The electronic structure of Fe3+ was studied in a mutant that has been modified to bind manganese or iron at a site corresponding to the manganese-binding site of photosystem II (Kálmán et al., Biochemistry 45:13869-13874, 2006). Using electron paramagnetic resonance spectroscopy, the presence of the oxidized state of the bacteriochlorophyll dimer, P·+, was detected in the light when no metal was added. When iron was bound to the modified reaction centers in the presence of bicarbonate, the contribution of P·+ was greatly reduced and a signal characteristic of Fe3+ was evident. To characterize the electronic structure of this ferric ion, the electron paramagnetic resonance spectrum was measured at X-band at temperatures from 4 to 200 K. The major contribution to the spectrum at 4 K is from Fe3+ with a spin 3/2 in a rhombic coordination and E/D ratio of 0.1914 and geff values of 6.0, 2.9, and 2.0. As the temperature increases from 4 to 200 K, the signal shifts, with the central geff value changing from 2.9 to 2.2. This change with temperature may result from alterations in the interaction with the bicarbonate coordinated to the iron as the temperature increases.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics