Low-temperature Electron Paramagnetic Resonance Spectra Research Articles

Electrical and structural characterization of shallow As acceptors in natural p-type 2H-MoS2

Hexagonal molybdenite (MoS2) is one of the most promising two-dimensional (2D) semiconductors, known with n-type and p-type conduction, with possible applications in electronic, opto-electronic, and spintronic devices. In this work, highly pure geological samples of 2H-MoS2 were investigated by temperature-dependent electron paramagnetic resonance (EPR) and Hall effect measurements. The low-temperature (<55 K) EPR spectra were consistently explained as holes (S = 1/2) trapped on shallow As acceptors on sulfur sites leading to a four-line hyperfine-split spectrum (75As: I = 3/2, 100%) in axial symmetry and with a concentration of 5(1) ppm (∼2 × 1017 cm−3). Electrical measurements indicate p-type conduction with a free carrier concentration of about 5 × 1017 cm−3 at room temperature and an ionization energy of 52 meV associated with the shallow As acceptors, which is consistent with the ionization energy determined from the hydrogenic model of shallow acceptors in 2H-MoS2. These values are in strong contrast to the unrealistic value of 0.7 meV reported in the literature obtained from the analysis of temperature-dependent EPR measurements. Possible explanations are related to temperature-dependent spin-lattice relaxation effects, affecting strongly EPR line intensities and making impossible their use in obtaining the ionization energy.

Photodarkening-resistance improvement of Yb3+/Al3+ co-doped silica fibers fabricated via sol-gel method.

Yb3+/Al3+ co-doped silica fibers (YDFs) with almost identical core glass compositions were prepared using the sol-gel and modified chemical vapor deposition (MCVD) methods. The photodarkening (PD) and laser performance before and after the PD process were tested under 974 nm pumping. The doping homogeneity of Yb3+ ions and clusters of Yb3+ ions in preform slices of these two fibers were investigated via optical absorption spectroscopy, photoluminescence emission spectra, electron probe microanalysis (EPMA), and low-temperature (4 K) electron paramagnetic resonance (EPR). It is known that the PD resistance of YDFs prepared via the sol-gel method is significantly better than that of YDFs prepared via MCVD under the same test conditions. EPMA mapping reveals that the doping homogeneity of Yb3+ ions in the sol-gel fiber core glass is better than that in the MCVD fiber. The low-temperature (4 K) EPR and cooperative luminescence spectra of Yb3+ ions indicate that the clustering degree of Yb3+ ions in the sol-gel fiber is lower than that in the MCVD fiber. In the absorption and emission spectra, small amounts of Yb2+ ions are observed in the preform slice from the sol-gel method. A model of the color-center generation in the PD process was proposed to explain the mechanism of PD resistance improvement for the YDFs fabricated via the sol-gel method.

Magnetic characterization of microcrystalline Na3Ln0.99–xEr0.01Crx(PO4)2 orthophosphates synthesized by Pechini method (Ln = La, Gd)

Abstract Na3Ln(PO4)2 orthophosphates (Ln = La, Gd) doped with Er3+ and co-doped with Cr3+ ions were synthesized by Pechini method and characterized by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Low temperature EPR spectra were detected and analyzed in terms of temperature dependence and the structure of the obtained materials. They show that erbium and chromium ions substitute Ln3+ and also Na+ ions or Na+ channels forming complex EPR spectra. Both kinds of ions reveal ferromagnetic type of interaction which shows some anomaly at the temperature between 10 K and 15 K. Magnetic susceptibility reveals a weak antiferromagnetic kind of interaction dominating in the whole temperature range, from 3.5 to 300 K.

Synthesis, crystal structure, spectroscopic, fluorescent, thermal properties and EPR spectra of doped Cu2+ ions in [Cd(sac)2(H2O)2(meim)2] single crystal

The crystal structures of the six-coordinate complexes [Cd(sac)2(H2O)2(meim)2] (complex 1) formed by reaction of 4-methylimidazole(meim) with [Cd(sac)2(H2O)4]·2H2O (saccharinate=sac), was synthesized and characterized by elemental analysis, infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy, thermal analysis and X-ray single crystal diffraction. X-ray diffraction analysis revealed that complex 1 crystallized in the monoclinic crystal system with space group P21/c. The Cd(II) center was six-coordinated with four nitrogen atoms from two sac and two 4-meim ligands, two oxygen atoms from two aqua ligands. Spectral and thermal analysis data for complex 1 was in agreement with the crystal structures. In addition complex 1 displayed blue fluorescent emission in the solid state at room temperature. Single crystal EPR spectra at room temperature are resolved and have exhibited that two different Cu2+ complexes were located in different chemical environments which contained two magnetically nonequivalent Cu2+ sites. In low temperature EPR spectra down to 110°C did show no considerable change. At higher temperatures, however, both thermo gravimetric analyses (TGA) and EPR spectra showed detectable changes around 140°C; the causes and the mechanisms of changes are discussed.

