A long-lived luminescence and EPR bimodal lanthanide-based probe for free radicals

We developed a novel spin-labeled terbium complex Tb(3+)/cs124-DTPA-TEMPO (1) by covalently labeling a nitroxide radical on the terbium complex for monitoring free radicals of various areas. This lanthanide complex probe shows a high EPR signal which resulted from the nitroxide radical moiety, and is weakly luminescent which resulted from the intramolecular quenching effect of the nitroxide radical on sensitised terbium luminescence. The intensity of both the EPR and luminescence can be modulated by eliminating the paramagnetism of the nitroxide radical through recognition of a carbon-centered radical analyte and thus gives a quantification of the analyte. We have preliminarily applied this probe in the luminescent detection of model carbon-centered radicals and hydroxyl radicals (·OH). This probe is water-soluble and contains lanthanide-luminescence properties, favorable for the time-resolved luminescence technique. The investigation of the intramolecular quenching process has showed that the labeled nitroxide radical quenches multiple excited states of the terbium complex, resulting in highly efficient quenching of terbium luminescence. This probe is the first example of intramolecular modulation of lanthanide luminescence by a nitroxide radical.     

Photochromic and luminescence properties of isomeric forms of the hydronaphthyl radicals

The optical properties of the hydronaphthyl radicals produced upon radiolysis and photolysis of rigid naphthalene solutions in boric acid at 300 K and alcohol, ether and methylcyclohexane at 77 K were investigated. The ESR, absorption and luminescence spectra of the two different isomeric forms of hydronaphthyl radicals have been observed. The estimated values of the lowest electronic transition in α- and β-hydronaphthyl radicals are, respectively: E_α = 2.42 ± 0.03 eV, E_β = 2.66 ± 0.05 eV. It has been found that a reversible photo-induced interconversion between two hydroradicals takes place as well as a dark conversion from the β- to the α-isomer. Analogous photochromic behaviour of the absorption bands of biphenyl, phenanthrene, chrysene and fluorene rigid solutions have also been identified as reversible interconversion between isomeric hydroradicals.     

Formation and reactions of radiation-induced free radicals in chemically modified cotton celluloses

With a view to assessing the role of chemical substitutions in the modification of radiation effects on cotton fibers, the induction and post-irradiation behavior of free radicals in γ-irradiated pure, mercerized, benzoylated, and allylated cottons have been investigated by ESR spectroscopy. While mercerization and allylation were found to influence both the nature and yield of radicals, benzoylation reduced only the yield. These substitutions, which have been found to protect cotton fibers against radiation damage, therefore act differently at the molecular level.     

The role of stable free radicals in the radiation-induced conductivity of low-density polyethylene

The radiation-induced conductivity (RIC) of low-density polyethylene (LDPE) under continuous irradiation with fast electrons (50 keV) was experimentally studied. The dose dependence of the concentration of stable paramagnetic centers was determined. The kinetics of RIC in LDPE was calculated on the basis of the Rose-Fowler-Vaisberg (RFV) model taking into account the buildup of radiation-induced traps. Good correlation between the experimental results and calculated data was found.     

Electrical properties of some free nitroxyl radicals

D.C. and a.c. electrical conductivity of some free radicals is measured, and a “hopping” mechanism for the conduction is proposed, on the basis of the low values of mobility and the frequency dependence of the conductivity.     

EPR study of free radicals in non- and gamma-irradiated nutritive supplements containing anthocyanins concentrate from lyophilized red wine

Nutritive supplements Enoviton, Enoviton C and Enoviton CE containing standardized anthocyanins from lyophilized red wine, vitamins (some of them) and excipients were investigated by EPR spectrometry before and after gamma-irradiation. Non-irradiated samples exhibit one singlet line with g=2.0039±0.0002, most probably due to free radicals from anthocyanins. After irradiation with 10 kGy gamma-rays, tablets of Еnoviton, Еnoviton С and Еnoviton СЕ, all exhibit complex EPR signals centered at a g-value of g=2.0034. The EPR spectrum of irradiated Enoviton is different from that of Еnoviton С or Еnoviton СЕ due to the overlap of the spectra of microcrystalline cellulose and the background singlet spectrum present in all tablets with the EPR resonance due to irradiated ascorbic acid (in Еnoviton С and Еnoviton СЕ). Gamma-induced free radicals exhibit long time stability—for a six months period the intensity of central peak decrease with 30–40%.     

