17O Hyperfine Research Articles

1H, 13C and 17O principal hyperfine tensor determination for the p-benzosemiquinone anion radical using hybrid density functional methods

The 1H, 13C and 17O hyperfine coupling tensors for the p-benzosemiquinone anion radical have been directly calculated using hybrid density functional methods. Excellent agreement between experimentally determined tensor values and theoretically calculated ones are reported. Proton tensors for solvent hydrogen bonding interactions with an alcohol solvent are also well reproduced.

Electron spin resonance rare gas matrix studies of 12CO2−, 13CO2−, and C17O−2: Comparison with ab initio calculations

The 12C16O2−, 13C16O2−, 12C17O2−, and 12C16,17O2− radical anions have been generated by four independent methods and isolated in neon and argon matrices for detailed ESR (electron spin resonance) investigations. Included with these experimental measurements for the various magnetic parameters of CO−2 are high level ab initio calculations (MR SD-CI and others) of the 13C and 17O hyperfine A tensors. Some of the calculations included the effects of a 42-atom neon cage on the electronic structure of CO−2. Previous ESR studies of CO−2 have been conducted in more perturbing environments, such as ionic crystals, where the close proximity of the counter cation can alter the anion’s properties. A comparison of the earlier measurements in more interactive materials with these theoretical and rare gas matrix results reveals a significantly different distribution of the spin density. The neon magnetic parameters (MHz) for CO−2 are gx=2.0018, gy=1.9964, gz=2.0010; for 13C, Ax=320.4, Ay=296.1, Az=394.5; for 17O, Ax=−81.6, Ay=−74.9 and Az=−151.8. The argon results are similar to these neon values; isotropic spectra in argon were also observed at elevated temperatures that yielded giso and Aiso parameters consistent with the low temperature (4 K) anisotropic spectra.

High Frequency (139.5 GHz) Electron Paramagnetic Resonance Characterization of Mn(II)−H217O Interactions in GDP and GTP Forms of p21 ras

As a molecular switch, the ras protein p21 undergoes structural changes that couple recognition sites on the protein surface to the guanine nucleotide-divalent metal ion binding site. X-ray crystallographic studies of p21 suggest that coordination between threonine-35 and the divalent metal ion plays an important role in these conformational changes. Recent ESEEM studies of p21 in solution, however, place threonine-35 more distant from the metal and were interpreted as weak or indirect coordination of this residue. We report high frequency (139.5 GHz) EPR spectroscopy of p21.Mn(II) complexes of two guanine nucleotides that probes the link between threonine-35 and the divalent metal ion. By analysis of high-frequency EPR spectra, we determine the number of water molecules in the first coordination sphere of the manganous ion to be four in p21.Mn(II).GDP, consistent with prior low-frequency EPR and X-ray crystallographic studies. In the complex of p21 with a GTP analog, p21.Mn(II).GMPPNP, we determine the hydration number to be 2, also consistent with crystal structures. This result rules out indirect coordination of threonine-35 in the solution structure of p21.Mn(II).GMPPNP, and implicates direct, weak coordination of this residue as suggested by Halkides et al. [(1994) Biochemistry 33,4019]. The 17O hyperfine coupling constant of H2(17)O is determined as 0.25 mT in the GDP from and 0.28 mT in the GTP form. These values are similar to reported values for 17O-enriched aquo ligands and some phosphato ligands in Mn(II) complexes. The high magnetic field strength (4.9 T) employed in these 139.5 GHz EPR measurements leads to a narrowing of the Mn(II) EPR lines that facilitates the determination of 17O hyperfine interactions.

Electron Magnetic Resonance of the Tyrosyl Radical in Ribonucleotide Reductase from Escherichia coli

The spin density distribution of the Y122 tyrosyl radical in the R2 subunit of ribonucleotide reductase from Escherichia coli has been determined. Incorporation of isotopically labeled tyrosine into the protein has allowed us to measure the 17O hyperfine coupling by using EPR, giving a direct measure of the tyrosine phenol oxygen spin density, 0.29 ± 0.02. The hyperfine tensors of six protons of the radical have been determined by using ENDOR. Magnetic field selection allows a determination of the orientation of the hyperfine tensors relative to the g tensor. Electron−nuclear−nuclear triple resonance has been applied to establish the relative signs of three hyperfine couplings. These measurements give a more precise and more accurate picture of the spin density distribution in a protein tyrosyl radical than has been available previously. The 17O hyperfine splitting in tyrosyl radicals in aqueous glasses has also been measured. The differences in hyperfine couplings indicate that addition of a hydrogen bon...

