Functionalized graphene quantum dots loaded with free radicals combined with liquid chromatography and tandem mass spectrometry to screen radical scavenging natural antioxidants from Licorice and Scutellariae

A novel screening method was developed for the detection and identification of radical scavenging natural antioxidants based on a free radical reaction combined with liquid chromatography with tandem mass spectrometry. Functionalized graphene quantum dots were prepared for loading free radicals in the complex screening system. The detection was performed with and without a preliminary exposure of the samples to specific free radicals on the functionalized graphene quantum dots, which can facilitate charge transfer between free radicals and antioxidants. The difference in chromatographic peak areas was used to identify potential antioxidants. This is a novel approach to simultaneously evaluate the antioxidant power of a component versus a free radical, and to identify it in a vegetal matrix. The structures of the antioxidants in the samples were identified using tandem mass spectrometry and comparison with standards. Fourteen compounds were found to possess potential antioxidant activity, and their free radical scavenging capacities were investigated. The order of scavenging capacity of 14 compounds was compared according to their free radical scavenging rate. 4′,5,6,7‐Tetrahydroxyflavone (radical scavenging rate: 0.05253 mL mg^?1 s^?1) showed the strongest capability for scavenging free radicals.     

Evaluation of the free radical scavenging capability of wheat extracts by capillary electrophoresis and multivariate curve resolution

In the current work, phenolic acids were analysed and their radical scavenging activity evaluated by CE, including the quantification of overlapping peaks by applying the multivariate curve resolution (MCR) method. To investigate radical scavenging activity, the solution of phenolic acids was treated with a stable free 2,2-diphenyl-1-picrylhydrazyl radical of different concentrations and the radical scavenging activity of the reaction mixture was monitored. The radical scavenging reaction was also applied to wheat bran extracts. The overlapping peaks on the electropherograms of both mixtures of standard compounds and extracts of different wheat bran varieties were resolved by using the MCR method, assuming that the overlapping compounds have different UV spectra. MCR enabled the insufficient resolution between syringic and ferulic acids to be effectively resolved.     

Synthesis,conformational analysis and free radical scavenging activity of some new spiropyranoquinolinones

A series of novel spiroadamantyl- and spirocyclical substituted pyranoquinolin-2-ones were synthesized and the conformation of the pyran ring was investigated. The free radical scavenging activity of the synthesized compounds was determined by their interaction with the stable free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH). All compounds tested scavenged the DPPH radical and among them derivatives possessing extended conjugation showed the highest activity.     

几种天然黄酮类化合物对二苯代苦味肼自由基的清除作用

采用循环伏安法、示差脉冲伏安法对四种天然黄酮类化合物清除二苯代苦味肼自由基（DPPH）的能力进行了探讨.研究表明：此法可直接观测样品对DPPH的清除作用,而且灵敏度高、准确可靠,是一种检测自由基及物质对其清除作用的有效方法.     

Flavonoids from Lupinus texensis and their free radical scavenging activity

Seventeen flavonoids including one new compound were isolated from Texas bluebonnet (Lupinus texensis), the state flower of Texas. Their structures were determined by extensive nuclear magnetic resonance and high-resolution electrospray ionization mass spectrometry analyses. High-performance liquid chromatography analytic method for simultaneous determination of the 17 compounds was established and validated. Eleven isolated flavonoids were first evaluated for their free radical scavenging activity using α,α-diphenyl-β-picrylhydrazyl scavenging assay and they showed activity with EC(50) 48.6-172.5 μg mL(-1).     

Component analysis and free radicals scavenging activity of Cicer arietinum L. husk pectin

A pectin (CAP) was extracted from the husk of Cicer arietinum L. Monosaccharide analysis of CAP revealed the dominance of galacturonic acid and smaller amounts of galactose, arabinose, rhamnose, glucose, xylose and mannose. Viscosimetric analysis showed that the intrinsic viscosity ([η]) and the molecular weight (MW) of CAP were 296 mL/g and 105 kDa, respectively. The degree of esterification (DE = 10%) was determined by FTIR spectroscopy. CAP exhibited a dose-dependent free radical scavenging activity, as shown by its DPPH radical inhibition. At 1.0 mg/mL CAP exhibited a scavenging rate of 29% on DPPH radicals. The evaluation of antioxidant activity suggested that CAP had good potential for DPPH radical scavenging activity and should be explored as a novel potential antioxidant.     

