Romana Sokolová

The oxidation of natural flavonoid quercetin

This study explains the controversies in the literature concerning the number of electrons involved in the oxidation of quercetin. This stems from inappropriate handling samples, which require strict anaerobic conditions. The redox potential of quercetin strongly depends on the pH and on the presence of dissociation forms in solution.

On the stability of the bioactive flavonoids quercetin and luteolin under oxygen-free conditions

AbstractThe natural flavonoid compounds quercetin (3,3′,4′,5,7-pentahydroxyflavone) and luteolin (3′,4′,5,7-tetrahydroxyflavone) are important bioactive compounds with antioxidative, anti-allergic, and anti-inflammatory properties. However, both are unstable when exposed to atmospheric oxygen, which causes degradation and complicates their analytical determinations. The oxidative change of these flavonoids was observed and followed by UV–visible spectrophotometry, both in aqueous and ethanolic solutions. The distribution of the degradation products in aqueous media was monitored by LC–MS and LC–DAD analysis. The amounts of oxidative reaction products increase with the exposure time. The oxidative degradation reduces the pharmacological efficiency of these antioxidants and renders analytical determination inaccurate. The oxidative changes in flavonoid test solutions can explain the inconsistent dissociation constants reported in the literature. Dissociation constants of quercetin and luteolin were determined both by alkalimetric titration and by UV–visible spectrophotometry under deaerated conditions. The values pK1 = 5.87 ± 0.14 and pK2 = 8.48 ± 0.09 for quercetin, and pK1 = 5.99 ± 0.32 and pK2 = 8.40 ± 0.42 for luteolin were found. FigureThe change of absorption spectra of quercetin during the exposure to the air oxygen

Flavonolignan 2,3-dehydroderivatives: Preparation, antiradical and cytoprotective activity

The protective constituents of silymarin, an extract from Silybum marianum fruits, have been extensively studied in terms of their antioxidant and hepatoprotective activities. Here, we explore the electron-donor properties of the major silymarin flavonolignans. Silybin (SB), silychristin (SCH), silydianin (SD) and their respective 2,3-dehydroderivatives (DHSB, DHSCH and DHSD) were oxidized electrochemically and their antiradical/antioxidant properties were investigated. Namely, Folin-Ciocalteau reduction, DPPH and ABTS(+) radical scavenging, inhibition of microsomal lipid peroxidation and cytoprotective effects against tert-butyl hydroperoxide-induced damage to a human hepatocellular carcinoma HepG2 cell line were evaluated. Due to the presence of the highly reactive C3-OH group and the C-2,3 double bond (ring C) allowing electron delocalization across the whole structure in the 2,3-dehydroderivatives, these compounds are much more easily oxidized than the corresponding flavonolignans SB, SCH and SD. This finding was unequivocally confirmed not only by experimental approaches, but also by density functional theory (DFT) calculations. The hierarchy in terms of ability to undergo electrochemical oxidation (DHSCH~DHSD>DHSB>>SCH/SD>SB) was consistent with their antiradical activities, mainly DPPH scavenging, as well as in vitro cytoprotection of HepG2 cells. The results are discussed in the context of the antioxidant vs. prooxidant activities of flavonolignans and molecular interactions in complex biological systems.

Single-Molecule Conductance in a Series of Extended Viologen Molecules.

Single-molecule conductance in a series of extended viologen molecules was measured at room temperature using a gold-molecule-gold scanning tunneling microscopy break junction arrangement. Conductance values for individual molecules change from 4.8 ± 1.2 nS for the shortest compound to 2.9 ± 1.0 nS for the compound with six repeating units and length of 11 nm. The latter value is almost 3 orders of magnitude higher than that reported for all-carbon-based aromatic molecular wires of comparable length. On the basis of the length of the molecules, an attenuation factor of only 0.06 ± 0.004 nm(-1) (0.006 ± 0.0004 Å(-1)) was obtained. To the best of our knowledge, this is the smallest value reported for the conductance attenuation in a series of molecular wires.

Voltammetry of hypoxic cells radiosensitizer etanidazole radical anion in water

Cytotoxic properties of radiosensitizers are due to the fact that, in the metabolic pathway, these compounds undergo one-electron reduction to generate radical anions. In this study we focused our interest on the electrochemical transfer of the first electron on radiosensitizer Etanidazole (ETN) and, consequently, on the ETN radical-anion formation in the buffered aqueous media. ETN was electrochemically treated in the broad pH range at various scan rates. Three reduction peaks and one oxidation peak were found. At strong alkaline pH the four-electron reduction peak was separated into one-electron and three-electron reductions. Under these conditions the standard rate constant k(0) for the redox couple ETN-NO(2)+e(-) <--> ETN-NO(2)(*-) was calculated. Moreover, the value of a so called E(7)(1) potential that accounts for the energy necessary to transfer the first electron to an electroactive group at pH=7 in aqueous medium to form a radical anion was also determined. The obtained value of E(7)(1) indicates that lower energy compared to the other possible chemical radiosensitizers is necessary for the system to transfer the first electron to ETN. On the other hand, the necessity of the strong alkaline pH may decrease the ability of ETN to act as hypoxic radiosensitizer in the human body.

