Previous Electron Paramagnetic Resonance Research Articles

Non-Henry’s Law behavior of REE partitioning between fluorapatite and CaF 2-rich melts: Controls of intrinsic vacancies and implications for natural apatites

The partitioning of rare-earth elements (REEs: Gd and multiple REEs), Sr, and Mn between fluorapatite and CaF 2-rich melts was investigated over a wide range of REE concentrations (i.e., from 0.8 ± 0.1 to 25,000 ± 2600 ppm Gd in fluorapatite) in two different sample assemblies (i.e., tightly covered Pt crucibles and sealed Pt capsules) at 1220 °C and atmospheric pressure. Attainment of equilibrium is indicated by selected reversal experiments. The partition coefficient D(Gd) decreases from ∼2 to ∼0.5 with increasing Gd in fluorapatite, hence a marked non-Henry’s Law behavior, but becomes independent of composition at and above ∼5000 and ∼1000 ppm Gd for experiments in Pt crucibles and Pt capsules, respectively. Non-Henry’s Law behavior is also observed in experiments involving multiple REEs. All REE patterns are convex upward in shape with maxima between Nd and Gd, and D(La)/D(Nd) and D(Nd)/D(Yb) decrease systematically with increasing total REEs in fluorapatite, suggesting that REE fractionations are partly related to non-Henry’s Law behavior. These experimental results and local structural data from previous electron paramagnetic resonance spectroscopic studies suggest that the non-Henry’s Law behavior of REE partitioning between fluorapatite and melt is controlled by intrinsic Ca 2+ vacancies in the c-axis channels. The D(Sr) and D(Mn) values are independent of composition and, therefore, do not deviate from the Henry’s Law in their respective compositional ranges investigated in this study. Nonstoichiometry, such as Ca 2+ and F − vacancies in the c-axis channels, is well known in natural apatites, particularly in biogenic apatites. Therefore, the observed non-Henry’s Law behavior of REE partitioning is expected to have important implications for REE geochemical modeling involving apatites and for the uptake of REEs by natural apatites. Particularly, the non-Henry’s Law behavior of REE partitioning is at least partly responsible for the commonly observed, bell-shaped REE patterns in fossil biogenic apatites.

X-ray absorption and resonance Raman studies of methyl-coenzyme M reductase indicating that ligand exchange and macrocycle reduction accompany reductive activation.

Methyl-coenzyme M reductase (MCR) catalyzes methane formation from methyl-coenzyme M (methyl-SCoM) and N-7-mercaptoheptanoylthreonine phosphate (CoBSH). MCR contains a nickel hydrocorphin cofactor at its active site, called cofactor F(430). Here we present evidence that the macrocyclic ligand participates in the redox chemistry involved in catalysis. The active form of MCR, the red1 state, is generated by reducing another spectroscopically distinct form called ox1 with titanium(III) citrate. Previous electron paramagnetic resonance (EPR) and (14)N electron nuclear double resonance (ENDOR) studies indicate that both the ox1 and red1 states are best described as formally Ni(I) species on the basis of the character of the orbital containing the spin in the two EPR-active species. Herein, X-ray absorption spectroscopic (XAS) and resonance Raman (RR) studies are reported for the inactive (EPR-silent) forms and the red1 and ox1 states of MCR. RR spectra are also reported for isolated cofactor F(430) in the reduced, resting, and oxidized states; selected RR data are reported for the (15)N and (64)Ni isotopomers of the cofactor, both in the intact enzyme and in solution. Small Ni K-edge energy shifts indicate that minimal electron density changes occur at the Ni center during redox cycling of the enzyme. Titrations with Ti(III) indicate a 3-electron reduction of free cofactor F(430) to generate a stable Ni(I) state and a 2-electron reduction of Ni(I)-ox1 to Ni(I)-red1. Analyses of the XANES and EXAFS data reveal that both the ox1 and red1 forms are best described as hexacoordinate and that the main difference between ox1 and red1 is the absence of an axial thiolate ligand in the red1 state. The RR data indicate that cofactor F(430) undergoes a significant conformational change when it binds to MCR. Furthermore, the vibrational characteristics of the ox1 state and red1 states are significantly different, especially in hydrocorphin ring modes with appreciable C=N stretching character. It is proposed that these differences arise from a 2-electron reduction of the hydrocorphin ring upon conversion to the red1 form. Presumably, the ring-reduction and ligand-exchange reactions reported herein underlie the enhanced activity of MCR(red1), the only form of MCR that can react productively with the methyl group of methyl-SCoM.

SITE PREFERENCE OF Gd IN SYNTHETIC FLUORAPATITE BY SINGLE-CRYSTAL W-BAND EPR AND X-RAY REFINEMENT OF THE STRUCTURE: A COMPARATIVE STUDY

Fluorapatite containing 1.2(2) wt% Gd2O3 (sample AP30–1), synthesized from a CaF2-rich melt in a Pt crucible, has been investigated by single-crystal W-band (~94 GHz) electron paramagnetic resonance (EPR) spectroscopy and X-ray structure refinement. The EPR spectrum of AP30–1 measured at ~287 K, with Zeeman field B // z’ and microwave field B // x’, gave a Gd site-occupancy ratio ( Ca2 Gd/ Ca1 Gd) of 1.4(4), indicating a weak preference between the two types of Ca sites. Similarly, X-ray refinement of the structure of AP30–1 suggests that Gd is essentially equally distributed between the two Ca sites: Ca2 Gd/ Ca1 Gd = 0.8(2), although the Gd content in this sample is close to the practical limit of detection of this method. This agreement between the W-band EPR and X-ray structural data confirms that EPR is capable of determining the distribution of paramagnetic trace elements between structurally nonequivalent positions in minerals. X-ray refinement of the structure of another sample of fluorapatite [AP40–0, containing 2.9(1) wt% Gd2O3] synthesized in a sealed Pt capsule, yielded a Ca2 Gd/ Ca1 Gd value of 3.1(6), indicating a marked preference for the Ca2 site, consistent with the site preference of Gd in hydrothermally grown Gd-rich fluorapatite. The marked preference of Gd for the Ca1 site observed in previous EPR studies of flux-grown fluorapatite samples [AP30–0, 0.8(1) ppm Gd and Ca2 Gd/ Ca1 Gd = 0.13; AP30–5, 57(4) ppm Gd and Ca2 Gd/ Ca1 Gd = 0.20] is now attributed to the presence of Ca 2+ vacancies at ppm concentrations in these crystals. These point defects are unlikely to reach appreciable (weight %) concentrations in fluorapatite crystals synthesized from CaF2-rich melts, and hence the normal preference of Gd for the Ca2 site in AP30–1 and AP40–0.

Force generation, but not myosin ATPase activity, declines with age in rat muscle fibers.

We tested the hypothesis that age-associated decline in muscle function is related to a change in myosin ATPase activity. Single, glycerinated semimembranosus fibers from young (8-12 mo) and aged (32-37 mo) Fischer 344 x Brown Norway male rats were analyzed simultaneously for force and myosin ATPase activity over a range of Ca2+ concentrations. Maximal force generation was ~20% lower in fibers from aged animals (P = 0.02), but myosin ATPase activity was not different between fibers from young and aged rats: 686 +/- 46 (n = 30) and 697 +/- 46 microM/s (n = 33) (P = 0.89). The apparent rate constant for the dissociation of strong-binding myosin from actin was calculated to be ~30% greater in fibers from aged animals (P = 0.03), indicating that the lower force produced by fibers from aged animals is due to a greater flux of myosin heads from the strong-binding state to the weak-binding state during contraction. This is in agreement with our previous electron paramagnetic resonance experiments that showed a reduced fraction of myosin heads in the strong-binding state during a maximal isometric contraction in fibers from older rats.

X and Q-band endor study of the Fe+ (I) center in chlorinated SrCl2 single crystals

Previous Electron Paramagnetic Resonance (EPR) studies have shown that Fe2+ doped SrCl2 crystals grown in a chlorine atmosphere exhibit new trapped electron Fe+ centers after irradiation with X or γ-rays at 77 K. Two types of such centers, called Fe+ (I) and Fe+ (II), both exhibiting axial ⟨001⟩ symmetry, have been identified. We present here the results of an Electron Nuclear Double Resonance (ENDOR) study in X and Q-band on the Fe+ (I) center, which has been found to be thermally stable up to 700K. The superhyperfine and quadrupole interactions of the paramagnetic Fe+ ion with the nearest Cl− ions have been determined from the ENDOR angular dependence data. The results strongly suggest that the Fe+ (I) center consists of a Fe+ ion in the center of a cube of eight Cl− ions, compressed along an ⟨001⟩ direction.

High spatial resolution multi-site EPR oximetry. The use of convolution-based fitting method.

We describe a new method to enhance the spatial resolution of multi-site electron paramagnetic resonance (EPR) oximetry. The method is suitable for any shape (density distribution function) of a solid paramagnetic material implanted in tissue. It corrects distortions of lineshapes caused by the gradient and thus overcomes limitations of previous multi-site EPR oximetry methods that restricted the ratio of the particle size to the distance between sites. The new method is based on consecutive applications of magnetic field gradients with the same direction but with a different magnitude and uses a convolution-based fitting algorithm to derive Lorentzian EPR linewidths of each individual peak of the EPR spectrum. The method is applicable for any particulate EPR oxygen sensitive materials whose EPR spectra can be approximated by a Lorentzian function or a superposition of Lorentzian functions. By incorporating this model of the lineshape in the data processing, we are able to decrease significantly the number of parameters needed for the calculations and to recover the oxygen concentration, even from quite noisy spectra. We (i) describe our method and the data-processing algorithm, (ii) demonstrate our approach in model and in vivo experiments, and (iii) discuss the limitations.

High Spatial Resolution Multi-Site EPR OximetryThe Use of a Convolution-Based Fitting Method

We describe a new method to enhance the spatial resolution of multi-site electron paramagnetic resonance (EPR) oximetry. The method is suitable for any shape (density distribution function) of a solid paramagnetic material implanted in tissue. It corrects distortions of lineshapes caused by the gradient and thus overcomes limitations of previous multi-site EPR oximetry methods that restricted the ratio of the particle size to the distance between sites. The new method is based on consecutive applications of magnetic field gradients with the same direction but with a different magnitude and uses a convolution-based fitting algorithm to derive Lorentzian EPR linewidths of each individual peak of the EPR spectrum. The method is applicable for any particulate EPR oxygen sensitive materials whose EPR spectra can be approximated by a Lorentzian function or a superposition of Lorentzian functions. By incorporating this model of the lineshape in the data processing, we are able to decrease significantly the number of parameters needed for the calculations and to recover the oxygen concentration, even from quite noisy spectra. We (i) describe our method and the data-processing algorithm, (ii) demonstrate our approach in model and in vivo experiments, and (iii) discuss the limitations.

Unraveling the Cu2+ Binding Sites in the C-Terminal Domain of the Murine Prion Protein: A Pulse EPR and ENDOR Study

