In order to account for the high efficiency of photosynthesis, it has been suggested that chlorophyll has long-lived excited states, in which absorbed light energy can be stored for photochemical action. Until now, the only known state of this kind has been the metastable triplet state. The existence of this state has been experimentally verified only in chlorophyll b, where, at low temperatures, a fluorescence band presumably originating in such a state was observed at 860 my[ by Becker and Kasha (1).Livingston (2) and Linschitz (3) have observed, in chlorophyll a and chlorophyll b solutions illuminated by a strong flash of light, the appearance of very broad absorption bands which they interpreted as originating in the metastable triplet state. We now find at low temperatures (-193?C) a new sharp fluorescence band at 720 m,i in Chlorella and Porphyridium cruentum; a similar band appeared at 715 mt in concentrated ethanolic solutions of chlorophyll a. The occurrence of this band suggests the existence of an excited state of chlorophyll in the cell situated slightly below the 16L 14L 12p 10L WU) z z W C.
The fluorescence lifetime of chlorophyll agives information about the primary photo-physical events in photosynthesis. Most of the light energy absorbed by chlorophylls is utilized for photochemistry. There are two main additional pathways competing for the absorbed light energy: fluorescence and radiationless internal conversion (heat). Only a few percent of the absorbed energy proceeds along these two pathways. This historical minireview focuses on the first direct measurements of the lifetime of chlorophyll fluorescence, the time it takes to transfer energy from phycoerythrin to chlorophyll a, and the discovery of the fluorescence band at 720 nm (F720; then attributed to a dimer of chlorophyll). These works were carried out during the the late 1950s to the early 1960s in the laboratory of Professor Eugene Rabinowitch at the University of Illinois, Urbana-Champaign [Brody (1995) Photosynth Res 43: 67-74].
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