Multifrequency High-field EPR Research Articles

High-field ELDOR-detected NMR study of a nitroxide radical in disordered solids: Towards characterization of heterogeneity of microenvironments in spin-labeled systems

The combination of high-field EPR with site-directed spin-labeling (SDSL) techniques employing nitroxide radicals has turned out to be particularly powerful in probing the polarity and proticity characteristics of protein/matrix systems. This information is concluded from the principal components of the nitroxide Zeeman (g), nitrogen hyperfine (A) and quadrupole (P) tensors of the spin labels attached to specific sites. Recent multi-frequency high-field EPR studies underlined the complexity of the problem to treat the nitroxide microenvironment in proteins adequately due to inherent heterogeneities which result in several principal x-components of the nitroxide g-tensor. Concomitant, but distinctly different nitrogen hyperfine components could, however, not be determined from high-field cw EPR experiments owing to the large intrinsic EPR linewidth in fully protonated guest/host systems. It is shown in this work that, using the W-band (95GHz) ELDOR- (electron–electron double resonance) detected NMR (EDNMR) method, different principal nitrogen hyperfine, Azz, and quadrupole, Pzz, tensor values of a nitroxide radical in glassy 2-propanol matrix can be measured with high accuracy. They belong to nitroxides with different hydrogen-bond situations. The satisfactory resolution and superior sensitivity of EDNMR as compared to the standard ENDOR (electron–nuclear double resonance) method are demonstrated.

High-field EPR

Among the numerous spectroscopic techniques utilized in photosynthesis research, high-field/high-frequency EPR and its pulse extensions ESE, ENDOR, ESEEM, and PELDOR play an important role in the endeavor to understand, on the basis of structure and dynamics data, dominant factors that control specificity and efficiency of light-induced electron- and proton-transfer processes in primary photosynthesis. Short-lived transient intermediates of the photocycle can be characterized by high-field EPR techniques, and detailed structural information can be obtained even from disordered sample preparations. The chapter describes how multifrequency high-field EPR methodology, in conjunction with mutation strategies for site-specific isotope or spin labeling and with the support of modern quantum-chemical computation methods for data interpretation, is capable of providing new insights into the photosynthetic transfer processes. The information obtained is complementary to that of protein crystallography, solid-state NMR and laser spectroscopy.

Multifrequency high-field EPR investigation of a mononuclear manganese(iv) complex

Electronic properties of [Mn(IV)(tacn)(OMe)(3)](+), especially the axial zero field splitting term D, which characterizes the magnetic anisotropy, have been investigated by multifrequency high-field EPR.

High-field EPR investigation of a series of mononuclear Mn(II) complexes doped into Zn(II) hosts

The five-coordinate mono-halide mononuclear Zn(II) complexes [Zn(tpa)X] + (tpa = tris(2-pyridylmethyl)amine; X = I ([Zn(tpa)I]I; 1a), Br ([Zn(tpa)Br](ZnBr 4) 0.5; 2a) and Cl ([Zn(tpa)Cl](ZnCl 4) 0.5; 3a)) and the six-coordinate mononuclear complex [Zn(tpa)(NCS) 2] ( 4a) have been synthesized and characterized by X-ray crystallography. The [Zn(tpa)X] + complexes doped with the corresponding [Mn(tpa)X 2] complexes (X = I ( 1b), Br ( 2b) and Cl ( 3b)) have been synthesized and their electronic properties investigated by multifrequency high field EPR (HF-EPR) (95–285 GHz). The magnetically diluted conditions allow the determination of the hyperfine coupling constant A (∣ A∣ = 68.10 −4 cm −1 for 1b– 3b). The zero-field splitting parameters ( D and E) found for 1b– 3b are comparable to those found for neat samples of the [Mn(tpa)X 2] complexes ( 1b: ∣ D∣ = 0.635 cm −1, ∣ E/ D∣ = 0.189; 2b: ∣ D∣ = 0.360 cm −1, ∣ E/ D∣ = 0.192; 3b: ∣ D∣ = 0.115 cm −1, ∣ E/ D∣ = 0.200). The efficacy of using multifrequency EPR under dilute conditions to precisely determine spin Hamiltonian parameters is discussed.

Multi-Frequency High-Field EPR Study of Iron Centers in Malarial Pigments

The multi-frequency high-field electron paramagnetic resonance (HFEPR) was used to study the magnetic properties of malarial pigment hemozoin and its synthetic analogue, beta-hematin. (FeIII-protoporphyrin-IX)2 dimers containing five-coordinate high-spin FeIII, S = 5/2, are the building blocks of these pigments. The fit of EPR spectra that were acquired in an unprecedented wide range of microwave frequencies of 34 and 94 GHz for hemozoin and 27-500 GHz for beta-hematin yielded a complete set of intrinsic spin Hamiltonian parameters: D = +5.85(1) cm-1, E = 0, g perpendicular = 1.95(1), g parallel = 2.00(1). These results point to the existence of largely axial symmetry of the iron environment in the bulk phase of hemozoin and beta-hematin.

