EPR Study of KO2 as a Source of Superoxide and •BMPO-OH/OOH Radical That Cleaves Plasmid DNA and Detects Radical Interaction with H2S and Se-Derivatives.

Superoxide radical anion (O2•-) and its derivatives regulate numerous physiological and pathological processes, which are extensively studied. The aim of our work was to utilize KO2 as a source of O2•- and the electron paramagnetic resonance (EPR) spin trapping 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO) technique for the preparation of •BMPO-OOH and/or •BMPO-OH radicals in water solution without DMSO. The method distinguishes the interactions of various compounds with •BMPO-OOH and/or •BMPO-OH radicals over time. Here, we show that the addition of a buffered BMPO-HCl mixture to powdered KO2 formed relatively stable •BMPO-OOH and •BMPO-OH radicals and H2O2, where the •BMPO-OOH/OH ratio depended on the pH. At a final pH of ~6.5-8.0, the concentration of •BMPO-OOH radicals was ≥20 times higher than that of •BMPO-OH, whereas at pH 9.0-10.0, the •BMPO-OH radicals prevailed. The •BMPO-OOH/OH radicals effectively cleaved the plasmid DNA. H2S decreased the concentration of •BMPO-OOH/OH radicals, whereas the selenium derivatives 1-methyl-4-(3-(phenylselanyl) propyl) piperazine and 1-methyl-4-(4-(phenylselanyl) butyl) piperazine increased the proportion of •BMPO-OH over the •BMPO-OOH radicals. In conclusion, the presented approach of using KO2 as a source of O2•-/H2O2 and EPR spin trap BMPO for the preparation of •BMPO-OOH/OH radicals in a physiological solution could be useful to study the biological effects of radicals and their interactions with compounds.

In situ simultaneous electrochemical ESR study of radicals generated from 2,2-dinitroethene-1,1-diamine (FOX-7). Intramolecular chemical exchange resulting in an alternation line-width effect.

The molecule of 2,2-dinitroethene-1,1-diamine (FOX-7) is one of the most interesting molecules with multiple redox centres stabilized by push-pull effect. To reveal the detailed mechanism of its electrochemical process radical intermediates formed in the course of its electrochemical reduction in organic aprotic media have been studied by in situ simultaneous electrochemical ESR measurements (SEESR). The radical generated on the second reduction step possesses an alternating line-width (AL) effect in the ESR spectra as a result of intramolecular dynamic processes in the timescale of ESR splitting constants. The spectra measured at different temperatures (230-335 K) were analysed with the help of a fitting program which includes a molecular dynamics. Observed dynamics describes well an asymmetric 2-site exchange model for the whole temperature range. With help of the optimized parameters and quantum chemical calculations this radical has been identified as 2,2-dinitroethane-1-amine-1-imine radical dianion, [(H2N)(HN)C=C(NO2)2]2-. The dynamic process responsible for the AL effect consists of mutual turning (changing of dihedral angle) of the both nitro groups, resulting in an intramolecular spin-density (electron) transfer. The dynamic parameters of the process have been established.

Charge density of 4-methyl-3-[(tetrahydro-2H-pyran-2-yl)oxy]thiazole-2(3H)-thione. A comprehensive multipole refinement, maximum entropy method and density functional theory study.

The structure of 4-methyl-3-[(tetrahydro-2H-pyran-2-yl)oxy]thiazole-2(3H)-thione (MTTOTHP) was investigated using X-ray diffraction and computational chemistry methods for determining properties of the nitrogen-oxygen bond, which is the least stable entity upon photochemical excitation. Experimentally measured structure factors have been used to determine and characterize charge density via the multipole model (MM) and the maximum entropy method (MEM). Theoretical investigation of the electron density and the electronic structure has been performed in the finite basis set density functional theory (DFT) framework. Quantum Theory of Atoms In Molecules (QTAIM), deformation densities and Laplacians maps have been used to compare theoretical and experimental results. MM experimental results and predictions from theory differ with respect to the sign and/or magnitude of the Laplacian at the N-O bond critical point (BCP), depending on the treatment of n values of the MM radial functions. Such Laplacian differences in the N-O bond case are discussed with respect to a lack of flexibility in the MM radial functions also reported by Rykounov et al. [Acta Cryst. (2011), B67, 425-436]. BCP Hessian eigenvalues show qualitatively matching results between MM and DFT. In addition, the theoretical analysis used domain-averaged fermi holes (DAFH), natural bond orbital (NBO) analysis and localized (LOC) orbitals to characterize the N-O bond as a single σ bond with marginal π character. Hirshfeld atom refinement (HAR) has been employed to compare to the MM refinement results and/or neutron dataset C-H bond lengths and to crystal or single molecule geometry optimizations, including considerations of anisotropy of H atoms. Our findings help to understand properties of molecules like MTTOTHP as progenitors of free oxygen radicals.

