FID detection of EPR and ENDOR spectra at high microwave frequencies

High-frequency pulsed EPR spectroscopy allows FID detection of EPR spectra owing to the short dead time that can be achieved. This FID detection is particularly attractive for EPR and ENDOR spectroscopy of paramagnetic species that exhibit inhomogeneously broadened EPR lines and short dephasing times. Experiments are reported for the metalloprotein azurin at 275 GHz.     

EPR Spectra of Transition-Metal Proteins: the Benefits of Data Deposition in Standard Formats

Electron paramagnetic resonance (EPR) spectroscopy can provide answers to significant questions about structure and function in biochemistry. Often EPR spectra of proteins containing transition-metal centers and radicals show the influence of electron spin–spin interactions between paramagnets. Analysis of these spectra provides information about distances and molecular orientations. For small proteins, the shape of the EPR spectra is sensitive to freezing effects, and to changes in the solvent environment; intermolecular spin–spin interactions are also observed. For large complex proteins, details of the EPR spectra are often highly conserved in evolution. Reference spectra of paramagnetic proteins are an aid to identification. The sharing of data is becoming a requirement in the public funding of research, and deposition in public databases is already standard for information such as protein structures and gene sequences. This would require the adoption of standard file formats, such as JCAMP-DX (Joint Committee on Atomic and Molecular Physical Data Exchange) or Bruker BES³T (Bruker EPR Standard for Spectrum Storage and Transfer). An archive of EPR spectra of different types of paramagnetic proteins would assist the identification of paramagnetic centers in biological materials. It would increase the profile of research data, allow comparison of different studies, and further interpretation of data in the light of subsequent discoveries.     

A New Q-Band EPR Probe for Quantitative Studies of Even Electron Metalloproteins

Existing Q-band (35 GHz) EPR spectrometers employ cylindrical cavities for more intense microwave magnetic fields B₁, but are so constructed that only one orientation between the external field B and B₁is allowed, namely the B B₁orientation, thus limiting the use of the spectrometer to measurements on Kramers spin systems (odd electron systems). We have designed and built a Q-band microwave probe to detect EPR signals in even electron systems, which operates in the range 2 K ≤ T ≤ 300 K for studies of metalloprotein samples. The cylindrical microwave cavity operates in the TE₀₁₁mode with cylindrical wall coupling to the waveguide, thus allowing all orientations of the external magnetic field B relative to the microwave field B₁. Such orientations allow observation of EPR transitions in non-Kramers ions (even electron) which are either forbidden or significantly weaker for B B₁. Rotation of the external magnetic field also permits easy differentiation between spin systems from even and odd electron oxidation states. The cavity consists of a metallic helix and thin metallic end walls mounted on epoxy supports, which allows efficient penetration of the modulation field. The first quantitative EPR measurements from a metalloprotein (Hemerythrin) at 35 GHz with B₁ B are presented.     

Growth and optical properties of BiI3 and PbI2 microcrystals

A study has been made of low-temperature absorption and luminescence spectra of bismuth iodate microcrystals in various hosts (layered cadmium iodate, microporous glasses and polymers), as well as of the spectra of mesoscopic domains formed in bulk bismuth iodate by mechanical strains. The structure of the spectra and the quantum shifts of the exciton levels yielded information on the dimensions and size dispersion of the microcrystals and domains, as well as on the effect of microcrystal size on the Stokes losses. The quantum exciton-line shifts in microcrystalline lead-iodate films grown in pores of the glass host have been used to calculate the film thicknesses. A photoinduced change in the mechanism of radiative recombination was observed in PbI₂ microcrystals. The spectrum of resonant Raman scattering in lead iodate microcrystals consists of broad bands corresponding to the branches of optical phonons undergoing strong angular dispersion. Fiz. Tverd. Tela (St. Petersburg) 40, 1724–1730 (September 1998)     

Structure and dynamics of annexin 12 bound to a planar lipid bilayer

Site directed spin labeling is used to investigate the protein annexin 12 absorbed on a single planar phospholipid bilayer of approximately 2-3 cm(2). Electron paramagnetic resonance spectra of nitroxide side chain at several topological sites reveal a conserved tertiary fold of the protein in the absorbed state, in agreement with earlier diffraction results. The angular dependent spectra of the two-dimensional microcrystals are shown to provide information on the degree of ordering of spin labels in a alpha-helix and in turn on the orientation of the alpha-helix with respect to the surface.     

