Anisotropic Hyperfine Splitting Research Articles

Structures and cis-to-trans Photoisomerization of Hexafluoro-1,3-butadiene Radical Cation: Electron Spin Resonance and Computational Studies

The s-cis and s-trans isomer radical cations of hexafluoro-1,3-butadiene (s-cis-HFBD+ and s-trans-HFBD+) were generated by a gamma-irradiated solid solution of the neutral HFBD molecule in solid matrix at 77 K and observed by means of electron spin resonance (ESR) and electronic spectroscopies. In comparing the experimental isotropic and anisotropic 19F hyperfine splittings with the computational ones by the DFT B3LYP and MP2 methods, the generated s-cis-HFBD+ and s-trans-HFBD+ radical cations were confirmed to be 2A2 and 2Bg electronic ground states in C2v and C2h symmetries, respectively. The present spectroscopic study revealed that the relative abundance of s-cis-HFBD+ to s-trans-HFBD+ was 4.0 immediately after being formed by gamma-irradiation, and subsequently most s-cis-HFBD+ was isomerized to s-trans-HFBD+ by visible-light illumination with 500-600 nm wavelength. The process of nonplanar HFBD ionizing to form stable planar s-cis- and s-trans-HFBD+ and the reaction mechanism of the cis-to-trans photoisomerization were discussed by (MS-)CASPT2//CASSCF calculated vertical excitation energies (Tv) and torsional potential energy curves (TPECs) of HFBD and HFBD+.

TOAC Spin Labels in the Backbone of Alamethicin: EPR Studies in Lipid Membranes

Alamethicin is a 19-amino-acid residue hydrophobic peptide that produces voltage-dependent ion channels in membranes. Analogues of the Glu(OMe) 7,18,19 variant of alamethicin F50/5 that are rigidly spin-labeled in the peptide backbone have been synthesized by replacing residue 1, 8, or 16 with 2,2,6,6-tetramethyl-piperidine-1-oxyl-4-amino-4-carboxyl (TOAC), a helicogenic nitroxyl amino acid. Conventional electron paramagnetic resonance spectra are used to determine the insertion and orientation of the TOAC n alamethicins in fluid lipid bilayer membranes of dimyristoyl phosphatidylcholine. Isotropic 14N-hyperfine couplings indicate that TOAC 8 and TOAC 16 are situated in the hydrophobic core of the membrane, whereas the TOAC 1 label resides closer to the membrane surface. Anisotropic hyperfine splittings show that alamethicin is highly ordered in the fluid membranes. Experiments with aligned membranes demonstrate that the principal diffusion axis lies close to the membrane normal, corresponding to a transmembrane orientation. Combination of data from the three spin-labeled positions yields both the dynamic order parameter of the peptide backbone and the intramolecular orientations of the TOAC groups. The latter are compared with x-ray diffraction results from alamethicin crystals. Saturation transfer electron paramagnetic resonance, which is sensitive to microsecond rotational motion, reveals that overall rotation of alamethicin is fast in fluid membranes, with effective correlation times <30 ns. Thus, alamethicin does not form large stable aggregates in fluid membranes, and ionic conductance must arise from transient or voltage-induced associations.

Identification of the Triplet State N-V Defect in Neutron Irradiated Silicon Carbide by Electron Paramagnetic Resonance

Two types of a new triplet centers labeled as N-V have been observed in heavily neutron irradiated (dose of 1021 cm-2) and high-temperature annealed (2000°C) 6H-SiC crystals. The centers have an axial symmetry along c-axis. Anisotropic hyperfine splitting due to the one 14N nucleus has been observed. The EPR spectra of N-V defects in the triplet state in 6H-SiC reveal strong temperature dependence. The parameters of these centers are similar to that for well-known N-V center in diamond. It seems to consist of silicon vacancy and carbon substitutional nitrogen in the adjacent lattice cites oriented along c-axis. Similar to the diamond N-V centers in SiC have been produced by neutron irradiation and high-temperature annealing of the crystals containing nitrogen. For the first shell the structure of the N-V defect in 6H-SiC is practically identical with that in diamond. The charge state of this defect seems to be +1 compare with neutral state for nitrogensilicon vacancy defect in 6H-SiC with S=1/2.