High spin to low spin change induced by reductive chemistry with iron-substituted Dawson polyoxometalate

Abstract Chemical reduction of an iron(III) substituted Dawson 5-molybdo-12-tungsto-2-phosphate α2-K7[Fe(H2O)P2Mo5W12O61]⋅ 17H2O complex, with dithionite in aqueous solution is reported alongside with similar data for the monolacunary Dawson molybdo-tungsto-diphosphates α2-[P2Mo5W12O61]10 −. UV–vis absorption and low-temperature EPR (electron paramagnetic resonance) spectra indicate several distinct species and an unprecedented change of spin state at the ferric center, induced indirectly by reduction of the polyoxometalate framework.

Manifestation of non-centrality effect in the EPR spectrum of Fe3+ ion in the polycrystalline materials

In a number of works [1–6], the study of the electron paramagnetic resonance (EPR) spectrum of polycrystalline non-organic and organic matters revealed presence of EPR spectrum of impurity Fe3+ ion which has essentially different forms at helium and room temperatures. At the helium temperature intensity of the resonance line with g-value close to g1 ≅ 4.3 prevails (Line 1 of the low temperature spectrum). The resonance line with g ≅ 4.3 corresponds to magnetic center with spin S = 5/2 by the spin Hamiltonian parameter D ≫ hv. The resonance line with g-value close to 2.0 (Line 2 of the high temperature spectrum) prevails at the room temperatures. The resonance line with g2 ≅ 2.0 corresponds to magnetic center with spin S = 5/2 by the spin Hamiltonian parameter D ≪ hv. The transition from the low- to high-temperature spectrum is gradual and it is accompanied by a redistribution of the absorption intensity. The observed properties of the temperature dependence of the EPR spectrum are characteristic of systems with a multiminimum potential. The described effect of change of integral intensity of low-temperature and high-temperature EPR spectrum of Fe3+ ion is investigated in substances Na[FeO6(C10H8N)3], C21H14Br4O5S, C25H30ClN3, Al2[Si2O5](OH)4, polyaniline and polyparaphenylene.

Thermal and EPR spectroscopic studies of novel coordination compound trans-bis(acesulfamato)tetraaquazinc(II) with Mn(II) impurity

The thermogravimetric analysis (TGA) and electron paramagnetic resonance (EPR) studies of powder and single crystals of bis(acesulfamato)tetraaquazinc(II), Zn(acs) 2(H 2O) 4, a novel coordination compound, are carried out. Previously synthesized bis(acesulfamato) tetraaquamanganese(II), Mn(acs) 2(H 2O) 4, is included into the host in trace amount as a paramagnetic probe for EPR analysis. Single crystal EPR spectra at room temperature are resolved and discussed. Low temperature EPR spectra down to 90 K do not show remarkable change. At higher temperatures, however, the TGA and EPR spectra show changes around 335 K and 395 K; the causes and the mechanisms of changes are discussed.

Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: An Investigation of Electron Transfer at Different Temperatures by Multifrequency Time-Resolved and CW EPR

The ambient temperature and low-temperature electron transfer properties of Photosystem I (PS I) from the M688NPsaA and M668NPsaB mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 are studied using transient electron paramagnetic resonance (EPR) and continuous-wave (CW) EPR. The two mutations are expected to alter the midpoint potentials of, and the reorganization energies around, the primary electron chlorophyll acceptors A0A and A0B, which should lead to a change in the yield and/or rate of electron transfer to the phylloquinone acceptors A1A and A1B, respectively. At ambient temperature it is known that both quinone acceptors are active in electron transfer. At low temperature there are at least two fractions that undergo either reversible or irreversible electron transfer. The EPR data of the two PS I variants are used to investigate the relationship between these low-temperature fractions and the ambient temperature electron transfer pathway. The results show that mutation in the PsaA-branch increases the rate of \( {\text{A}}_{{1{\text{A}}}}^{. - } \) to FX electron transfer at ambient temperature, while the corresponding mutation in the PsaB-branch has no effect on the electron transfer rate observable by transient EPR. An analysis of the complete time/field datasets from both variants suggests that the yield of electron transfer in the branch carrying the mutation is reduced. The mutations have no effect on the low-temperature CW EPR spectra of the iron–sulfur clusters if the samples are frozen under illumination but they both cause a decrease in the yield of reduced FA and FB if the samples are frozen in the dark and then illuminated. The PsaA-branch mutation greatly reduces the intensity and changes the polarization pattern of the radical pair \( {\text{P}}_{700}^{ + } {\text{A}}_{1}^{. - } \). Possible causes of the changes in the polarization pattern are discussed and it is suggested that the mutations introduce structural heterogeneity in the vicinity of the A0 binding site. No clear correlation between the yield of electron transfer in a particular branch and the yield of stable charge separation is found.

