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  1. Free, publicly-accessible full text available May 1, 2023
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  3. Metal-metal bonding interactions can engender outstanding magnetic properties in bulk materials and molecules, and examples abound for the transition metals. Extending this paradigm to the lanthanides, herein we report mixed-valence dilanthanide complexes (Cp iPr5 ) 2 Ln 2 I 3 (Ln is Gd, Tb, or Dy; Cp i Pr5 , pentaisopropylcyclopentadienyl), which feature a singly occupied lanthanide-lanthanide σ-bonding orbital of 5 d z 2 parentage, as determined by structural, spectroscopic, and computational analyses. Valence delocalization, wherein the d electron is equally shared by the two lanthanide centers, imparts strong parallel alignment of the σ-bonding and f electrons on both lanthanides according to Hund’s rules. The combination of a well-isolated high-spin ground state and large magnetic anisotropy in (Cp iPr5 ) 2 Dy 2 I 3 gives rise to an enormous coercive magnetic field with a lower bound of 14 tesla at temperatures as high as 60 kelvin.
  4. The radicalS-adenosylmethionine (rSAM) enzyme SuiB catalyzes the formation of an unusual carbon–carbon bond between the sidechains of lysine (Lys) and tryptophan (Trp) in the biosynthesis of a ribosomal peptide natural product. Prior work on SuiB has suggested that the Lys–Trp cross-link is formed via radical electrophilic aromatic substitution (rEAS), in which an auxiliary [4Fe-4S] cluster (AuxI), bound in the SPASM domain of SuiB, carries out an essential oxidation reaction during turnover. Despite the prevalence of auxiliary clusters in over 165,000 rSAM enzymes, direct evidence for their catalytic role has not been reported. Here, we have used electron paramagnetic resonance (EPR) spectroscopy to dissect the SuiB mechanism. Our studies reveal substrate-dependent redox potential tuning of the AuxI cluster, constraining it to the oxidized [4Fe-4S]2+state, which is active in catalysis. We further report the trapping and characterization of an unprecedented cross-linked Lys–Trp radical (Lys–Trp•) in addition to the organometallic Ω intermediate, providing compelling support for the proposed rEAS mechanism. Finally, we observe oxidation of the Lys–Trp• intermediate by the redox-tuned [4Fe-4S]2+AuxI cluster by EPR spectroscopy. Our findings provide direct evidence for a role of a SPASM domain auxiliary cluster and consolidate rEAS as a mechanistic paradigm for rSAM enzyme-catalyzed carbon–carbon bond-forming reactions.

  5. Ligand-based mixed valent (MV) complexes of Al( iii ) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I 2 P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I 2 P − )(I 2 P 2− )Al and [(I 2 P 2− )(I 2 P 3− )Al] 2− charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I 2 P − )(I 2 P 2− )Al and [(I 2 P 2− )(I 2 P 3− )Al] 2− show intervalence charge transfer (IVCT) transitions in the regions 6850–7740 and 7410–9780 cm −1 , respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I 2 P 2− )(I 2 P 3− )Al] 2− complexes containing –PhNMe 2 or –PhF 5 substituents. Minor localization of charge in [(I 2 P 2− )(I 2 Pmore »3− )Al] 2− was observed when –PhOMe substituents are included.« less