High frequency (139.5 GHz) electron paramagnetic resonance spectroscopy of the GTP form of p21 ras with selective 17O labeling of threonine.

Electron paramagnetic resonance spectroscopy at 139.5 GHz has been used to study p21 ras complexed with Mn(II) and guanosine 5'-(beta, gamma-imidotriphosphate), an analog of GTP. The p21 sample studied was selectively labeled with [17O gamma]threonine to a final enrichment of 30%. A Mn(II)-17O hyperfine interaction was observed, but the value of the coupling constant, 0.11 +/- 0.04 mT, is the smallest such value yet reported. Ab initio calculations indicate that this value is consistent with direct coordination of the threonine hydroxyl group and provide an estimate for the Mn(II)-17O bond length of 2.7 A. The measured hyperfine coupling constant and associated bond length starkly contrast with typical values for Mn(II)-17O coordination complexes, namely, approximately 0.25 mT and approximately 2.2 A, respectively. This contrast underscores the peculiar weakness of this Mn(II)-O interaction in p21 and persuasively argues that the nucleotide-induced conformational change, which is known to encompass the region of p21 involving Thr35, is not driven by Mn(II) coordination of the Thr35 hydroxyl group.

ESR and IR Study of ClOO Exchange-Coupled to O2

Photolysis below 100 K of the exchange-coupled complex [OClO,O2] in KClO4 at λ ≈ 365 nm yields a metastable product, already identified earlier as the complex [ClO,O3] and also a stable product that we identify as an exchange-coupled complex of ClOO and O2. The 17O hyperfine splitting of the ESR spectrum serves to verify the presence of O2 and the overall spin-doublet ground state of the complex, while the O−O stretching frequency ν1 is used as a fingerprint of ClOO. The observed value of ν1, the principal values of the spin Hamiltonian parameter matrices, and the structural clues conveyed by the direction of the principal axes with respect to the lattice all indicate that ClOO retains its integrity in the complex [ClOO,O2] despite the extreme weakness of the Cl−O bond.

Atom positions from hyperfine data for germanium centers in α‐quartz

AbstractGermanium 3+ occurs in α‐quartz substitutionally for silicon 4+, either as complex anions [GeO4]− or these anions charge‐compensated with interstitial alkali cations, i.e. as two primary types of species denoted by [GeO4/M], where M = Li, Na. A brief review of the relevant literature is presented. Using the molecular‐orbital spin population for the unpaired electron in each center, available from 73Ge and 17O hyperfine data, an evaluation of the likely M+ positions within the quartz structure has been performed. In part, the present calculations were done to test the well known McConnell‐Strathdee approach with well defined experimental hyperfine data. A generalized discussion of atom‐position determination by EPR, and problems encountered in the process, is included.

EPR and ENDOR characterization of semiquinone anion radicals related to photosynthesis

AbstractEPR and ENDOR experiments were performed on the radical anions of 1,4‐benzoquinone, 2,3‐dimethyl‐1,4‐benzoquinone, 2,3,5,6‐tetramethyl‐1,4‐benzoquinone, 2,3‐dimethyl‐5‐ethyl‐1,4‐benzoquinone, plastoquinone‐1, plastoquinone‐9 and ubiquinone‐10. In liquid solutions the isotropic hyperfine coupling constants could be determined. In the frozen state hyperfine tensors for α‐protons and the freely rotating methyl groups were measured whereas other β‐protons (CH2) were broadened beyond detection probably caused by several superimposed chain conformations. In the immobilized radicals proton hyperfine couplings of hydrogen bonds formed between the alcoholic solvent and the CO groups of the quinones could be determined by experiments in protonated and deuterated solvents. Two different types of strong hydrogen bonds exist in these systems: in‐plane with aiso > 0 and out‐of‐plane with aiso ≈︁ 0. The latter type is observed if substituents at both carbons adjacent to the CO groups force the hydrogen bond out of the molecular plane. By use of the point‐dipole model and oxygen spin densities obtained from 17O hyperfine couplings, hydrogen bond lengths could be determined from the measured dipolar proton hyperfine tensors. These findings are very important for the binding and function of quinones in biological systems such as the photosynthetic reaction centres of plants and bacteria.

