Resolution of the paradox of ammonia and hydroxylamine as substrate analogues for the water-oxidation reaction catalyzed by photosystem II

Journal Article · Wed Mar 02 04:00:00 EST 1988 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00213a026· OSTI ID:7148149

Beck, W F; Brudvig, G W

The O/sub 2/-evolving center of photosystem II (PSII) contains a Mn tetramer complex that serves as the catalyst for the H/sub 2/O-oxidation reaction. In a ligand-substitution reaction thought to be analogous to the substrate-binding reaction, NH/sub 3/ coordinates to the Mn complex only after formation of the S/sub 2/ state, which then exhibits an altered S/sub 2/-state multiline electron paramagnetic resonance (EPR) signal. The free-base form of hydroxylamine binds to a Cl/sup -/-binding site prior to reacting with the Mn complex; subsequently, hydroxylamine probably reduces the Mn complex from the S/sub 1/ state to the S..sqrt../sub 1/ state through an outer-sphere electron-transfer mechanism. The rate of hydroxylamine's reaction with the Mn complex is reduced about 17-fold with each added N-methyl substituent, showing that the binding site is sterically selective for small ligands. NH/sub 3/ also binds to the O/sub 2/-evolving center in the S/sub 1/ state in a manner inversely dependent on the Cl/sup -/ concentration, causing the Mn complex to exhibit a g = 4.1 EPR signal in lieu of the multiline EPR signal in the S/sub 2/ state. The results suggest that primary amines and hydroxylamines bind to the O/sub 2/-evolving center in the S/sub 1/ state at the same Cl/sup -/-binding site. Thus, two ligand-binding sites exist in the O/sub 2/-evolving center. One site is located on the Mn complex and is restricted to binding NH/sub 3/ and presumably H/sub 2/O. This site is assigned to the substrate-binding site. A second site is identified as the Cl/sup -/-binding site; primary amines and hydroxylamines compete with Cl/sup -/ for coordination to this site. In view of the finding that hydroxylamines bind to the Cl/sup -/-binding site, they conclude that hydroxylamines should not be considered as substrate analogues for the H/sub 2/O-oxidation reaction catalyzed by PSII.

• Research Organization: Yale Univ., New Haven, CT (USA)
• OSTI ID: 7148149
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 110:5; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

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ALKALI METAL COMPOUNDS
AMINES
AMMONIA
AQUEOUS SOLUTIONS
CATALYSIS
CATALYSTS
CATALYTIC EFFECTS
CHALCOGENIDES
CHEMICAL REACTION YIELD
CHEMICAL REACTIONS
CHLORIDES
CHLORINE COMPOUNDS
DATA
DECOMPOSITION
DISPERSIONS
ELECTRON SPIN RESONANCE
EXPERIMENTAL DATA
FOOD
HALIDES
HALOGEN COMPOUNDS
HYDRIDES
HYDROGEN COMPOUNDS
HYDROXYLAMINE
INFORMATION
MAGNETIC RESONANCE
MANGANESE COMPOUNDS
MANGANESE OXIDES
MEMBRANES
MIXTURES
NITROGEN COMPOUNDS
NITROGEN HYDRIDES
NUMERICAL DATA
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOLYSIS
PHOTOSYNTHETIC MEMBRANES
PHOTOSYNTHETIC REACTION CENTERS
PLANTS
RESONANCE
SODIUM CHLORIDES
SODIUM COMPOUNDS
SOLUTIONS
SPINACH
TRANSITION ELEMENT COMPOUNDS
VEGETABLES
WATER
YIELDS