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Hongyu Zhang - One of the best experts on this subject based on the ideXlab platform.

  • substituent effects on o h bond dissociation enthalpies and ionization potentials of catechols a dft study and its implications in the rational design of phenolic Antioxidants and elucidation of structure activity relationships for Flavonoid antioxid
    Chemistry: A European Journal, 2003
    Co-Authors: Hongyu Zhang, Xiuli Wang
    Abstract:

    Density functional theory (DFT) on B3LYP/6-31G(d,p) level was employed to investigate the substituent effects on O-H bond dissociation enthalpies (BDEs) and ionization potentials (IPs) of catechols. It was revealed that the ortho hydroxyl of catechol was effective for the reduction of the O-H BDE; however, the group had little influence on the IP. The para substituent effects upon O-H BDEs and IPs for catechols were roughly the same as those for monophenols, and this gave the catechol moiety more potential than monophenol to be used as a lead compound in rational design of phenolic Antioxidants. In addition, the 1,4-pyrone effects on O-H BDEs of catecholic rings A or B of Flavonoids were also investigated. Although 1,4-pyrone extended the conjugation system of Flavonoids, it was not beneficial to reduce the O-H BDE as a result of its electron-withdrawing property. Thus, 1,4-pyrone was unlikely to be favorable to enhance the H-abstraction activity of Flavonoids.

  • theoretical elucidation on activity differences of ten Flavonoid Antioxidants
    Acta Biochimica et Biophysica Sinica, 2000
    Co-Authors: Hongyu Zhang, Dezhan Chen
    Abstract:

    : Theoretical methods including structure-activity relationships (SAR) and quantum chemical calculations were used to elucidate the free radical scavenging activity differences of 10 Flavonoid Antioxidants. SAR could give a qualitative explanation on the antioxidant activity differences. And a further elucidation was performed by a theoretical parameter, the difference of heat of formation between antioxidant and its free radical (deltaHOF), calculated by Austin Model 1 (AM1) method. Besides, deltaHOF showed a linear correlationship with the logarithm of the relative antioxidant efficiency (lgRAE, r = 0.7523), indicating that the theoretical methods were effective to elucidate the differences of antioxidant activity. On the other hand, the ineffectiveness of another theoretical parameter, the highest occupied molecular orbit (HOMO) energy level, was verified further.

  • theoretical elucidation of structure activity relationship of Flavonoid Antioxidants
    Science China-chemistry, 1999
    Co-Authors: Hongyu Zhang
    Abstract:

    AM1 method was emyloyed to calculate Flavonoid Antioxidants, and the results obtained are as follows. Firstly, Flavonoid hydroxyls atortho position were more active than the hydroxyls atmeta position in scavenging oxygen-free raidicals, which resulted from the facts that (i) the former were stabilized by forming intramolecular hydrogen bond and (ii)ortho benzocluinone formed in the former structures through resonance, which resulted in large percentage of distribution of spin density or1ortho oxygen and low internal energy. Secondly, electron-attracting effect of ring C of chromone-Flavonoids showed some passive efftrts on hydroxyls of ring A, making the OH less active. As ring C had little effect on ring B and hydroxyls of ring B in most Flavonoids were atortho position, the rule summarized from experiments showing that hydroxyls of ring B were more active in scavenging oxygen-free radicals was elucidated.

Manfred Saran - One of the best experts on this subject based on the ideXlab platform.

  • Flavonoid Antioxidants rate constants for reactions with oxygen radicals
    Methods in Enzymology, 1994
    Co-Authors: Wolf Bors, Christa Michel, Manfred Saran
    Abstract:

    Publisher Summary This chapter describes the Flavonoids that are plant secondary metabolites having a polyphenol structure, occurring mostly as glycosides or methoxylated derivatives, and sometimes as aglycones. It has been assumed for years that they act as Antioxidants, primarily based on the fact that they extend the shelf-life of fat-containing foodstuffs. In contrast, an antioxidative function in plants themselves is still a matter of debate, even though protective effects during plant photooxidative processes. The biochemical background for the antioxidative effect of Flavonoids is inhibition of lipid peroxidation, which has been observed on numerous occasions. Owing to the polyphenol structure, this inhibition can be brought about either by chelating of transition metals or by scavenging of free radicals with the formation of less reactive Flavonoid aroxyl radicals. At present, radical scavenging is clearly the favored mechanism as evidenced by the lopsided ratio of reports on scavenging versus chelating properties of Flavonoids. The chapter discusses the methods for oxygen radical generation. Radiolytic, photolytic, chemical, and enzymatic systems may be used as sources of oxygen radicals.