Finite-temperature properties of the muonium substituted ethyl radical CH2MuCH2: nuclear degrees of freedom and hyperfine splitting constants*

The finite-temperature (T) properties of the muonium substituted ethyl radical CH2MuCH2 have been theoretically studied by Feynman path integral quantum Monte Carlo (PIMC) simulations. To derive the ensemble averaged expectation values we have combined the PIMC formalism with an efficient tight-binding (TB) Hamiltonian and a density functional operator of the B3LYP type in the EPRIII basis. The TB operator has been used to calculate the potential energy surface (PES) of the ethyl radical in the doublet ground state, the harmonic and anharmonic vibrational wave numbers as well as several probability density functions of the nuclei. The harmonic linear response approximation, which makes use of the Feynman centroid density, has been adopted to evaluate the anharmonic wave numbers. The large anharmonicities in the nuclear potential lead to bond lengths in thermal equilibrium which exceed the vibrationless parameters at the PES minimum. This enhancement is particularly strong for the C–Mu bond. It is responsible for the suppression of the intramolecular rotation for temperatures below room temperature. In C2 H5 the rotation is allowed down to 10 K. The dissimilar rotational dynamics for H2MuCH2 and C2 H5 has been studied with the help of TB-based probability density functions. The nuclear configurations of CH2MuCH2 and C2 H5, which are populated in thermal equilibrium, have been used to evaluate the isotropic and anisotropic hyperfine splitting (hfs) constants under explicit consideration of the nuclear vibrations and the internal rotation. The hfs constants have been determined with the help of the B3LYP-EPRIII Hamiltonian. The hindered low-temperature rotation in the Mu isomer is responsible for roto-vibrational corrections to the isotropic hfs constants which are smaller than the corrections in C2 H5. The shortcomings of single-configuration approaches for the evaluation of isotropic hfs constants have been demonstrated for both radicals. The ensemble corrections to the isotropic hfs parameters are correlated with fluctuations in the atomic spin densities. Differences in the absolute values of the isotropic hfs parameters in CH2MuCH2 and C2 H5 can be traced back to differences in the nuclear degrees of freedom. The ensemble shift for each isotropic hfs parameter can be explained by characteristic nuclear motions. For this discussion we make use of the distribution functions of the isotropic hfs constants. Roto-vibrational corrections to the anisotropic hfs constants are rather small. PIMC simulations have been performed between 25 and 1000 K, i.e. in a T interval that is large enough to consider nuclear effects beyond zero-point motions. The TB and B3LYP-EPRIII based physical quantities of CH2MuCH2 and C2 H5 have been compared with experimental findings whenever possible.

Electron Spin Resonance Study on Dynamic Heterogeneity in Miscible Blend of Poly(cyclohexyl methacrylate) and Poly(cyclohexyl acrylate)

Electron spin resonance (ESR) was applied to estimate the segmental dynamics of poly(cyclohexyl methacrylate) (PCHMA) and poly(cyclohexyl acrylate) (PCHA) in their miscible blend. The PCHMA and PCHA chains were labeled with nitroxide radicals, and the temperature-dependent ESR spectra of the spin-labels selectively reflected the segmental dynamics of the labeled chains in the blend. The ESR result showed different mobility between PCHMA and PCHA chains even though their blend showed a single glass transition in any composition by a calorimetric measurement. The WLF treatment demonstrated that a transition temperature, T5.0mT, at which the extreme separation width due to 14N anisotropic hyperfine splitting was 5.0 mT, reflected the glass transition observed at a frequency of the ESR. The T5.0mT of poly(CHMA-random-CHA) was dependent on the composition; namely, the mobility of the spin-labels was sensitive to the composition around them. The self-concentration model suggested by Lodge and McLeish [Lodge, T....

