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Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts
Published on: August 7, 2018
Ca(2+) function in photosynthetic oxygen evolution studied by alkali metal cations substitution
1Laboratory for Photo-Biology, RIKEN Photodynamics Research Center, The Institute of Physical and Chemical Research, Aoba, Aramaki, Sendai 980-0845, Japan. takaaki@postman.riken.go.jp
Biophysical Journal
|September 22, 2001
Summary
Monovalent cations affect the oxygen-evolving complex in photosystem II. Larger cations like K+ inhibit oxygen evolution and alter spectroscopic signals, indicating ionic radius is crucial for manganese cluster properties.
Area of Science:
- Biochemistry
- Spectroscopy
- Photosynthesis Research
Background:
- Photosystem II (PSII) is crucial for oxygenic photosynthesis.
- The oxygen-evolving complex (OEC) within PSII is responsible for water oxidation.
- Calcium ions (Ca2+) are known cofactors for OEC activity.
Purpose of the Study:
- To investigate the impact of monovalent alkali metal cations on Ca2+-depleted PSII membranes.
- To understand how cation ionic radii influence the biochemical and spectroscopic properties of the OEC.
- To elucidate the role of the Ca2+-binding site in modulating manganese cluster function.
Main Methods:
- Preparation of Ca2+-depleted PSII membranes.
- Addition of various monovalent alkali metal cations (Li+, Na+, K+, Rb+, Cs+).
- Assays for oxygen evolution.
- Spectroscopic analysis using electron paramagnetic resonance (EPR) and thermoluminescence (TL).
Main Results:
- Oxygen evolution was inhibited by K+, Rb+, and Cs+ (larger ionic radii than Ca2+), but not by Li+ and Na+ (smaller radii).
- Ca2+-depleted membranes supplemented with Li+ or Na+ retained normal S2 EPR signals and S2QA- TL bands.
- K+, Rb+, and Cs+ induced a shift in the TL peak temperature and abolished the S2 EPR signal, suggesting Ca2+ site substitution.
- The K+-induced high-temperature TL band and the S2QA- TL band were interconvertible by dark incubation with K+ or Ca2+.
- Both Ca2+-depleted and K+-substituted membranes showed an S2YZ+ EPR signal under multiple turnover conditions.
Conclusions:
- The ionic radius of cations occupying the Ca2+-binding site critically influences the properties of the manganese cluster in PSII.
- Monovalent cations with ionic radii larger than Ca2+ can competitively bind and disrupt OEC function.
- Spectroscopic techniques like EPR and TL are valuable for probing cation-dependent mechanisms in PSII.
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