A Dinuclear Ni(I) System Having a Diradical Ni2N2Diamond Core Resting State: Synthetic, Structural, Spectroscopic Elucidation, and Reductive Bond Splitting Reactions

One-electron reduction of the square-planar nickel precursor (PNP)NiCl ( 1) (PNP (-) = N[2-P(CHMe 2) 2-4-methylphenyl] 2) with KC 8 effects ligand reorganization of the pincer ligand to assemble a Ni(I) dimer, [Ni(mu 2-PNP)] 2 ( 2), containing a Ni 2N 2 core structure, as inferred by its solid-state X-ray structure. Solution magnetization measurements are consistent with a paramagnetic Ni(I) system likely undergoing a monomer <--> dimer equilibrium. The room-temperature and 4 K solid-state X-band electron paramagnetic resonance (EPR) spectra display anisotropic signals. Low-temperature solid-state X-band EPR data at 4 K reveal rhombic values g z = 1.980(4), g x = 2. 380(4), and g y = 2.225(4), as well as a forbidden signal at g = 4.24 for the Delta M S = 2 half field transition, in accord with 2 having two weakly interacting metal centers. Utilizing an S = 1 model, full spin Hamiltonian simulation of the low-temperature EPR spectrum on the solid sample was achieved by applying a nonzero zero-field-splitting parameter ( D = 0.001 cm (-1)), which is consistent with an S = 0 ground state with a very closely lying S = 1 state. Solid-state magnetization data also corroborate well with our solid-state EPR data and reveal weak antiferromagnetic behavior ( J = -1.52(5) cm (-1)) over a 2-300 K temperature range at a field of 1 Tesla. Evidence for 2 being a masked "(PNP)Ni" scaffold originates from its reaction with N 2CPh 2, which traps the Ni(I) monomer in the form of a T-shaped species, Ni(PNPNNCPh 2), a system that has been structurally characterized. The radical nature of complex 2, or its monomer component, is well manifested through the plethora of cooperative H-X-type bond cleavage reactions, providing the nickel(II) hydride (PNP)NiH and the corresponding rare functionalities -OH, -OCH 3, -PHPh, and -B(catechol) integrated into the (PNP)Ni moiety in equal molar amounts. In addition to splitting H 2, compound 2 can also engage in homolytic X-X bond cleavage reactions of PhXXPh to form (PNP)Ni(XPh) (X = S or Se).

Dielectric constant and EPR spectra of Fe doped TlInS2 crystal near the structural phase transitions

Dielectric constant and electron paramagnetic resonance (EPR) spectra of Fe-doped TlInS2 crystal has been investigated in the temperature range of 5–300 K. The influence of Fe impurities on dielectric properties of TlInS2 crystal has been considered in comparison with earlier observed results from pure TlInS2 compounds. Considerable decrease of the dielectric constant, as well as the change of the shape of its temperature dependence in a result of influence of doped iron atoms, has been observed. It has been established that EPR lines exhibit remarkable splitting and appearance of additional resonance lines at the temperatures lower than 200 K, which is attributed earlier as the temperature of the ferroelectric phase transition. The simulation for low-temperature EPR spectra has been performed taking into account the appearance of four additional centers in addition to four equivalent centers observed from EPR spectra at room temperature. Such transformations are considered as result of nonequivalent displacements of different groups of Tl atoms during the structural phase transitions.

