Reconsideration of X, the diiron intermediate formed during cofactor assembly in E. coli ribonucleotide reductase

Bradley E. Sturgeon, Doug Burdi, Shuxian Chen, Boi Hanh Huynh*, Dale E. Edmondson, Jo Anne Stubbe, Brian M Hoffman

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

218 Scopus citations


The R2 subunit of Escherichia coli ribonucleotide reductase (RNR) contains a stable tyrosyl radical (.Y122) diferric cluster cofactor. Earlier studies on the cofactor assembly reaction detected a paramagnetic intermediate, X, that was found to be kinetically competent to oxidize Y122. Studies using rapid freeze-quench (RFQ) Mossbauer and EPR spectroscopies led to the proposal that X is comprised of two high spin ferric ions and a S = 1/4 ligand radical, mutually spin coupled to give a S = 1/4 ground state (Ravi, N.; Bollinger, J.M., Jr.; Huyhn, B.H.; Edmondson, D.E.; Stubbe, J., J. Am. Chem. Soc. 1994, 116, 8007-8014). An extension of RFQ methodology to Q-band ENDOR spectroscopy using 57Fe has shown that one of the irons has a very nearly isotropic hyperfine tensor (A(Fe(A)) = -[74.2(2), 72.2(2), 73.2(2)] MHz) as expected for Fe(III), but that the other iron site displays considerable anisotropy (A(Fe(B)) = +[27.5(2), 36.8(2), 36.8(2)] MHz), indicative of substantial Fe(IV) chracter. Reanalysis of the Mossbauer data using these results leads to isomer shifts of δ(Fe(A)) = 0.56(3) mm/s and δ(Fe(B)) = 0.26(4) mm/s. Based on the hyperfine anisotropy of Fe(B) plus the reduced isomer shift, X is now best described as a spin-coupled Fe(III)/Fe(IV) center without a radical, but with significant spin delocalization onto the oxygen ligand(s).

Original languageEnglish (US)
Pages (from-to)7551-7557
Number of pages7
JournalJournal of the American Chemical Society
Issue number32
StatePublished - Aug 14 1996

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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