Electron-nuclear double resonance spectroscopy (and electron spin-echo envelope modulation spectroscopy) in bioinorganic chemistry

Abstract

This perspective discusses the ways that advanced paramagnetic resonance techniques, namely electron-nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) spectroscopies, can help us understand how metal ions function in biological systems.T he invitation to write this article explained, ''Perspective articles are intended to survey, from the author's distinctive perspective, some frontier aspects of the featured field and to highlight important recent developments and challenges.'' Within that definition, I shall discuss the ways that advanced paramagnetic resonance techniques, namely electron-nuclear double resonance (ENDOR) spectroscopy (1) and its junior partner electron spin-echo envelope modulation (ESEEM) spectroscopy (2), help us understand how metal ions function in biological systems and provide information in all of the critical categories listed in Table 1. In these early days of proteomics, where the primary sequence of all proteins in an organism are knowable in principle, and increasingly in fact, and where large-scale efforts are under way to determine the resting-state x-ray crystal structures of these proteins, this article is a representative response to the broader question: what roles do spectroscopies play? The answer, of course, is many, and important ones. BackgroundKey information about the composition, bonding, and structure of a paramagnetic metal center can be obtained by analysis of the electron-nuclear hyperfine and nuclear quadrupole couplings (3) of nuclei associated with endogenous and exogenous metal ligands, enzymebound substrates, inhibitors, and products, as well as the metal ions themselves. In principle, these couplings can be derived from splittings observable in a center's EPR spectrum. For most metallo-biomolecules, however, a variety of factors make these interactions unresolvable for most (or any) nuclei, and the chemical information is lost.ENDOR and ESEEM spectroscopies (3-7) retrieve the missing information. They provide an NMR spectrum of those nuclei that interact with the electron spin of the paramagnetic center, and such spectra display orders-ofmagnitude-better resolution than the

EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

“…As shown in Fig. 27a, [Fe 3 (CO) 12 ] in THF without photosensitized irradiation is EPR silent. In contrast, in combination with the IrPS in THF/TEA/H 2 O (8:2:1) even without irradiation (Fig.…”

“…An important example of water reduction in which EPR spectroscopy has been used in combination with Raman Spectroscopy and DFT studies has been published by Ludwig et al [103]. The catalytic system consists of the iridium complex [Ir(ppy) 2 -(bpy)]PF 6 (ppy = 2-phenylpyridine, bpy = 2,2 0 -bipyridine) used as a photosensitizer (IrPS), [Fe 3 (CO) 12 ] as the water-reduction catalyst (WRC), and triethylamine (TEA) as a sacrificial reductant. To gain insight in the catalytic cycle, the reaction was monitored by in situ EPR/Raman spectroscopy.…”

“…Inter-spin distances can be measured using double electron-electron resonance (DEER) and pulsed electron-electron double resonance (PELDOR) [11]. While such advanced hyperfine methodologies are useful and have been successfully applied in organometallic chemistry, they are more commonly encountered in the study of biological systems [12,13], with some notable examples including [Ni,Fe]hydrogenase [14], ribonucleotide reductase [15], the catalytic molybdenum-iron cofactor of the nitrogenase [16], and electron transfer processes occurring in green leaves of plants during photosynthesis [17]. For most metallo-enzymes, g-tensor and HFI information is (partially) lost due to g-strain and A-strain caused by a distribution of geometries with similar spectral features that overlap, leading to additional signal broadening.…”

Analysis of two stacked cylindrical dielectric resonators in a TE102 microwave cavity for magnetic resonance spectroscopy

“…Pulsed and continuous-wave (CW) electron resonance techniques, such as electron paramagnetic resonance (EPR) [1][2][3][4][5][6], electron-nuclear double resonance (ENDOR) [4,[7][8][9], electron-electron double resonance (ELDOR) [4,6,10,11], electron spin echo envelope modulation (ESEEM) [9,12,13], double quantum coherence (DQC) [14][15][16] and double electron-electron resonance (DEER) [12,[17][18][19][20][21], are powerful spectroscopic methods for studying the magneto-structural properties of molecules containing unpaired electrons. They are becoming the experimental methods of choice to determine spin-spin distances, geometry, structures and gyromagnetic, fine, and hyperfine tensors of paramagnetic molecules of biological and medicinal significance.…”

Probing the Hydrogen Bonding of the Ferrous–NO Heme Center of nNOS by Pulsed Electron Paramagnetic Resonance

“…Electron paramagnetic resonance (EPR) is a powerful tool for elucidating protein/enzyme structures both at the level of local environment of the enzyme active site(s) 5 and at the level of the overall protein geometry and folding (using site-directed spin labeling). 6 In addition to complementing X-ray crystallography by providing structural information in solution, EPR also can directly detect the presence of protons in the vicinity of a paramagnetic center.…”