Superhyperfine Coupling Constants Research Articles

Hyperfine Decoupling of ESR Spectra Using Wavelet Transform.

The objective of spectral analysis is to resolve and extract relevant features from experimental data in an optimal fashion. In continuous-wave (cw) electron spin resonance (ESR) spectroscopy, both values of a paramagnetic center and hyperfine splitting caused by its interaction with neighboring magnetic nuclei in a molecule provide important structural and electronic information. However, in the presence of - and/or -anisotropy and/or large number of resonance lines, spectral analysis becomes highly challenging. Either high-resolution experimental techniques are employed to resolve the spectra in those cases or a range of suitable ESR frequencies are used in combination with simulations to identify the corresponding and values. In this work, we present a wavelet transform technique in resolving both simulated and experimental cW-ESR spectra by separating the hyperfine and super-hyperfine components. We exploit the multiresolution property of wavelet transforms that allow the separation of distinct features of a spectrum based on simultaneous analysis of spectrum and its varying frequency. We retain the wavelet components that stored the hyperfine and/or super-hyperfine features, while eliminating the wavelet components representing the remaining spectrum. We tested the method on simulated cases of metal-ligand adducts at L-, S-, and X-band frequencies, and showed that extracted values, hyperfine and super-hyperfine coupling constants from simulated spectra, were in excellent agreement with the values of those parameters used in the simulations. For the experimental case of a copper(II) complex with distorted octahedral geometry, the method was able to extract and hyperfine coupling constant values, and revealed features that were buried in the overlapped spectra.

Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation.

Stoichiometric electron-transfer (ET) oxidation of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examined to scrutinize the reaction mechanism of O2 evolution from the peroxo complexes, as seen in the final step in water oxidation by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystallization under different conditions. Although cyclic voltammograms of the two isomers in aqueous solutions showed one reversible wave at 1.1 V vs NHE at pH 2.0, two oxidation waves were observed at 1.0 and 1.4 V at pH 7.0 in the aqueous solutions, the latter of which is responsible for the O2-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O2 evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. In order to clarify the O2-evolving mechanism, we performed electron paramagnetic resonance (EPR) and resonance Raman (RR) measurements for characterizing one-electron oxidized species: The observed EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was observed between two isomers in the EPR parameters including g values and superhyperfine coupling constants. ET-oxidation rate constants of the isomers were determined to be much faster than the O2-evolving rate constants, indicating that the O2-releasing step is the rate-determining step in the O2 evolution through the stoichiometric ET oxidation of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O2 evolution for the two isomers should be derived from the thermodynamic stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.

Well-Known Mediators of Selective Oxidation with Unknown Electronic Structure: Metal-Free Generation and EPR Study of Imide-N-oxyl Radicals.

Nitroxyl radicals are widely used in chemistry, materials sciences, and biology. Imide-N-oxyl radicals are subclass of unique nitroxyl radicals that proved to be useful catalysts and mediators of selective oxidation and CH-functionalization. An efficient metal-free method was developed for the generation of imide-N-oxyl radicals from N-hydroxyimides at room temperature by the reaction with (diacetoxyiodo)benzene. The method allows for the production of high concentrations of free radicals and provides high resolution of their EPR spectra exhibiting the superhyperfine structure from benzene ring protons distant from the radical center. An analysis of the spectra shows that, regardless of the electronic effects of the substituents in the benzene ring, the superhyperfine coupling constant of an unpaired electron with the distant protons at positions 4 and 5 of the aromatic system is substantially greater than that with the protons at positions 3 and 6 that are closer to the N-oxyl radical center. This is indicative of an unusual character of the spin density distribution of the unpaired electron in substituted phthalimide-N-oxyl radicals. Understanding of the nature of the electron density distribution in imide-N-oxyl radicals may be useful for the development of commercial mediators of oxidation based on N-hydroxyimides.