17O hyperfine and quadrupole interactions for water ligands in frozen solutions of high spin Mn2+

The magnetic couplings of 17O in H217O coordinated to high spin Mn2+ in a frozen aqueous solution were determined using the complementary magnetic resonance techniques of pulsed and continuous wave (cw) ENDOR (electron nuclear double resonance), ESEEM (electron spin echo envelope modulation), and PFSEPR [pulse field sweep electron paramagnetic resonance (EPR)]. Several complications arise from the high electron spin multiplicity of the d5, Mn2+ ion and the high nuclear spin multiplicity, I=5/2, of the 17O nucleus. At the applied magnetic field strengths in 9 GHz EPR studies, the zero-field splitting of the S=5/2 Mn2+ ion in aqueous frozen solution is small relative to the electron spin Zeeman interaction so that the MS=±1/2,±3/2,±5/2 electron spin states all contribute to the ENDOR spectrum. This results in a complex spectrum in which the 17O ENDOR powder pattern arising from the MS=±1/2 manifolds are separately resolved but the powder patterns from the MS=±3/2,±5/2 manifolds overlap the multiple 1H ENDOR lines arising from all six MS manifolds [X. Tan, M. Bernardo, H. Thomann, and C. P. Scholes, J. Chem. Phys. 98, 5147 (1993)]. Given this complexity, a combination of complementary spectroscopic techniques and numerical simulations are used to deconvolute the overlapping spectra and to assign the spectral lines. The ENDOR spectra provided an experimental description of H217O hyperfine couplings to high spin Mn2+ in a frozen solution. The ESEEM results are consistent with the first-order assignments of the ENDOR lines and demonstrate the feasibility of ESEEM measurements of 17O ligand hyperfine couplings to Mn2+. Simulations of the 17O ENDOR hyperfine patterns of aqueous frozen solutions of Mn2+, especially those near 20 MHz, indicated an A-tensor anisotropy of A⊥=−6.5±0.5 MHz and A∥=−9.5±0.5 MHz, consistent with couplings observed by single crystal ENDOR of H217O ligated to Mn2+ doped in [LaMg(NO2)12⋅24(H2O)]. More detailed simulations of the ENDOR pattern below 10 MHz indicated the need for quadrupole couplings consistent with those measured by single crystal ENDOR and with those determined by gas phase measurements on H17OD. Simulations of the ENDOR spectra recorded by the cw and pulsed techniques have delineated important features of the techniques which must be taken into account for a quantitative analysis of the ENDOR amplitudes. It is expected that the general ENDOR conditions employed and the theory developed will be useful in frozen solution studies of 17O involved as a ligand to Mn2+ in enzymes.

Rotational Spectra of 14N17O and 15N18O

Abstract The mm-wave spectra of 15N18 O and 14N 17 O in both X2 Π1/2 and X2 Π3/2 states have been measured and assigned for the first time. Improved rotational and, for the first time, 17O hyperfine coupling constants have been derived from the analysis. The 14N17 O data yield parameters of the unpaired electron spatial distribution near both nuclei. Furthermore, the deviation from the Born-Oppenheimer approximation is confirmed on the basis of the rotational constants of five isotopomers. Finally, when comparing the nuclear spin-rotation constants to the ones of other molecules containing 14N or 17O nuclei, a roughly linear proportionality with the rotational constant B is found.

Interpretation of the 17O magnetic inequivalency in 14N 17O 2. A theoretical and MRSD-CI study

The NO 2X 2A 1 state is in investigated using ab initio multireference single and double excitation configuration interaction (MRSD-CI). The 14N and 17O hyperfine tensor components are in very good agreement with the corresponding experimental values. The MRSD-CI results show that although the two 17O centers are spatially equivalent they are magnetically inequivalent. The difference in resonance field positions between the two 17O centers, Δ, is derived. This computed Δ, from the MRSD-CI wavefunction, reproduces the previously unexplained experimental differences. The orientation of the hyperfine tensor principal axes are in excellent agreement with those obtained from EPR experiments.

Electron spin resonance characterization of self-trapped holes in amorphous silicon dioxide

The electron spin resonance spectra of radiation-induced self-trapped holes (STHs) in amorphous silicon dioxide are isolated by isochromal annealing experiments and computer simulation analyses. Two distinct components, denoted STH 1 and STH 2 (plus a third component intermediate between the two), have been identified. The g matrices and 29Si and 17O hyperfine matrices of STH 1 and STH 2 are determined and form the bases for proposing the following models for these defects: STH 1 comprises a hole trapped in the 2p orbital of a normal bridging oxygen in the glass network; it is a small polaron. STH 2 probably consists of a hole rapidly tunneling between degenerate valence-band-edge states on a pair of adjacent oxygens; it is argued to be a peculiar type of Anderson localized state. Among the various radiation-induced paramagnetic centers in silica, the STHs exhibit unique isochronal anneal characteristics; these characteristics closely mimic the corresponding behavior of a component of trapped positive charge in thermally grown SiO 2 films.

