Protein and Substrate Coordination to the Manganese Cluster in the Photosynthetic Water Oxidizing Complex: 15N and 1H ENDOR Spectroscopy of the S2 State Multiline Signal in the Thermophilic Cyanobacterium Synechococcus elongatus

X. S. Tang, M. Sivaraja, G Charles Dismukes

Research output: Contribution to journalArticlepeer-review

60 Citations (Scopus)


The hyperfine constants have been measured by ENDOR spectroscopy for 1H and 15N nuclei located within magnetic contact to the tetranuclear manganese cluster of the photosynthetic water oxidizing site in the thermophilic cyanobacterium Synechococcus el. The Mn cluster was examined in the S2 oxidation state using the “multiline” EPR signal. The data were compared to model dimanganese(III,IV) complexes possessing both N and O ligand atoms and µ-oxo and µ-carboxylato bridging ligands. This revealed that the photosynthetic Mn cluster is coordinated predominantly by nonmagnetic O atoms having no covalently bound protons at α or β positions. This is indirect evidence for protein-derived carboxylato type ligands. Two 15N hyperfine constants were resolved at 0.7 and 3.7 MHz. These values are comparable to the range predicted for coordination to π type sites on MnIII or MnIV and exclude σ type coordination sites on MnIII which yield much larger hyperfine constants. Either a single class of protein N (imidazole) ligands with coupling to both N1 and N3 atoms or possibly two coordination sites could be involved. An unexpectedly simple 1H ENDOR spectrum was observed with two well resolved hyperfine splittings of 2.4 and 1.0 MHz and two poorly resolved or weak splittings of 4.9 and 0.5 MHz. All of these were removed by incubation in 2H2O. Positive assignment to the Mn cluster was established by ENDOR-induced EPR. The 1H ENDOR results differ greatly from those reported for the Mn cluster in spinach (Kawamori, Inui, Ono, Inoue FEBS Lett. 1989, 254, 219–224). The three largest splittings could be accounted for by a simple model involving a single rhombic 1H tensor, with four possible sign choices for the principal values. One of these choices, Ax,y,z(dipolar) = −4.5, 2.9, 1.5 and A(isotropic) = −0.5 (MHz), coincided with the predicted dipolar and isotropic hyperfine terms obtained from a spin-coupled point-pair model used to describe the ligand dipolar hyperfine interaction with a pair of spin-coupled paramagnetic ions. The experimental ENDOR line shape could be approximately simulated by location of a proton nearly equidistant between the Mn ions along the normal to the Mn-Mn vector (R). The closest approach is predicted for the case of antiferromagnetically coupled MnIII and MnIV ions, for which the proton would be located at R = 3.65 Å. At the present level of sensitivity, the hyperfine data suggest there may be no un-ionized water or hydroxo ligands directly bound to the Mn cluster but instead a “dry” environment with rather long (weak) H-bonds from solvent exchangeable protons to µ-oxo bridges or possibly to terminally coordinated ligands.

Original languageEnglish
Pages (from-to)2382-2389
Number of pages8
JournalJournal of the American Chemical Society
Issue number6
Publication statusPublished - Mar 1 1993


  • N-label
  • deuterium substitution
  • manganese
  • photosynthesis
  • S2-state multiline signal
  • water coordination
  • water oxidation

ASJC Scopus subject areas

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

Fingerprint Dive into the research topics of 'Protein and Substrate Coordination to the Manganese Cluster in the Photosynthetic Water Oxidizing Complex: <sup>15</sup>N and <sup>1</sup>H ENDOR Spectroscopy of the S<sub>2</sub> State Multiline Signal in the Thermophilic Cyanobacterium Synechococcus elongatus'. Together they form a unique fingerprint.

Cite this