Tuning Redox Properties and Self-Assembly of Thienoacene-Extended Tetrathiafulvalenes.

Turning on and off associations between molecules by a reversible change in their redox states is a convenient way of controlling self-assembly and hence for advancing supramolecular chemistry. Here we present systematic studies on a selection of extended tetrathiafulvalenes with thienoacene spacers. By cyclic and differential pulse voltammetry and in situ EPR/UV-Vis-NIR spectroelectrochemistry, in combination with computations, we have elucidated how the number and orientations of thiophene rings in the spacer between the two dithiafulvene rings influence both the donor strength and association properties. The radical cations and their associates were found to cover a remarkable large region of the UV-Vis-NIR spectrum, but the appearance of the absorption spectrum of the radical cations as well as of the unassociated dications also depended strongly on the exact molecular structure.

Polyradical PROXYL/TEMPO-Derived Amides: Synthesis, Physicochemical Studies, DFT Calculations, and Antimicrobial Activity.

A series of polynitroxide amides possessing 2,2,5,5-tetramethyl-1-pyrrolidinyloxy (PROXYL) and/or 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) units connected through various bridges were synthesized and their properties were analyzed. EPR spectroscopy provided detailed insight into their paramagnetic character and related properties. A thorough examination of the EPR spectra of dinitroxides in organic solvents provided valuable information on the intramolecular motions, thermodynamics, and spin-exchange mechanisms. Analysis of low-temperature X- and Q-band EPR spectra of the dissolved dinitroxides provided spin-spin distances that were comparable with the theoretical values obtained by DFT. Cyclic voltammetry investigations revealed (quasi)reversible electrochemical behavior for PROXYL-derived biradicals, whereas significant loss of the reversibility was found for TEMPO-containing bi- and polyradicals. The inhibitory activities of the nitroxides against model bacteria, yeasts, and filamentous fungi were assessed.

On the association of neutral and cationic tris(tetrathiafulvaleno)dodecadehydro[18]annulenes.

Here, we report the first X-ray crystal structure of a tetrathiafulvalene-fused dehydroannulene with peripheral ethylthio substituents. In addition, we have subjected this compound to electrochemical and UV-Vis-NIR/ESR spectroelectrochemical studies to elucidate the degree to which the oxidised species associate.

Interactions between tetrathiafulvalene units in dimeric structures - the influence of cyclic cores.

A selection of cyclic and acyclic acetylenic scaffolds bearing two tetrathiafulvalene (TTF) units was prepared by different metal-catalyzed coupling reactions. The bridge separating the two TTF units was systematically changed from linearly conjugated ethyne, butadiyne and tetraethynylethene (trans-substituted) units to a cross-conjugated tetraethynylethene unit, placed in either acyclic or cyclic arrangements. The cyclic structures correspond to so-called radiaannulenes having both endo- and exocyclic double bonds. Interactions between two redox-active TTF units in these molecules were investigated by cyclic voltammetry, UV-vis-NIR and EPR absorption spectroscopical methods of the electrochemically generated oxidized species. The electron-accepting properties of the acetylenic cores were also investigated electrochemically.

Stable radical trianions from reversibly formed sigma-dimers of selenadiazoloquinolones studied by in situ EPR/UV-vis spectroelectrochemistry and quantum chemical calculations.

The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-h]quinolines and 8-substituted 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-f]quinolines with R(7), R(8) = H, COOC(2)H(5), COOCH(3), COOH, COCH(3), and CN has been studied by in situ EPR and EPR/UV-vis spectroelectrochemistry in dimethylsulfoxide. All selenadiazoloquinolones undergo a one-electron reduction process to form the corresponding radical anions. Their stability strongly depends on substitution at the nitrogen atom of the 4-pyridone ring. The primary generated radical anions from N-ethyl-substituted quinolones are stable, whereas for the quinolones with imino hydrogen, the initial radical anions rapidly dimerize to produce unusually stable sigma-dimer (σ-dimer) dianions. These are reversibly oxidized to the initial compounds at potentials considerably less negative than the original reduction process in the back voltammetric scan. The dimer dianion can be further reduced to the stable paramagnetic dimer radical trianion in the region of the second reversible reduction step. The proposed complex reaction mechanism was confirmed by in situ EPR/UV-vis cyclovoltammetric experiments. The site of the dimerization in the σ-dimer and the mapping of the unpaired spin density both for radical anions and σ-dimer radical trianions with unusual unpaired spin distribution have been assigned by means of density functional theory calculations.