Novel redox label for proteins.: Electron transfer properties of (η5-cyclopentadienyl) tricarbonyl manganese bound to bovine serum albumin

Abstract (η 5 -Cyclopentadienyl) tricarbonyl manganese (cymantrenyl) derivatives of the protein bovine serum albumin (BSA) with coupling ratios ranging from 7 to 20 were prepared by reaction of cymantrenyl methyl imidate with some of the lysine residues of BSA. Electrochemical measurements based on the AC voltammetric detection of the reduction of cymantrene bound to BSA protein were successfully set up in aqueous solutions and the detection limit of 2×10 −7 M BSA was achieved. This limit is almost two orders of magnitude lower than that measured by IR spectroscopy. Electrochemical detection of cymantrene label will be used for immunoassay analysis of water-soluble analytes in our future work.

On the adsorption of extended viologens at the electrode|electrolyte interface.

Extended viologens represent a group of organic molecules intended to be used as molecular wires in molecular electronic devices. Adsorption properties of a novel series of extended viologen molecules were studied at the mercury electrode|electrolyte interface. These compounds form compact monolayers around the potential of zero charge with a constant differential capacitance value of 2.5 ± 0.2 μF cm(-2) independent of temperature, length of the molecule, and its bulk concentration. At more negative potentials their reduction in the adsorbed state takes place. We showed that the adsorption process is diffusion controlled and time needed to fully cover the electrode surface is independent of the electrode potential. A modified Koryta equation was employed for the calculation of the surface concentration of the adsorbates leading to the value of 5.3 × 10(-11) mol cm(-2) for the shortest wire and to 1.6 × 10(-11) mol cm(-2) for the longest one. Based on the space filling model and the differential capacitance value in the compact film region, it was postulated that these molecules lay flat on the electrode surface.

Characterization of cadmium ion transport across model and real biomembranes and indication of induced damage of plant tissues

The present study is focused on characterization and elucidation of transport processes of the environmentally important element cadmium across model and real membranes. Basic steps of these processes were clarified on model synthetic membranes, which were characterized by atomic force microscopy and by electrochemical impedance spectroscopy. To gain the cytoplasmic membranes containing specific transport systems, the technique for isolation of protoplasts from barley cells was optimized (cell wall-degrading enzyme, physiological state of plant leaves, type of osmotic stabilizers, and the composition of reaction solution). The obtained real parts of membranes were mixed with lecithin membranes and were characterized by electrochemical impedance spectroscopy, and the transported species were determined by voltammetric methods. Further, influence of cadmium on damage of plant tissues was investigated. Young barley plants were subjected to different doses of cadmium chloride and its presence affected the tissue damage, which could be characterized via conductivity of the electrolyte leakage.Graphical abstract

Atrazine-based self-assembled monolayers and their interaction with anti-atrazine antibody: building of an immunosensor.

As a part of our objective to build an immunosensor for the detection of the pesticide atrazine (ATZ) in environmental samples, we studied the self-assembling process of the disulfide derivative of the pesticide atrazine on a gold substrate. Atrazine-based self-assembled monolayers were characterized by ellipsometry, scanning tunneling microscopy, polarization-modulation infrared reflection-absorption spectroscopy (PM IRRAS), X-ray photoelectron spectroscopy and quartz crystal microbalance (QCM) measurements. Two different time constants for the adsorption process were observed, depending on the experimental method used. The QCM data reflect adsorption kinetics of the original disulfide compound, whereas ellipsometry and ex situ PM IRRAS refer to the formation of thiolate (ATZS) monolayers. In situ QCM data demonstrated the suitability of such monolayers for the detection of atrazine in aqueous samples. Exposure of the ATZS sensing surface to an anti-atrazine antibody (anti-ATZ IgG) resulted in complete coverage of the surface by antibody, whereas approximately half of the antibody molecules were displaced from the QCM sensor surface by further addition of atrazine into the solution.

Electrochemical properties of three dicarboximide-type pesticides : Vinclozoline, iprodione and procymidone

Abstract The reduction mechanism of vinclozoline, 3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dione, iprodione, 3-(3,5-dichlorophenyl)- N -(1-methylethyl)-2,4-dioxo-1-imidazolidinecarboxamide, and procymidone, 3-(3,5-dichlorophenyl)-1,5-dimethyl-3-azabicyclo[3.1.0]hexane-2,4-dione, was studied in acetonitrile by electrochemical methods combined with GC/MS identification of products. Irreversible reduction was observed at about −2.2 V. The electron transfer reaction was coupled with subsequent chemical reactions yielding several final products. The main decomposition pathways included the elimination of the hetero-ring (producing chloroanilines and most likely diketones) and the cleavage of one or both chlorine atoms. Vinclozoline also yielded compounds that derive from the reductive elimination of the OCO fragment (carbon dioxide) from the heterocyclic ring.