Magnetic field effect on the luminescence from gamma-irradiated solutions of fluorene

The luminescence from gamma-irradiated solutions of fluorene increases when a magnetic field is applied, in agreement with recent theoretical predictions. The magnitude of the field effect is shown to be solvent dependent due to the changes in the solute ion recombination rates.     

Characterization and luminescence properties of Gd2O3 phosphor

Gd₂O₃ phosphor was synthesized by combustion synthesis using gadolinium nitrate hexahydrate as precursor and urea as fuel. Structural and surface morphology were studied by X-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Chemical composition analysis of the phosphor was performed by Fourier-transform infrared spectroscopy, and tts optical properties were characterized by use of photoluminescence (PL) and thermoluminescence (TL) techniques. In PL spectra, feeble emission at 490 nm (blue) and intense emission at approximately 545 nm (green) are observed after excitation at 300 nm. TL measurement was performed on the Gd₂O₃ phosphor by irradiating it with γ-rays (1 kGy). A well resolved glow peak at 226.4 °C was observed. Kinetic data were estimated from the TL glow curve by use of Chen’s peak-shape method; the results are discussed in detail. The average particle size of the Gd₂O₃ phosphor was 41 nm; a monoclinic phase was formed at a firing temperature of 500 °C. This was in agreement with SEM and TEM results.     

Electron paramagnetic resonance, photo-acoustic spectroscopy and thermally stimulated luminescence for identification of thermally stimulated processes in gamma-irradiated LaBa2F7

Prostate specific antigen (PSA) is a reliable biochemical marker used in screening for prostate carcinoma. Immunoradiometric assays (IRMA) are generally used for the estimation of total/free PSA in serum samples. Radiolabeled antibody, an important reagent of IRMA was prepared and characterized using an in-house anti-PSA monoclonal antibody (Mab), Mab-2S. Mab-2S was radiolabeled with ¹²⁵I and characterized for immunoreactivity and radiochemical purity. The usability of the radioiodinated Mab as tracer in IRMA was ascertained using authentic reagents for IRMA of PSA.     

Thermo-stimulated luminescence analysis of powder of the gamma-irradiated tetrafluoroethylene/ethylene copolymer

The thermo-stimulated luminescence of Γ-irradiated commercial copolymers of tetrafluoroethylene with ethylene were explored. There are five intense maxima in the luminescence curves. Position, halfwidth and number of maxima remain constant at doses of irradiation from 10 kGy up to 30 kGy. The first peak at 112 K to 119 K has the greatest intensity and is found on the luminescence curves of all of the investigated copolymers except for Tefzel®. For Tefzel®, the maximum occurs at 141 K. The differences of the glow curves in the different copolymers may be the result of impurities capable of reacting with ions or radicals formed under irradiation and by differences in the copolymer’s topological structures.     

Electric properties of non-irradiated and gamma-irradiated keratin

Dielectric measurements as a function of temperature and frequency are reported for non-irradiated and γ-irradiated keratin, the irradiation doses being 5 and 50 kGy. The effect of γ-irradiation on the dielectric permittivity of keratin is not observed up to 190°C. In this temperature range, the values of the relaxation time and dipole moment are similar for non-irradiated and irradiated keratin at the same temperature. The influence of irradiation is manifested as a shift of the parameter (_s−_∞) peaks associated with the process of denaturation, towards lower temperatures. This fact is supported by lower values of the dipole moment for irradiated than for non-irradiated keratin, as a result of a decrease in the number of polar groups in the side and main chains of the macromolecule in the irradiated samples.     

