photosynthesis

SummaryStarting with a discussion of the dependence of the activation energy of electron transport on reorganization energy, conclusions that apply to charge transfer in photosynthetic membrane proteins are: (i) a relatively low reorganization energy is anticipated from the low dielectric constant of the protein and membrane interior; (ii) the rate of intraprotein electron transfer in theory depends on the detailed pathway, but empirically a first-order rate-distance dependence for electron transfer holds over twelve orders of magnitude in rates, a factor of ten decrease for each 1.7 Å increase in center-center separation. Most of the data apply to the photosynthetic reaction center. (iii) The theories of the distance dependence of intraprotein electron transfer do not provide an explanation for the asymmetry of electron transfer, i.e., the far greater rate through the ‘L’ compared to the ‘M’ branch of the photosynthetic reaction center. (iv) The existence of long distance intraprotein electron transfer implies that the redox poise between centers separated by as much as 20 Å must be carefully regulated in vivo to avoid ‘promiscuous’ transfer events. (v) Special aspects of intramembrane electron transfer include (a) consideration of a three layer membrane model that includes a 10–15 Å thick interfacial layer of intermediate (ε=10–20) dielectric constant; and (b) in the case of the reaction center, a time-dependent ps→μs) increase in effective dielectric constant that is needed to keep the reaction activationless. (vi) Long distance transmembrane H+ translocation is likely to involve extended water chains, as inferred from recent atomic structure data on the n-side peripheral domain of the reaction center and the p-side peripheral domain of cytochrome f of the integral cytochrome b 6 f complex.