Phase transitions and proton ordering in hemimorphite: new insights from single-crystal EPR experiments and DFT calculations

Single-crystal electron paramagnetic resonance spectra of gamma-ray-irradiated hemimorphite (Mapimi, Durango, Mexico) after storage at room temperature for 3 months, measured from 4 to 275 K, reveal a hydroperoxy radical HO2 derived from the water molecule in the channel. The EPR spectra of the HO2 radical confirm that hemimorphite undergoes two reversible phase transitions at ~98 and ~21 K and allow determinations of its spin Hamiltonian parameters, including superhyperfine coupling constants of two more-distant protons from the neighboring hydroxyl groups, at 110, 85, 40 and 7 K. These EPR results show that the HO2 radical changes in site symmetry from monoclinic to triclinic related to the ordering and rotation of its precursor water molecule in the channel at <98 K. The monoclinic structure of hemimorphite with completely ordered O–H systems at low temperature has been evaluated by both the EPR spectra of the HO2 radical at <21 K and periodic density functional theory calculations.

Room-Temperature Ionic Liquids Discerned Via Nitroxyl Spin Probe Dynamics

The temperature dependence of the rotational correlation times, τ(c), of the nitroxide spin probes TEMPO, TEMPOL, TEMPAMINE, and Fremy's salt in the ionic liquids 1-ethyl-3-methylimidazolium ethylsulfate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium tetrafluoroborate, and 1-butyl-3-methylimidazolium tetrafluoroborate is scrutinized. The rotation correlation times vary between 54 and 1470 ps at 300 K. Within a temperature range of 280-380 K, the rotational tumbling is well described by the extended Debye-Stokes-Einstein law. The hydrodynamic radii are smaller than the geometrical radii though. This discrepancy can partly be accounted for by microviscosity effects and deviations from the spherical shape. This study is distinguished from similar studies by the fact that proton superhyperfine coupling constants could be resolved for all nitroxides in the ionic liquids by carefully optimizing the experimental protocol. As a consequence, many rotational correlation times reported here are smaller than those found previously. Furthermore, the temperature dependence of the nitrogen ESR coupling constants is reported and discussed in detail. A surprising effect of adventitious water is reported for TEMPAMINE.

Electron Paramagnetic Resonance Studies of Nitrosyl and Thionitrosyl and Density Functional Theory Studies of Nitrido, Nitrosyl, Thionitrosyl, and Selenonitrosyl Complexes of Chromium

The novel S = 1/2 thionitrosyl complexes Cr(NS)(CN)(5)(3-), Cr(NS)(dmso)(5)(2+), and Cr(NS)(nmf)(5)(2+) (dmso = dimethyl sulfoxide, nmf = N-methylformamide) have been prepared, and their optical and electron paramagnetic resonance (EPR) spectra were studied. The values of the isotropic and anisotropic hyperfine and superhyperfine coupling constants A((53)Cr), A((14)N), and A((13)C) and of g were determined from the EPR spectra at room temperature and at 66 K. The values of A(⊥) and A(iso) in the thionitrosyl complexes were slightly higher than in the analogous nitrosyl complexes. A common feature in the optical absorption spectra of the thionitrosyl complexes in solution at 298 K is an absorption band around 600 nm with a vibronic structure whereas such a band is located around 450 nm in the analogous nitrosyl complexes. Density functional theory (DFT) studies of the series of complexes Cr(N)(H(2)O)(5)(2+), Cr(NO)(H(2)O)(5)(2+), Cr(NS)(H(2)O)(5)(2+), and Cr(NSe)(H(2)O)(5)(2+) show that the unpaired electron resides in a metal-based d(xy) orbital and that the electronic structure in the equatorial plane is similar in all four complexes and similar to Cr(3+). The σ donating ability was found in the order N(3-) ≫ NO < NS ≈ NSe and the π accepting ability in the order NO > NS ≈ NSe. Time dependent DFT calculations gave in all four complexes a d(x(2)-y(2)) ← d(xy) transition energy around 17 500 cm(-1).