The mammalian prion protein (PrPC) is a cell surface protein consisting of a flexibly disordered N-terminal segment (residues 23−120) and a structured C-terminal domain (residues 121−231). PrPC is supposed to bind Cu2+ in vivo, and several studies have recently focused on the ability of this protein to bind divalent cations. In a previous continuous wave electron paramagnetic resonance (CW EPR) study, we showed that Cu(II) binds both to the N- and C-terminal parts of PrPC. Here we present a pulse EPR and electron nuclear double resonance (ENDOR) study of the three different Cu(II) binding sites observed in the structured, C-terminal part of the murine prion protein, mPrP(121−231). It was found that the three complexes are distinguished by a different number of nitrogen atoms directly involved in the Cu(II) ligation. For one of the Cu(II) binding sites that is observed at low pH (3−6), no directly coupled nitrogens could be observed. For a second type of Cu(II) complex, observed at pH 3−8, Davies-ENDOR and hyperfine sublevel correlation (HYSCORE) spectroscopy revealed that histidine is one of the binding ligands. Furthermore, the presence of a nonexchangeable proton close to a copper ion could be demonstrated in a sample containing mainly the second Cu(II) complex. For the third mode of Cu(II) complexation, which can be detected at pH 7−8, Davies-ENDOR spectra indicate that more than one nitrogen atom is directly bound to the copper ion. The observed EPR parameters suggest the involvement of backbone nitrogens in this copper(II) complex.

Theoretical analysis of alkali metal trapping sites in rare gas matrices

The rare gas (Ne, Ar, Kr, Xe)–alkali metal (Li, Na) ground-state pair interaction potentials and distance-dependent isotropic hyperfine coupling constants are evaluated by coupled-cluster approaches at the van der Waals region of the dimers. The computed properties are further utilized in classical molecular dynamics simulations of rare gas lattices doped with alkali atoms. Atomic trajectories and time averaged hyperfine constants are obtained from the simulations and exploited to provide theoretical insights into experimentally observed atomic trapping and dynamics of alkali metal atoms in rare gas matrices. The simulations support our previous electron paramagnetic resonance (EPR) data [Chem. Phys. Lett, 310, 245 (1999)], suggesting that alkali metal atoms, while generated by laser vaporization, do trap in single substitutional sites, whereas thermal atom sources yield trapping in multiple substitutional sites. In order to theoretically reproduce the EPR spectra for the latter case, more than six neighboring vacancies had to be included in the model system. Based on the simulations, the trapped atoms are able to move rather freely within the extended cage.

Carbon-related defects in proton-irradiated, n-type epitaxial Si1−xGex

C i and CiCs defects, created by proton irradiation of n-type, strain-relaxed, epitaxial Si1−xGex of 0.005⩽x⩽0.5, have been studied using deep-level transient spectroscopy (DLTS). The ionization enthalpies of the two defects relative to the conduction band edge, ΔH, are found to increase linearly with increasing Ge content. It is shown that the corresponding levels are not pinned to any of the band edges. Furthermore, it is shown that, for both defects, the slopes, δΔH/δx, as well as the full width at half maximum (FWHM) of the corresponding DLTS peaks, are similar. These observations are in agreement with conclusions deduced from previous electron-paramagnetic resonance (EPR) measurements in pure silicon, stating that, for both defects, the trapped electron is preferentially located at the Ci atom because of its larger electronegativity as compared to those of silicon and germanium. The anneal temperature of the Ci defect, and correspondingly the in-growth temperature of the CiCs complex, increase with increasing Ge content. This is equivalent to an increasing retardation of the diffusion of Ci in Si1−xGex with increasing Ge content.

Identification of Two Important Heme Site Residues (Cysteine 75 and Histidine 77) in CooA, the CO-Sensing Transcription Factor of Rhodospirillum rubrum

The CO-sensing mechanism of the transcription factor CooA from Rhodospirillum rubrum was studied through a systematic mutational analysis of potential heme ligands. Previous electron paramagnetic resonance (EPR) spectroscopic studies on wild-type CooA suggested that oxidized (FeIII) CooA contains a low-spin heme with a thiolate ligand, presumably a cysteine, bound to its heme iron. In the present report, electronic absorption and EPR analysis of various substitutions at Cys residues establish that Cys75 is a heme ligand in FeIII CooA. However, characterization of heme stability and electronic properties of purified C75S CooA suggest that Cys75 is not a ligand in FeII CooA. Mutational analysis of all CooA His residues showed that His77 is critical for CO-stimulated transcription. On the basis of findings that H77Y CooA is perturbed in its FeII electronic properties and is unable to bind DNA in a site-specific manner in response to CO, His77 appears to be an axial ligand to FeII CooA. These results imply a ligand switch from Cys75 to His77 upon reduction of CooA. In addition, an interaction has been identified between Cys75 and His77 in FeIII CooA that may be involved in the CO-sensing mechanism. Finally, His77 is necessary for the proper conformational change of CooA upon CO binding.

