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Quaternary Ammonium Oxidative Demethylation: X-ray Crystallographic, Resonance Raman, and UV–Visible Spectroscopic Analysis of a Rieske-Type Demethylase

Daughtry, Kelly D., Xiao, Youli, Stoner-Ma, Deborah, Cho, Eunsun, Orville, Allen M., Liu, Pinghua, Allen, Karen N.
Journal of the American Chemical Society 2012 v.134 no.5 pp. 2823-2834
Raman spectroscopy, X-radiation, X-ray diffraction, absorption, active sites, crystal structure, data collection, electrons, energy, ligands, oxidation, photons, proline, spectral analysis, ultraviolet-visible spectroscopy, wavelengths
Herein, the structure resulting from in situ turnover in a chemically challenging quaternary ammonium oxidative demethylation reaction was captured via crystallographic analysis and analyzed via single-crystal spectroscopy. Crystal structures were determined for the Rieske-type monooxygenase, stachydrine demethylase, in the unliganded state (at 1.6 Å resolution) and in the product complex (at 2.2 Å resolution). The ligand complex was obtained from enzyme aerobically cocrystallized with the substrate stachydrine (N,N-dimethylproline). The ligand electron density in the complex was interpreted as proline, generated within the active site at 100 K by the absorption of X-ray photon energy and two consecutive demethylation cycles. The oxidation state of the Rieske iron–sulfur cluster was characterized by UV–visible spectroscopy throughout X-ray data collection in conjunction with resonance Raman spectra collected before and after diffraction data. Shifts in the absorption band wavelength and intensity as a function of absorbed X-ray dose demonstrated that the Rieske center was reduced by solvated electrons generated by X-ray photons; the kinetics of the reduction process differed dramatically for the liganded complex compared to unliganded demethylase, which may correspond to the observed turnover in the crystal.