Determination of reactions between free radicals and selected Chilean wines and transition metals by ESR and UV-vis technique

Four different types of Chilean wines (Cabernet Sauvignon, Merlot, Carmenere and Syrah) were selected and examined in their free radical scavenging capacities by electron spin resonance (ESR) and spectrophotometric methods. The free radical scavenging properties were evaluated against 2,2-diphenyl-1-picrylhydrazyl (DPPH*) radical, 2,6-di-tert-butyl-alpha-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy (Galvinoxyl) radical and hydroxyl radical (HO*). The possible effect on these scavenging properties of added transition metals to these wines was evaluated. Among the wines evaluated, Cabernet Sauvignon was the one with the highest activity against all radicals tested. The presence of added copper or iron to wines resulted in a reduced free radical scavenging capacity for all type of wines studied. The formation of redox inactive complexes between polyphenols of wine and transition metals is the possible cause of this reduction in antioxidant activity.     

Separation and evaluation of free radical-scavenging activity of phenol components of Emblica officinalis extract by using an HPTLC-DPPH* method

A new procedure has been developed to separate and quantify the free radical-scavenging activity of individual compounds from an Emblica officinalis extract based on the combination of HPTLC with a diode array detector (DAD) and postchromatographic DPPH* radical derivatization. Free gallic and ellagic acids and emblicanins A and B in the E. officinalis extract were separated by TLC and identified. All the compounds of the extract were capable of scavenging of DPPH* radicals. It was established that the DPPH* scavenging activity of emblicanins A and B was 7.86 and 11.20 times more than that of ascorbic acid and 1.25 and 1.78 times more than gallic acid, respectively. From the estimated ID50 values, it can be seen that the increasing order of activity was emblicanin B > emblicanin A > gallic acid > ellagic acid > ascorbic acid. Probably, the antioxidant activity of E. officinalis extract is associated with the presence of hydrolyzable tannins having ascorbic acid-like action.     

Chemical constituents of two sages with free radical scavenging activity

From the aerial parts of Salvia trichoclada Bentham and S. verticillata L. one new and two known phenolic acids, 3-(3',4'-dihydroxyphenyl)-2-hydroxymethyl propionic acid (1), 3-(3',4'-dihydroxyphenyl) lactic acid (2), and rosmarinic acid (3); two flavonoids, apigenin 4'-methyl ether 7-O-glucuronide (4), and luteolin 7-O-beta glucuronide (5); two lupan type triterpene aglycones, lupeol (6), and 30-hydroxylup-20 (29)-en-3-on (7); an oleanane-type triterpene acid, oleanolic acid (8); and an ursan-type triterpene acid, ursolic acid (9) were isolated. The structures of the compounds were elucidated by spectroscopic analysis. Different extracts of the plants were examined for their free radical scavenging activities by DPPH (2,2-Diphenyl-1-picrylhydrazyl) assay. Some of the polar extracts showed high free radical scavenging activity.     

Flavonoids: hemisynthesis, reactivity, characterization and free radical scavenging activity

Phenolic compounds form one of the main classes of secondary metabolites. They display a large range of structures and they are responsible for the major organoleptic characteristics of plant-derived-foods and beverages, particularly color and taste properties and they also contribute to the nutritional qualities of fruits and vegetables. Phenolic compounds are also highly unstable compounds which undergo numerous enzymatic and chemical reactions during postharvest food storage and processing thus adding to the complexity of plant polyphenol composition. Among these compounds flavonoids constitute one of the most ubiquitous groups of all plant phenolics. Owing to their importance in food organoleptic properties and in human health, a better understanding of their structures, their reactivity and chemical properties in addition to the mechanisms generating them appears essential to predict and control food quality. The purpose of this work is an overview of our findings concerning the hemisynthesis, the reactivity and the enzymatic oxidation of some flavonoids and shed light on the mechanisms involved in some of these processes and the structures of the resulting products. The free radical scavenging activity of some of the synthesized compounds is also presented and a structure-activity relationship is discussed. The first part of this review concerns the synthesis and structural characterization of modified monomeric flavanols. The use of these compounds as precursor for the preparation of natural and modified dimeric procyanidin derivatives was then explored through different coupling reactions. The full characterization of the synthesized compounds was achieved by concerted use of NMR and ESI-MS techniques. The free radical scavenging activity of some of the synthesized compounds was investigated. The second part of this review concerns the enzymatic oxidation of several flavonols by Trametes versicolor laccase. Most of the major oxidation products have been isolated as pure compounds and their structures unambiguously established through spectroscopic methods. Correlation between the structure of the oxidation product and the substitution pattern of the starting materials allows mechanistic features of this transformation to be elucidated.     