Dynamic Heterogeneity in Interfacial Region of Microphase-Separated Polystyrene-block-poly(methyl acrylate) Studied by the ESR Spin-Label Technique

Molecular motion in an interfacial region of microdomains of polystyrene-block-poly(methyl acrylate) (PS-block-PMA) was studied by electron spin resonance (ESR) technique. The junction points between the blocks, which are located in the interfacial region, were labeled with stable nitroxide radicals. Mobility of the spin-labels reflected the dynamic environments in the interfacial region. The transition temperature of the motion of the spin-labels, T5.0mT, at which the extreme separation width due to 14N anisotropic hyperfine splitting is 5.0 mT, was estimated, and it reflects a glass transition of the region around the labels. The T5.0mT of the PS-block-PMA labeled at the junction point was almost the mean value of those of the spin-labeled PS and PMA homopolymers, and the distribution of the motional correlation times (τc) in the interfacial region was much broader than that in the homopolymers. These results are considered to be caused by the heterogeneous mixture of the each segment in the interfacial region at a certain length scale. The molecular weight strongly influenced the interfacial thickness and the segmental mobility and the width of the distribution of the τc in the interfacial region as well as on the glass transition temperatures (Tg's) of the microdomains. It was revealed that the width of the distribution of the τc depended on not the interfacial thickness so much as the difference between the mobilities of the block chains in the microdomains. On the other hand, extremely small effects of the overall composition and the morphology of the PS-block-PMA on the segmental mobility in the interfacial region were observed. From these results, it was considered that the dynamic environment in the interfacial region was strongly affected by the gradient of the segmental concentration in the interfacial region and the mobility of the block chains in the microdomains.

Phosphorylation-Dependent Changes in Structure and Dynamics in ERK2 Detected by SDSL and EPR

Mitogen-activated protein kinases are regulated by occupancy at two phosphorylation sites near the active site cleft. Previous studies using hydrogen exchange to investigate the canonical mitogen-activated protein kinase, extracellular signal-regulated protein kinase-2, have shown that phosphorylation alters backbone conformational mobility >10 Å distal to the site of phosphorylation, including decreased mobility within amino acids 102–105 and increased mobility within 108–109. To further describe changes after enzyme activation, site-directed spin labeling at amino acids 101, 105–109, 111, 112 and electron paramagnetic resonance spectroscopy were used to investigate this region. The anisotropic hyperfine splitting of the spin labels in glassy samples was unchanged by phosphorylation, consistent with previous crystallographic studies that indicate no structural change in this region. At positions 101, 111, and 112, the mobility of the spin label was unchanged by diphosphorylation, consistent with little or no conformational change. However, diphosphorylation caused small but significant changes in rotational diffusion rates at positions 105–108 and altered proportions of probe in a motionally constrained state at positions 105, 107, and 109. Thus, electron paramagnetic resonance indicates reproducible changes in nanosecond side-chain mobilities at specific residues within the interdomain region, far from the site of phosphorylation and conformational change.

Influence of Chain End and Molecular Weight on Molecular Motion of Polystyrene, Revealed by the ESR Selective Spin-Label Method

A local segmental mobility was determined by electron spin resonance (ESR) spin-label method for a series of polystyrene (PS) with various molecular weights. Each PS specimen was selectively spin-labeled with stable nitroxide radicals at a chain end or inside sites. Molecular motion at the inside of the chain was compared with that at the chain end from the temperature dependence of ESR spectra of the nitroxide radicals. The transition temperature of molecular motion, T5.0mT, at which the extreme separation width due to 14N anisotropic hyperfine splitting is 5.0 mT, increased with an increase in molecular weight. The WLF equation confirmed that the T5.0mT correlated with a glass transition temperature, Tg, of PS. The T5.0mT for the spin-labeled PS at the chain end was ca. 5 K lower than that for the spin-labeled PS at the inside sites due to the enrichment of the specific free volume around the chain end. The transition temperature, T5.0mT, for both labeled PS depended on the molecular weight in accordance with the Unberreiter−Kanig equation for a glass transition. The T5.0mT for the spin-labeled PS at the chain end had a strong dependence on the molecular weight as compared with that at the inside sites because the molecular motion of the chain end was accelerated by an encounter of more than two chain ends. From the molecular weight dependence, we determined the short correlation time for segmental motion of the chain end, ca. 40 s, and the segment size undergoing the segmental motion at the Tg. The obtained segment size agreed well with the general segment size reported by others, 5−10 monomeric unit size.

