Fluorescence and photochemical quenching in photosynthetic reaction centers.

Photosynthetic bacteria contain bacteriochlorophyll (BChl) that absorbs light and delivers the resulting singlet excitation energy to photochemical reaction centers.' The reaction centers, which have been isolated in a form free of lightharvesting BChl,'-3 contain a photochemically specialized BChl which in Rhodopseudomonas spheroides is called P870 after its absorption maximum near 870 nm. Excitation energy reaching the reaction centers causes oxidation (bleaching) of P870 and, presumably, reduction of an electron acceptor. Another specialized BChl, P800, is associated with P870 in the reaction centers and shows a band shift to shorter wavelengths when P870 is bleached. The oxidation of P870 occurs with a quantum efficiency close to 100 per cent whether the light is absorbed by light-harvesting BChl in chromatophores4 5 or by either P800 or P870 in reaction center preparations.6 Cells or chromatophores of photosynthetic bacteria emit BChl fluorescence when illuminated. This fluorescence comes from the light-harvesting BChl and competes with the utilization of energy at the reaction centers." 7 The yield of fluorescence from P870, measured in the absence of light-harvesting BChl, is small compared with that from light-harvesting BChl measured in chromatophores.8 The low fluorescence yield of P870 shows that energy reaching this pigment is quenched rapidly, and the high quantum efficiency for P870 oxidation shows that most of this quenching is due to the initiation of photochemistry. The lifetime of an excited state of a molecule is indicated by the yield of fluorescence corresponding to that state. If the only avenue for de-excitation were the radiative one, the yield would be 100 per cent and the lifetime would assume a maximum value called the intrinsic lifetime, To. If de-excitation can occur by other competing processes such as a photochemical quenching, both the lifetime r and the fluorescence quantum yield of will be reduced in proportion: