We used variegated Plectranthus coleoides as a model plant with the aim of clarifying whether the effects of realistic ultraviolet-B (UV-B) doses on phenolic metabolism in leaves are mediated by photosynthesis. Plants were exposed to UV-B radiation (0.90 W m(-2) ) combined with two photosynthetically active radiation (PAR) intensities [395 and 1350 μmol m(-2) s(-1) , low light (LL) and high light (HL)] for 9 d in sun simulators. Our study indicates that UV-B component of sunlight stimulates CO2 assimilation and stomatal conductance, depending on background light. UV-B-specific induction of apigenin and cyanidin glycosides was observed in both green and white tissues. However, all the other phenolic subclasses were up to four times more abundant in green leaf tissue. Caffeic and rosmarinic acids, catechin and epicatechin, which are endogenous peroxidase substrates, were depleted at HL in green tissue. This was correlated with increased peroxidase and ascorbate peroxidase activities and increased ascorbate content. The UV-B supplement to HL attenuated antioxidative metabolism and partly recovered the phenolic pool indicating stimulation of the phenylpropanoid pathway. In summary, we propose that ortho-dihydroxy phenolics are involved in antioxidative defence in chlorophyllous tissue upon light excess, while apigenin and cyanidin in white tissue have preferentially UV-screening function.
Antioxidant (AO) activity of extracts of hop cones (Serbian domestic varieties) and commercial hop products (Saaz, Spalter, Spalter select, and Magnum pellets) was determined by parallel application of recently developed direct current (DC) polarographic and widely used DPPH assay. Correlations between 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) scavenging and total phenolics (TPC) determined by the Folin-Ciocalteu assay (FC) (0.99), and between H2O2 scavenging (HPS) and humulone content (H) determined by conductometric method (0.94), total resins (TR) (0.85), and hop storage index (HIS) (-0.90), were found statistically significant at p < 0.05 level while complete lack of HPS correlation with TPC and DPPH was observed. To obtain an insight into differences between results of AO assays applied, activity of individual compounds, prevalent hop phenolics, and bitter acids was determined. By far superior HPS activity of humulone was followed by catechin, quercetin, xanthohumol, lupulone, and rutin. In contrast, DPPH scavenging activity of phenolics (quercetin > catechin > rutin > xantohumol) was found substantially higher than activity of bitter acids. According to ferric reducing antioxidant power (FRAP) and scavenging of 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS), higher AO activity was ascribed to phenolics, while almost neglecting humulone. Besides reliability, low cost, and an easy-to-handle procedure, an ability to recognize humulone as the major contributor of hop AO activity could allow DC polarographic assay to be applied in analysis of various hop-derived products.
Herein we compared antioxidative activities (AA) of 25 free L-amino acids (FAA) against Fenton system-mediated hydroxyl radical (HO(•)) production in aqueous solution, and examined the relation between AA and a set of physicochemical properties. The rank order according to AA was: Trp > norleucine > Phe, Leu > Ile > His >3,4-dihydroxyphenylalanine, Arg > Val > Lys, Tyr, Pro > hydroxyproline > α-aminobutyric acid > Gln, Thr, Ser > Glu, Ala, Gly, Asn, Asp. Sulfur-containing FAA generated different secondary reactive products, which were discriminated by the means of electron paramagnetic resonance spin-trapping spectroscopy. AA showed a general positive correlation with hydrophobicity. However, when taken separately, uncharged FAA exhibited strong positive correlation of AA with hydrophobicity whereas charged FAA showed negative or no significant correlation depending on the scale applied. A general strong negative correlation was found between AA and polarity. Steric parameters and hydration numbers correlated positively with AA of nonpolar side-chain FAA. In addition, a decrease of temperature which promotes hydrophobic hydration resulted in increased AA. This implies that HO(•)-provoked oxidation of FAA is strongly affected by hydrophobic hydration. Our findings are important for the understanding of oxidation processes in natural and waste waters.
The behavior of chlorogenic acid (CGA) and its interaction with Hg(II), Pb(II), Cu(II), Cd(II) and Zn(II) has been studied using direct current (DC) and differential-pulse polarography (DPP). Adsorptive and kinetic/catalytic anodic waves of CGA have been detected at À0.02 and À0.18 V vs. SCE, respectively; Hg(I)-CGA surface disproportionation has been assumed. Reduction of Hg(II)-ions, added in the buffered solution of CGA (pH 7.5), occurs rather than complexation, while the complex forming ability of CGA towards Pb(II) and Cu(II) has been observed. Stoichiometry 1 : 1 for Pb(II)-CGA, and 1 : 1 and 1 : 2 for Cu(II)-CGA has been established applying Jobs method of continuous variation on DPP data. UV-vis spectrophotometric measurements additionally confirmed the existence and stoichiometry of Cu(II) complexes.
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