Our efforts to model the oxygen activation chemistry of nonheme iron enzymes have yielded transient intermediates with novel properties. These properties can be dramatically affected by the introduction of a 6-methyl substituent on the pendant pyridines of the tetradentate ligand TPA (TPA = tris(2-pyridylmethyl)amine). A series of Fe(TPA) complexes has thus been synthesized and characterized to provide the structural basis for these dramatic effects. The following complexes have been obtained:  [Fe(L)(CH3CN)2](ClO4)2 (1, L = TPA; 2, L = 6-MeTPA; 3, L = 6-Me2TPA; 4, L = 6-Me3TPA) and [Fe(L)(acac)](ClO4)2 (5, L = TPA; 6, L = 5-Me3TPA; 7, L = 6-MeTPA). As indicated by 1H NMR and/or EPR, 1, 5, and 6 with no 6-methyl substituent are low spin, while complexes 2, 3, 4, and 7 with at least one 6-methyl substituent are all high spin, with higher redox potentials than their low-spin counterparts. The ligands with 6-methyl substituents thus favor a metal center with a larger ionic radius, i.e., FeII over FeIII and high spin over low spin. Careful scrutiny of the crystal structures of 1, 4, 6, and 7 reveals that one hydrogen of the 6-methyl group is only 2.7 Å away from the metal center in the high-spin complexes. Its presence thus prevents the pyridine nitrogen from forming an Fe−N bond shorter than 2.1 Å as required for an iron center to adopt a low-spin configuration. This steric effect of the 6-methyl substituent serves as a simple but very useful ligand design tool to tune the electronic properties of the metastable alkylperoxoiron(III) species derived from the reactions of 1−4 with tert-butyl hydroperoxide. These intermediates serve as models for low-spin and high-spin peroxoiron(III) species in the reaction cycles of the antitumor drug bleomycin and lipoxygenase, respectively. Similar principles apply in the design of nonheme diiron(II) complexes that reversibly bind dioxygen and of high-valent bis(μ-oxo)diiron complexes that model the high-valent intermediates in the redox cycles of nonheme diiron enzymes such as methane monooxygenase and ribonucleotide reductase.

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SummaryTrichothecenes are isoprenoid mycotoxins produced in wheat infected with the filamentous fungus Fusarium graminearum. Some fungal genes for trichothecene biosynthesis (Tri genes) are known to be under control of transcription factors encoded by Tri6 and Tri10. Tri6 and Tri10 deletion mutants were constructed in order to discover additional genes regulated by these factors in planta. Both mutants were greatly reduced in pathogenicity and toxin production and these phenotypes were largely restored by genetic complementation with the wild-type gene. Transcript levels for over 200 genes were altered Ն twofold for Dtri6 or Dtri10 mutants including nearly all known Tri genes. Also reduced were transcript levels for enzymes in the isoprenoid biosynthetic pathway leading to farnesyl pyrophosphate, the immediate molecular precursor of trichothecenes. DNA sequences 5Ј to isoprenoid biosynthetic genes were enriched for the Tri6p DNA binding motif, YNAGGCC, in F. graminearum but not in related species that do not produce trichothecenes. To determine the effect of trichothecene metabolites on gene expression, cultures were treated with trichodiene, the first metabolic intermediate specific to the trichothecene biosynthetic pathway. A total of 153 genes were upregulated by added trichodiene and were significantly enriched for genes likely involved in cellular transport. Differentially regulated genes will be targeted for functional analysis to discover additional factors involved in toxin biosynthesis, toxin resistance and pathogenesis.

In our efforts to model high-valent intermediates in the oxygen activation cycles of nonheme diiron enzymes such as methane monooxygenase (MMOH-Q) and ribonucleotide reductase (RNR R2-X), we have synthesized and spectroscopically characterized a series of bis(µ-oxo)diiron(III,IV) complexes, [Fe 2 (µ-O) 2 -(L) 2 ](ClO 4 ) 3 , where L is tris(2-pyridylmethyl)amine (TPA) or its ring-alkylated derivatives. We now report the crystal structure of [Fe 2 (µ-O) 2 (5-Et 3 -TPA) 2 ](ClO 4 ) 3 (2), the first example of a structurally characterized reactive iron(IV)-oxo species, which provides accurate metrical parameters for the diamond core structure proposed for this series of complexes. Complex 2 has Fe-µ-O distances of 1.805(3) Å and 1.860(3) Å, an Fe-Fe distance of 2.683(1) Å, and an Fe-µ-O-Fe angle of 94.1(1)°. The EXAFS spectrum of 2 can be fit well with a combination of four shells: 1 O at 1.82 Å, 2-3 N at 2.03 Å, 1 Fe at 2.66 Å, and 7 C at 2.87 Å. The distances obtained are in very good agreement with the crystal structure data for 2, though the coordination numbers for the first coordination sphere are underestimated. The EXAFS spectra of MMOH-Q and RNR R2-X contain features that match well with those of 2 (except for the multi-carbon shell at 2.87 Å arising from pyridyl carbons which are absent in the enzymes), suggesting that an Fe 2 (µ-O) 2 core may be a good candidate for the core structures of the enzyme intermediates. The implications of these studies are discussed.