Mössbauer Spectroscopy

Summary

⁵⁷Fe-Mössbauer spectroscopy (MS) has played an important role in the elucidation of the iron centers in the photosynthetic apparatus. In 1975, G. Feher and collaborators demonstrated that the single iron of bacterial reaction centers (RC) was high-spin ferrous irrespective of the state of Q_A. Moreover, they showed that reduction of Q_A broadened the Mössbauer lines at 4.2K, indicative of spin coupling between the semiquinone and the iron, related to the broadening observed in the EPR signal of the semiquinone (Debrunner et al., 1975). Photosystem I (PSI) of green plants and algae contains three iron-sulfur centers labeled F_A, F_B and F_X that have originally been identified by EPR. Evans et al. (1977, 1979, 1981) showed that the Mössbauer spectra of PSI were practically identical with those of the well understood bacterial 4Fe-4S centers. The low-potential center F_X remained controversial, however, as others suggested a 2Fe-2S center instead. The controversy was resolved by Petrouleas et al. (1989), who studied a mutant lacking centers F_A and F_B and found that F_X had indeed all the properties of a 4Fe-4S center.

The more difficult task of analyzing Photosystem II(PSII) of green plants was undertaken by Petrouleas and Diner (1982, 1986, 1990), who identified the redox center known as Q₄₀₀ with the iron-quinone complex and showed the iron to be redox active in contrast to the Fe(II) of the bacterial RC. The same group demonstrated, by MS, that formate affected the iron-quinone complex (Diner and Petrouleas, 1987), and finally that the Fe(II) formed an NO derivative. After a review of the methodology, the various iron sites of bacterial RC, of PSI and PSII will be discussed in detail to illustrate the application of the method.