The highly resolved electronic spectrum of the square planar CuCl42− ion

The low temperature magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectra of Cu(II) doped Cs(2)ZrCl(6) are reported. The Cu(II) ion is incorporated as the square planar copper tetrachloride ion, CuCl(4) (2-), which substitutes at the Zr(IV) site in the Cs(2)ZrCl(6) lattice, with a complete absence of axial coordination. Both the EPR and MCD show highly resolved spectra from which it is possible to determine the superhyperfine coupling constants and excited state geometries respectively. The Franck-Condon intensity patterns suggest that there is a substantial relaxation of the host lattice about the impurity ion. For the lowest energy (2)B(1g)(x(2)-y(2))-->(2)B(2g)(xy) transition, both the magnetic dipole allowed electronic origin as well as vibronic false origins are observed. The high resolution of the spectra allowed the accurate determination of the odd parity vibrations that are active in the spectra. The opposite sign of the MCD of the two components of the (2)E(g)(xz,yz) excited state allows this splitting to be determined for the first time. Accurate and unambiguous spectral parameters for the CuCl(4) (2-) ion are important as it has become a benchmark transition metal complex for theoretical electronic structure calculations.

Chromium(V) complexes generated in Arthrobacter oxydans by simulation analysis of EPR spectra

Chromium(V) is an intermediate formed during the reduction of Cr(VI) to Cr(III) compounds by various bacteria. However, little is known about the nature, localization and reactivity of Cr(V) species in microbial systems. Electron paramagnetic resonance (EPR) spectroscopy was used to study the nature of Cr(V) complexes generated in basalt-inhabiting Gram-positive Arthrobacter oxydans bacteria after exposure to high concentrations of Cr(VI). Numerical simulations of the EPR spectroscopic data provide strong evidence for at least two different diolato-type oxoCr(V) complexes (I, giso=1.9801; II, giso=1.9796) involving bacterial cell wall macromolecules in the Cr(VI)–A. oxydans system. The relative concentrations of the two oxoCr(V)–diolato species differ when Cr(VI) is incubated with either untreated A. oxydans cells (I:II ∼50:50) or lyophilized cells (I:II ∼10:90). Based upon the magnitudes of the proton superhyperfine coupling constants (1H aiso) for species I and II, the EPR simulation model is unable to distinguish unambiguously whether the oxoCr(V)–diolato species are linear alkoxides or cyclic diols (carbohydrates). The oxygen-containing functional groups associated with teichoic acids are the most likely candidates for complexation with the Cr(V) ion.

Computation of the Stark effect in P impurity states in silicon

We compute within the effective-mass theory and without adjustable parameters the Stark effect for shallow P donors in Si with anisotropic band structure. Valley-orbit coupling is taken into account in a nonperturbative way and scattering effects of the impurity core are included to properly describe low-lying impurity states. The ground-state energy slightly decreases with increasing electric field up to a critical value ${E}_{\mathit{cr}}\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{cm}$, at which the donor can be ionized by tunneling due to a field-induced mixing of the ``$1s$-like'' singlet ground state with a ``$2{p}_{0}$-like'' excited state in zero field. The resulting ground-state wave function at high field extends significantly outside the potential barrier surrounding the impurity. Calculations of the hyperfine splitting and of the $A$-shell superhyperfine coupling constants as a function of the electric field complete the work.

Parallel-Stranded Guanine Quadruplex Interactions with a Copper Cationic Porphyrin

G-quadruplexes are formed by association of DNA strands containing multiple contiguous guanines. The capability of drugs to induce formation of or stabilize G-quadruplexes is an active area of investigation. We report the interactions of CuTMpyP4, the Cu(2+) derivative of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine, with the parallel-stranded G-quadruplexes formed by d(T(4)G(4)T(4)) (1) and d(T(4)G(8)T(4)) (3). Absorption titrations of CuTMpyP4 with (1)(4) or (3)(4) cause both bathochromicity and hypochromicity of the porphyrin Soret band, with larger changes observed for the longer oligonucleotide. An approximate binding constant for (1)(4) and CuTMpyP4 according to the Scatchard model is 5.6 x 10(6) M(-)(1) in terms of quadruplexes and according to the McGhee-von Hippel model is 1.3 x 10(6) M(-)(1) in terms of potential binding sites. An approximate binding constant for (3)(4) and CuTMpyP4 according to the Scatchard model is 5.2 x 10(7) M(-)(1) in terms of quadruplexes and in terms of the McGhee-von Hippel model is 2.4 x 10(6) M(-)(1) in terms of potential binding sites. The site size for CuTMpyP4 and (1)(4) is four using the McGhee-von Hippel model. We find a 2:1 binding stoichiometry for CuTMpyP4 and (1)(4) and a 3:1 binding stoichiometry for CuTMpyP4 and (3)(4) using the method of continuous variation analysis. Induced emission spectra of CuTMpyP4 with (1)(4) or (3)(4) indicate a mode of binding in which the ligand is protected from the solvent. Electron paramagnetic resonance spectra of CuTMpyP4 with added oligonucleotide show an increase in the Cu-N superhyperfine coupling constant as the length of the oligonucleotide increases. On the basis of these data, we propose that for both (1)(4) and (3)(4), CuTMpyP4 molecules externally stack at each end of the run of guanines, similar to other planar G-quadruplex ligands. For (3)(4), our data are consistent with intercalation of a CuTMpyP4 molecule into the G-quadruplex.

Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by cofacial dicobalt porphyrins.

The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin [Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt porphyrin [Co(2)(DPX)]. The other cofacial dicobalt porphyrins [Co(2)(DPA), Co(2)(DPB), and Co(2)(DPD)] also catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the case of Co(2)(DPX). The micro-superoxo species of cofacial dicobalt porphyrins were produced by the reactions of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-electron oxidation of the resulting micro-peroxo species by iodine. The superhyperfine structure due to two equivalent cobalt nuclei was observed at room temperature in the ESR spectra of the micro-superoxo species. The superhyperfine coupling constant of the micro-superoxo species of Co(2)(DPX) is the largest among those of cofacial dicobalt porphyrins. This indicates that the efficient catalysis by Co(2)(DPX) for the four-electron reduction of dioxygen by Fe(C(5)H(4)Me)(2) results from the strong binding of the reduced oxygen with Co(2)(DPX) which has a subtle distance between two cobalt nuclei for the oxygen binding. Mechanisms of the catalytic two-electron and four-electron reduction of dioxygen by ferrocene derivatives will be discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The turnover-determining step in the Co(OEP)-catalyzed two-electron reduction of dioxygen is an electron transfer from ferrocene derivatives to Co(OEP)(+), whereas the turnover-determining step in the Co(2)(DPX)-catalyzed four-electron reduction of dioxygen changes from the electron transfer to the O-O bond cleavage of the peroxo species of Co(2)(DPX), depending on the electron donor ability of ferrocene derivatives.

Molecular Structure, Spin Density Distribution, and Hyperfine Coupling Constants of the η{CuNO}11 Adduct in the ZSM-5 Zeolite: DFT Calculations and Comparison with EPR Data

DFT calculations of the molecular structure of the intrazeolite η1{CuNO}11 adduct and the 14N and 17O hyperfine and 63Cu superhyperfine coupling constants were performed and compared with previous EPR results. The calculations confirmed the choice of signs adopted in the previous analysis of the experimental data and the character of the SOMO. The influence of the basis set and the exchange-correlation functional on the HFCC and the spin-density distribution was investigated and briefly discussed. The global repartition of the spin density over Cu (ρ = 0.11), N(ρ = 0.58), and O (ρ = 0.34) atoms determined from the Mulliken population analysis compared well with the experiment. The 14N hyperfine tensor was successfully reproduced with the LanL2DZ basis and BPW91 functional, whereas in the case of the 63Cu superhyperfine dipolar tensor T the agreement, except for that of the Tzz component, was less satisfactory because of an overestimated polarization of the 3dyz orbital, regardless of the computation level...

Optical Spectroscopy of Mn2+ Ions in CaF2

Excitation and emission spectra of Mn2+ ions in calcium fluoride have been studied using the photoluminescence technique. An analysis of the positions of the Mn2+ crystal-field transitions with aid of the model developed by Curie et al. was performed to determine the relevant crystal-field parameters and the covalency parameters f_s and fp of the Mn2+ ions entering substitutionally for Ca2+ ions in an Oh symmetry cubic environment. The As isotropic and Ap anisotropic superhyperfine coupling constants of Mn2+ in CaF2 are also calculated and compared with those previously measured through the use of EPR technique.

