Electron paramagnetic resonance complexes, structural characterization

Electron paramagnetic resonance spectroscopy (HER), also called electron spin resonance spectroscopy (ESR), may be used for direct detection and conformational and structural characterization of paramagnetic species. Good introductions to F.PR have been provided by Fischer8 and I.effler9 and most books on radical chemistry have a section on EPR. EPR detection limits arc dependent on radical structure and the signal complexity. However, with modern instrumentation, radical concentrations > 1 O 9 M can be detected and concentrations > I0"7 M can be reliably quantified. 

DETECTION AND STRUCTURAL CHARACTERIZATION OF OXO-CHROMIUM(V)-SUGAR COMPLEXES BY ELECTRON PARAMAGNETIC RESONANCE... 

Three membrane-bound adenosine triphosphatase enzymes have been characterized using Mn(II) and Gd(III) electron paramagnetic resonance (EPR) and a variety of NMR techniques. Mn(II) EPR studies of both native and partially delipidated (Na+ + K+)-ATPase from sheep kidney indicate that the enzyme binds Mn2+ at a single, catalytic site with Kq = 0.21 x 10- M. The X-band EPR spectrum of the binary Mn(II)-ATPase complex exhibits a powder line shape consisting of a broad transition with partial resolution of the 55 n nuclear hyperfine structure, as well as a broad component to the low field side of the spectrum. ATP, ADP, AMP-PNP and Pj all broaden the spectrum, whereas AMP induces a substantial narrowing of the hyperfine lines of the spectrum. 

As yet, no X-ray crystal structures are available for any of the molybdenum enzymes in Table I. Therefore, present descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The two most powerful techniques for directly probing the molybdenum centers of enzymes are electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS), especially the extended X-ray absorption fine structure (EXAFS) from experiments at the Mo K-absorption edge. Brief summaries of techniques are presented in this section, followed by specific results for sulfite oxidase (Section III.B), xanthine oxidase (Section III.C), and model compounds (Section IV). 

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