Magnetically aligned phospholipid bilayers in weak magnetic fields: optimization, mechanism, and advantages for X-band EPR studies

Our lab is developing a spin-labeled EPR spectroscopic technique complementary to solid-state NMR studies to study the structure, orientation, and dynamics of uniaxially aligned integral membrane proteins inserted into magnetically aligned discotic phospholipid bilayers, or bicelles. The focus of this study is to optimize and understand the mechanisms involved in the magnetic alignment process of bicelle disks in weak magnetic fields. Developing experimental conditions for optimized magnetic alignment of bicelles in low magnetic fields may prove useful to study the dynamics of membrane proteins and its interactions with lipids, drugs, steroids, signaling events, other proteins, etc. In weak magnetic fields, the magnetic alignment of Tm 3+-doped bicelle disks was thermodynamically and kinetically very sensitive to experimental conditions. Tm 3+-doped bicelles were magnetically aligned using the following optimized procedure: the temperature was slowly raised at a rate of 1.9 K/min from an initial temperature being between 298 and 307 K to a final temperature of 318 K in the presence of a static magnetic field of 6300 G. The spin probe 3β-doxyl-5α-cholestane (cholestane) was inserted into the bicelle disks and utilized to monitor bicelle alignment by analyzing the anisotropic hyperfine splitting for the corresponding EPR spectra. The phases of the bicelles were determined using solid-state 2H NMR spectroscopy and compared with the corresponding EPR spectra. Macroscopic alignment commenced in the liquid crystalline nematic phase (307 K), continued to increase upon slowly raising the temperature, and was well-aligned in the liquid crystalline lamellar smectic phase (318 K).

Silicon vacancy containing two hydrogen atoms studied with electron paramagnetic resonance and infrared absorption spectroscopy

Float-zone and Czochralski-grown silicon crystals have been implanted with protons or deuterons. Electron-paramagnetic-resonance measurements performed during illumination with light at 1064 nm reveal a signal, labeled $\mathrm{DK}5,$ in addition to the well-known signal from ${\mathrm{VO}}^{*}$---the excited spin-triplet state of the oxygen-vacancy defect. The $\mathrm{DK}5$ signal originates from a spin-triplet state of a vacancy-type defect with monoclinic-I (near-orthorhombic-I) symmetry. In contrast to the ${\mathrm{VO}}^{*}$ signal, $\mathrm{DK}5$ has about the same intensity in the spectra recorded on oxygen-lean and oxygen-rich samples, which indicates that the $\mathrm{DK}5$ defect is not oxygen related. However, the close resemblance between the D tensors of $\mathrm{DK}5$ and ${\mathrm{VO}}^{*}$ strongly suggests that the electron-spin distributions are similar in the two defects. Moreover, anisotropic hyperfine splittings from two proton spins are partially resolved in the $\mathrm{DK}5$ signal. The signal is assigned to ${\mathrm{VH}}_{2}^{*},$ the excited spin-triplet state of the silicon vacancy containing two hydrogen atoms, which is the simplest defect consistent with the observed properties. The isochronal annealing behavior of $\mathrm{DK}5$ coincides with that of two infrared-absorption lines at 2063 and 2077 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which, like $\mathrm{DK}5,$ are observable only during illumination. These lines are assigned to Si-H stretch modes of ${\mathrm{VH}}_{2}^{*}.$

Molecular motion of poly(methyl methacrylate) chains tethered on a polyethylene surface by the spin-label method

Methyl methacrylate monomer was graft-copolymerized on a polyethylene (PE) surface and the tethered poly(methyl methacrylate) (PMMA) chains were spin-labelled to study the molecular motion of the PMMA chains by electron spin resonance (e.s.r.). Molecular mobility of the PMMA chains is strongly affected by physical structures of both PE and PMMA phases. For example, the transition temperature of molecular motion, T 5.0 mT , at which the extreme separation width due to 14N anisotropic hyperfine splitting is 5.0 mT, increases with increases in the grafting ratio and the crystallinity of the PE phase. The transition temperature for the grafted sample is lower than that for PMMA homopolymer bulk and the mobility of the PMMA chains in the mobile region of the grafted sample is much higher than that of PE chains on the PE surface. It is concluded that the structure and molecular motion of the PMMA chains tethered on the PE surface are strongly affected by the PE chains in the amorphous phase, the structure and molecular motion of which are also closely related to the crystallinity and/or the size of crystalline PE.