Reactions with dye free radicals reveal weak redox properties of drugs

The calcium release channel (CRC) of the skeletal sarcoplasmic reticulum is rich in thiol groups and is strongly regulated by covalent modification of these thiols. Oxidizing reagents activate the release channel, whereas reducing reagents inhibit the channel. However, most CRC regulators are not thiol reagents. Here, we propose that reversible redox interactions are involved in regulation of the CRC by nonthiol reagents. This hypothesis was tested with several CRC regulators. The local anesthetics tetracaine, procaine and QX-314, which block the CRC, behaved as electron donors in reactions with dye free radicals. In contrast, ryanodine, caffeine, doxorubicin and daunorubicin, compounds known to activate the release channel, all accepted electrons from dye anion radicals. Moreover, release of Ca2+ from SR initiated by doxorubicin (acceptor) was antagonized by local anesthetics (donors). Based on the redox characterization of CRC modulators, we propose a functional model in which channel inhibitors and activators act as weak electron donors and acceptors, respectively, and shift the thiol-disulfide balance within the release protein. The consequence of this model is that, in spite of the chemical diversity of CRC modulators, a common mechanism of channel regulation involves the transient exchange of electrons between the activator/inhibitor and the CRC.     

Entropies and heat capacities of free radicals

Methods are presented for rapidly estimating the entropies and heat capacities of free radicals from the known S⁰ and C of structurally similar compounds. The methods consist of estimating the differences due to changes in mass, vibration frequencies, spin, symmetry, and changes in rotational barriers. Tables of contributions to S⁰ and C by different frequencies over the temperature range 300–1500°K are presented to facilitate the tabulation of the above differences. Conjugated radicals, such as benzyl and allyl, are included. It is shown that the greatest uncertainties in the estimates arise from uncertainties in the barriers to rotation in the radicals. The results are applied to kinetic data on the pyrolysis of branched hydrocarbons and the reverse reactions of radical recombination. Major discrepancies exist in these data which can be nearly reconciled by postulating improbably high rotational barriers of 8 kcal for CH₃ rotation in isopropyl and t-butyl radicals. It is shown that radical thermochemistry can be fitted into group schemes and tables of groups values are given for the rapid estimation of ΔH, S⁰, and C for different organic radicals, including those containing sulfur, oxygen, and nitrogen.     

Thermochemical properties of free radicals from G3MP2B3 calculations

For a set of 32 selected free radicals, energy minimum structures, harmonic vibrational wave numbers ω_e, principal moments of inertia I_A, I_B, and I_C, heat capacities C°_p(T), entropies S°(T), thermal energy contents H°(T) ? H°(0), and standard enthalpies of formation Δ_fH°(T) were calculated at the G3MP2B3 level of theory in the temperature range 200–3000 K. In this article, thermodynamic functions at T = 298.15 K are presented and compared with recent experimental values. The mean absolute deviation between calculated and experimental Δ_fH°(298.15) values resulted in 3.91 kJ mol^?1, which is close to the average experimental uncertainty of ± 3.55 kJ mol^?1. The influence of hindered rotation on thermodynamic functions is studied for isopropyl and tert‐butyl radicals. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 550–560, 2002     

Accurate vibrational spectra and magnetic properties of organic free radicals: the case of H2CN

We present the structural, magnetic, and vibrational properties of H2CN computed using a second-order perturbative approach in which equilibrium values and harmonic frequencies evaluated at the coupled-cluster level are combined with anharmonic and vibrational averaging contributions obtained by hybrid Hartree-Fock/Kohn-Sham methods. Our computations lead to remarkably accurate results and suggest some revision of the experimental vibrational assignments.     

Relation of the decay of free radicals in irradiated polyethylene to the molecular motion of the polymer and the configurations of the free radicals

Decay reactions of the free radicals produced in irradiated polyethylene (high-density and low-density materials) were examined in connection with the molecular motion of the matrix polymer. Three temperature regions, in which the free radicals decay very rapidly, at around 120, 200, and 250°K, were designated T_A, T_L, and T_B, respectively. The decay of the free radicals at these temperatures had activation energies in high-density polyethylene of 0.4 kcal/mole for T_A, 9.4 kcal/mole for T_L, and 18.4 kcal/mole for T_B. In low-density polyethylene these quantities were 0.7 kcal/mole for T_A, 23.1 kcal/mole for T_L, and 24.8 kcal/mole for T_B. Comparison of time constants for the decay reactions and for molecular motion of the matrix polymer indicate that the decay in T_A and T_B is closely related to molecular motion in the amorphous regions of the polymer. The decay of the free radicals at T_L in high-density polyethylene is due to molecular motion associated with local mode relaxation at lamellar surfaces, while that of low-density polyethylene is due to local mode relaxation in the completely amorphous region. Steric configurations of the free radicals which decay in the respective temperature regions were also investigated.