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

Electron spin echo envelope modulation (ESEEM) investigations were carried out on samples of the low-pH (IpH) 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 IpH 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).