Amperometric glucose biosensor utilizing FAD-dependent glucose dehydrogenase immobilized on nanocomposite electrode.

Amperometric glucose biosensors utilizing commercially available FAD-dependent glucose dehydrogenases from two strains of Aspergillus species are described. Enzymes were immobilized on nanocomposite electrode consisting of multi-walled carbon nanotubes by entrapment between chitosan layers. Unlike the common glucose oxidase based biosensor, the presented biosensors appeared to be O(2)-independent. The optimal amount of enzymes, working potential and pH value of working media of the glucose biosensors were determined. The biosensor utilizing enzyme isolated from Aspergillus sp. showed linearity over the range from 50 to 960 µM and from 70 to 620 µM for enzyme from Aspergillus oryzae. The detection limits were 4.45 µM and 4.15 µM, respectively. The time of response was found to be 60 s. The biosensors showed excellent operational stability - no loss of sensitivity after 100 consecutive measurements and after the storage for 4 weeks at 4 °C in phosphate buffer solution. When biosensors were held in a dessicator at room temperature without use, they kept the same response ability at least after 6 months. Finally, the results obtained from measurements of beverages and wine samples were compared with those obtained with the enzymatic-spectrophotometric and standard HPLC methods, respectively. Good correlation between results in case of analysis of real samples and good analytical performance of presented glucose biosensor allows to use presented concept for mass production and commercial use.

The charging of the carbon nanotube/oligothiophene interphase as studied by in situ electron spin resonance/UV-Vis-NIR spectroelectrochemistry.

A detailed in situ Electron Spin Resonance (ESR)/UV-Vis-NIR spectroelectrochemical study of the oligothiophene/single walled carbon nanotube (SWCNT) interphase is presented to provide an insight into the interaction of nanotubes with oligothiophenes. Used as electrode materials these composites are followed in situ with respect to the paramagnetic and diamagnetic states formed upon electrochemical charging. The variation of the oligomer chain length and the type, position and number of substituents at the oligomer is used to understand the structural influence on the formation of the charged states in the material upon electrochemical reaction. For ß,ß'-dihexylsexithiophene (ß,ß'-DHST)-SWCNT the enlarged current in the composite and a decreased radical cation concentration can be explained by the formation of π-dimers. By interaction with SWCNTs the π-dimerization of oligothiophenes and the formation of multi π-stack structures occur. For α,ω-dicyano-ß,ß'-dibutylquaterthiophene (DCNDBQT)-SWCNT a new paramagnetic structure of the oligomer is formed as an intermediate which undergoes follow-up reactions. Using different substituted oligothiophenes their interaction with nanotubes can be understood with respect to the structure of the oligomer.

Syntheses, electronic structures, and EPR/UV-vis-NIR spectroelectrochemistry of nickel(II), copper(II), and zinc(II) complexes with a tetradentate ligand based on S-methylisothiosemicarbazide.

Template condensation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde S-methylisothiosemicarbazone with pentane-2,4-dione and triethyl orthoformate at elevated temperatures resulted in metal complexes of the type M(II)L, where M = Ni and Cu and H(2)L = a novel tetradentate ligand. These complexes are relevant to the active site of the copper enzymes galactose oxidase and glyoxal oxidase. Demetalation of Ni(II)L with gaseous hydrogen chloride in chloroform afforded the metal-free ligand H(2)L. Then by the reaction of H(2)L with Zn(CH(3)COO)(2)·2H(2)O in a 1:1 molar ratio in 1:2 chloroform/methanol, the complex Zn(II)L(CH(3)OH) was prepared. The three metal complexes and the prepared ligand were characterized by spectroscopic methods (IR, UV-vis, and NMR spectroscopy), X-ray crystallography, and DFT calculations. Electrochemically generated one-electron oxidized metal complexes [NiL](+), [CuL](+), and [ZnL(CH(3)OH)](+) and the metal-free ligand cation radical [H(2)L](+•) were studied by EPR/UV-vis-NIR and DFT calculations. These studies demonstrated the interaction between the metal ion and the phenoxyl radical.