Resonance Raman (RR) spectra are reported for (Et4N)2[Fe4S4(SCH 2Ph)4] in polycrystalline samples and in dimethylacetamide solution. RR spectra are also reported for high-potential iron protein (HiPIP) from Chromatium vinosum and for ferredoxin (Fd) from Clostridium pasteurianum, the latter being reconstituted with 34S and 54Fe for isotope shift measurements. The Fe-S stretching modes of polycrystalline (Et4N)2[Fe4S4(SCH 2Ph)4] have been completely assigned via the 34S spectral shifts and the descent in symmetry from Td to D2d. These assignments are supported by the normal mode calculation, using a Urey-Bradley force field, which accurately reproduces the frequencies and isotope shifts. In solution, the RR spectrum of (Et4N)2[Fe4S4(SCH 2Ph)4] shows a tetrahedral pattern. The strongest evidence against significant symmetry lowering is the lack of activation of the cluster E mode, which is associated with the D2d distortion, and is seen clearly in the crystal spectrum. The RR spectrum of oxidized Fd is very similar to that of crystalline (Et4N)2[Fe4S4(SCH 2Ph)4], and it shows clear evidence for a D2d distortion, consistent with the X-ray crystal structure of the Peptococcus aerogenes Fd. On the other hand, the RR spectrum of HiPIP is nearly the same as that of (Et4N)2[Fe4S4(SCH 2Ph)4] in solution, but some evidence for the D2d distortion is indicated by weak activation of the E cluster mode. The functional significance, if any, of the observed Fd versus HiPIP structural difference is discussed. Excitation profiles for terminal and bridging Fe-S modes of (Et4N)2-[Fe4S4(SCH 2Ph)4] show maxima at ∼450 nm, in a rising portion of the electronic absorption band and a decline at the absorption maximum, ∼420 nm. This fall-off is suggested to result from interference effects among the charge-transfer transitions contained in the absorption band.
ASJC Scopus subject areas
- Colloid and Surface Chemistry