Distortion and 17O hyperfine interaction in the centres [GeO 4] I,II- in α-quartz

Hyperfine splitting arising from 17O for the centres [GeO 4] − I, II has been measured in an isotopically enriched α-quartz crystal. The 50 K X-band EPR results indicate that the GeO 4 quasi-tetrahedron is distorted for Ge(I), so that all four oxygen atoms are symmetry-inequivalent. In contrast, for Ge(II), the expected local symmetry ( C 2) is present and only two types of inequivalent oxygens were observed. These results are consistent with the site symmetries determined from 73Ge hyperfine structure. A preliminary account of the observed 17O hyperfine splittings for the centres [AlO 4/Li] + and [GeO 4/Li] A,C 0 is also given.

Oxygen-17, proton, and deuterium electron nuclear double resonance characterization of solvent, substrate, and inhibitor binding to the iron-sulfur [4Fe-4S]+ cluster of aconitase

17O electron nuclear double resonance (ENDOR) studies at X-band (9-GHz) and Q-band (35-GHz) microwave frequencies reveal that the [4Fe-4S]+ cluster of substrate-free aconitase [citrate (isocitrate) hydro-lyase, EC 4.2.1.3] binds solvent, HxO (x = 1, 2). Previous 17O ENDOR studies [Telser et al. (1986) J. Biol. Chem. 261, 4840-4846] had disclosed that Hx17O binds to the enzyme-substrate complex and also to complexes of enzyme with the substrate analogues trans-aconitate and nitroisocitrate (1-hydroxy-2-nitro-1,3-propanedicarboxylate). We have used 1H and 2H ENDOR to characterize these solvent species. We propose that the fourth ligand of Fea in substrate-free enzyme is a hydroxyl ion from the solvent; upon binding of substrate or substrate analogues at this Fea site, the solvent species becomes protonated to form a water molecule. Previous 17O and 13C ENDOR studies [Kennedy et al. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 8854-8858] showed that only a single carboxyl, at C-2 of the propane backbone of cis-aconitate or at C-1 of the inhibitor nitroisocitrate, coordinates to the cluster. Together, these results imply that enzyme-catalyzed interconversion of citrate and isocitrate does not involve displacement of an endogenous fourth ligand, but rather addition of the anionic carboxylate ligand and a change in protonation state of a solvent species bound to Fea. We further report the 17O hyperfine tensor parameters of the C-2 carboxyl oxygen of substrate bound to the cluster as determined by the field dependence of the 17O ENDOR signals. 17O ENDOR studies also show that the carboxyl group of the inhibitor trans-aconitate binds similarly to that of substrate.

Geometrical arrangement of water molecules in copper–aquoion complex—Influence on shifts and relaxivities in nuclear magnetic resonance

The unrestricted Hartree–Fock cluster procedure has been applied to the Cu2+–(H2O)6 complex to study both its geometry and the hyperfine properties of protons and 17O nuclei in the water molecules of the first hydration sphere. The geometry was studied to understand the Jahn–Teller effect expected for this complex and to explain the observed tetragonal distortion of an elongated nature from extended x-ray absorption fine structure measurements involving four of the H2O molecules being closer to the Cu2+ ion than the other two. The hyperfine properties were studied to attempt to explain observed data in this respect from nuclear magnetic resonance measurements. Our results for the situation in which there is a hole in the dx2–y2 type of state do explain the elongated nature of the tetragonal distortion. The calculated electronic wave functions provide a satisfactory explanation of the 17O hyperfine interaction. For the protons, our results demonstrate the validity of the point ion model commonly used for the dipolar interaction between protons of the hydrated H2O molecules and the unpaired spins in explaining proton nuclear magnetic resonance relaxation data for aquoions. For the proton contact interaction however, theory appears to lead to a higher value than experiment. Possible sources that can lead to an improvement in the predicted proton hyperfine interaction constant are discussed.

