First characterisation of rotational conformers in a chiral nitroxide by EPR spectroscopy

The detection and characterisation in liquid solution by EPR spectroscopy of the rotational conformers of a nitroxide radical containing a chiral centre is reported for the first time.     

Calculation of anisotropically broadened EPR spectra. Shape of the EPR spectrum of a nitroxide radical in the two-millimeter range

Institute of Catalysis, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 3, pp. 67–70, May–June, 1989.     

Satellite lines in the EPR spectra of nitroxide radicals: experimental data and DFT calculations

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The binding behavior of cyclodextrins toward a nitroxide spin probe in the presence of different alcohols as studied by EPR

Stability constants, rates of association and dissociation, and thermodynamic and activation parameters for the formation of inclusion complexes between the radical guest, N-benzyl- tert-butyl- d 9-nitroxide and beta- or 2,6- O-dimethyl-beta-cyclodextrin (CDs), have been determined by EPR spectroscopy in water in the presence of 14 different alcohols, differing in size and lipophilicity. In all cases, it was found that addition of alcohol, depending on its structure and concentration, causes a reduction of the stability of the paramagnetic complex. Global analysis of EPR data allowed us to explain the CDs binding behavior: we discarded the formation of a ternary complex, where alcohol and radical guest are coincluded into CD cavity, while data were found more consistent with the formation of a binary complex alcohol:CD competing with the monitored complex nitroxide:CD. Both kinetic and thermodynamic analysis of the experimental results have revealed that the presence of alcohols affects to a larger extent the dissociation rather then the association of radical probe and CD and that the former process is of greater importance in determining the stability of the complex, this confirming the reliability of the competition model proposed. This competition has been used for the indirect determination of the stability constants of complexes between CD and examined alcohols. By using a similar approach, we showed EPR spectroscopy can be considered a rapid and accurate technique to investigate the CDs binding behavior toward different nonradical guest.     

Synthesis of N-labeled 4-oxo-2,2,6,6-tetraethylpiperidine nitroxide for EPR brain imaging

A scalable synthetic route for ¹⁵N-labeled 4-oxo-2,2,6,6-tetraethylpiperidine nitroxide (¹⁵N-TEEPONE) is described. This ¹⁵N-labeled nitroxide is suitable for electron paramagnetic resonance imaging of brain, and its higher sensitivity compared with that of its ¹⁴N-counterpart is an important advantage of the labeled derivative.     

Intermolecular interactions on photolytically generated anion radicals of maleic anhydride measured by EPR

This paper deals with the results of EPR measurements and semi-empirical INDO calculations concerning the influence of intramolecular vibrations and specific intermolecular interactions between the maleic anhydride (MA) anion radical and polar solvents on the EPR coupling constants. The solvent dependence of the EPR coupling is caused by hydrogen bonding interactions with the solvent molecules.The lower the absolute values of the coupling constants, the stronger the solvent-radical anion interaction. The probable solvation structure of the two radical states, A and B, of the anion radical are discussed.     

An EPR and NMR study of supramolecular effects on paramagnetic interaction between a nitroxide incarcerated within a nanocapsule with a nitroxide in bulk aqueous media

A 15N-labeled nitroxide was incarcerated into an octa acid nanocapsule, which was confirmed by 1H NMR and EPR spectroscopy. Electron paramagnetic interaction between the 15N-labeled incarcerated nitroxide and a 14N-labeled free nitroxide in the external aqueous solution was observed by EPR spectroscopy. The observation of spin-spin interaction, through the walls of the cancer and is reflected in the simultaneous line-broadening of both the 15N-labeled and 14N-labeled nitroxides. The computer-assisted analysis of the EPR data further provides direct information on the motion and the polarity of both the incarcerated paramagnetic nitroxide and the nitroxides in the external bulk aqueous phase. We also show how communication between an incarcerated guest and molecules in the bulk solvent can be enhanced or inhibited by supramolecular factors such as Coulombic attraction or repulsion between a charged guest@host complex (incarcerated 15N nitroxide) and charged molecules in the aqueous phase.     

Site-directed electrostatic measurements with a thiol-specific ph-sensitive nitroxide: differentiating local pK and polarity effects by high-field EPR

This communication describes the use of a methanethiosulfonate derivative of an imidazolidine nitroxide, methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethyl-imidazolidin-4-ylmethyl) ester, IMTSL, for site-directed pKa determination of peptides by electron paramagnetic resonance. This spin label is covalently attached to the thiol group of unique cysteines incorporated into peptide structures. The tertiary amine nitrogen N3 of the label readily participates in proton exchange reactions, which are monitored through changes in EPR spectra of nitroxide moiety. Using EPR at 95 GHz (W-band) isotropic magnetic parameters of this nitroxide, both Aiso and giso, were calibrated in solvents of different polarity and pH. Two different linear correlations between Aiso and giso for acidic and basic forms of IMTSL were observed, making it possible to differentiate effects of local polarity from N3 protonation on nitroxide EPR spectra. Titration of a synthetic P11 peptide fragment of the laminin B1 chain illustrates the utility of this method.     

