Mössbauer spectroscopy on the reaction center of Rhodopseudomonas viridis

Proteins called “reaction centers” (RC) can be isolated from many photosynthetic bacteria. They have one non-heme iron in a quinone acceptor region. The RC of Rhodopseudomonas viridis contains an additional tightly bound tetra-heme cytochrome c subunit. The electronic configuration of both cytochrome and the non-heme iron has been studied in the crystallized protein by Mössbauer spectroscopy at different redox potentials, pH-values, and with an addition of o-phenanthroline. At high potentials (E_h=+500mV) all heme irons are in the low spin Fe³⁺-state, and at low potential (E_h=−150mV) they are low spin Fe²⁺ with the same Mössbauer parameters for all hemes independent of pH. Redox titrations change the relative area of the reduced and oxidized states in agreement with other methods. The non-heme iron shows a high spin Fe²⁺ configuration independent of E_h and pH with parameters comparable to those of Rhodopseudomonas sphaeroides. Surprisingly, there is strong evidence for another non-heme iron species in part of the molecules with a Fe²⁺ low spin configuration. Incubation with o-phenanthroline decreases the relative Fe²⁺ hs-area and increases the contribution of Fe²⁺ ls-area. Above 210K the mean square displacement, <x²>, of the RC-crystals increases more than linearly with temperature. This may be correlated with the increase of the electron transfer rate and indicates that intramolecular mobility influences the functional activity of a protein.