The State of Iron in the Oxygen-Evolving Core Complex of the Cyanobacterium Phormidium laminosum: Mossbauer Spectroscopy

The state of iron in a purified oxygen-evolving core complex from Phormidium laminosum was characterized using Mössbauer spectroscopy at 77 K. Mössbauer spectra of the sample, as prepared, were fitted by two quadrupole pairs corresponding to Fe of high spin (2+) and low spin (2+). The signal of the high-spin (2+) species disappeared in the presence of the oxidant K₃Fe(CN)₆. After dialysis of the oxidized sample and addition of the reductant Na₂S₂O₄, the spectra recovered the original shape, and the signals corresponding to the high-spin (2+) and low-spin (2+) species reappeared. Furthermore, light excitation of the sample at 200 K, previously oxidized with K₃Fe(CN)₆ at 4°C, induced the accumulation of the high-spin (2+) species. Based on Mössbauer and optical spectroscopy, we ascribe the high-spin and low-spin species to the Fe of the iron-quinone complex and to both cytochrome b-559 and c-549, respectively. The Mössbauer results also indicate that in this cyanobacterium the Fe of the iron-quinone complex can undergo redox changes induced either chemically or by light. Moreover, we observe approximately one cytochrome b-559 per reaction center in this preparation.