Peter Rapta

Redox intermediates of flavonoids and caffeic acid esters from propolis: An EPR spectroscopy and cyclic voltammetry study

The redox properties of flavonoids: chrysin (1), tectochrysin (2), galangin (3), isalpinin (4), pinostrobin (5), pinobanksin (6), pinobanksin-3-acetate (7), and of caffeic acid ester (8) and diacetylcaffeic acid ester (9), all isolated from propolis, were investigated by cyclic voltammetry in acetonitrile. The choice of aprotic solvent lowered the reactivity of the radical intermediates and made possible to identify redox steps and intermediates not detected so far. The oxidation potentials (vs. saturated calomel electrode) of the investigated compounds were in the region of 1.5 V for 3 and 4; 1.9 V for 1, 2, and 5; 2.0 V for 6 and 7; 1.29 V for 8; and 2.3 V for 9. These oxidation potentials were mainly influenced by the presence of a double bond in 2,3-position and substituent R1 in position 3. Comparison with our earlier data revealed that flavonoids, 1-4, and caffeic acid ester 8 with lower oxidation potentials showed the maximal lipid antioxidant activity, whereas those with higher potentials (5, 6, 7, and 9) are less active. On reduction of 1-9 several one-electron-steps were typically observed in the potential regions: -1.5 V, -1.8 V, and -2 V. where in simultaneous EPR experiments anion radicals of 1 and 3 were observed with the center of unpaired spin density on ring A. Upon oxidation of flavonoids 1-4 carbonyl carbon-centered radicals, .C(O)R, were identified as consecutive products using the EPR spin trapping technique.

In situ Raman and Vis-NIR spectroelectrochemistry at single-walled carbon nanotubes

Abstract The population of valence-band electronic states of single-walled carbon nanotubes (SWCNT) was tuned electrochemically in aqueous electrolyte solution. Depending on the applied potential, reversible changes of intensity and frequency of the Raman-active tangential mode and radial breathing mode were observed. The maximum intensity occurs at ca. −0.2 V vs. Ag/AgCl independent of the electrode material used as a support (Pt, Au, Hg). SWCNT are sensitive to photoanodic breakdown upon prolonged exposure to anodic bias and laser light. Reversible bleaching of the first band-gap transition in semiconducting tubes (at 0.7 eV) occurs after anodic polarization.

Charged states of Sc3N@C68: an in situ spectroelectrochemical study of the radical cation and radical anion of a non-IPR fullerene.

The redox behavior of Sc 3N@C 68 is studied systematically by means of electrochemistry, in situ ESR/Vis-NIR spectroelectrochemistry, and detailed theoretical treatment. Formation of the negatively and positively charged paramagnetic species for the same trimetallic nitride endohedral fullerene is demonstrated for the first time. The electrochemical study of Sc 3N@C 68 exhibits two electrochemically irreversible but chemically reversible reduction steps and two reversible oxidation steps. A double-square reaction scheme is proposed to explain the observed redox reaction at cathodic potentials involving the reversible dimerisation of the Sc 3N@C 68 monoanion. The spin state of the radical cation and the radical anion is probed by ESR spectroscopy, indicating that in both states, the large part of the unpaired spin is delocalized on the fullerene cage. The charged states of the non-isolated pentagon rule fullerene are characterized furthermore by in situ absorption spectroscopy. The interpretation of experimental data is supported by the density functional theory (DFT) calculations of the spin distribution in the anion and cation radicals of Sc 3N@C 68 and time-dependent DFT calculations of the absorption spectra of the charged species.

The spin state of a charged non-IPR fullerene: the stable radical cation of Sc3N@C68

In-situ ESR/UV-vis-NIR spectroelectrochemistry was implemented to probe the spin state of the radical cation of a non-IPR cluster-fullerene Sc(3)N@C(68), which represents the first study on the stable paramagnetic cation of an endohedral fullerene.

Ruthenium- and osmium-arene-based paullones bearing a TEMPO free-radical unit as potential anticancer drugs

A modified paullone ligand bearing a TEMPO free-radical unit (HL) and its ruthenium(II) and osmium(II)-arene complexes [M(p-cymene)(HL)Cl]Cl·nH(2)O (M = Ru, Os) exhibit high antiproliferative activity in human cancer cell lines.