  • structural principles of Flavonoid Antioxidants
    1992
    Co-Authors: Wolf Bors, Werner Heller, Christa Michel, Manfred Saran
    Abstract:

    Flavonoids are the most ubiquitous and structurally evolved class of plant phenolic compounds. Based on a few principal structures (see Fig. 1), multitudinous hydroxylation, methoxylation, and glycosylation patterns exist. At present more than 4000 individual substances are known [31]. Plants contain both glycosylated compounds and aglycones [74]. As can be expected from the structural diversity, a variety of biological functions have been attributed to Flavonoids: photo reception, light screening, visual attraction, feeding repellance, phytoalexin function, etc. [30]. Antioxidative properties were first suggested when it was discovered that the Flavonoid content of food contributes to an extended shelf life and retards spoilage [43]. Owing to the rapid degradation of Flavonoids in the digestive tract [28, 43], pharmacological effects in mammals are limited [32, 50]. Only few Flavonoid derivatives have so far been shown to be of therapeutic value: rutoside [68], cyanidanol (catechin) [20], silybin [69, 70]; all act predominantly as detoxicants after liver injury.

  • radical chemistry of Flavonoid Antioxidants
    Advances in Experimental Medicine and Biology, 1990
    Co-Authors: Wolf Bors, Werner Heller, Christa Michel, Manfred Saran
    Abstract:

    Flavonoid aglycones, members of an ubiquitous class of plant phenols, have often been proposed to act as Antioxidants.1,2 More recently this activity has been specifically attributed to their radical-scavenging capabilities.3–8 Compounds of various structural features have already been tested,4,7,9,10 but only qualitative conclusions could be drawn.

Hamid Reza Moallem - One of the best experts on this subject based on the ideXlab platform.

  • effect of solute solvent interactions on dpph radical scavenging efficiency of some Flavonoid Antioxidants in various binary water methanol mixtures
    Canadian Journal of Chemistry, 2015
    Co-Authors: Morteza Jabbari, Hamid Reza Moallem
    Abstract:

    Scavenging ability of three kinds of natural Flavonoid Antioxidants including chrysin, naringenin, and quercetin against the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical was evaluated by the UV-vis spectrophotometric technique in different aqueous mixtures of methanol (50%–90% v/v) at physiological pH. The studied Flavonoids showed their activity to be comparable with ascorbic acid (vitamin C), which is used as a standard reference material in most testing methods. Our findings indicated that an increase in the organic solvent percentage (v/v) has different effects on the radical scavenging efficiency of Flavonoids presumably due to solute–solvent interactions. These variations were analyzed in light of various simple and multiple regression equations using the normalized polarity parameter (ETN ) and Kamlet, Abboud, and Taft solvatochromic parameters. Moreover, The IC50 values of the samples were then obtained by the Yasuda–Shedlovsky extrapolation procedure in pure water.

  • Effect of solute–solvent interactions on DPPH radical scavenging efficiency of some Flavonoid Antioxidants in various binary water–methanol mixtures
    Canadian Journal of Chemistry, 2015
    Co-Authors: Morteza Jabbari, Hamid Reza Moallem
    Abstract:

    Scavenging ability of three kinds of natural Flavonoid Antioxidants including chrysin, naringenin, and quercetin against the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical was evaluated by the UV-vis spectrophotometric technique in different aqueous mixtures of methanol (50%–90% v/v) at physiological pH. The studied Flavonoids showed their activity to be comparable with ascorbic acid (vitamin C), which is used as a standard reference material in most testing methods. Our findings indicated that an increase in the organic solvent percentage (v/v) has different effects on the radical scavenging efficiency of Flavonoids presumably due to solute–solvent interactions. These variations were analyzed in light of various simple and multiple regression equations using the normalized polarity parameter (ETN ) and Kamlet, Abboud, and Taft solvatochromic parameters. Moreover, The IC50 values of the samples were then obtained by the Yasuda–Shedlovsky extrapolation procedure in pure water.

Dezhan Chen - One of the best experts on this subject based on the ideXlab platform.

