Domain sizes in chloroplasts and chlorophyll-protein complexes probed by fluorescence yield quenching induced by singlet-triplet exciton annihilation

Utilizing about 1 μs duration laser pulse excitation at 650 nm, it is demonstrated that the phenomenon of singlet-triplet exciton annihilation can provide information on the domain size of photosynthetic systems (number of chlorophyll molecules connected to each other by energy transfer). The domain sizes are estimated from the shapes of the fluorescence yield vs. excitation-energy curves, from the number of photon hits per molecule per pulse, and from the triplet quantum yield utilizing the master equation theory developed by Paillotin, Geacintov and Breton (Paillotin, G. et al. (1983) Biophys J. 44, 65-77). The photosynthetic systems investigated in this manner include light-harvesting chlorophyll-protein complexes (LHCP) isolated by sodium dodecyl sulfate solubilization of spinach chloroplasts followed by polyacrylamide gel electrophoresis, aggregates of light-harvesting complexes (LHC), and Photosystem I particles (both prepared by Triton X solubilization and sucrose gradient centrifugation), as well as chloroplasts and free chlorophyll a in pyridine solution or polyacrylamide gels. The LHCP particles are characterized by approx. five connected chlorophyll molecules per domain and a triplet quantum yield of approx. 0.2. The LHC complexes and chloroplasts are examples of large domains (more than 240 molecules) with triplet quantum yields of approx. 0.08 and 0.10-0.15, respectively. The Photosystem I particles similarly constitute large domains, but the fluorescence quenching is relatively inefficient, because of the apparently low triplet quantum yield of approx. 0.01.