Radical scavenging ability of polyphenolic compounds towards DPPH free radical

Free radical scavenging activity of different polyphenolic compounds commonly present in wine has been evaluated using DPPH method. The experiments were performed with different amounts of phenols within the linear interval of response and with an excess of DPPH in all cases. In these conditions, for most of the compounds tested, the reaction was biphasic. Total stoichiometry values n confirm the implication of more than one step in the process. Flavan-3-ol compounds showed the highest values, especially procyanidins B1 (9.8) and B2 (9.1). In this family, n values coincide with the number of hydroxyl groups available. EC₅₀ and TEC₅₀ parameters have been calculated. EC₅₀ values are extremely diverse, being the procyanidins B1 and B2 the most potent scavenging compounds and resveratrol the less one. TEC₅₀ considers the rate of reaction towards the free radical. (+)-Catechin and (−)-epicatechin are the phenolic compounds that need more time to react. In contrast, caftaric and caffeic acids are the phenolic acids that react more rapidly. Antioxidant efficacy (AE) is a parameter that combines both factors. Compounds as kaempferol, with a high EC₅₀ value, could be considered as an antioxidant with low relevance, but instead shows the highest AE value of the phenolic compounds tested, due to its fast rate of reaction, what is of great biological importance.     

Comparison of the simple cyclic voltammetry (CV) and DPPH assays for the determination of antioxidant capacity of active principles

Antioxidant activity of a number of small (low molecular weight) natural compounds found in spices, condiments or drugs (gallic acid, sesamol, eugenol, thymol, carvacrol, vanillin, salicylaldehyde, limonene, geraniol, 4-hexylresorcinol, etc.) has been evaluated using electrochemical and DPPH? radical scavenging measurements. Structural analysis of the tested compound suggest a remarkable activity for phenol derivatives and the importance of the -R groups located on the phenolic ring in the molecule's ability to act as free radical scavenging as well as their influence in the electrochemical behavior. The voltammetric method can be used for the determination of the antioxidant capability in the same manner as the DPPH? radical scavenging because of the correlation found between oxidation potentials and anti-radical power (ARP = 1/EC??). Such electrochemical determination is fast and cheap and allows making measurements under a variety of experimental conditions. The accuracy of the electrochemical measurements is the same for all the compounds, irrespective of their scavenging activity, the opposite of what occurs in the DPPH? test.     

A novel series of thromboxane A2 synthetase inhibitors with free radical scavenging and anti-peroxidative activities

A novel series of indoline derivatives with imidazole and carboxyl moieties were synthesized and evaluated for their thromboxane A2 (TXA2) synthetase inhibiting, radical scavenging and anti-peroxidative activities. Among the compounds synthesized, 3-[5-substituted-3-[2-(imidazol-1-yl)ethyl]indolin-1-yl]propionic acids showed free radical scavenging activity and inhibitory effects on lipid-peroxidation of rat brain homogenate and on arachidonate-induced TXA2-dependent aggregation of rabbit platelets. The anti-platelet and anti-peroxidative activities were related to the lipophilicity of the 5-substituent. The 5-hexyloxy derivative (13) showed about 35-fold higher inhibitory activity on TXA2 synthesis than that of ozagrel and about 100-fold higher activity on lipid peroxidation than that of alpha-tocopherol. Compound 13 showed in vivo anti-thrombotic effect in mice and ex vivo anti-peroxidative activity in rats.     

Optimization and validation of post-column assay for screening of radical scavengers in herbal raw materials and herbal preparations

On-line method, which combines HPLC distribution and post-column reaction, was designed for the search of individual antioxidants. Optimization of the assay was performed evaluating optimal ABTS(+) radical cation concentration in the reactor, reaction time, impact of flow rate, reaction coil length. HPLC-ABTS assay validation in this work was performed by assessing reference antioxidant negative peak areas in radical scavenging chromatogram. Sample free radical scavenging activity is expressed as trolox equivalent antioxidant capacity (TEAC). Optimized and validated method was applied in detection of compounds possessing free radical scavenging ability in complex mixtures. Antioxidant compounds were studied in perilla (Perilla frutescens (L.) Britton var. crispa f. viridis) herbal raw material and its preparations. The HPLC-separated antioxidant compounds were identified using HPLC-photodiode array coupled to mass spectrometer, using a reference mass for determining accurate masses. Radical scavenging characteristics of rosmarinic acid, which is the dominant phenolic compound in medicinal herbal raw material of perilla and its preparations, were confirmed by the calculated TEAC values. Compounds responsible for antioxidant effect in herbal raw materials and herbal preparations were identified, evaluated and compared.     

Azo compounds for free radical crosslinking of polysiloxane stationary phases

There has been much effort spent in recent years developing the technology for free radical crosslinking of polysiloxane polymers to prepare capillary columns coated with thermally stable and nonextractable stationary phases. Organic peroxides have been used extensively as the free radical initiators for the in situ polymerization of the stationary phases. However, these peroxides adversely affect the phase polarity and column activity. Seven azo compounds were studied for crosslinking efficiency. Azo compounds can be used as free radical initiators to prepare nonextractable stationary phases without the adverse effects caused by peroxides.     