Steady state and time-resolved studies of pyrene in solution and in single microcrystals

Single microcrystals of pyrene have been studied by steady state and time resolved fluorescence microscopy. The fluorescence spectra of microcrystals exhibit vibrational structure unlike the broad spectrum observed in pyrene excimer. A risetime is observed in the decay curves of the concentrated solutions, indicating the excimer formation. In contrast, the fluorescence decay profiles of the single microcrystals are nonexponential in nature and the decay times vary with their size and the wavelength of emission. This behaviour has been explained mainly by considering the pyrene dimer stabilized in the ground state.     

糖与氨基酸反应产物结构的EPR和~(13)C NMR研究(英文)

应用EPR和~(13)C NMR技术系统地研究了糖与氨基酸反应的产物。EPR波谱分析表明,反应产物的结构主要取决于氨基酸的结构。~(13)C NMR波谱的化学位移表明、反应产物的结构应该是2,5-哌嗪二酮,糖是作为1,4位取代的配位基。本文还提出了一种可能的反应机制。     

Fast right-angle spinning EPR on organic radicals: Resolution enhancement and angle determination

Fast right-angle sample spinning (RAS) with rotation frequencies up to 17 kHz at temperatures down to 205 K is applied to electron paramagnetic resonance (EPR) experiments on organic radicals. Echo-detected RAS EPR provides substantial resolution enhancements for the range of anisotropies between 10 and 100 MHz which is not accessible with either magic-angle sample spinning EPR or anisotropy-resolved EPR on the basis of slow rotation. The larger reorientation angles in experiments with fast spinning cause strong phase shifts of the echo, which manifest themselves as regions with negative intensity in the spectrum. These phase shifts and thus the lineshape in echo-detected RAS EPR depend significantly on the relative orientation of theg and hyperfine tensor. For the determination of anisotropies in poorly resolved spectra of organic radicals in disordered solids, we introduce the two-dimensional fixed-angle rotation experiment as an alternative to anisotropy-resolved EPR.     

Twinned EPR spectra of Fe centers in ternary layered TlGaS2 crystal

TlGaS₂ single crystal doped by paramagnetic Fe³⁺ ions has been studied by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra reveal a nearly orthorhombic symmetry of the crystal field (CF) on the Fe³⁺ ions. Two groups each consisting of four equivalent Fe³⁺ centers were observed in the EPR spectra. The local symmetry of the crystal field on the Fe³⁺ centers and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of the GaS₄ tetrahedrons. The rhombic distortion of the sulfur ligand CF is attributed to the effect of Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed twinning of the resonance lines indicates a presence of two non-equivalent positions of Tl ions that confirms their zigzag alignment in the TlGaS₂ crystal structure.     

糖与氨基酸反应产物结构的EPR和13C NMR研究

应用EPR和¹³C NMR技术系统地研究了糖与氨基酸反应的产物。EPR波谱分析表明,反应产物的结构主要取决于氨基酸的结构。¹³C NMR波谱的化学位移表明、反应产物的结构应该是2,5-哌嗪二酮,糖是作为1,4位取代的配位基。本文还提出了一种可能的反应机制。     

Electron paramagnetic resonance detection of the giant concentration of nitrogen vacancy defects in sintered detonation nanodiamonds

A giant concentration of nitrogen vacancy defects has been revealed by the electron paramagnetic resonance (EPR) method in a detonation nanodiamond sintered at high pressure and temperature. A high coherence of the electron spins at room temperature has been observed and the angular dependences of the EPR spectra indicate the complete orientation of the diamond system.     