EPR oximetry of tumors in vivo in cancer therapy

The partial oxygen pressure (pO2) in tumors is considered to be one of important factors that affect the response of tumors to different treatment. Therefore, we anticipate that the information about the variation of oxygen concentration in tumors can be used as a guide for individualizing radiotherapy, chemotherapy, and especially the combined therapies. There is thus a need to obtain quantitative data on the effects of different therapies on tumor oxygenation under in vivo conditions. One of the methods, which enable these measurements is EPR oximetry. In this work basic principles of the method will be described as well as some examples of tumor oxygenation changes after application of chemotherapeutic drugs (vinblastine, cisplatin, bleomycin) or electric pulses in combination with cisplatin or bleomycin to fibrosarcoma SA-1 tumors in mice. A paramagnetic probe, a char of Bubinga tree, was implanted into the tumor (center and periphery) and in the muscle or subcutis. EPR spectra line-width, which is proportional to oxygen concentration, was measured with time after the treatments. Tumor oxygenation was reduced for 58% of pretreatment value 1 h after intraperitoneal injection of 2.5 mg kg(-1) VLB and returned to pretreatment level within 24 h. Reduction in oxygenation of muscle and subcutis was much smaller and returned to pretreatment value faster as in tumors. With cisplatin (4 mg kg(-1)) and bleomicyn (1 mg kg(-1)) the reduction was less than 15%, but increases in combined therapy to 70%. Similar reduction was observed also with electric pulses alone (eight pulses, 1300 V cm(-1), 100 micros, 1 Hz) with fast recovery of 8h. After electrochemotherapy the recovery was slower and occurs only after 48 h. This study demonstrates that EPR oximetry is a sensitive method for monitoring changes in tissue oxygenation after different treatments, which may have implications in controlling side effects of therapy and in the planning of combined treatments.     

Absorption spectra of human skin in vivo in the ultraviolet wavelength range measured by optoacoustics

Knowledge of the optical properties of human skin in the ultraviolet range is fundamental for photobiologic research. However, optical properties of human skin in the ultraviolet spectral range have so far mainly been measured ex vivo . We have determined the absorption spectra of human skin in vivo in the wavelength range from 290 to 341 nm in 3 nm steps using laser optoacoustics. In this technique, optical properties are derived from the pressure profile generated by absorbed light energy in the sample. In a study on 20 subjects belonging to phototypes I–IV, we studied the optical properties at the volar and dorsal aspect of the forearm as well as on the thenar. Analysis of the measured absorption spectra shows that comparable skin areas—like different sides of the forearm—have qualitatively similar optical characteristics. Still, the optical properties may vary substantially within the same area, probably due to the skin structure and inhomogeneities. Comparison of the spectra from different skin sites indicates that the spectral characteristics of the stratum corneum and its chromophores play an important role for the optical properties of human skin in vivo in the ultraviolet B range.     

Paramagnetic probe for molecular interactions: I. EPR and electronic absorption studies of lithium cation solvation by a nitroxide radical

The 2,2,6,6-tetramethylpiperidine nitroxide radical (TMPN) was studied by EPR and electronic absorption spectroscopy in LiClO₄ solutions in various organic solvents. The¹⁴N hyperfine structure, together with the exchange broadening of its components in EPR and the electronic n^* transition in the visible region, provide useful information about Li⁺-TMPN complexes. Both spectroscopic methods prove that the oxygen atom of TMPN was involved in the Li⁺-TMPN interaction. The complex formation constants 15.61, 2.50, 0.75, 0.61, 0.75, and 0.33 dm³-mol^–1 were found in nitromethane, benzonitrile, acetonitrile, propylene carbonate, acetone and tetrahydrofurane, respectively. These formation constants were correlated with donor and acceptor numbers of the solvents and interpreted in terms of competitive Li⁺-TMPN, solvent-Li⁺, and solvent-TMPN interactions.     

INVESTIGATION OF DAMAGE TO FOREST BY EPR SPECTROSCOPY in vivo

Spruce needles collected from several trees of the Black Forest were investigaled by EPR spectroscopy. These needles show in the g = 2.00 region a signal II_S(Tyr D ₊) and a light-induced signal I(P700₊) and a Mn₂₊ hyperfine structure which superimposes the other absorptions. Difference spectra, light minus dark, partly eliminate the manganese hyperfine structure, and P700₊ can be observed. By comparison of these EPR signals with those of spinach chloroplast or thylakoid membranes described in the literature, significant deviations were observed, whereas several trees grown in the vicinity of Tubingen exhibit the well known D₊ and P700₊ EPR spectra. After treatment of branches of these 'normal' trees with herbicides like Amitrol and Roundup or chemicals like toluene or trichlormethane the EPR signals obtained are comparable with those observed with needles of the Black Forest.     

Electrocatalytic oxidation of L-tyrosine by a nitroxide

The electrochemistry of L-tyrosine was studied by cyclic voltammetry at a glassy carbon electrode. Although L-tyrosine itself showed very poor electrochemical response, the response could be greatly enhanced by using 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy (HO-TEMPO) as a mediator that enables a sensitive determination of the substrate. An electrocatalytic EC' mechanism with the oxoammonium ion of HO-TEMPO as the active oxidant is proposed. The catalytic rate constant was determined to be 2.0 x 10(4) M(-1) s(-1) by using chronoamperometry.