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Due to the negative impact of reactive species (including free radicals) on humans and animals, the investigations to find effective substances (antioxidants), which protect living organisms against their damaging influence are carried out throughout the world. As most widespread synthetic antioxidants are suspected of having a noxious effect on the human body, more and more attention is paid to natural antioxidant compounds found in plants (especially phenolic compounds). The aim of this paper is to present the data about antioxidant activity of polyphenolic compounds with the emphasis on the main factors having influence on their antioxidant activity: chemical structure, ability to form hydrogen bonds, capability of metal ions chelation and reduction, adduct formation, kinetic solvents effect, mechanism of antioxidant reaction, capability of antioxidant enzyme activation and reduction potential.

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The great prevalence of thin-layer chromatography over high-performance liquid chromatography is connected with the possibility of analyzing many samples in parallel. Therefore, the method is often used in screening and/or effect directed analysis to compare composition and chemical/biological properties of many samples in one run. It was already proved, that high performance thin-layer chromatography, in many cases, can replace high-performance liquid chromatography for quantitative analysis. The main aim of the paper is to show that simple thin-layer chromatography can also be used as a quantitative or at least as a semi-quantitative method, even when it concerns effect directed analysis e.g. direct bioautography. Chlorogenic acid content was measured in four methanol extracts of various green coffees and in one extract of black coffee using thin-layer chromatography with ultraviolet detection and thin-layer chromatography with effect directed detection. High-performance liquid chromatography was used as a reference method. Additionally, total contents of polyphenols and antioxidants were estimated using thin-layer chromatography or dot-blot on chromatography plates. These results were compared to spectrophotometric methods. It was proved that thin-layer chromatography can be used as a quantitative (using densitometry) or semi-quantitative method (using other detection methods including effect directed detection) as well as for estimating total antioxidants or polyphenols content.

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The presented paper shows the influence of temperature increase in the range typically used during antioxidant measurements (15-35 °C) on the estimation of antioxidant properties of phenolic compounds (caffeic acid, ferulic acid, gallic acid, trolox, butylhydroxyanisole, butylhydroxytoluene and 2,6-diisopropylphenol) in associating and non-associating solvents. A significant influence of temperature on the DPPH(•)/antioxidant reaction kinetic is observed for strongly associating solvents (e.g. methanol). This trend is less marked for non-associating solvents (e.g. dioxane, ethyl acetate). The performed experiments prove that the change of solvent structure, caused by temperature increase, influences the estimation of antioxidant properties of phenolic compounds much more than the increase of kinetic energy of reacting molecules and/or the increase of the dissociation degree of hydroxyl groups occurring in antioxidants.

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This study discusses the similarities and differences between the antioxidant activities of some essential oils: thyme (Thymus vulgaris), basil (Ocimum basilicum), peppermint (Mentha piperita), clove (Caryophyllus aromaticus), summer savory (Satureja hortensis), sage (Salvia hispanica) and lemon (Citrus limon (L.) Burm.) and of their main components (thymol or estragole or menthol or eugenol or carvacrol or camphor or limonene) estimated by using 2,2'-Diphenyl-1-picrylhydrazyl, 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt and ß-carotene bleaching assays. The obtained data show that the antioxidant properties of essential oil do not always depend on the antioxidant activity of its main component, and that they can be modulated by their other components. The conclusions concerning the interaction of essential oil components depend on the type of method applied for assessing the antioxidant activity. When comparing the antioxidant properties of essential oils and their main components, the concepts of synergism, antagonism and additivity are very relevant.

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RESUMEN

The 2,2'-diphenyl-1-picrylhydrazyl (DPPH(•)) method is commonly applied for the estimation of antioxidant activity of single compounds and plant extracts. In this method, the amount of disappeared DPPH(•) in the examined system, determined spectrophotometrically, is a measure of the antioxidative (hydrogen-donating) activity of compounds. The present paper discusses the influence of buffer components on the estimation of antioxidant activity of phenolic compounds by this method. According to the obtained results, the change of hydrogen ion concentration changes the mechanism of scavenging process of DPPH radicals by phenolic antioxidants, and the introduction of metal ions into measuring system blocks the scavenging process of DPPH radicals. Both factors depend on the anion type used in the measuring system. The presented results may be especially important for the researches examining plant extract which differ in the content and composition of natural acids and metal ions, and for those who investigate the mechanisms of the reaction applied for the estimation of antioxidant properties.

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