Reaction of electrochemically generated dinitromethyl radical with aromatic hydrocarbons

Conclusions Dinitromethyl radicals generated by electrochemical oxidation of dinitromethane anion can react with aromatic hydrocarbons (benzene and toluene) to give the corresponding aryldinitromethanes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2747–2751, December, 1982.     

A novel method for flow injection analysis of total antioxidant capacity using enzymatically produced ABTS and biamperometric detector containing interdigitated electrode

Application of interdigitated array microelectrodes as electrochemical sensors for determination of antioxidant capacity is reported. Electrochemical measurements with interdigitated electrodes (IDE) were studied in both stationary solutions and the flow system. The method is based on biamperometric measurements using ABTS⁺|ABTS redox couple in phosphate buffer solution, pH 7.40. During analysis, the ABTS radical cation was enzymatically produced by peroxidase in a tubular flow-through reactor. The performance of bioreactor was tested at different concentrations of immobilized enzyme, ABTS and hydrogen peroxide. The influence of flow rate on proper operation of the bioreactor was also studied. The results of antioxidant activity were determined using Trolox as a standard. The applied IDE detector accomplished good sensitivity of 0.3 nA/μM of Trolox and offered linear range between 20 to 500 μM of Trolox.The comparison of results (R² = 0.9915) for antioxidant activity between spectroscopic and FIA biamperometric measurements by interdigitated electrodes confirmed the applicability of the proposed method for determination of antioxidant capacity.     

A rapid assay method for catechol-O-methyltransferase activity by flow injection analysis

A rapid assay employing flow injection analysis (FIA) to determine the activity of purified catechol-O-methyltransferase (COMT) from porcine liver is described. The method was based on the determination of normetanephrine, the 3-O-methyl metabolite of the substrate norepinephrine. Excess norepinephrine was removed from the incubation mixture by alumina extraction twice to allow normetanephrine to be subjected to flow injection analysis, coulometrical oxidation, fluorogenic reaction with ethylenediamine and fluorescence detection. K(m) and V(max) values for COMT obtained with the system were 503 microM and 4.51 nmol/min/mg protein, respectively. The method is suitable for screening of COMT inhibitors or activators, as a large number of samples, up to 200, can be processed in one working day.     

The antioxidant activity of wines determined by the ABTS(+) method: influence of sample dilution and time

The free radical scavenging activity of 42 Spanish commercial wines was determined using the 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS⁺). The ABTS⁺ radical was generated enzymatically using a horseradish peroxidase and hydrogen peroxide. The presence of wine phenolic compounds caused the absorbance of the radical to decay at 414 nm. The measurement conditions were optimised. The total phenolic content of wines ranged from 1262 to 2389 mg l⁻¹ for red wines and 70 to 407 mg l⁻¹ for white wines, expressed as gallic acid equivalents. The phenolic content of Sherry wines was similar to that of white wines. Optimum dilutions for white and Sherry wines were set up as a function of their total phenolic content (for total phenol index, TPI<300 mg gallic acid per liter, dilution 2.5:10 to 5:10; for TPI>300 mg gallic acid per liter, dilution 1:10 to 3:10). Red wines absorb at the wavelength of measurement and dilutions between 0.35:10 and 0.1:10 are advisable. Reaction kinetics were also monitored and the antioxidant activity, expressed as Trolox Equivalent Antioxidant Capacity (TEAC), was determined at 2 and 15 min of reaction. The mean values for TEAC_2 min were 5.01±1.40 mM for red wines, 0.46±0.32 mM for white wines and 0.26±0.19 mM for Sherry wines. At 15 min, mean values were 6.93±2.41 mM for red wines, 0.67±0.47 mM for white wines and 0.26±0.19 mM for Sherry wines. The correlation coefficients were better at 2 min (r=0.9012) than at 15 min (r=0.8462) when compared with TPI. Hence, TEAC_2 min seems to be a more appropriate measure.     

Reactivity of electrochemically generated radical cations of alkylnaphthalenes interpreted by AM1 calculations

Electro-oxidations of alkylnaphthalenes lead mainly to polymeric products and show a radical cation mechanism. Mechanisms are proposed and discussed based on energetic arguments. Enthalpies of formation and the energies of the HOMO and SOMO, calculated using the AM1 program, are presented. It is shown that the prevailing polymeric products are obtained by way of the intermediate formation of naphthols.     