Hydrothermal synthesis of Co-doped In2S3 micropompons and their physical properties

In2−xCoxS3 (x = 0 to 0.1) micropompons (diameters ∼3–4 μm) consisting of ∼10–15-nm-thick randomly self-assembled nanoflakes were synthesized hydrothermally. X-ray study indicated a steady variation of lattice parameter ratio up to 5% Co. Detailed investigations of the Co incorporation in In2S3 were carried out by optical absorbance, room temperature photoluminescence (PL), and electron paramagnetic resonance (EPR) studies. Significant blue shift in the absorbance spectra was noticed due to the crystal-field splitting of Co2+ ions in the host lattice structure. Unlike the visible emission found in undoped In2S3, PL spectra of the Co-doped samples were recognized by a strong ultraviolet emission peak at ∼335 nm, introduced by the t2g level of Co2+ ions, with maximum intensity for 5% Co. Room-temperature and low-temperature EPR spectra revealed octet paramagnetic bands up to 5% Co beyond which a single resonance band appeared.

Peculiarities of neutron-transmutation phosphorous doping of Si30 enriched SiC crystals: Electron paramagnetic resonance study

We have obtained a high concentration of P donor dopants in 6H-SiC enriched with Si30 and irradiated with thermal neutrons. It was established that annealing at a relatively low temperature of 1300°C, i.e., 500–600°C lower than that used for annealing SiC with the natural isotope composition after neutron-transmutation doping, gives rise to an electron paramagnetic resonance (EPR) signal corresponding to three different shallow P (sP) donors with large hyperfine interactions. The correlated changes of these sP centers in all the annealing experiments and the similarities to the spectra of shallow N donors demonstrate that these sites have shallow donor levels and a similar electronic structure and that they belong to different lattice sites: two quasicubic and hexagonal. The phosphorus at these three sites is suggested to occupy the C position. Simultaneously the low-temperature EPR signal from another set of P-related donor centers having a small, strongly anisotropic hyperfine interaction is observed. It is suggested that phosphorus in these centers occupies the Si position. Annealing at 1800°C yields opposite changes in the concentrations of the two types of P-related donor centers: The EPR signals of sP centers disappear, while the intensity of the low-temperature EPR spectra of P donors considerably increases. Thus, the phosphorus at the C position is established to be unstable and annealing above 1700°C causes P at the C site to move to the Si site. This process is vacancy mediated, as the temperature of this process is shown to depend on the thermal stability of intrinsic defects produced by neutron irradiation, whose concentration is proportional to the neutron irradiation dose.

Phosphorus doping of Si nanocrystals: Interface defects and charge compensation

Using electron paramagnetic resonance (EPR), Fourier-transform infrared absorption (FTIR) and temperature programmed desorption (TPD), we have investigated the doping of silicon nanocrystals (Si-ncs) and the interaction between intrinsic defects and dopants. Si-ncs were produced in a low-pressure microwave plasma reactor using silane as precursor gas. Phosphorus doping was achieved by addition of phosphine to the precursor gas. The low temperature EPR spectra of all P-doped samples display a line at g = 1.998 , which is the fingerprint of substitutional P in crystalline silicon for [ P ] > 10 18 cm - 3 . In addition, the characteristic hyperfine signature of P in Si is also observed for samples with nominal P doping levels below 10 19 cm - 3 . Two more features are observed in our EPR spectra: a broad band located at g = 2.0056 , due to isotropic Si dangling bonds (Si-dbs), and an axially symmetric powder pattern ( g ⊥ = 2.0087 , g ∥ = 2.0020 ) arising from Si-dbs at the interface between the crystalline Si core and a native oxide shell. The formation of this oxide layer and the presence of different H-termination configurations are revealed by FTIR spectroscopy. The density of Si-dbs is reduced in P-doped samples, indicating a sizable compensation of the doping by Si-dbs. This compensation effect was verified by H desorption, which enhances the density of Si-dbs, in combination with TPD and FTIR.

Trapped Tyrosyl Radical Populations in Modified Reaction Centers from Rhodobacter sphaeroides