A multifrequency high-field EPR (9–285 GHz) investigation of a series of dichloride mononuclear penta-coordinated Mn(II) complexes

The electronic structure of a series of 11 penta-coordinated dichloride mononuclear Mn(II) complexes [Mn(L)Cl 2] (L = Cl-terpy, Br-terpy, OH-terpy, phenyl-terpy, tolyl-terpy, mesityl-terpy, EtO-terpy, Me 2N-terpy, tBu 3-terpy, py-phen, and dpya) has been investigated by a multifrequency EPR study (9–285 GHz). The X-ray structures of [Mn(Br-terpy)Cl 2], [Mn(EtO-terpy)Cl 2], [Mn(Me 2N-terpy)Cl 2] and [Mn(tolyl-terpy)Cl 2] are described. The spin Hamiltonian parameters have been determined for all complexes and show that the steric and electronic effects of the N-tridentate ligand L do not induce appreciable variations on the zero field splitting parameters. The magnitude of D, close to 0.3 cm −1, is governed by the chloride anion. High-field EPR spectroscopy allows the determination of electronic parameters of mononuclear Mn(II) complexes characterized by relatively large magnitudes of D and the unambiguous interpretation of the X-band spectra of these kinds of complexes.

Multi-frequency high-field EPR studies on metal-substituted xylose isomerase

The bacterial enzyme D-xylose isomerase (XI) catalyses the conversion of D-xylose to D-xylulose. Each subunit of the homotetrameric protein contains a bimetallic active centre requiring divalent metal ions such as Mg2+, Mn2+ or Co2+ for catalytic activity. We report here on XI in which the metal binding site 1 is specifically loaded with EPR active Mn2+, while binding site 2 is occupied by Co2+ or Cd2+, rendering a catalytically active or inactive species respectively. The Q-band (34 GHz) EPR spectra of these mixed-metal samples (Co2+/Mn2+ and Cd2+/Mn2+ XI) show a clear influence of the metal in site 2 on the Mn2+ EPR parameters. Likewise, a systematic increase of the zero field splitting parameters (zfs) of Mn2+ in site 1 upon incubation with the inhibitor xylitol or substrates for both mixed-metal samples is found. For Co2+/Mn2+ XI complexed with substrate, a drastic line broadening of the central -1/2 <--> +1/2 transition is observed in Q-band EPR, which was not amenable to analysis so far. By means of a multi-frequency approach at frequencies beyond Q-band, the relevant zfs parameters were derived from spectral simulations of EPR spectra measured at 94, 285 and 670 GHz. It is shown that parallel to the increase of the D-value its distribution also grows considerably in going from free Co2+/Mn2+ XI to the species complexed with inhibitor or substrate. For XI with bound substrate, D-values in the range of 70-90 mT and a distribution of about 30 mT were found from simulation trials. The large distribution in zfs values is thought to be correlated to the structural disorder induced by the shift of the metal ion of site 2 into a location necessary for the isomerisation reaction. The results are discussed with respect to high-resolution crystal data.

Multifrequency High-Field EPR Study of the Interaction between the Tyrosyl Z Radical and the Manganese Cluster in Plant Photosystem II

The light-driven oxidation of water to dioxygen is catalyzed by the enzyme photosystem II. A four-manganese ion cluster and a tyrosine, YZ, are present in the catalytic site. In preparations inhibited by addition of acetate or removal of the calcium cofactor, it is possible to trap the tyrosyl radical in interaction with the metal cluster. The coupled species is characterized by a broad split EPR signal at 9 GHz. In this work, high-field EPR has been used for further characterization of the coupling. The 285, 190 and 95 GHz EPR spectra of the interacting system are reported. Analysis of these spectra yielded exchange and dipolar couplings of the same magnitude as those found with 9 GHz EPR. However, the high-field spectra show that the coupling between the radical and the manganese cluster has opposite sign in acetate-treated compared to calcium-depleted samples. The sign difference indicates differences in the electronic structure of the radical−metal center pair. Comparisons are made between photosystem...

Multifrequency High-Field EPR Study of Binuclear Mn(III)Mn(IV) Complexes

The results from a multifrequency high-field EPR study of five di-μ-oxo bridged mixed-valence binuclear Mn(III)Mn(IV) complexes are reported. Spectra were obtained at 9, 95, and 285 GHz. The g anisotropy was unambiguously observable at 285 GHz. Hyperfine and g tensor values were estimated using spectral simulation procedures that cyclically and simultaneously fit the multifrequency data. In all five cases, the g tensors of the mixed-valence complexes were found to be rhombic. The g tensors were analyzed using the vector projection model. Most, but not all, of the g anisotropy originates from the Mn(III) center. The rhombic g tensors result from the low symmetry of the manganese centers. The size of the effective g anisotropy for a given complex was found to be a linear function of the average bond distance between the manganese and axial nitrogens. This relationship can be understood in terms of the influence of tetragonal distortion on the electronic levels of the Mn(III) center. The frequency-dependent ...