Electron nuclear double resonance investigation of iron-acceptor pairs in silicon

With stationary electron nuclear double resonance (ENDOR) the two - pairs having trigonal and orthorhombic symmetry, respectively, and the trigonal - pair have been investigated. The hyperfine and quadrupole interactions with the interstitial and the acceptor nuclei and , respectively, have been determined with higher precision than possible with previous electron paramagnetic resonance (EPR) investigations. Also the interactions with several shells of neighbour nuclei around the pairs have been determined. For all three pair defects `negative' stationary ENDOR signals from and have been observed, where the saturation of the partially saturated EPR signal is enhanced upon induction of a nuclear magnetic transition in contrast to the usual positive stationary ENDOR signals where it is diminished.

Spectroscopic investigations on the highly purified lactoperoxidase Fe(III)-heme catalytic site

Purification of the lactoperoxidase (LPO) major cationic isoenzyme was significantly improved by the use of preparative chromatographic and electrophoretic methods combined with analytical electrophoretic techniques and image processing. A detailed report is given of the experimental procedure. Furthermore, electron paramagnetic resonance has played a fundamental role in evaluating the enzyme purity against lactoferrin and minor LPO isoenzyme components in setting the final steps of the purification. With the aim to completely clarify the Fe(III)-heme high-spin nature of the native LPO, two samples of lactoperoxidase, LPO1 and LPO2 ( RZ = 0.95) from farm and commercial milk, respectively, were purified and characterized in particular by electron paramagnetic resonance (EPR) spectroscopy, in comparison with a commercial preparation (LPOs). The LPO1 EPR spectrum, at physiological pH, is clearly indicative of the presence of an iron(III)-heme high-spin catalytic site in the native enzyme. On the contrary, in the LPO2 spectrum a thermal equilibrium between high- and low-spin iron(III)-heme species is present. The low-spin component of the spectrum has been assigned to an LPO-NO − 2 adduct due to the presence of some nitrite impurities originating either from commercial unpasteurized milk or from external sources. The LPOs EPR spectrum shows the presence of some spurious lines in the g ≊ 6 and 4 regions due to the minor LPO isoenzyme components and to lactoferrin, respectively. The LPO EPR spectra previously reported in the literature contain a variable number of spurious lines in the g ≊ 4 and 2 regions as a consequence of lactoferrin impurity and LPO low-spin adducts with endogenous or exogenous anions. Furthermore, the interaction of LPO with its native substrate (the thiocyanate anion), which previously was shown by NMR and EPR (at high substrate concentration) spectroscopies, has been confirmed by EPR at low temperature and low substrate concentration and by optical spectroscopy at room temperature and high substrate concentration as a function of pH. The LPO activity at optimum pH ( ≊ 4–5) has been measured in phosphate and acetate buffer using as an oxidizable substrate the system dimethylamino benzoic acid 3-methyl-2-benzothiazolinone hydrazone hydrochloride monohydrate (DMAB-MBTH), which was considered a good chromogen for other peroxidases such as HRP and zucchini peroxidases. The LPO vs. SCN − activity at optimum pH ( ≊5.5) has been measured in phosphate and acetate buffer. Kinetic studies in the presence of the DMAB-MBTH chromogen show a noncompetitive inhibition between hydrogen peroxide and thiocyanate, in particular at optimum SCN − substrate activity around pH 5.5. Spectroscopic results together with activity and kinetic data seem to indicate the insertion of thiocyanate as a ligand bridge between Fe(III)-heme and N 3H imidazole of the distal histidine to give a small Fe(III) interaction at room temperature, which became stronger at low temperature. Previous NMR and preliminary EPR measurements suggested the heme pocket site close to the Fe(III) ion as the SCN − binding site.

