Dynamics of an [Fe{sub 4}S{sub 4}(SPh){sub 4}{sup 2-} cluster explored via IR, Raman, and nuclear resonance vibrational spectroscopy (NRVS)-analysis using {sup 36}S substitution, DFT calculations, and empirical force fields.
We have used four vibrational spectroscopies--FT-IR, FT-Raman, resonance Raman, and {sup 57}Fe nuclear resonance vibrational spectroscopy (NRVS)--to study the normal modes of the Fe-S cluster in [(n-Bu){sub 4}N]{sub 2}[Fe{sub 4}S{sub 4}(SPh){sub 4}]. This [Fe{sub 4}S{sub 4}(SR){sub 4}]{sup 2-} complex serves as a model for the clusters in 4Fe ferredoxins and high-potential iron proteins (HiPIPs). The IR spectra exhibited differences above and below the 243 K phase transition. Significant shifts with {sup 36}S substitution into the bridging S positions were also observed. The NRVS results were in good agreement with the low temperature data from the conventional spectroscopies.The NRVS spectra were interpreted by normal mode analysis using optimized Urey-Bradley force fields (UBFF) as well as from DFT theory. For the UBFF calculations, the parameters were refined by comparing calculated and observed NRVS frequencies and intensities. The frequency shifts after {sup 36}S substitution were used as an additional constraint. A D{sub 2d} symmetry Fe{sub 4}S{sub 4}S{sub 4} model could explain most of the observed frequencies, but a better match to the observed intensities was obtained when the ligand aromatic rings were included for a D{sub 2d} Fe{sub 4}S{sub 4}(SPh){sub 4} model. The best results were obtained using the low temperature structure without symmetry constraints. In addition to stretching and bending vibrations, low frequency modes between 50 and 100 cm{sup -1} were observed. These modes, which have not been seen before, are interpreted as twisting motions with opposing sides of the cube rotating in opposite directions. In contrast with a recent paper on a related Fe{sub 4}S{sub 4} cluster, we find no need to assign a large fraction of the low frequency NRVS intensity to rotational lattice modes. We also reassign the 430 cm{sup -1} band as primarily an elongation of the thiophenolate ring, with 10% terminal Fe-S stretch character. This study illustrates the benefits of combining NRVS with conventional Raman and IR analysis for characterization of Fe-S centers. DFT theory is shown to provide remarkable agreement with the experimental NRVS data. These results provide a reference point for the analysis of more complex Fe-S clusters in proteins.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); National Institutes of Health (NIH)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 914931
- Report Number(s):
- ANL/XSD/JA-58424; TRN: US200817%%18
- Journal Information:
- Dalton Trans., Journal Issue: 2006
- Country of Publication:
- United States
- Language:
- ENGLISH
Similar Records
Normal mode analysis of pyrococcus furiosus rubredoxin via nuclear resonance vibrational spectroscopy (NRVS) and resonance raman spectroscopy.
Structural Characterization of CO-Inhibited Mo-Nitrogenase by Combined Application of Nuclear Resonance Vibrational Spectroscopy, Extended X-ray Absorption Fine Structure, and Density Functional Theory: New Insights into the Effects of CO Binding and the Role of the Interstitial Atom