Evaluation of the antioxidant power of honey, propolis and royal jelly by amperometric flow injection analysis

In this paper is described the applicability of a flow injection system, operating with an amperometric detector, for measurement in rapid and simple way the antioxidant power of honey, propolis and royal jelly. The proposed method evaluates the reducing power of selected antioxidant compounds and does not require the use of free radicals or oxidants. Twelve honey, 12 propolis and 4 royal jelly samples of different botanical and geographical origin were evaluated by the electrochemical method and the data were compared with those obtained by the DPPH assay. Since a good correlation was found (R² = 0.92) the proposed electrochemical method can be successfully employed for the direct, rapid and simple monitoring of the antioxidant power of honeybee products. Furthermore, the total phenolic content of samples was determined by the Folin-Ciocalteau procedure and the characteristic antioxidant activities showed a good correlation with phenolics (R² = 0.96 for propolis and 0.90 for honey).     

流动注射化学发光法测定DL-酪氨酸

在甲醛存在下 ,高锰酸钾与DL 酪氨酸能够发生化学发光反应 ,产生很强的化学发光。据此采用流动注射技术 ,建立了一种测定DL 酪氨酸的化学发光分析法。方法的检出限为 2 .9× 1 0 - 8g/mL ,相对标准偏差为 1 .5 % ( 1 .0× 1 0 - 6g/mLDL 酪氨酸 ,n =1 1 ) ,线性范围为 1 .0× 1 0 - 7g/mL～ 5 .0× 1 0 - 6g/mL。     

The spectral characterization of thiophene radical cation generated by pulse radiolysis

Pulse radiolysis matched with kinetic spectrometry has been employed to produce and characterize the radical cations of the series of unsubstituted oligothiophenes from one to six rings in dichloromethane dilute solutions. The concentration of radicals has been tuned to low values so as to slow down dimerization processes. The spectra obtained for the radical cations with 2 to 6 rings agree with previous literature reports. The novel isolation of the radical cation of thiophene itself and its spectral characterization, both experimental and theoretical, stresses its close relationship to the electronic structure of oligothiophenes. This fact strongly recommends that the charge carrier properties of oligothiophenes be interpreted on the basis of the molecular orbital theory rather than of the polaron model. The state responsible for the main UV absorption band is found to be described by a mixture of configurations where the HOMO→LUMO transition is present with different spin couplings.     

Formation and decay of the ABTS derived radical cation: A comparison of different preparation procedures

Bleaching of a preformed solution of the blue‐green radical cation 2,2′‐azinobis (3‐ethylbenzothizoline‐6‐sulfonic acid) (ABTS^+·) has been extensively used to evaluate the antioxidant capacity of complex mixtures and individual compounds. The reaction of the preformed radical with free‐radical scavengers can be easily monitored by following the decay of the sample absorbance at 734 nm. The ABTS radical cation can be prepared employing different oxidants. Results obtained using MnO₂ as oxidant show that the presence of manganese ions increases the rate of [ABTS]^+· autobleaching in a concentration‐dependent manner. The radicals can also be obtained by oxidizing ABTS with 2,2^′‐azobis(2‐amidinopropane)hydrochloride (AAPH) or peroxodisulfate (PDS). The oxidation by AAPH takes place with a large activation energy and a low reaction order in ABTS. The data support a mechanism in which the homolysis of AAPH is the rate‐limiting step, followed by the reaction of ABTS with the peroxyl radicals produced after the azocompound thermolysis. On the other hand, the low activation energy measured employing PDS, as well as the kinetic law, are compatible with the occurrence of a bimolecular reaction between the oxidant and ABTS. Regarding the use of ABTS‐based methodologies for the evaluation of free radical scavengers, radical cations obtained employing AAPH as oxidant can be used only at low temperatures, conditions where further decomposition of the remaining AAPH is minimized. The best results are obtained with ABTS derived radicals generated in the reaction of PDS with an ABTS/PDS concentration ratio equal (or higher) to two. However, even with radicals prepared by this procedure, stoichiometric coefficients considerably larger than two are obtained for the consumption of the radical cation employing tryptophane or p‐terbutylphenol as reductants. This casts doubts on the use of ABTS‐based procedures for the estimation of antioxidant capacities. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 659–665, 2002     