Structure and molecular motion of polystyrene chains adsorbed on porous silica surfaces by the spin label method

The spin-label method has been used to study the structure and molecular motion of polystyrene chains adsorbed on porous silica. Spin-labelled polystyrene (PS) with a narrow molecular weight distribution, having M w (weight average molecular weight) = 1.9 × 10 5 was adsorbed on the surface of porous silica with two different pore diameters, 28.5 nm (MB-300) and 81.3 nm (MB-800), in cyclohexane (C 6H 12) and carbon tetrachloride (CCl 4) solutions at 35°C. E.s.r. spectra were observed at various temperatures after the samples were completely dried. The anisotropic hyperfine splitting due to nitrogen nucleus observed at −196°C, which is a good measure of the magnitude of the PS-silica interaction, is larger for the adsorbed PS from the CCl 4 solution and the PS on the silica with the larger pore diameter. The molecular mobility of the PS chains was estimated from temperature dependence of the e.s.r. spectra. It is found that the conformations of the PS chains in the ⊖ (C 6H 12) and good (CCl 4) solvents affect the structures of adsorbed PS on the silica surface and the molecular mobility decreases with increasing PS-silica interaction.

Interactions of 77Se and 13CO with Nickel in the Active Site of Active F420-nonreducing Hydrogenase from Methanococcus voltae

The selenium-containing F420-nonreducing hydrogenase from Methanococcus voltae was prepared in the Nia(I) middle dotCO state. The effect of illumination on this light-sensitive species was studied. EPR studies were carried out with enzyme containing natural selenium or with enzyme enriched in 77Se. Samples were prepared with either CO or 13CO. In the Nia(I) middle dotCO state, the nuclear spins of both 77Se (I = 1/2) and 13C (I = 1/2) interacted with the nickel-based unpaired electron, suggesting that they are positioned on opposite sites of the nickel ion. In the light-induced signal, the interaction with 13CO was lost. The 77Se nuclear spin introduced an anisotropic hyperfine splitting in both the dark and light-induced EPR signals. The data on the active enzyme of M. voltae are difficult to reconcile with the crystal structure of the inactive hydrogenase of Desulfovibrio gigas (Volbeda, A., Charon, M. H., Piras, C., Hatchikian, E. C., Frey, M., and Fontecilla Camps, J. C. (1995) Nature 373, 580-587) and suggest a structural change in the active site upon activation of the enzyme.

Spin Label Study of the Factors Affecting Molecular Motion of Poly(methyl methacrylate)

The electron spin resonance (ESR) line shape of nitroxide radical labels attached to poly(methyl methacrylate) (PMMA) chains was studied as a function of temperature. In order to clarify various factors affecting molecular motions of PMMA chains, molecular mobility of a spin-labeled PMMA chain (L-chain) in another PMMA (M-chain) matrix was estimated by the ESR method. Isotactic and atactic PMMA having narrow molecular weight distribution were used as L- and M-chains. Strong dependencies of molecular mobility on molecular weights (ML and MM) of the chains and tacticity of M-chains were found. For example, the transition temperature, T50G at which the extrema separation width due to anisotropic hyperfine splitting drops sharply increases with increasing molecular weight of M-chains. The correlation times at the transition temperatures, calculated from the ESR spectra are compared with those in so-called relaxation map and the nitroxide rotational motion is concluded to strongly correlate with micro-Brownian motion of the PMMA chains. It was also found that the correlation times estimated by the ESR method is proportional to ML0.5 and MM when MM<2Me, and independent of MM when MM>2Me. Here, Me is the molecular Me weight between entanglements. These expetimental facts are reflected on Stokes-Einstein and reptation types of diffusions of L-chains.

Anisotropic hyperfine interactions of xenon nuclei surrounding a copper atom

The electronic structure and magnetic properties of a Cu atom trapped in an octahedral substitutional site of twelve Xe atoms are computed by the multiple scattering local-density-functional method. The experimental Cu and Xe hyperfine splittings agree within 25% with those computed by this technique. The Xe hyperfine tensor is found to be anisotropic and arises from a weak sigma-type bonding between the Cu and Xe atoms. These findings are in agreement with Adrian's theory that predicts anisotropic Xe hyperfine splittings. The isotropic appearance of the experimental Xe splittings is suggested to arise from an isotropic g tensor and the numerous overlapping lines of the two Xe isotopes.

