2School of Physics, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 932-8820; E-mail: email@example.com
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Received April 13, 1998; Revision received June 30, 1998
A mathematical model of the Mössbauer spectrum (80K) of native membranes of Synechococcus elongatus was constructed on the basis of values of the quadruple splitting (Delta) and the isomeric shift (delta) of the iron-containing components of the photosynthetic apparatus obtained from the literature. Thermally induced changes in the intensity of the spectral components of membranes and isolated preparations of photosystem (PS) I were studied using this model. It was shown that exposure of membranes to 70-80°C causes a decrease in the intensity of the components related to the FX, FA, and FB centers and surface-located ferredoxins of PS I, an increase in the intensity of the doublets of oxidized iron clusters that are nonspecifically absorbed by the membranes, and formation of a new doublet. Spectral parameters of this doublet (Delta = 3.10 mm/sec and delta = 1.40 mm/sec) are typical of inorganic hydrated forms of reduced iron. Heating of PS I preparations also causes a decrease in the intensity of doublets of the FX, FA, and FB centers and an increase in the intensity of doublets of nonspecifically bound oxidized iron. However, this does not cause formation of inorganic reduced iron. Comparison between the intensities of the Mössbauer spectral components in intact and heated samples suggests that the main source of reduced iron in membranes is surface-located ferredoxins. Nonspecifically bound oxidized iron is formed at the expense of the FX, FA, and FB centers. Disappearance of spectral components associated with ferredoxins and accumulation of reduced iron in membranes occur within the temperature range critical for inhibition of electron transport through PS I to oxygen. These findings suggest that the thermally induced processes of accumulation of reduced iron and inhibition of electron transport in PS I in membranes of thermophilic cyanobacteria are interrelated and caused mainly by degradation of the Fe--S centers of ferredoxins. The possible role of reduced iron accumulation in the degradation of the photosynthetic apparatus induced by heat and other extreme physical and chemical factors is discussed.
KEY WORDS: Mössbauer spectroscopy, thermophilic cyanobacteria, photosystem I, iron-sulfur centers, thermal destruction, reduced iron