Kinetics of oxidation of benzyl alcohols by the dication and radical cation of ABTS. Comparison with laccase-ABTS oxidations: an apparent paradox

Laccase, a blue copper oxidase, in view of its moderate redox potential can oxidise only phenolic compounds by electron-transfer. However, in the presence of ABTS (2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) as a redox mediator, laccase reacts with the more difficult to oxidise non-phenolic substrates, such as benzyl alcohols. The role of ABTS in these mediated oxidations is investigated. Redox interaction with laccase could produce in situ two reactive intermediates from ABTS, namely ABTS++ or ABTS*+. These species have been independently generated by oxidation with Ce(iv) or Co(iii) salts, respectively, and their efficiency as monoelectronic oxidants tested in a kinetic study towards a series of non-phenolic substrates; a Marcus treatment is provided in the case of ABTS++. On these grounds, intervention of ABTS++ as a reactive intermediate in laccase-ABTS oxidations appears unlikely, because the experimental conditions under which ABTS++ is unambiguously generated, and survives long enough to serve as a diffusible mediator, are too harsh (2 M H2SO4 solution) and incompatible with the operation of the enzyme. Likewise, ABTS*+ seems an intermediate of limited importance in laccase-ABTS oxidations, because this weaker monoelectronic oxidant is unable to react directly with many of the non-phenolic substrates that laccase-ABTS can oxidise. To solve this paradox, it is alternatively suggested that degradation by-products of either ABTS++ or ABTS*+ are formed in situ by hydrolysis during the laccase-ABTS reactions, and may be responsible for the observed oxidation of non-phenolics.     

A study of vitamin A radical cation generated by chemical and radiolytic oxidation

Electron transfer reaction between vitamin A (1) and tris(p-bromophenyl)aminium hexachloroautimonate (2) in dichloromethane (DCM) has been investigated by means of UV-VIS absorption and ESR spectroscopy. The title radical cation formed in the reaction was characterized by a new absorption band around 600 nm and a singlet unresolved ESR spectrum with g factor of 2.0038-2.0039 and line width of 20 G. Further studies indicated that ESR pattern and parameters of the radical cation generated by 7-irradiation of 1 in CFCl3 matrix at 77 K are consistent with that resulted in the chemical oxidation in DCM at ambient temperature.     

Determination of serum alkaline phosphatase activity by electrochemical detection with flow injection analysis

Summary In this enzyme assay for kinetic measurement of alkaline phosphatase (EC 3.1.3.1) activity in human serum by flow injection analysis with electrochemical detection (FIA-EC), the substrate phenyl phosphate is enzymatically hydrolyzed and the phenol produced is quantified by FIA-EC. The standard curve is linear over the range 40 to 500 U/l, and the predicted linearity is up to 1000 U/l. The CV is 5% over the range of the assay. Correlation with the Sclavo Diagnostics nitrophenyl phosphate spectrophotometric technique gave an r of 0.98. This new technique requires as little as 1 l of serum. The FIA-EC method is not subject to interference by hemolysis, icteria, or lipemia or by electroactive substances such as urate, ascorbate, or acetaminophen. Sensitive measurements free from spectrophotometric interferences are thus possible.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

The novel dimerisation of radical anions generated electrochemically in aqueous solution from maleimide

The one-electron reduction of maleimide yields the novel compound 1,2,3,4-butane-tetracarboxylic 1,2:3,4 diimide exclusively. Substrate-substrate hydrogen bonding was found to have a profound influence upon the pathways and kinetics of this unusual reaction.     

Automatic sequential determination of the hydrogen peroxide scavenging activity and evaluation of the antioxidant potential by the 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation assay in wines by sequential injection analysis

