Free radicals in biological energy conversion: EPR studies of model systems. Final report. [Mechanism of chlorophyll participation in photosynthesis]
Energy conversion in photosynthesis is known to proceed via light-induced one-electron transfer reactions involving chlorophyll and electron donors and acceptors. Although the chemical identities of all of the components have not as yet been elucidated, considerable evidence has been accumulated which points to quinones (Q) as primary electron acceptors in both green plants and bacterial photosynthesis. Furthermore, it has been established that the initial photoprocess leads to the formation of a chlorophyll cation radical (C./sup +/). The research described in this report has as its goal the elucidation of the molecular-electronic mechanism of chlorophyll participation in photosynthesis. The following reactions have been observed: (a) Photoproduction of C./sup +/ in solution in the absence of added electron acceptors. This is a low quantum yield reaction which proceeds via the lowest excited singlet state. Bacteriochlorophyll also undergoes this reaction, whereas pheophytin does not. (b) One-electron phototransfer between the chlorophyll lowest triplet state and quinones to yield a radical pair (C./sup +/ - Q./sup +/). This may either recombine or separate. The C./sup +/ formed upon separation is unstable and reacts with hydroxylic compounds to regenerate chlorophyll. The Q./sup -/ species partly reacts with oxidized solvent and partly disproportionates. Both bacteriochlorophyll and pheophytin are also able to react with quinones in this manner. The quenching of the chlorophyll lowest singlet state by quinones does not, however, lead to detectable radical formation. These reactions seem to provide acceptable models for certain aspects of photosynthetic energy conversion, and thus elucidation of their detailed mechanisms should lead to useful insights into the nature of the biological process.
- Research Organization:
- Arizona Univ., Tucson (USA). Dept. of Chemistry
- DOE Contract Number:
- E(11-1)-908
- OSTI ID:
- 7145484
- Report Number(s):
- COO-908-50
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
09 BIOMASS FUELS
PHOTOSYNTHESIS
BIOCHEMICAL REACTION KINETICS
CHLOROPHYLL
PLANTS
QUINONES
RADICALS
SOLAR ENERGY CONVERSION
AROMATICS
BIOMASS
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
CONVERSION
ENERGY CONVERSION
ENERGY SOURCES
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
KINETICS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHYTOCHROMES
PIGMENTS
PORPHYRINS
REACTION KINETICS
RENEWABLE ENERGY SOURCES
SYNTHESIS
400500* - Photochemistry
140504 - Solar Energy Conversion- Biomass Production & Conversion- (-1989)