High-multiplicity spin states of 2[4Fe-4Se]+ clostridial ferredoxins

Abstract

The electron paramagnetic resonance (EPR) spectra of the reduced selenium-substituted 2-[4Fe-4Se]+ ferredoxins from three bacteria of the Clostridium genus display low-field signals at g = 5.17, g = 10.11, and g = 12.76. The positions, shapes, and temperature dependencies of these signals have allowed their assignments to the three excited states of an S = 7/2 spin multiplet, the fundamental state of which is observed as unusual features in low-temperature (T less than or equal to 20 K) Mössbauer spectra. The S = 7/2 spin state is present in 2[4Fe-4Se]+ clostridial ferredoxins together with the classical S = 1/2 state and with a S = 3/2 state, the fundamental doublet of which is observed as a broad signal in the g = 3-4 region. The relative intensities of the EPR signals corresponding to these spin states depend on the species of Clostridium that the ferredoxin is extracted from. In contrast with clostridial ferredoxins, the reduced selenium-substituted ferredoxin from Bacillus stearothermophilus, which differs significantly from the clostridial proteins by its primary structure and by its containing only one tetranuclear cluster, displays only the S = 1/2 state. Thus, the high-multiplicity spin states arise from a specific interaction between the clostridial ferredoxin polypeptide chain and the reduced [4Fe-4Se]+ clusters.

Quantitative EPR of an S= 7/2 system in thionine‐oxidized MoFe proteins of nitrogenase

“…There is only one example of such a calculation in the biochemical literature [34] and its numerical results are not fully consistent with our results. The present account is an attempt to resolve these discrepancies.…”

“…Meyer and co-workers have applied this method in calculating the effective g values of an S = 7/2 system in a 2[4Fe-4Se] ferredoxin [34]. We have now calculated these g values with the exact-calculation method, using the value of I D 1 = 2.1 cm-' that was also determined by Meyer et al [34]. In Table 3, we comparc our results with those previously calculated by Meyer et al (given in parentheses).…”

Ligand Field Strength Mediates Electron Delocalization in Octahedral [(HL)2Fe6(L′)m]n+ Clusters

“…The spectral behavior of 7 was then studied over the temperature range 3.6–100 K. As the temperature was raised to 30 K, the broad absorption at g ≈ 1.4 and the weak one at g = 4.3 disappeared, while the rest persisted to 100 K. The absorption at g ≈ 11 suggests population of a high-spin state $false(S>+1\raisebox{1ex}{$5$}\!\left/ \!\raisebox{-1ex}{$2$}\right.false)$. 2a,22 The theoretically predicted effective g values for each Kramers doublet of high-multiplicity noninteger spin states under the assumption that the system’s real g values are 2.00 have been described and plotted against the rhombicity parameter | E / D |. 23 On the basis of the g value of ~11, two potential spin states to consider are $S=+1\raisebox{1ex}{$9$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ and $S=+1\raisebox{1ex}{$7$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$.…”

“…The other two expected effective g values would be at ~4.5 and ~1.5. It has been reported that higher doublets for $S\ge +1\raisebox{1ex}{$7$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ are not seen even at low temperatures because of their very anisotropic g tensors 22 and low transition probabilities. 2a The broad low-field ( g ≈ 4.5) and high-field ( g ≈ 1.5) absorptions seem to follow the same temperature dependence, where they become unobservable at and above 30 K. In contrast, the low-field feature at g ≈ 11 does not follow this temperature behavior and persists until 100 K. Additionally, the rhombic g tensor at g = 2 appears to be independent of the already-discussed transitions.…”

Benzoyl‐CoA Reductase (Dearomatizing), A Key Enzyme of Anaerobic Aromatic Metabolism

“…There are only a few reports on high-spin systems iron-sulfur-proteins, especially for systems with S>512: for S = 712 only two 124, 251 and for S = 912 only a few examples are known [26-281. In most cases the iron-sulfur clusters have a complex structure as, for example, the P-cluster of MoFe protein of nitrogenase [24] and the hypothetical [6Fe-6S]-cluster-containing protein [26]. In clostridial ferredoxin the high-spin state was obtained by exchanging the acid-labile sulfur for selenium [25]. The dithionite-reduced selenium-substituted ferredoxin exhibits EPR spectra very similar to those of dithionite-reduced benzoyl-CoA reductase in the presence of MgATP with a typical isotropic signal at g = 5.17 with E/D = 0.12.…”