Calculation of the hyperfine splitting constants for diatomic molecules using numerical wavefunctions

Isotropic and anisotropic hyperfine splitting constants have been calculated for the 2Σ states of BeH, MgH, CN, and CP. All wavefunctions were calculated numerically using our partial-wave procedure. Spin polarization has been introduced with selected single excitations from the restricted Hartree-Fock (RHF) wavefunctions, followed by complete or partial orbital optimization. For comparison the results of spin-unrestricted Hartree-Fock (SUHF) calculations are presented. Comparison is also made with RHF results as well as with available configuration interaction (CI) and experimental results.

ENDOR of perylene radicals adsorbed on alumina and silica-alumina powders. I. The ring protons

Complex electron-nuclear double-resonance (ENDOR) spectra were obtained from the radicals formed on activated alumina and silica-alumina powders exposed to solutions of perylene in benzene. Use of deuterated solvent to suppress spectral contributions from proton matrix effects enabled more detailed analysis of resonances associated with hyperfine interactions of ring protons of the perylene radical ion. A model of isotropic Zeeman interactions (g) and very anisotropic hyperfine splittings (A) is supported by multifrequency EPR measurements and ENDOR simulations devised for this work. All three proton hyperfine interactions of the perylene radical ion were resolved and the absolute values of the principal elements of the hyperfine coupling matrices were obtained. The fact that these values for radicals formed on alumina and silica-alumina were nearly identical implies that the same radical species forms on both surfaces. These findings are discussed and the radical is assigned as the cation.

Further study of the BeH radical by UHF-type and configuration interaction methods

The UHF, PUHF, EHF and CI methods have been used for calculating the isotropic and anisotropic hyperfine splitting constants and electronic energy for the BeH radical for 12 different basis sets of increasing flexibility. As the quality of the basis improves (based on an energy criterion) there is a convergence of the theoretical hyperfine splitting constants with the experimental values, except for the anisotropic h.f.s. constant for the H atom, for both the UHF and CI methods. The isotropic h.f.s. constants calculated by the EHF method converge to more positive values than observed experimentally. The EHF wavefunction is analysed in terms of a CI expansion using the EHF natural orbitals and reasons for the discrepancies are investigated.

Study of the BeH radical by UHF-type and complete configuration interaction methods

The UHF, PUHF, EHF and CCI methods have been used for calculating the isotropic and anisotropic hyperfine splitting constants and electronic energy of the BeH radical employing 6 different basis sets. In all cases, except for anisotropic h.f.s. constants for the H atom, as the quality of the basis improves (based on the energy criterion) the theoretical values approach the experimental ones.

ESR studies of the silver(2+) ion produced in Ag-X zeolite by .gamma.-irradiation

The silver-X zeolite was prepared and ..gamma..-irradiated by /sup 60/Co at room temperature. ESR measurements were made at -196/sup 0/C. A figure shows the ESR spectrum observed for silver-X ..gamma..-irradiated after evacuation at 350/sup 0/C. The anisotropic g factors, and the anisotropic hyperfine splittings obtained agree well with those of the divalent silver ion. The Ag/sup 2 +/ spectrum was broadened by exposing the sample to 150 Torr of oxygen, suggesting that the Ag/sup 2 +/ ions are located at the sites in the supercage. Upon the introduction of water vapor, the Ag/sup 2 +/ disappeared, suggesting that the polarization of water may result in the transfer of negative charge from anion center irradiation, and reduce the ion to Ag/sup +/. The effect of pretreatment temperature on the intensity of the Ag/sup 2 +/ spectrum increases up to 350/sup 0/, then it gradually decreases as the temperature increases, indicating that the zeolite structure can no longer stabilize the Ag/sup 2 +/ ion. The exposure of ..gamma..-irradiated Ag-X to pyridine changed the ESR spectrum showing 8 superhyperfine lines due to nitrogen nuclei, indicating 2 or 3 pyridine molecules adsorbed on one silver ion. The ESR spectrum of Ag/sup 2 +/ disappearedmore » upon heating the ..gamma..-irradiated sample to 150/sup 0/C, while the spectrum with nitrogen superhyperfine lines disappeared at 100/sup 0/C.« less