  • theoretical elucidation on activity differences of ten Flavonoid Antioxidants
    Acta Biochimica et Biophysica Sinica, 2000
    Co-Authors: Hongyu Zhang, Dezhan Chen
    Abstract:

    : Theoretical methods including structure-activity relationships (SAR) and quantum chemical calculations were used to elucidate the free radical scavenging activity differences of 10 Flavonoid Antioxidants. SAR could give a qualitative explanation on the antioxidant activity differences. And a further elucidation was performed by a theoretical parameter, the difference of heat of formation between antioxidant and its free radical (deltaHOF), calculated by Austin Model 1 (AM1) method. Besides, deltaHOF showed a linear correlationship with the logarithm of the relative antioxidant efficiency (lgRAE, r = 0.7523), indicating that the theoretical methods were effective to elucidate the differences of antioxidant activity. On the other hand, the ineffectiveness of another theoretical parameter, the highest occupied molecular orbit (HOMO) energy level, was verified further.

Maurizio Guerra - One of the best experts on this subject based on the ideXlab platform.

  • a critical evaluation of the factors determining the effect of intramolecular hydrogen bonding on the o h bond dissociation enthalpy of catechol and of Flavonoid Antioxidants
    Chemistry: A European Journal, 2004
    Co-Authors: Marco Lucarini, Gian Franco Pedulli, Maurizio Guerra
    Abstract:

    : New experimental results on the determination of the bond dissociation enthalpy (BDE) value of 3,5-di-tert-butylcatechol, a model compound for Flavonoid Antioxidants, by the EPR radical equilibration technique are reported. By measurement of the equilibrium constant for the reaction between 3,5-di-tert-butylcatechol and the 2,6-di-tert-butyl-4-methylphenoxyl radical, in UV irradiated isooctane solutions at different temperatures, it has been shown that the thermodynamic parameters for this reaction are DeltaH degrees = -2.8+/-0.1 kcal mol(-1) and DeltaS degrees = +1.3+/-0.2 cal mol(-1) K(-1). This demonstrates that the entropic variations in the hydrogen exchange reaction between phenols and the corresponding phenoxyl radicals are also negligible when one of the reacting species is a polyphenol and that the EPR radical equilibration technique also allows the determination of the Obond;H BDEs in intramolecularly hydrogen-bonded polyphenols. The BDE of 3,5-di-tert-butylcatechol (78.2 kcal mol(-1)) was determined to be identical to that of alpha-tocopherol. Through use of the group additivity rule, this piece of data was also used to calculate the strength of the intramolecular hydrogen bond between the hydroxyl proton and the oxygen radical centre in the corresponding semiquinone radical (5.6 kcal mol(-1)), which is responsible both for the excellent antioxidant properties of catechols and for the BDE of catechol (81.8 kcal mol(-1)). These values are in poor agreement with those predicted by DFT calculations reported in the literature (9.5 kcal mol(-1) and 77.6 kcal mol(-1), respectively). Extensive theoretical calculations indicate that the BDE of catechol is reproduced well (81.6 kcal mol(-1)) by use of diffuse functions on oxygen and the CCSD method.

  • A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the O ? H Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants
    Chemistry: A European Journal, 2004
    Co-Authors: Marco Lucarini, Gian Franco Pedulli, Maurizio Guerra
    Abstract:

    : New experimental results on the determination of the bond dissociation enthalpy (BDE) value of 3,5-di-tert-butylcatechol, a model compound for Flavonoid Antioxidants, by the EPR radical equilibration technique are reported. By measurement of the equilibrium constant for the reaction between 3,5-di-tert-butylcatechol and the 2,6-di-tert-butyl-4-methylphenoxyl radical, in UV irradiated isooctane solutions at different temperatures, it has been shown that the thermodynamic parameters for this reaction are DeltaH degrees = -2.8+/-0.1 kcal mol(-1) and DeltaS degrees = +1.3+/-0.2 cal mol(-1) K(-1). This demonstrates that the entropic variations in the hydrogen exchange reaction between phenols and the corresponding phenoxyl radicals are also negligible when one of the reacting species is a polyphenol and that the EPR radical equilibration technique also allows the determination of the Obond;H BDEs in intramolecularly hydrogen-bonded polyphenols. The BDE of 3,5-di-tert-butylcatechol (78.2 kcal mol(-1)) was determined to be identical to that of alpha-tocopherol. Through use of the group additivity rule, this piece of data was also used to calculate the strength of the intramolecular hydrogen bond between the hydroxyl proton and the oxygen radical centre in the corresponding semiquinone radical (5.6 kcal mol(-1)), which is responsible both for the excellent antioxidant properties of catechols and for the BDE of catechol (81.8 kcal mol(-1)). These values are in poor agreement with those predicted by DFT calculations reported in the literature (9.5 kcal mol(-1) and 77.6 kcal mol(-1), respectively). Extensive theoretical calculations indicate that the BDE of catechol is reproduced well (81.6 kcal mol(-1)) by use of diffuse functions on oxygen and the CCSD method.