Dynamics of an [Fe4S4(SPh)4]2− cluster explored viaIR, Raman, and nuclear resonance vibrational spectroscopy (NRVS)-analysis using 36S substitution, DFT calculations, and empirical force fields

Abstract

We have used four vibrational spectroscopies—FT-IR, FT-Raman, resonance Raman, and ⁵⁷Fe nuclear resonance vibrational spectroscopy (NRVS)—to study the normal modes of the Fe–S cluster in [(n-Bu)₄N]₂[Fe₄S₄(SPh)₄]. This [Fe₄S₄(SR)₄]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 ³⁶S 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 ³⁶S substitution were used as an additional constraint. A D_2d symmetry Fe₄S₄S′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_2d Fe₄S₄(SPh)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⁻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 Fe₄S₄ 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⁻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.