Electronic structure of the non-heme iron center in photosynthetic apparatus of plants and bacteria. Computational DFT study.

Jiri Mrazek, Jaroslav V. Burda

Charles University in Prague, Department of Chemical Physics and Optics, Ke Karlovu 3, 121 16 Praha 2, Czech Republic

jirimrazek@atlas.cz

The non-heme iron center (Fe-center) is a part of electron-transfer (ET) chain located in Photosystem II (PS II) and bacterial reaction centers (BRC), which is responsible for primary charge separation in photosynthesis. The Fe-center represents a bridge between two quinone molecules: Qa and the terminal electron acceptor, Qb. However, its role in the ET process remains unclear [1]. Although the structure of Fe-center is highly conserved among photosynthetic organisms, experimental studies revealed that, in case of PS II, various ligands can bind reversibly to the Fe-center (the "native" ligand being bicarbonate).

We present a density functional theory investigation for molecular models of the Fe-center. Calculations on the small model system predict that the high-spin (quintuplet) state is the ground state in both PS II and BRC. However, much smaller energy differences of the spin states were found in case of BRC. Disctinct local minima were found for both intermadiate- (triplet) and low-spin (singlet) configurations. These minima differ only in the lengths of Fe-N and Fe-O bonds. This can explain why both low- and high-spin states were observed in PS II. From this point, more accurate x-ray structures would be required.

Spin states energies are only slightly affected when the bicarbonate ligand is removed in PS II. Unlike in the “native” Fe-center, electron affinity of the center without bicarbonate appears sufficient to accept electron from Qa- in our small model. In this way we assume the blocking of the possible electron transfer.

The extended Fe-center+Qa model shows that the reduction of Qa is accompanied by proton transfer from the H-bonded histidine residue, in accord with previous studies on simplified Zn-replaced model [3].

1. L. M. Utschig, M. C. Thurnauer, Acc. Chem. Res., 37 (2004) 439-447.

2. C. Goussias, Y. Deligiannakis, Y. Sanakis et al., BIOCHEMISTRY, 41 (2002) 15212.

3. A. Peluso, M. Di Donato, R. Improta, G. A. A. Saracino, J. theor. Biol., 207 (2000) 101.