Sep 19, 2012

Alternating electron and proton transfer steps in photosynthetic water oxidation

Proceedings of the National Academy of Sciences of the United States of America
André KlaussHolger Dau

Abstract

Water oxidation by cyanobacteria, algae, and plants is pivotal in oxygenic photosynthesis, the process that powers life on Earth, and is the paradigm for engineering solar fuel-production systems. Each complete reaction cycle of photosynthetic water oxidation requires the removal of four electrons and four protons from the catalytic site, a manganese-calcium complex and its protein environment in photosystem II. In time-resolved photothermal beam deflection experiments, we monitored apparent volume changes of the photosystem II protein associated with charge creation by light-induced electron transfer (contraction) and charge-compensating proton relocation (expansion). Two previously invisible proton removal steps were detected, thereby filling two gaps in the basic reaction-cycle model of photosynthetic water oxidation. In the S(2) → S(3) transition of the classical S-state cycle, an intermediate is formed by deprotonation clearly before electron transfer to the oxidant (Y Z OX). The rate-determining elementary step (τ, approximately 30 µs at 20 °C) in the long-distance proton relocation toward the protein-water interface is characterized by a high activation energy (E(a) = 0.46 ± 0.05 eV) and strong H/D kinetic isotope effect...Continue Reading

  • References54
  • Citations58

Mentioned in this Paper

Calcium [EPC]
Calcium
Electron Transport
Contraction (Finding)
Protons
Oxygen Evolving Activity
Oxidation
Catalytic Domain
Oxidation-Reduction
Manganese Measurement

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