Effect of nitrogen dioxide on the EPR property of lithium octa-n-butoxy 2,3-naphthalocyanine (LiNc-BuO) microcrystals

Lithium octa-n-butoxy-naphthalocyanine (LiNc-BuO) is a stable free radical that can be detected by electron paramagnetic resonance (EPR) spectroscopy. Previously we have reported that microcrystals of LiNc-BuO exhibit a single sharp EPR peak, whose width varies linearly with the partial pressure of paramagnetic molecules such as oxygen and nitric oxide. In this report, we present the effect of nitrogen dioxide (NO2), which is also a paramagnetic molecule, on the EPR properties of LiNc-BuO. The gas-sensing property of LiNc-BuO is attributed to the open molecular framework of the crystal structure which is arranged with wide channels capable of accommodating large molecules such as NO2. The EPR linewidth of LiNc-BuO was highly sensitive to the partial pressure of NO2 in the gas mixture. The line-broadening was quick and reversible in the short-term for low concentration of NO2. However, the EPR signal intensity decreased with time of exposure, apparently due to a reaction of NO2 with LiNc-BuO crystals to give diamagnetic products. The results suggested that LiNc-BuO may be a useful probe for the determination of trace amounts of NO2 using EPR spectroscopy.     

Spin-labeled photosynthetic reaction centers fromRhodobacter sphaeroides studied by electron paramagnetic resonance spectroscopy and molecular dynamics simulations

A new strategy has been applied that combines molecular dynamics (MD) simulations and electron paramagnetic resonance (EPR) spectroscopy to study the structure and conformational dynamics of the spin-labeled photosynthetic reaction center (RC) ofRhodobacter sphaeroides. This protein serves here as a model system to demonstrate the applicability of this new methodology. The RC contains five native cysteines and EPR experiments show that only one cysteine, located on the H subunit, is accessible for spin labeling. The EPR spectra calculated from MD simulation trajectories of spin labels bound to the native cysteines C156 and C234 in subunit H reveal that only the spin label side chain at position 156 provides a spectrum which agrees with the experimental EPR spectrum.     

EPR studies of photoinduced electron transfer in triad model compounds of photosynthesis

Time-resolved EPR spectra are reported for porphyrin-quinone-quinone and porphyrin-porphyrin-quinone triads obtained after photoexcitation in the nematic and soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the triplet states of the porphyrin created by spin-selective intersystem crossing (ISC) from the excited singlet state and those of the charge-separated radical pair states (RP) generated by electron transfer (ET) processes. The EPR polarization patterns of the RP are discussed in terms of the favored decay channel of the photoexcited singlet state of the porphyrin donor. The decay pathway may either be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene, significantly influences the ET mechanism and thus the polarization patterns of the RP spectra. Using liquid crystals, information about the orientation of the guest molecules in the liquid crystal matrix with respect to the long axes of the liquid crystal molecules can be obtained. In the porphyrin-porphyrin-quinone triads the energy and ET processes strongly depend on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal or both porphyrins bear a zinc atom.     

Analysis of nitroxide spin label motion in a protein-protein complex using multiple frequency EPR spectroscopy

X- and W-band EPR spectra, at room and low temperatures, are reported for nitroxide spin labels attached to cysteine residues selectively introduced into two proteins, the DNase domain of colicin-E9 and its immunity protein, Im9. The dynamics of each site of attachment on the individual proteins and in the tight DNase-Im9 complex have been analysed by computer simulations of the spectra using a model of Brownian dynamics trajectories for the spin label and protein. Ordering potentials have been introduced to describe mobility of labels restricted by the protein domain. Label mobility varies with position from completely immobilised, to motionally restricted and to freely rotating. Bi-modal dynamics of the spin label have been observed for several sites. We show that W-band spectra are particularly useful for detection of anisotropy of spin label motion. On complex formation significant changes are observed in the dynamics of labels at the binding interface region. This work reveals multi-frequency EPR as a sensitive and valuable tool for detecting conformational changes in protein structure and dynamics especially in protein-protein complexes.     

