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Ligated chlorophyll cation radicals: their function in photosystem II of plant photosynthesis

Journal Article · · Proc. Natl. Acad. Sci. U.S.A.; (United States)
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Magnesium tetraphenylchlorin, a synthetic model for chlorophyll, exhibits significant variations in the unpaired spin densities of its cation radicals with concomitant changes in oxidation potentials as a function of solvent and axial ligand. Similar effects are observed for chlorophyll (Chl)a and its cation radicals. Oxidation potentials for Chl ..-->.. Chl/sup +/ as high as +0.9 V (against a normal hydrogen electrode) are observed in nonaqueous solvent, with linewidths of the electron spin resonance signals of monomeric Chl/sup +/ ranging between 9.2 and 7.8 G in solution. These changes in electronic configuration and ease of oxidation are attributed to mixing of two nearly degenerate ground states of the radicals theoretically predicted by molecular orbital calculations. Comparison of the properties of chlorophyll in vitro with the optical, redox, and magnetic characteristics attributed to P-680, the primary donor of photosystem II which mediates oxygen evolution in plant photosynthesis, leads us to suggest that P-680 may be a ligated chlorophyll monomer whose function as a phototrap is determined by interactions with its (protein) environment.

Research Organization:
Brookhaven National Lab., Upton, NY
OSTI ID:
5368053
Journal Information:
Proc. Natl. Acad. Sci. U.S.A.; (United States), Journal Name: Proc. Natl. Acad. Sci. U.S.A.; (United States) Vol. 76:9; ISSN PNASA
Country of Publication:
United States
Language:
English

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Related Subjects

550200* -- Biochemistry
59 BASIC BIOLOGICAL SCIENCES
BIOCHEMICAL REACTION KINETICS
CARBOXYLIC ACIDS
CATIONS
CHARGED PARTICLES
CHEMICAL REACTIONS
CHEMISTRY
CHLOROPHYLL
ELECTRON SPIN RESONANCE
ELECTRONIC STRUCTURE
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
IONS
KINETICS
LIGANDS
MAGNETIC RESONANCE
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOCHEMISTRY
PHOTOSYNTHESIS
PHYTOCHROMES
PIGMENTS
PORPHYRINS
RADICALS
REACTION KINETICS
REDOX POTENTIAL
RESONANCE
STRUCTURAL CHEMICAL ANALYSIS
SYNTHESIS