Investigation of Solid and Solution Structures of N-Substituted Cu(II) Salicyldimines by X-Ray Absorption Spectroscopy

A series of bis(N-alkylsalicylaldiminato)copper(II) complexes that serve as models for mononuclear, nontetragonal copper(II) sites in copper proteins are examined by X-ray absorption spectroscopy in order to investigate their structural characteristics in crystalline, room temperature solution, and frozen solution forms. Previous electron paramagnetic resonance (EPR) studies of frozen solution samples of bis(tert-butylsalicylaldiminato)copper(II) (Cu{sup II}(t-butSal){sub 2}) suggest solvent dependent variation in site symmetry. In addition, linear electric field effect (LEFE) investigations in frozen solution reveal a deviation from centrosymmetry greater than expected based on X-ray crystal structure. X-ray edge results indicate that a more tetrahedral coordination geometry is assumed by Cu{sup II}(t-butSal){sub 2} in solution preparations, both at room temperature or frozen, when compared with crystalline samples. Extended X-ray absorption fine structure (EXAFS) studies of the Cu{sup II}(t-butSal){sub 2} species show no difference in metal - ligand distances between crystalline or frozen solution samples. In contrast, no differences in X-ray edge spectra were detected between crystalline and solution forms of bis(methyl-) and bis-(propylsalicylaldiminato)copper(II) complexes nor for bis(salicylaldehyde)copper(II), which suggests that bulkiness of the N-alkyl group in salicyldimines mediates the geometric sensitivity to local environmental conditions.

Identification of the Iron-Boron Line Spectrum in Silicon

A new line spectrum at about 860 meV has been observed in silicon co-doped with iron and boron. The isochronal annealing behaviour of this line spectrum and the FeB centre was studied by transmission spectroscopy and electron paramagnetic resonance (EPR), respectively. The good agreement between the two measurements strongly suggests that the line spectrum originates from excitations at the trigonal FeB pair complex, well known from previous EPR studies. The positions of the FeB donor and acceptor levels were estimated to be at Ev+110 meV and Ec-275 meV respectively, in close agreement with previously reported values for the FeB pair. Furthermore, the energy difference between the ground and first excited states of this centre was determined to be 1.09 meV, which compares favourably with the 0.75 meV EPR value previously obtained.

Identification of the iron-boron line spectrum in silicon

A new line spectrum at about 860 meV has been observed in silicon co-doped with iron and boron. The isochronal annealing behaviour of this line spectrum and the FeB centre was studied by transmission spectroscopy and electron paramagnetic resonance (EPR), respectively. The good agreement between the two measurements strongly suggests that the line spectrum originates from excitations at the trigonal FeB pair complex, well known from previous EPR studies. The positions of the FeB donor and acceptor levels were estimated to be at Ev+110 meV and Ec-275 meV respectively, in close agreement with previously reported values for the FeB pair. Furthermore, the energy difference between the ground and first excited states of this centre was determined to be 1.09 meV, which compares favourably with the 0.75 meV EPR value previously obtained.

Energy levels of the SbGa heteroantisite defect in GaAs:Sb

A transient capacitance study of antimony-doped bulk GaAs has led to the identification of two energy levels related to the SbGa heteroantisite defect. The levels with electron emission activation energies of 0.54 and 0.70 eV are typically overshadowed by omnipresent EL3 and EL2 traps related to oxygen defect and the arsenic antisite, respectively. Positive identification of the levels, and determination of their emission rate signatures, was made possible employing GaAs crystals with a defect structure especially engineered to achieve very low concentrations of background traps. Relationship of the levels to the SbGa defect is deduced from excellent agreement with previous electron paramagnetic resonance results.