The photosynthetic reaction center from the purple bacterium Rhodobacter sphaeroides has been modified such that the bacteriochlorophyll dimer, when it becomes oxidized after light excitation, is capable of oxidizing tyrosine residues. One factor in this ability is a high oxidation-reduction midpoint potential for the dimer, although the location and protein environment of the tyrosine residue appear to be critical as well. These factors were tested in a series of mutants, each of which contains changes, at residues L131, M160, M197, and M210, that give rise to a bacteriochlorophyll dimer with a midpoint potential of at least 800 mV. The protein environment was altered near tyrosine residues that are either present in the wild type or introduced by mutagenesis, focusing on residues that could act as acceptors for the phenolic proton of the tyrosine upon oxidation. These mutations include Ser M190 to His, which is near Tyr L162, the combination of His M193 to Tyr and Arg M164 to His, which adds a Tyr-His pair, and the combinations of Arg L135 to Tyr with Tyr L164 to His, Arg L135 to Tyr with Tyr L144 to Glu, and Arg L135 to Tyr with Tyr L164 to Phe. Radicals were produced in the mutants by using light to initiate electron transfer. The radicals were trapped by freezing the samples, and the relative populations of the oxidized dimer and tyrosyl radicals were determined by analysis of low-temperature electron paramagnetic resonance spectra. The mutants all showed evidence of tyrosyl radical formation at high pH, and the extent of radical formation at Tyr L135 with pH differed depending on the identity of L144 and L164. The results show that tyrosine residues within approximately 10 A of the dimer can become oxidized when provided with a suitable protein environment.

Inhibition of bacteriophage lambda protein phosphatase by organic and oxoanion inhibitors.

Bacteriophage lambda protein phosphatase (lambdaPP) with Mn(2+) as the activating metal cofactor was studied using phosphatase inhibition kinetics and electron paramagnetic resonance (EPR) spectroscopy. Orthophosphate and the oxoanion analogues orthovanadate, tungstate, molybdate, arsenate, and sulfate were shown to inhibit the phosphomonoesterase activity of lambdaPP, albeit with inhibition constants (K(i)) that range over 5 orders of magnitude. In addition, small organic anions were tested as inhibitors. Phosphonoacetohydroxamic acid (PhAH) was found to be a strong competitive inhibitor (K(i) = 5.1 +/- 1.6 microM) whereas phosphonoacetic acid (K(i) = 380 +/- 45 microM) and acetohydroxamic acid (K(i) > 75 mM) modestly inhibited lambdaPP. Low-temperature EPR spectra of Mn(2+)-reconstituted lambdaPP in the presence of oxoanions and PhAH demonstrate that inhibitor binding decreases the spin-coupling constant, J, compared to the native enzyme. This suggests a change in the bridging interaction between Mn(2+) ions of the dimer due to protonation or replacement of a bridging ligand. Inhibitor binding also induces several spectral shifts. Hyperfine splitting characteristic of a spin-coupled (Mn(2+))(2) dimer is most prominent upon the addition of orthovanadate (K(i) = 0.70 +/- 0.20 microM) and PhAH, indicating that these inhibitors tightly interact with the (Mn(2+))(2) form of lambdaPP. These EPR and inhibition kinetic results are discussed in the context of establishing a common mechanism for the hydrolysis of phosphate esters by lambdaPP and other serine/threonine protein phosphatases.

Spectroscopic characterization of a manganese–lignin peroxidase hybrid isozyme produced by Bjerkandera adusta in the absence of manganese: evidence of a protein centred radical by hydrogen peroxide

Electronic absorption and electron paramagnetic resonance (EPR) spectra are reported for a novel manganese–lignin peroxidase (MnLiP) hybrid isozyme produced by Bjerkandera adusta in the absence of manganese at pH 5. The room temperature absorption and the low temperature (10 K) EPR spectra indicate that the same coordination and spin states are present at both temperatures: mainly six coordinate high spin containing low percentage six coordinate low spin ferric heme, the latter probably with a bis-imidazole coordination. A protein centred radical was detected in the presence of an excess of hydrogen peroxide and assumed to be a tryptophanyl radical. The catalytic significance of this site was addressed by specific chemical modification of the tryptophan residues that revealed a marked effect on the specific activity of the enzyme. It is proposed that substrate oxidation might proceed through a long range-electron transfer process.

A mixed-valence manganese(III)–manganese(IV) di-μ-oxo complex, [(cyclam)MnO]2(ClO4)2(NO3)

The title dinuclear di-mu-oxo-bis[(1,4,8,11-tetraazacyclotetradecane-kappa(4)N)manganese(III,IV)] diperchlorate nitrate complex, [Mn(2)O(2)(C(10)H(24)N(4))(2)](ClO(4))(2)(NO(3)) or [(cyclam)MnO](2)(ClO(4))(2)(NO(3)), was self-assembled by the reaction of Mn(2+) with 1,4,8,11-tetraazacyclotetradecane in aqueous media. The structure of this compound consists of a centrosymmetric binuclear [(cyclam)MnO](3+) unit, two perchlorate anions and one nitrate anion. While the low-temperature electron paramagnetic resonance spectra show a typical 16-line signal for a di-mu-oxo Mn(III)/Mn(IV) dimer, the magnetic susceptibility studies also confirm a characteristic antiferromagnetic coupling between the electronic spins of the Mn(IV) and Mn(III) ions.