An Improved HPLC Post-Column Methodology for the Identification of Free Radical Scavenging Phytochemicals in Complex Mixtures

The importance of natural antioxidants principally relates to their health-promoting properties. The discovery of new sources of established compounds or more potent compounds is a costly exercise and any technique capable of aiding this procedure would be highly significant. An approach combining chromatographic separation, component analysis and post-column identification of free radical scavenging has been reported. However, its effectiveness is dependent upon various factors and the number of samples analyzed without operator intervention is restricted. A more applicable technique using a 10% to 100% methanol with 2% acetic acid mobile phase and a citric acid-sodium citrate buffered methanolic 1,1-diphenyl-2-picrylhydrazyl free radical reagent is presented, whereby free radical scavengers can be detected within crude extracts of variable polarity without special considerations. As an illustration, acetone, 70% (aq.) methanol and acetonitrile-soluble extracts were prepared from sage (Salvia officinalis L.) for extraction of polar and non-polar radical scavengers and analyzed. Radical scavenging components were identified as carnosic, caffeic and rosmarinic acids and luteolin-7-O-glycoside. Others radical scavengers were tentatively identified as benzoic and hydroxycinnamic acid derivatives, flavonoids and diterpenoids. Through the application of this technique, carnosic acid and rosmarinic acid were identified as the principal free radical scavengers.     

Antioxidant activities of extracts and metabolites isolated from the fungus Antrodia cinnamomea

Three different solvent partitions (n-hexane, ethyl acetate [EtOAc] and n-BuOH) of the culture broth from Antrodia cinnamomea were assayed with two different radical scavenging methods: 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging and superoxide radical scavenging (SOD) assay. The EtOAc layer exhibited the best antioxidant activity. Two major antioxidant metabolites were isolated from the active EtOAc layer. The antioxidant activities of compounds 1-6 were further evaluated by DPPH, SOD and trolox equivalent antioxidant capacity (TEAC) assays. Compounds 3 and 5 showed stronger free radical scavenging than the reference BHA, ED???=?1.36 and 34.24 μM. Compound 5 displayed moderate SOD activity (ED???=?310.0?μM), and its antioxidant capacity of TEAC value was 2.2 mM trolox equivalency.     

Synthesis,characterization and antioxidant activities of highly functionalized cyclopentadienes catalyzed by ZnO-nanorod as economic and efficient heterogeneous nano catalyst

A three-component condensation was applied for the preparation of cyclopentadiene derivatives through the reaction of primary amines, alkyl propiolate and dialkyl acetylenedicarboxylate in the presence of catalytic amount of ZnO-nanorods (Zn-NR) under solvent-free conditions at 50℃. The method has proved to be synthetically green, simple, and effective with high atom economy and yield. The catalyst also revealed significant reusability. Moreover, the antioxidant activity and free radical scavenging capacity of the newly synthesized derivatives 4a, 4b, 4c, and 4d was screened using free radical scavenging 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays and compared with hydroxytoluene (BHT) and tert-butylhydroquinone (TBHQ). These compounds do not exhibit good DPPH radical scavenging, but they have a desirable FRAP.     

Chemical studies on antioxidant mechanisms and free radical scavenging properties of lignans

The antioxidant activity, in terms of radical scavenging capacity, of altogether 15 different lignans was measured by monitoring the scavenging of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). The effect of differences in skeletal arrangement or the degree of oxidation of the lignans was investigated in a structure-activity relationship study. A large variety in the radical scavenging capacities of the different lignans was observed and related to some structural features. Lignans with catechol (3,4-dihydroxyphenyl) moieties exhibited the highest radical scavenging capacity, while the corresponding guaiacyl (3-methoxy-4-hydroxyphenyl) lignans showed a slightly weaker scavenging capacity. In addition, the butanediol structure was found to enhance the activity, whereas a higher degree of oxidation at the benzylic positions decreased the activity. Additionally, the readily available lignans (-)-secoisolariciresinol, a mixture of hydroxymatairesinol epimers and (-)-matairesinol were studied in more detail, including kinetic measurements and identification of oxidation products in the reactions with DPPH and ABAP (2,2-azobis(2-methylpropionamidine) dihydrochloride. The identification of reaction products, by GC-MS, HPLC-MS and NMR spectroscopy, showed that dimerisation of the two aromatic moieties was the major radical termination reaction. Also, the formation of adducts was a predominant reaction in the experiments with ABAP. The kinetic data obtained from the reactions between the lignans and DPPH indicated a complex reaction mechanism.