Non-Kramers ESEEM of integer-spin diferrous carboxylate-bridged clusters in proteins

Bradley E. Sturgeon, Peter E Doan, Katherine E. Liu, Doug Burdi, Wing H. Tong, Judith M. Nocek, Nishi Gupta, Jo Anne Stubbe, Donald M. Kurtz, Stephen J. Lippard, Brian M Hoffman*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

Abstract

The diferrous oxidation level of carboxylate-bridged non-heme diiron proteins is pivotal because only it reacts directly with dioxygen. In this state each ferrous ion has S(T) = 2. Such integer-spin, so-called 'Non-Kramers' (NK) systems typically are difficult to access by paramagnetic resonance techniques, and this property has precluded the application of ENDOR and ESEEM spectroscopies to this state. Integer-spin systems frequently exhibit a ground state 'Non-Kramers doublet', however, which is split in zero applied field by an energy within the microwave range, thus rendering the center EPR active. Recently, we showed that ENDOR and ESEEM spectroscopies also can be applied to NK doublets, in preliminary measurements on diferrous methane monoxygenase hydroxylases (MMOH(red)) Methylococcus capsulatus (Bath) and Methylosinus trichosporium OB3b and azido-hemerythrin (N3-Hr(red)) (Hoffman, B.M.; Sturgeon, B.E.; Doan, P.E.; DeRose, V.J.; Liu, K.E.; Lippard, S.J., J. Am. Chem. Soc. 1994, 116, 6023-6024). Building on this study plus subsequent theoretical work (Hoffman, B.M., J. Phys. Chem. 1994, 98, 11657-11665), we now present a comparative 3-pulse ESEEM study of the diiron(II) centers of the two MMOH(red), N3Hr(red), N3Ribonucleotide reductase (N3R2(red)), N3Rubrerythrin (N3Rr(red)), and N3-'Chopped' Rubrerythrin (N3CRr(red)), which lacks the 39-residue C-terminal FeS4 domain. This report first presents the theory necessary for quantitative interpretations of the frequency-domain (ENDOR-like) spectra obtained by three-pulsed ESEEM measurements performed in the limit where the external magnetic field (B(o)) oriented parallel to the microwave field is small: B(o) ≤ 20 G. It then presents experimental NK-ESEEM results for 14N nuclei from coordinated histidine of each of the five diferrous proteins and from azide of the three azido-proteins. The measurements yield 14N quadrupole tensors to high precision as well as estimates of hyperfine couplings. In addition they provide otherwise unavailable information about the orientations of the tetragonal crystal fields of the individual ferrous ions. The data suggest that the histidyl ligands to the cluster play a major role in determining the electronic structure of the functionally important diferrous state of these proteins. They disclose a surprising reorientation of azide in the Rr(red) pocket upon deletion of the C-terminal domain.

Original languageEnglish (US)
Pages (from-to)375-386
Number of pages12
JournalJournal of the American Chemical Society
Volume119
Issue number2
DOIs
StatePublished - Jan 15 1997

ASJC Scopus subject areas

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

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