Electron spin echo envelope spectroscopy

EPR = Electron paramagnetic resonance EXAFS = Extended X-ray fine stmctnre analysis ESEEM = Electron spin echo envelope spectroscopy XANES = X-ray absorption near edge stmctnre analysis NMR = Nnclear magnetic resonance. 

Advanced EPR techniques such as CW and pulsed ENDOR, electron spin-echo envelope modulation (ESEEM), and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) have been successfully used to examine complexation and electron transfer between carotenoids and the surrounding media in which the carotenoid is located. 

G.R. Eaton and S.S. Eaton, Electron-nuclear double resonance spectroscopy and electron spin echo envelope modulation spectroscopy, Comprehensive Coordination Chemistry II, Elsevier, Boston, 2004, 49. 

Y. Deligiannakis, M. Louloudi and N. Hadjiliadis, Electron spin echo envelope modulation (ESEEM) spectroscopy as a tool to investigate the coordination environment of metal centers, Coord. Chem. Rev., 2000, 204, 1. 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

Since the phenoxyls possess an S = ground state, they have been carefully studied by electron paramagnetic spectroscopy (EPR) and related techniques such as electron nuclear double resonance (ENDOR), and electron spin-echo envelope modulation (ESEEM). These powerful and very sensitive techniques are ideally suited to study the occurrence of tyrosyl radicals in a protein matrix (1, 27-30). Careful analysis of the experimental data (hyperfine coupling constants) provides experimental spin densities at a high level of precision and, in addition, the positions of these tyrosyls relative to other neighboring groups in the protein matrix. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy... 

Morrissey, S. R., Horton, T. E., Grant, C. V., Hoogstraten, C. G., Britt, R. D., and DeRose, V. J. (1999). Mn2+-nitrogen interactions in RNA probed by electron spin-echo envelope modulation spectroscopy Application to the hammerhead ribozyme. 

Valuable spectroscopic studies on the dithiolene chelated to Mo in various enzymes have been enhanced by the knowledge of the structure from X-ray diffraction. Plagued by interference of prosthetic groups—heme, flavin, iron-sulfur clusters—the majority of information has been gleaned from the DMSO reductase system. The spectroscopic tools of X-ray absorption spectroscopy (XAS), electronic ultraviolet/visible (UV/vis) spectroscopy, resonance Raman (RR), MCD, and various electron paramagnetic resonance techniques [EPR, electron spin echo envelope modulation (ESEEM), and electron nuclear double resonance (ENDOR)] have been particularly effective probes of the metal site. Of these, only MCD and RR have detected features attributable to the dithiolene unit. Selected results from a variety of studies are presented below, chosen because their focus is the Mo-dithiolene unit and organized according to method rather than to enzyme or type of active site. 

To determine whether PLP was actually associated with the lysine radical, [4 - H]PLP was synthesized and exchanged into the enzyme, and the [4 - H]PLP-enzyme was used to prepare a sample of the putative product radical 3. The EPR spectrum of the sample containing [4 - H]PLP proved to be identical with that of a matched sample containing PLP. The two samples were submitted to electron spin echo envelope modulation spectroscopy (ESEEM). The ESEEM spectra revealed a signal corresponding to the Larmor frequency for deuterium in the sample containing [4 - H]PLP (Fig. 5) and no signal in the PLP sample. This meant that the deuterium in [4 - H]PLP must be... 

Britt, R. D., Sauer, K., Klein, M. P., Knaff, D. B., Kriauciunas, A., Yu, C. A., Yu, L., and Malkin, R., 1991, Electron spin echo envelope modulation spectroscopy supports the suggested coordination of two histidine ligands to the Rieske Fe-S centers of the cytochrome b6f complex of spinach and the cytochrome bcl complexes of Rhodospirillum rubrum, Rhodobacter sphaeroides, and bovine heart mitochondria. Biochemistry 30 1892nl901. 

ESEEM spectroscopy is a time-domain (i.e. pulsed) analog of EPR see Electron Spin Echo Envelope Modulation Spectroscopy). In principle, ESEEM contains the same information as is found in EPR and ENDOR, although in practice ESEEM is much more sensitive to weakly coupled nuclei that are not easily detected by ENDOR. On the other hand, strongly coupled nuclei can be undetectable by ESEEM, thus the combination of both techniques is often useful. 

As an example of how to determine the electronic ground state of a low-spin iron(lll) compound, we present work on the ferric low-spin heme complex [TPPFe(NH2PzH)2]Cl, which has been shown to have a (dxy) (dxz, dj, ) electronic ground state. The field-dependent Mossbauer spectra of [TPP Fe(NH2PzH)2]Cl displayed in Fignre 12 are well reproduced by simulations, which yield 5 =0.25mms , AEq = ( )2.50imns, an asymmetry parameter rj = —3, and an anisotropic A tensor of" / nMn = (-47.6, 6.7, 18.3)T. The g values necessary for the 5" = 1 /2 spin Hamiltonian (g z = 2.39, gyy = 2.28, and gxx = L87) have been taken from a combined EPR and electron spin echo envelope modulation spectroscopy ESEEM analysis. 

CEMS = conversion electron Mossbauer spectroscopy DFT = density functional theory EFG = electric field gradient EPR = electron paramagnetic resonance ESEEM = electron spin echo envelope modulation spectroscopy GTO = Gaussian-type orbitals hTH = human tyrosine hydroxylase MIMOS = miniaturized mossbauer spectrometer NFS = nuclear forward scattering NMR = nuclear magnetic resonance RFQ = rapid freeze quench SAM = S -adenosyl-L-methionine SCC = self-consistent charge STOs = slater-type orbitals TMP = tetramesitylporphyrin XAS = X-ray absorption spectroscopy. 

Electron Paramagnetic Resonance (EPR) Spectroscopy, Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy Electron-Nuclear Double Resonance (ENDOR) Spectroscopy Nuclear Magnetic Resonance (NMR) Spectroscopy of Inorganic/Organometallic Molecules. 

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