The dynamics of the non-heme iron in bacterial reaction centers from Rhodobacter sphaeroides

We investigate the dynamical properties of the non-heme iron (NHFe) in His-tagged photosynthetic bacterial reaction centers (RCs) isolated from Rhodobacter (Rb.) sphaeroides. Mössbauer spectroscopy and nuclear inelastic scattering of synchrotron radiation (NIS) were applied to monitor the arrangement and flexibility of the NHFe binding site. In His-tagged RCs, NHFe was stabilized only in a high spin ferrous state. Its hyperfine parameters (IS = 1.06 ± 0.01 mm/s and QS = 2.12 ± 0.01 mm/s), and Debye temperature (θD0 ~ 167 K) are comparable to those detected for the high spin state of NHFe in non-His-tagged RCs. For the first time, pure vibrational modes characteristic of NHFe in a high spin ferrous state are revealed. The vibrational density of states (DOS) shows some maxima between 22 and 33 meV, 33 and 42 meV, and 53 and 60 meV and a very sharp one at 44.5 meV. In addition, we observe a large contribution of vibrational modes at low energies. This iron atom is directly connected to the protein matrix via all its ligands, and it is therefore extremely sensitive to the collective motions of the RC protein core. A comparison of the DOS spectra of His-tagged and non-His-tagged RCs from Rb. sphaeroides shows that in the latter case the spectrum was overlapped by the vibrations of the heme iron of residual cytochrome c2, and a low spin state of NHFe in addition to its high spin one. This enabled us to pin-point vibrations characteristic for the low spin state of NHFe.

► Only high spin NHFe occurs in His-tagged bacterial RCs.
► Its hyperfine parameters and flexibility are similar to NHFe in non-His-tagged RCs.
► Pure vibrational modes characteristic of NHFe are observed.
► Some vibrations sensitive to different spin states of NHFe are indicated.