Optical measurements on semiconductor microcrystal beams generated by gas evaporation

We have developed a simple measurement method for measuring the optical spectra of free microcrystals, by combining the time-resolved and space-resolved spectroscopies with the gas evaporation method. The structure of the obtained spectra and their time- and space-dependencies indicate well the quantum-confinement effects of carriers and excitons in free semiconductors microcrystals.     

Electron paramagnetic resonance of scandium in silicon carbide

The EPR spectra of scandium acceptors and Sc²⁺(3d) ions are observed in 6H-SiC crystals containing a scandium impurity. The EPR spectra of scandium acceptors are characterized by comparatively small hyperfine interaction constants, whose values are consistent with the constants for other group III elements in SiC: boron, aluminum, and gallium acceptors. The EPR spectra of scandium acceptors undergo major changes in the temperature interval 20–30 K. In the low-temperature phase the EPR spectra are characterized by orthorhombic symmetry, whereas the high-temperature phase has higher axial symmetry. The EPR spectra observed at temperatures above 35 K and ascribed by the authors to Sc²⁺(3d) ions, or to the A ²⁻ state of scandium, have significantly larger hyperfine structure constants and narrower lines in comparison with the EPR spectra of scandium acceptors. The parameters of these EPR spectra are close to those of Sc²⁺(3d) in ionic crystals and ZnS, whereas the parameters of the EPR spectra of scandium acceptors correspond more closely to the parameters of holes localized at group III atoms, in particular, at scandium atoms in GeO₂. It is concluded that in all centers the scandium atoms occupy silicon sites. Fiz. Tverd. Tela (St. Petersburg) 39, 52–57 (January 1997)     

Morphology and optical spectra of metal iodate microcrystals in porous matrices

Microcrystals of iodates of mercury, lead, thallium, and bismuth were grown in pores of glass and polymer matrices from solutions or through sublimation in vacuum. Images of these microcrystals embedded in matrices were obtained with an electron microscope. Exciton absorption and luminescence spectra of the microcrystals revealed a strong difference in the temperature regions of stability of the various structural modfications between bulk crystals and microcrystals of some iodates. The absorption and luminescence spectra are broadened inhomogeneously because of considerable size dispersion of the microcrystals and exhibit quantum confinement effects. The exciton emission spectra show that exciton interaction in microcrystals becomes significant at very low optical pumping levels because of efficient excitation transfer from the matrix.     

EPR of Er3+, Nd3+, and Ce3+ ions in YAlO3 single crystals

EPR spectra of the Er³⁺, Nd³⁺, and Ce³⁺ ions substituting for the Y³⁺ ion in the YAlO₃ yttrium orthoaluminate lattice are studied. The EPR spectra of these rare-earth ions are described by a spin Hamiltonian of rhombic symmetry with an effective spin S=1/2. The principal values of the g tensors were determined from an analysis of the angular dependences of the EPR spectra. The orientation of the local magnetic axes of paramagnetic centers relative to the YAlO₃ crystallographic directions are shown to depend on the actual rare-earth species. The EPR spectra exhibit a hyperfine structure due to the ¹⁶⁷Er, ¹⁴³Nd, and ¹⁴⁵Nd odd isotopes, which permitted unambiguous identification of these spectra. The hyperfine coupling constants for the odd erbium and neodymium isotopes are determined.     

A correlation between manifestations of the magnetoplastic effect and changes in the electron paramagnetic resonance spectra after quenching of NaCl: Eu single crystals

The electron paramagnetic resonance (EPR) spectra of Eu²⁺ impurity ions in NaCl: Eu single crystals are investigated. It is found that the intensity of the EPR spectra undergoes prolonged (～200 h) multistage variations after quenching of NaCl: Eu single crystals. The variations observed in the EPR signal intensity are explained by the aggregation of impurity-vacancy dipoles into complexes. It is revealed that the magnetoplastic effect (a change in the microhardness in a magnetic field with an induction of 6 T) in these crystals manifests itself at an intermediate stage of impurity aggregation when all individual impurity-vacancy dipoles are temporarily stabilized in the sample. This can be associated with the thermally activated transformation of the internal atomic structure in the majority of already existing complexes.