Electron Spin Echo Envelope Modulation Spectroscopy Reveals and Distinguishes Equatorial and Axial Nitrogen Ligands Bound to VO2+

ESEEM (electron spin echo envelope modulation) spectra of a series of vanadyl complexes have for the first time revealed the spectral features that arise from amine nitrogen donors trans to the vanadyl oxo moiety. The complexes [VO(H2O)ada] (1) and [VO(H2O)Hheida] (2) (H2ada, N-(2-acetamido)iminodiacetic acid; H3heida, N-(2-hydroxyethyl)iminodiacetic acid) have recently been synthesized and crystallographically characterized (Hamstra, B. J.; Houseman, A. L. P.; Colpas, G. J.; Kampf, J. W.; LoBrutto, R.; Frasch, W. D.; Pecoraro, V. L. Inorg. Chem. 1997, 36, 4866−4874). In each structure, the ligand is disposed so that the tertiary nitrogen is bound trans to the vanadyl oxo. In solution, the structural integrity of these compounds is maintained as observed by UV/vis and EPR spectroscopies. ESEEM spectra of 1, 2, and [VO(H2O)nta]- (3) (H3nta, nitrilotriacetic acid) reveal that the superhyperfine coupling constants for axial amines are ≅1.3 MHz as opposed to the ≅5.0 MHz superhyperfine couplings typically observed for equatorially coordinated amines. The data indicate that not only can axial nitrogen donors to VO2+ be detected, but they may also be distinguished from nitrogen donors in the equatorial plane of the vanadyl ion. The implications of this study with respect to understanding the coordination environment of VO2+ in the vanadyl-substituted chloroplast F1-ATPase (CF1) and the reduced, inactive form of vanadium bromoperoxidase (VBrPO) are discussed.

Evaluation of the distance between the hydrogen atom and hydrogen molecule in the tunneling reaction HD + D → H + D 2 in an argon matrix at 20 K

A hydrogen atom-hydrogen molecule pair entirely isolated in an Ar matrix has been investigated by high-resolution ESR spectroscopy from 4.2 to 23 K. A doublet ESR spectrum of the hydrogen atom was separated by 51.18 mT and further split isotropically into nine lines with superhyperfine coupling constants of ca. 0.06 mT and peak intensities of 1:1:2:1:2:1:2:1:1 in a solid Ar matrix containing HD (3.3 mol%) at 20 K. The superhyperfine structure can be attributed to HHD and HD 2 pairs. The HD 2 pair is a product of the tunneling reaction HD + D → HD 2. The mean distance between the hydrogen atom and the hydrogen molecule in the tunneling reaction in the Ar matrix at 20 K is experimentally determined to be 1.73 Å.

Electron Spin Resonance Super-Hyperfine Splitting Study of Polycrystalline Niobocene and Vanadocene Dichlorides

The electron spin resonance (ESR) spectra of niobocene and vanadocene dichlorides were studied on the title compounds prepared as magnetically diluted species in polycrystalline form. The resolved super-hyperfine splitting of anisotropic ESR spectra of both studied compounds was firstly observed. Computer simulation confirmed that this super-hyperfine splitting is due to interaction of unpaired electron with nuclear spin of two chlorine ligands. Average value of super-hyperfine coupling constant is 13.3 MHz for niobocene dichloride. For vanadocene dichloride, it is estimated to be in the range of 6-10 MHz. It corresponds approximately to 10% delocalization of metal unpaired electron spin density onto chlorine ligands in case of niobocene dichloride and to 4-7% delocalization in case of vanadocene dichloride.

ESR demonstration of nitric oxide production from nitroglycerin and sodium nitrite in the blood of rats

Electron spin resonance (ESR) spectra of iron-metal complexes formed by the reaction between nitric oxide ('NO) and hemoglobin (Hb), referred to as nitrosylhemoglobin (HB-NO), were observed in rat blood treated in vitro and in vivo with nitroglycerin (GTN) at 77K. The same types of spectra were also detected in rats treated with sodium nitrite (NaNO 2). Two types of Hb-NO, which were identified by ESR parameters of g values and superhyperfine coupling constants (shfcc), were the 6- and 5-coordinated complexes. These two types of Hb-NO were generated in a dose-dependent manner in the blood after intraperitoneal administration of 1.5–6 mg of GTN. At the higher dose of GTN (6 mg), the 6-coordinated complex was the major species generated initially, but within 10 min, the 5-coordinated complex increased time-dependently. Quantitative analysis of Hb-NO revealed that when GTN 0.3 mg and 0.6 mg was administered sublingually in rats, the concentration of Hb-NO observed in rat blood was 30% higher than the estimated concentration of GTN. The methemoglobin and peroxide complex of hemoglobin were observed in the blood incubated with GTN at 37°C. These results suggest that the function of GTN was related to oxidative stress with the generation of Hb-NO. Therefore, monitoring of Hb-NO levels may be useful as an indicator of the function of various vasodilators.

