title: 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.
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 (35)Cl- and (37)Cl-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 ((35)Cl) 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).
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona, 1306 E. University Boulevard, Tucson, Arizona 85721, USA.
authors.forename: Eric L
authors.initials: EL
authors.lastname: Klein
authors.forename: Andrei V
authors.initials: AV
authors.lastname: Astashkin
authors.forename: Dmitry
authors.initials: D
authors.lastname: Ganyushin
authors.forename: Christoph
authors.initials: C
authors.lastname: Riplinger
authors.forename: Kayunta
authors.initials: K
authors.lastname: Johnson-Winters
authors.forename: Frank
authors.initials: F
authors.lastname: Neese
authors.forename: John H
authors.initials: JH
authors.lastname: Enemark
mesh_terms: D020134:Catalytic Domain; D002712:Chlorides / Q000032:analysis*; D003198:Computer Simulation; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007553:Isotope Labeling; D008024:Ligands; D008956:Models, Chemical; D008982:Molybdenum / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry; D050876:Sulfite Oxidase / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D002712:Chlorides; D008024:Ligands; D009942:Organometallic Compounds; D008982:Molybdenum; D050876:Sulfite Oxidase