Direct Detection and Characterization of Chloride in the Active Site of the 
Low-pH Form of Sulfite Oxidase Using Electron Spin Echo Envelope Modulation 
Spectroscopy, Isotopic Labeling, and Density Functional Theory Calculations

Klein, Eric L.; Astashkin, Andrei V.; Ganyushin, Dmitry; Riplinger, Christoph; Johnson-Winters, Kayunta; Neese, Frank; Enemark, John H.

doi:10.1021/ic801787s

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Direct Detection and Characterization of Chloride in the Active Site of the Low-pH Form of Sulfite Oxidase Using Electron Spin Echo Envelope Modulation Spectroscopy, Isotopic Labeling, and Density Functional Theory Calculations

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Citation

Klein, E. L., Astashkin, A. V., Ganyushin, D., Riplinger, C., Johnson-Winters, K., Neese, F., et al. (2009). Direct Detection and Characterization of Chloride in the Active Site of the Low-pH Form of Sulfite Oxidase Using Electron Spin Echo Envelope Modulation Spectroscopy, Isotopic Labeling, and Density Functional Theory Calculations. Inorganic Chemistry, 48(11), 4743-4752. doi:10.1021/ic801787s.

Cite as: https://hdl.handle.net/21.11116/0000-0008-3311-B

Abstract

Electron spin echo envelope modulation (ESEEM) investigations were carried out on samples of the low-pH (lpH) form of vertebrate sulfite oxidase (SO) prepared with ³⁵Cl- and ³⁷Cl-enriched buffers, as well as with buffer containing the natural abundance of Cl isotopes. The isotope-related changes observed in the ESEEM spectra provide direct and unequivocal evidence that Cl⁻ is located in close proximity to the Mo(V) center of lpH SO. The measured isotropic hyperfine interaction constant of about 4 MHz (³⁵Cl) suggests that the Cl⁻ ion is either weakly coordinated to Mo(V) at its otherwise vacant axial position, trans to the oxo ligand, or is hydrogen-bonded to the equatorial exchangeable OH ligand. Scalar relativistic all-electron density functional theory (DFT) calculations of the hyperfine and nuclear quadrupole interaction parameters, along with steric and energetic arguments, strongly support the possibility that Cl⁻ is hydrogen-bonded to the equatorial OH ligand rather than being directly coordinated to the Mo(V).