ESR studies of low-spin [N 1,N 4-bis(salicylidene)-S-alkyl-isothiosemicarbazonato]cobalt(II) complexes—II. Adducts of nitrogen-, phosphorus-, arsenic-, oxygen- and sulphur-donor bases

ESR spectra of [Co(Me,H,H)·H 2O] [Me,H,H = N 1,N 4-bis(salicylidene)-S-methyl-isothiosemicarbazone 2−] and its adducts with ligands containing nitrogen (pyridine, 4-CN pyridine, α- and γ-picoline, N-methyl- and N-ethylimidazole), phosphorus (tributyl-, triphenyl-, diphenylmethyl-, diphenylethylphosphine, triphenyl- and triethylphosphite), arsenic (triphenyl arsine), oxygen (DMF, THF) and sulphur (dimethyl-, diethyl-, diphenyl- and methylphenylsulphide) donor bases were studied in frozen solutions. All spectra were indicative of low-spin cobalt(II) centres with predominantly d z 2 character of the ground state. The determined values of A Co and g tensor components and superhyperfine coupling constants observed for base donor atoms indicated the formation of five-coordinate monoadducts. For some nitrogen bases (pyridine, 4-CN pyridine and γ-picoline) the formation of 1 : 2 six-coordinate bis-adducts was also observed. The bonding parameters were calculated. The activity of the N-donor adducts towards dioxygen is also reported.

EPR Study of Uranium(V) Species in Photo-and Electrolytic Reduction Processes of UO2(NO3)2-2TBP.

Abstract Electron Paramagnetic Resonance (EPR) and optical spectra of uranium(V) species were observed in both processes of photo- and electrolytic reduction of UO2(NO3)2-2tributylphosphate (TBP) in 80%TBP-n-dodecane solution. The formation of U(V) species was detected by an optical spectrum (λmax: 770, 970 and 1,420 nm). EPR signal with the value of ff-factor –2.3 and a linewidth of approximately 1,100 Gs was observed during the electrolytic reduction. On the other hand, during the photoreduction the signal with the value of fil-factor –1.94 was observed and there was found a superhyperfine structure with the intensity ratio of 1:2:1, that is caused by the superhyperfine coupling with nuclear spin, I=1/2, of the strongly coordinated 31P to the central uranium through oxygen atom. The superhyperfine coupling constant was estimated to be 27 Gs. Moreover, the signal with the value of g-factor –2.00 due to an organic radical was observed. The residue after the thermo-gravimetric analysis of UO2(NO3)2-2TBP wa...

Electron paramagnetic resonance studies of a ras p21-MnIIGDP complex in solution.

The number of water molecules bound to Mn2+ in the complex with a variant of Ha ras p21 and GDP has been determined by electron paramagnetic resonance (EPR) measurements in 17O-enriched water. A resolution enhancement method has been used to improve quantitation of the spectral data. These spectroscopic measurements show that Mn2+ has four water ligands in this complex, a result in agreement with the conclusions of a previous paper [Smithers, G. W., Poe, M., Latwesen, D. G., & Reed, G. H. (1990) Arch. Biochem. Biophys. 280, 416-420]. The resolution enhancement method has also been applied in a measurement of the 17O-Mn2+ superhyperfine coupling constant of 17O in the beta-phosphate of the GDP in the ras p21 complex. The intrinsically narrow EPR signals of Mn2+ in the complex with ras p21 and GDP in 2H2O respond to resolution enhancement such that the superhyperfine splitting from the 17O nuclear spin (I = 5/2) becomes visible in the EPR signals. An 17O-Mn2+ superhyperfine coupling constant is obtained from simulation of the resolution-enhanced EPR spectrum.