Electron paramagnetic spin trapping detection of free radicals generated in direct photolysis of 4-bromophenol in aqueous solution

The photolysis of 4-bromophenol in aqueous solution in the presence of the spin trap 5,5-dimethylpyrroline- N-oxide (DMPO) results in the formation of four distinct electron paramagnetic resonance (EPR) spectra. Three of these have been identified as arising from the DMPO spin adducts of an aryl radical (probably the 4-hydroxyphenyl radical, spectrum A), a hydrated electron plus later protonation (spectrum B) and the hydroxyl radical (spectrum C). Spectrum D is that of the p-benzosemiquinone anion. Spectra A and B appear immediately on initiation of photolysis in both deoxygenated and oxygenated solutions. Spectra C and D arise from secondary products and require the presence of oxygen. The component EPR spectra do not depend on the pH value in the pH range 7.0 × 10.5, but the overall appearance of the spectra changes with increasing pH, showing a larger contribution of both adducts A and B compared with adduct C. The results are compared with those of previous EPR investigations of 4-chlorophenol and with photochemical studies of 4-substituted halogenophenols using conventional sources of irradiation and the flash photolysis technique. The mechanism of aqueous photolysis of the compounds is discussed.

Structure of a substrate radical intermediate in the reaction of lysine 2,3-aminomutase.

Electron paramagnetic resonance (EPR) spectroscopy has been used to characterize an organic radical that appears in the steady state of the reaction catalyzed by lysine 2,3-aminomutase from Clostridium SB4. Results of a previous electron paramagnetic resonance (EPR) study [Ballinger, M. D., Reed, G. H., & Frey, P. A. (1992) Biochemistry 31, 949-953] demonstrated the presence of EPR signals from an organic radical in reaction mixtures of the enzyme. The materialization of these signals depended upon the presence of the enzyme, all of its cofactors, and the substrate, lysine. Changes in the EPR spectrum in response to deuteration in the substrate implicated the carbon skeleton of lysine as host for the radical center. This radical has been further characterized by EPR measurements on samples with isotopically substituted forms of lysine and by analysis of the hyperfine splittings in resolution-enhanced spectra by computer simulations. Changes in the hyperfine splitting patterns in EPR spectra from samples with [2-2H]lysine and [2-13C]-lysine show that the paramagnetic species is a pi-radical with the unpaired spin localized primarily in a p orbital on C2 of beta-lysine. In the EPR spectrum of this radical, the alpha-proton, the beta-nitrogen, and the beta-proton are responsible for the hyperfine structure. Analysis of spectra for reactions initiated with L-lysine, [3,3,4,4,5,5,6,6-2H8]lysine, [2-2H]lysine, perdeuteriolysine, [alpha-15N]lysine, and [alpha-15N,2-2H]lysine permit a self-consistent assignment of hyperfine splittings.(ABSTRACT TRUNCATED AT 250 WORDS)

Free Radical Mechanisms in High Density Nitrocompounds: Hexanitroisowurtzitane, A New High-Energy Nitramine

Abstract A new polycyclic nitramine, hexanitroisowunzitane (HNIW), has physical properties which differ from monocyclic nitramines. One unique property of HNIW is the formation of inclusion sites in the a-polymorph which can trap solvents or other impurities, forming clathrate complexes. Trapped solvents such as methanol and H2O have been observed directly by crystal structures of α-HNIW. Free radicals such as (i) nitric oxide and nitrogen dioxide, (ii) nitroxides, (iii) radical pairs, and (iv) ionic or non-ionic paramagnetic fragments can be formed in crystalline nitramines by photolytic and thermal decomposition. In a previous EPR (electron paramagnetic resonance) study, trapped NO2 radicals have been analyzed in a single crystal of HNIW at 77 K before (I) and after (II) annealing an HNIW crystal at 298 K. In this study, the formation of a free radical product from thermal decomposition of HNIW is reported. The radical is detected in the temperature range from 403–443 K by heating HNIW in sulfolane (b.p...