Nuclear Forward Scattering (NFS) on Iron-Sulfur Proteins

Introduction Iron-Sulfur (Fe-S) proteins are among the most common and ancient enzymes and electron-transfer agents in nature. They play key roles in photosynthesis, respiration, and the metabolism of small molecules, such as H2, CO and N2. Fe-S proteins contain Fe-S clusters as active centers of the proteins, where chemical reactions happen. Conventional Mossbauer spectroscopy has achieved a great success to determine electronic structures of Fe-S clusters in Fe-S proteins [1]. Nuclear forward scattering (NFS) has been used to study minerals in earth’s mantle, also it has been used to study model compounds in heme proteins [2]. Compared with conventional Mossbauer spectroscopy, instead of using radioactive sources, NFS uses synchrotron radiation to excite nuclei and monitor the decay of nuclear excited state in time domain; quantum beat pattern can be observed if the nuclear states contain nuclear hyperfine structures. NFS eliminates the contribution of radioactive source to the resolution of the spectrum, combining with long nuclear decay time observation windows, NFS could yield better spectroscopic resolution than conventional Mossbauer spectroscopy, which is important in studying complicated Fe-S proteins, such as nitrogenases and hydrogenases. Scheme 1 shows the experimental setup, Figure 1 shows the typical FeS cluster structures. In this study, we used NFS for the first time on several Fe-S proteins under liquid N2 and liquid He temperatures as well as magnetic field up to 7 Tesla. Nuclear decay signal has been observed up to 500 ns. Rubredoxin (Rd) and 4Fe ferredoxin (Fd) samples were provided by Prof. Mike Adams group in University of Georgia, US; 2Fe Fd sample was provided by Prof. Jacques Meyer in CEA, France, and Fe protein sample was provide by Prof. John Peters in Montana State University, US.