The Formation and Structure of the Sulfoxyl Radicals RSO·, RSOO·, RSO2·, and RSO2OO· from the Reaction of Cysteine, Glutathione and Penicillamine Thiyl Radicals with Molecular Oxygen

This work reports an electron spin resonance study of the reactions of cysteine, glutathione and penicillamine thiyl radicals with molecular oxygen in frozen aqueous solutions at low temperatures. For all three thiols, the thiyl radical, RS., is found to react with oxygen to form the thiol peroxyl radical, RSOO(.). On the absorption of visible light, RSOO(.) photoisomerizes to the sulfonyl radical, RSO2(.), which subsequently reacts with molecular oxygen to form RSO2OO(.), the sulfonyl peroxyl radical. The identities of the sulfonyl and sulfonyl peroxyl radicals were confirmed by their production by a different route, from sulfinic acid. Sulfinyl radicals, RSO(.), are found as the final radical species in the reactions of thiyl radicals and oxygen. Parallel 17O hyperfine couplings (A parallel) are reported for each sulfoxyl radical and a correlation between the spin density on oxygen and the reactivity of the radical is suggested. As a result of this correlation sulfonyl peroxyl radicals are predicted to be far more reactive than thiol peroxyl radicals. We also report molecular orbital calculations on the nature of the spin density distribution and the molecular geometry of the model radicals CH3SO2(.) and CH3SO2OO(.).

Hyperfine interactions and relaxivities in divalent and trivalent aquoion systems

The electronic structures of aquoions involving the divalent paramagnetic ion Mn+2 and the two trivalent ions Fe+3 and Cr+3 have been studied using the unrestricted Hartree–Fock procedure, with six H2O molecules surrounding the metal ion. From the results of our investigations, good agreement is found between the metal–oxygen distances obtained from the present calculation and those from hydrated ionic crystals and aqueous solutions. From the analysis of the charges on the various atoms of the cluster, the covlaent bonding between the metal ion and water molecules is found to be negligible in the Mn+2 system and significant for Fe+3 and Cr+3. The spin populations however indicate strong localization on the metal ions in all three cases. Very good agreement is found between the calculated contact proton hyperfine constants for all three systems and measured values and for the 17O hyperfine constant in the Mn+2 aquoion system, the only one of those systems for which it is available. The contributions of the dipolar hyperfine fields at the proton sites to the proton relaxivities are studied and it is shown that in all three cases, the use of the point dipole model in earlier literature to explain the data on these relaxivities is well justified. Physical reasons for the various features of our results are discussed.

The Microwave Spectrum and Nuclear Quadrupole Coupling Constants of HNC17O

Abstract The microwave spectrum of HNC17O is reported for the first time. The measurements were made with both Stark modulated and microwave Fourier transform spectrometers. The 17O and 14N quadrupole hyperfine structure has been resolved and rotational, centrifugal distortion and 17O and 14N quadrupole coupling constants have been evaluated. The 17O coupling constants are compared with those from an ab initio prediction.

A critical assessment of the overcoordination model for the P b center at the 〈111〉 Si/SiO 2 interface

In this paper we will review our recent calculations on two models for the P b center at the 〈111〉 Si/SiO 2 interface. The first model is the Poindexter model, the trivalent silicon atom. The second and more recently proposed model is one in which an oxygen atom that is part of the SiO 2 network interacts with a surface silicon atom to form a three-fold coordinated oxygen atom (and a four-fold silicon atom). We compare our theoretical results to experimental 29Si, and 17O hyperfine results, electrical levels as obtained from DLTS and C V measurements, and to recent high pressure DLTS results. In all cases, the Poindexter model is in excellent agreement, while the overcoordination model is in striking disagreement. The most recent experiment is the pressure-dependent DLTS. This imposes severe constraints on the sign and magnitude of interaction between a network oxygen atom and an otherwise trivalent silicon atom.

ChemInform Abstract: ESR Studies of Nitrogen‐Centered Free Radicals. Part 31. Electron Spin Resonance Spectroscopic Study of Cyclic Thiocarboxamidyl Radicals, 3‐Oxo‐1,2‐benzisothiazolin‐2‐yls: Complete Evaluation of ESR Parameters by Measuring 17O and 33S Hyperfine Splittings and Comparison of the ESR Parameters with Acyclic Analogues.

ChemInformVolume 19, Issue 49 Physical Organic Chemistry ChemInform Abstract: ESR Studies of Nitrogen-Centered Free Radicals. Part 31. Electron Spin Resonance Spectroscopic Study of Cyclic Thiocarboxamidyl Radicals, 3-Oxo-1,2-benzisothiazolin-2-yls: Complete Evaluation of ESR Parameters by Measuring 17O and 33S Hyperfine Splittings and Comparison of the ESR Parameters with Acyclic Analogues. Y. MIURA, Y. MIURA Dep. Appl. Chem., Fac. Eng., Osaka City Univ., Osaka 558, JapanSearch for more papers by this author Y. MIURA, Y. MIURA Dep. Appl. Chem., Fac. Eng., Osaka City Univ., Osaka 558, JapanSearch for more papers by this author First published: December 6, 1988 https://doi.org/10.1002/chin.198849042AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume19, Issue49December 6, 1988 RelatedInformation