Spore photoproduct lyase is a radical S-adenosyl-l-methionine (SAM) enzyme with the unusual property that addition of SAM to the [4Fe-4S]1+ enzyme absent substrate results in rapid electron transfer to SAM with accompanying homolytic S–C5′ bond cleavage. Herein, we demonstrate that this unusual reaction forms the organometallic intermediate Ω in which the unique Fe atom of the [4Fe-4S] cluster is bound to C5′ of the 5′-deoxyadenosyl radical (5′-dAdo•). During catalysis, homolytic cleavage of the Fe–C5′ bond liberates 5′-dAdo• for reaction with substrate, but here, we use Ω formation without substrate to determine the thermal stability of Ω. The reaction of Geobacillus thermodenitrificans SPL (GtSPL) with SAM forms Ω within ∼15 ms after mixing. By monitoring the decay of Ω through rapid freeze–quench trapping at progressively longer times we find an ambient temperature decay time of the Ω Fe–C5′ bond of τ ≈ 5–6 s, likely shortened by enzymatic activation as is the case with the Co–C5′ bond of B12. We have further used hand quenching at times up to 10 min, and thus with multiple SAM turnovers, to probe the fate of the 5′-dAdo• radical liberated by Ω. In the absence of substrate, Ω undergoes low-probability conversion to a stable protein radical. The WT enzyme with valine at residue 172 accumulates a Val•; mutation of Val172 to isoleucine or cysteine results in accumulation of an Ile• or Cys• radical, respectively. The structures of the radical in WT, V172I, and V172C variants have been established by detailed EPR/DFT analyses.
Reaction of octakis(dimethylamino) porphyrazine 1 with [60]fullerene gives a crystalline 2:1 charge-transfer (CT) complex as a three-dimensional network of chains; in contrast, reaction of 1 with tetracyanoquinodimethane (TCNQ) gives a crystalline 1:1 charge transfer salt as one-dimensional integrated stacks.
Nitrogen lifts iron to hexavalence The myriad ways that iron can interact with oxygen have been amply studied in biochemical and geochemical contexts. More recently, chemists have explored the extent to which nitrogen can likewise stabilize iron in high oxidation states. Martinez et al. now report that an iron center coordinated by carbene ligands can react with an organic azide to form a pentavalent bis(imido) complex with two Fe=N bonds. One-electron oxidation then accessed the Fe(VI) oxidation state. Both compounds were sufficiently stable for crystallographic characterization. Science , this issue p. 356
Abstract A series of quasi one-dimensional molecular metals based on metallomacrocycle building blocks is compared. The group of compounds Ni(L)I, where L = dian-ion of phthalocyanine (PC), tetrabenzporphyrin (TBP), or triazatetrabenzporphyrin (TATBP), are isoionic and isostructural but the nature of the charge carrier is quite different in each material. Oxidation in Ni(PC)l is exclusively from ligand π-orbitals so that conduction is associated with delocalized π-orbitals. Ni(TBP)I and Ni(TATBP)I display epr spectra that demonstrate that oxidation has occurred from both ligand ir-orbitals and Ni d-orbitals so that these compounds display a novel, doubly mixed-valence state. The compounds Ni(PC)I, Ni(TMP)I, and Ni(OMTBP)I (TMP = tetramethylporphyrin, OMTBP = octamethyltetrabenzporphyrin) form an isoionic series where Coulomb correlations are of progressively greater importance.
Mo-dependent nitrogenase is a major contributor to global biological N2 reduction, which sustains life on Earth. Its multi-metallic active-site FeMo-cofactor (Fe7MoS9C-homocitrate) contains a carbide (C4−) centered within a trigonal prismatic CFe6 core resembling the structural motif of the iron carbide, cementite. The role of the carbide in FeMo-cofactor binding and activation of substrates and inhibitors is unknown. To explore this role, the carbide has been in effect selectively enriched with 13C, which enables its detailed examination by ENDOR/ESEEM spectroscopies. 13C-carbide ENDOR of the S = 3/2 resting state (E0) is remarkable, with an extremely small isotropic hyperfine coupling constant, Ca = +0.86 MHz. Turnover under high CO partial pressure generates the S = 1/2 hi-CO state, with two CO molecules bound to FeMo-cofactor. This conversion surprisingly leaves the small magnitude of the 13C carbide isotropic hyperfine-coupling constant essentially unchanged, Ca = −1.30 MHz. This indicates that both the E0 and hi-CO states exhibit an exchange-coupling scheme with nearly cancelling contributions to Ca from three spin-up and three spin-down carbide-bound Fe ions. In contrast, the anisotropic hyperfine coupling constant undergoes a symmetry change upon conversion of E0 to hi-CO that may be associated with bonding and coordination changes at Fe ions. In combination with the negligible difference between CFe6 core structures of E0 and hi-CO, these results suggest that in CO-inhibited hi-CO the dominant role of the FeMo-cofactor carbide is to maintain the core structure, rather than to facilitate inhibitor binding through changes in Fe-carbide covalency or stretching/breaking of carbide−Fe bonds.
The conversion of porphyrazine-diamine derivatives into either seco-porphyrazines or porphyrazine palladium or platinum complexes has a profound effect on the photophysical and photochemical properties which has relevance to the design of switchable agents for photodynamic therapy and other applications.