An automatic system that performs two analytical procedures, allowing the evaluation of the relative antioxidant capacity of wine samples, was developed. Automation was carried out using a sequential injection analysis (SIA) system that allowed, thanks to its versatility, the development of two methodologies. One is based on the decolorization assay of the 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical, using a spectrophotometric detector. A second methodology allowed the evaluation of the hydrogen peroxide scavenging activity by measuring the oxidation of homovanylic acid (HVA) to its fluorescent dimer, using a fluorescent detector.The developed automatic methodologies were evaluated using trolox as standard and subsequently using other antioxidant substances as gallic acid, caffeic acid, ascorbic acid, catechin and taxifolin which are abundant in wine and whose antioxidant activities were compared to that shown by trolox. The spectrophotometric and fluorimetric assays showed linearity intervals between 0.001 and 0.01 mM, and 0.001 and 0.008 mM of trolox, respectively.The evaluation of the antioxidant power of 20 white and red wine samples, from different Portuguese wine producing regions, was carried out sequentially, in the automatic system. The results were expressed in trolox equivalent antioxidant capacity (TEAC) and presented, for the ABTS and hydrogen peroxide scavenging activity methodologies, detection limits of 8.4 × 10⁻⁷ and 1.4 × 10⁻⁴ mM and relative standard deviation (R.S.D. (%)) in the range 0.6-2.4 and 1-1.8, respectively.     

Evaluation of antioxidative activity of Croatian propolis samples using DPPH* and ABTS*+ stable free radical assays

Propolis is one of the richest sources of plant phenolics (flavonoids and phenolic acids), which are widely recognized as rather strong antioxidants. The aim of our work was to use colored stable free radical (DPPH* and ABTS*+) spectrophotometric and thin-layer chromatographic (TLC) assays to study the antioxidative behavior of the phenolics (caffeic acid, galangin and pinocembrin) most commonly present in Croatian propolis samples obtained from different Croatian regions. We propose a mathematical model providing a more sophisticated interpretation of the obtained results and a new parameter named antioxidative efficiency (AOE) is introduced. The kinetic behaviour of chosen standards determined by spectrophotometric assays follows the exponential decrease of the absorption curve. Explained numerically, AOE represents the absolute value of the first derivative of an absorbance curve in the point A0/e (where A0 is the absorbance measured at t = 0 and e is the natural logarithm base). The advantage of this newly introduced parameter is that it provides an easy and accurate mutual comparison between the rates of antioxidative efficiency of different propolis samples. A TLC assay was only applicable in the case of the DPPH* radical. Dose-response curves were described using a linear function with AOE expressed as a coefficient of the slope. The chromatographic method was shown to be very rapid, reliable and easy-to-perform.     

A highly selective rearrangement of a housane-derived cation radical: an electrochemically mediated transformation

To utilize housane-derived cation radicals as intermediates for the synthesis of the bicyclo (n.3.0) framework of natural products, a highly regioselective [1,2] shift of carbon to either a radical or an electron-deficient site is required. Herein we describe how this has been accomplished, provide a set of guidelines to assess housane oxidizability prior to its synthesis, and describe a synthesis of housane 18 that capitalizes upon the facility of [1,5] hydrogen shifts in substituted cyclopentadienes. The catalytic electrochemically mediated oxidation of 18 leads to a cation radical that engages in a rearrangement leading to the (4.3.0) adduct 23. The appearance of a catalytic current in the cyclic voltammogram of a solution containing the tris(aryl)amine and housane 18 is an excellent indicator that the amminium cation radical 14*+ is able to oxidize the housane and return the mediator to the original redox couple. DFT calculations show electron density on both the aryl and strained sigma framework in the HOMO of housane 18. From the spin density and electrostatic potential map for the cation radical, a picture where the spin resides on the side that is distal to the substituent emerges, while the hole is proximal to it. Both experiment and theory show that the rearrangement is best characterized as a [1,2] carbon shift toward an electron-deficient site and that migration toward the substituent-bearing carbon is much preferred over the alternative pathway.     

Monitoring the changes of 5-caffeoylquinic acid during its reaction with ABTS cation radicals by LC-MS

Abstract

Reactive oxygen species including free radicals are responsible for noxious changes in living organisms which can be associated with many diseases for e.g. cardiovascular, neurodegenerative, and inflammatory diseases or cancer. In the struggle against free radicals the organism is supported by antioxidants. As they are often provided by nature, they can be considered safe. 5-Caffeoylquinic acid is one of main chlorogenic acids found in vegetables and fruits. The main objective of the presented article was monitoring the changes of 5-caffeoylquinic acid during the reaction with colorful radicals. The experiments were conducted at different molar ratios of radicals to antioxidant using liquid chromatography–mass spectrometry. Irrespective of the applied molar ratios, the obtained results proved that the antioxidant takes part in the reaction for three days. Additionally, during the radical neutralization the formation of semiquinones and/or quinones, methoxy adduct of caffeoylquinic acid, and quinones adduct with methanol was observed.