Cobalt hexammine inhibition of the hammerhead ribozyme.

The effects of Co(NH(3))(6)(3+) on the hammerhead ribozyme are analyzed using several techniques, including activity measurements, electron paramagnetic resonance (EPR), and circular dichroism (CD) spectroscopies and thermal denaturation studies. Co(NH(3))(6)(3+) efficiently displaces Mn(2+) bound to the ribozyme with an apparent dissociation constant of K(d app) = 22 +/- 4.2 microM in 500 microM Mn(2+) (0.1 M NaCl). Displacement of Mn(2+) coincides with Co(NH(3))(6)(3+) inhibition of hammerhead activity in 500 microM Mn(2+), reducing the activity of the WT hammerhead by approximately 15-fold with an inhibition constant of K(i) = 30.9 +/- 2.3 microM. A residual 'slow' activity is observed in the presence of Co(NH(3))(6)(3+) and low concentrations of Mn(2+). Under these conditions, a single Mn(2+) ion remains bound and has a low-temperature EPR spectrum identical to that observed previously for the highest affinity Mn(2+) site in the hammerhead ribozyme in 1 M NaCl, tentatively attributed to the A9/G10.1 site [Morrissey, S. R. , Horton, T. E., and DeRose, V. J. (2000) J. Am. Chem. Soc. 122, 3473-3481]. Circular dichroism and thermal denaturation experiments also reveal structural effects that accompany the observed inhibition of cleavage and Mn(2+) displacement induced by addition of Co(NH(3))(6)(3+). Taken together, the data indicate that a high-affinity Co(NH(3))(6)(3+) site is responsible for significant inhibition accompanied by structural changes in the hammerhead ribozyme. In addition, the results support a model in which at least two types of metal sites, one of which requires inner-sphere coordination, support hammerhead activity.

In vivo formation of electron paramagnetic resonance-detectable nitric oxide and of nitrotyrosine is not impaired during murine leishmaniasis.

Recent studies have provided evidence for a dual role of nitric oxide (NO) during murine leishmaniasis. To explore this problem, we monitored the formation of NO and its derived oxidants during the course of Leishmania amazonensis infection in tissues of susceptible (BALB/c) and relatively resistant (C57BL/6) mice. NO production was detected directly by low-temperature electron paramagnetic resonance spectra of animal tissues. Both mouse strains presented detectable levels of hemoglobin nitrosyl (HbNO) complexes and of heme nitrosyl and iron-dithiol-dinitrosyl complexes in the blood and footpad lesions, respectively. Estimation of the nitrosyl complex levels demonstrated that most of the NO is synthesized in the footpad lesions. In agreement, immunohistochemical analysis of the lesions demonstrated the presence of nitrotyrosine in proteins of macrophage vacuoles and parasites. Since macrophages lack myeloperoxidase, peroxynitrite is likely to be the nitrating NO metabolite produced during the infection. The levels of HbNO complexes in the blood reflected changes occurring during the infection such as those in parasite burden and lesion size. The maximum levels of HbNO complexes detected in the blood of susceptible mice were higher than those of C57BL/6 mice but occurred at late stages of infection and were accompanied by the presence of bacteria in the cutaneous lesions. The results indicate that the local production of NO is an important mechanism for the elimination of parasites if it occurs before the parasite burden becomes too high. From then on, elevated production of NO and derived oxidants aggravates the inflammatory process with the occurrence of a hypoxic environment that may favor secondary infections.

The EPR spectrum of the general Ċ–CX 3 quantum rotor

The quantum mechanical Liouville equation for the general Ċ–CX 3 rotor is employed to obtain low temperature electron paramagnetic resonance (EPR) spectra exhibiting E-line splittings arising from the spin-rotational coupling in the hyperfine interaction. Analytic expression for the low temperature EPR spectrum for the general C 3-symmetry rotor is presented. Analytic spectra for I=1/2, 1 and 3/2 nuclei-containing rotors are discussed in detail. Intensities and positions of first order transition sidebands are given for the general system and explicitly discussed for I=1/2 and 1 systems, with high accuracy determination of tunnelling frequencies and potential twist angles from low temperature experimental data as possible applications. Anomalous A/E mixing effects on the EPR spectrum in the transition region between tunnelling and stopped rotor conditions are demonstrated for I=1/2 and 1 systems and a particular choice of twist angle.