Mössbauer study of iron centers in D1/D2/Cyt b₅₅₉ complexes isolated from photosystem II of spinach

Mössbauer spectroscopy was applied to study the properties of cytochrome b₅₅₉ (Cyt b₅₅₉) in isolated D1/D2/Cyt b₅₅₉ preparations (from spinach) that are completely deprived of non-heme iron. In these samples, all Cyt b₅₅₉ exists as low-potential form(s) with the iron center attaining the low-spin ferric state in the absence of a strong reductant. The Mössbauer spectra were analyzed using isomer shift and quadrupole splitting parameters below 100 K, gathered from an extrapolation of the temperature dependence of experimental data of photosystem II membrane fragments from spinach. The calculations, based on the Griffith model, lead to the conclusion that the crystal field around the heme iron of Cyt b₅₅₉ is characterized by a strong rhombic distortion. The g-values obtained are in agreement with recently published EPR results. The use of an extended theoretical approach permits the description of the relaxation changes of the Mössbauer spectra in the temperature region from 5 K to 60 K. It shows that the environment of the heme iron in D1/D2/Cyt b₅₅₉ is not homogeneous but most likely reflects the existence of two different forms. We assume that factors other than changes of the first coordination sphere are responsible for the drastic negative shift in the redox potential of Cyt b₅₅₉ that takes place during the isolation procedure of D1/D2/Cyt b₅₅₉ complexes. Possible implications of these findings are discussed.