In situ EPR/UV–VIS spectroelectrochemistry of polypyrrole redox cycling

The redox behaviour of polypyrrole layers prepared by the electrochemical oxidation of pyrrole in acetonitrile solutions was studied using insitu EPR/UV–VIS spectroelectrochemistry. Various supporting electrolytes: p-toluene sulfonic acid, tetrabutylammonium perchlorate (TBAP) and tetraethylammonium p-toluene sulfonate (TEATos), were used. Optically transparent indium tin oxide (ITO)-coated glass and gold mesh electrodes were used as electrode materials. Using the calibrated manganese EPR standard, the quantitative time dependences of the three oxidation states in the polymer layer were obtained during redox cycling. The separation of the superimposed UV–VIS spectra was carried out by a least-squares method. The different mechanisms of the polymer oxidation, the polaron/bipolaron route and polaron/dimer route, are compared.

Thermal generation of stable spin trap adducts with super-hyperfine structure in their EPR spectra: An alternative EPR spin trapping assay for radical scavenging capacity determination in dimethylsulphoxide

Thermal decomposition of potassium persulphate (K2S2O8) was studied in detail by the EPR spin trapping technique in dimethylsulphoxide (DMSO), employing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 5-ethoxycarbonyl-5-methyl-1-pyrroline-N-oxide (EMPO) and 5-(diisopropoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DIPPMPO) as spin traps. DMPO and/or its derivatives exclusively trapped the primary formed SO4• − radical anions producing stable spin adducts with half-lives exceeding 2 h at room temperature. High-resolution EPR spectra of these adducts showed unusually rich hyperfine structure due to the interaction of the unpaired electron with all magnetically active nuclei of the spin trap moiety. In contrast to aprotic DMSO solvent, •DMPO-OH adducts dominated in mixed DMSO/water solutions with water content higher than 50%. The thermal decomposition of K2S2O8 in DMSO represents an effective source of free radicals for the radical scavenging capacity (RSC) determination assay, applicable to hydrophilic as well as hydrophobic antioxidants. Efficiency of the assay is demonstrated with a series of cereal samples.

Dimerisation of organic radical ions and redox reactions of dimers as studied by temperature-dependent in situ ESR/UV-vis-NIR spectroelectrochemistry

Abstract The kinetic and structural consequences of dimerisation as one of the main chemical follow-up reactions in electron transfer in organic molecules is very often not understood. As electrochemical methods result in an indirect description of dimerisation processes following electron transfer steps, spectroelectrochemical methods are needed for direct evidence of dimerisation. The triple in situ experimental technique for the simultaneous measurement of ESR and UV–vis–NIR spectra in electrochemical experiments at different temperatures, including a special TE102 optical ESR cavity and Bruker temperature control system, is presented in its application to redox reactions of dimeric structures and dimerisation reactions following an electron transfer. As redox systems, Wursters reagent and thianthrene are used for reference work. The triple spectroelectrochemical method is successfully applied to extended organic π-conjugated systems like end-capped quarter-thiophene and dimeric fullerene C120 where the dimeric structures have been characterised in detail.

Charged States of 1,3,5-Triazine Molecules as Models for Star-shaped Molecular Architecture: A DFT and Spectroelectrochemcial Study

The density functional theory calculations of molecular structure and spectroscopic and electronic properties of charged states in four triazine-heteroaryl star-shaped compounds are presented. The molecules end-capped with different heteroaryl groups, i.e., thiophene (TTT1), furan (TFT2), 3,4-ethylenedioxythienyl (TET3), and thiazole (TSNT4), were studied by electrochemical and in situ ESR/UV-vis-NIR spectroelectrochemical techniques in tetrahydrofuran/(TBA)BF(4). The charged states of monomers were created by cathodic reduction and the stability of generated anion radicals was characterized in detail both experimentally and theoretically. The TET3 molecule resulted in a star-shaped hyperbranched polymer formed anodically which in turn can be reversibly oxidized up to the tetracation. The nature and charge distribution of charge carriers at each redox state in the as-prepared polymer was clarified by calculations for the model (TET3)(3) trimer structure.

The Extended View on the Empty C2(3)‐C82 Fullerene: Isolation, Spectroscopic, Electrochemical, and Spectroelectrochemical Characterization and DFT Calculations

An extended study of the spectroscopic and redox properties of the C(82) fullerene is presented. Among the nine isolated-pentagon-rule (IPR) isomers of the C(82) fullerene the C(82)(3) isomer with C(2) symmetry is the only stable, empty fullerene structure formed in the arc burning process that can be isolated in an isomerically pure form. Here, its formation and isolation are described and its structure is confirmed by experimental spectroscopic studies as well as time-dependent DFT calculations. The electrochemistry of the C(82)(3) isomer is studied in detail by cyclic voltammetry and spectroelectrochemistry. The anionic species of C(82) with the charge ranging from C(82) (-) to C(82) (4-) were successively generated in o-dichlorobenzene solution at room temperature and characterized by in situ ESR and visible/near-infrared (Vis/NIR) spectroscopy. The data give new insights into the charged states of the C(82)(3) fullerene.