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The Central Photochemical Event: Light-Driven Electron Flow: - Kinetic and Thermodynamic Factors Prevent the Dissipation of Energy by Internal Conversion

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  P723-733

2026-06-22

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The Central Photochemical Event: Light-Driven Electron Flow: - Kinetic and Thermodynamic Factors Prevent the Dissipation of Energy by Internal Conversion

The complex construction of reaction centers is the product of evolutionary selection for efficiency in the photosynthetic process. The excited state (Chl)₂ * could in principle decay to its ground state by internal con version, a very rapid process (10 picoseconds; 1 ps=10^-12 s) in which the energy of the absorbed photon is converted to heat (molecular motion). Reaction centers are constructed to prevent the inefficiency that would result from internal conversion. The proteins of the re action center hold the bacteriochlorophylls, bacterio pheophytins, and quinones in a fixed orientation relative to each other, allowing the photochemical reactions to take place in a virtually solid state. This accounts for the high efficiency and rapidity of the reactions; nothing is left to chance collision or random diffusion. Exciton transfer from antenna chlorophyll to the special pair of the reaction center is accomplished in less than 100 ps with >90% efficiency. Within 3 ps of the excitation of P870, pheophytin has received an electron and become a negatively charged radical; less than 200 ps later, the electron has reached the quinone QB (Fig. 19–48b). The electron-transfer reactions not only are fast but are thermodynamically “downhill”; the excited special pair (Chl)2 * is a very good electron donor (E⁰ -1 V), and each successive electron transfer is to an acceptor of substantially less negative E⁰. The standard free-energy change for the process is therefore negative and large; recall from Chapter 13 that ΔG⁰=-n ΔE⁰; here, ΔE⁰ is the difference between the standard reduction potentials of the two half-reactions

The combination of fast kinetics and favorable thermo dynamics makes the process virtually irreversible and highly efficient. The overall energy yield (the percentage of the photon’s energy conserved in QH₂) is>30%, with the remainder of the energy dissipated as heat.