杨梅酮的抗氧化活性

采用密度泛函理论(DFT)方法, 研究了杨梅酮的分子结构、电子结构和羟基离解焓, 并探讨了杨梅酮与OOCH₃·自由基发生反应的抗氧化机理. 自由基发生反应的抗氧化机理. 在M06-2X/6-31++G(d,p)的计算水平下, 得到了杨梅酮脱氢后各种自由基的相对能量、羟基离解焓、氢原子提取的活化能垒和速率常数. 计算结果表明杨梅酮的4'-OH位置具有最高的抗氧化活性. 杨梅酮4'-OH位置的高活性, 主要是由于4'位上脱氢后生成的羰基与相邻的羟基之间形成了稳定的氢键. 分子中的原子(AIM)理论分析表明, 这种氢键相互作用能够稳定氢提取过程中产生的自由基. 对杨梅酮抗氧化性机理的理论研究, 可为今后设计合成新型的具有更强活性的抗氧化物提供坚实的理论基础.     

Antioxidant properties of phenolic Schiff bases: structure–activity relationship and mechanism of action

Phenolic Schiff bases are known for their diverse biological activities and ability to scavenge free radicals. To elucidate (1) the structure–antioxidant activity relationship of a series of thirty synthetic derivatives of 2-methoxybezohydrazide phenolic Schiff bases and (2) to determine the major mechanism involved in free radical scavenging, we used density functional theory calculations (B3P86/6-31+(d,p)) within polarizable continuum model. The results showed the importance of the bond dissociation enthalpies (BDEs) related to the first and second (BDE_d) hydrogen atom transfer (intrinsic parameters) for rationalizing the antioxidant activity. In addition to the number of OH groups, the presence of a bromine substituent plays an interesting role in modulating the antioxidant activity. Theoretical thermodynamic and kinetic studies demonstrated that the free radical scavenging by these Schiff bases mainly proceeds through proton-coupled electron transfer rather than sequential proton loss electron transfer, the latter mechanism being only feasible at relatively high pH.     

Theoretical Study on the Antioxidant Activity of Curcumin

The computational results for curcumin at the B3LYP/6-31G(d,p) level show that the enol form of curcumin is more stable than the diketo form because of an intramolecular hydrogen bond, which extends the conjugation effect in the enol chain, formed in the enol structure. Cis-diketone form can not be obtained, presumably due to the strong repulsion between the carbonyl dipoles aligned in parallel. According to the phenolic O-H bond dissociation enthalpy, curcumin in its most stable form can be suggested to be a relatively good antioxidant. In order to avoid overcoming H-bond interaction and to improve the antioxidant activity of curcumin, a catechol moiety was incorporated into curcumin for designing a novel antioxidant. It is found that the designed molecule is much more efficient to scavenge radical than curcumin, comparable to vitamin E. Moreover, the ionization potential of the designed molecule is similar to that of curcumin, indicating that the designed molecule can not display the prooxidant effect.     

The "somersault" mechanism for the p-450 hydroxylation of hydrocarbons. The intervention of transient inverted metastable hydroperoxides

A series of model theoretical calculations are described that suggest a new mechanism for the oxidation step in enzymatic cytochrome P450 hydroxylation of saturated hydrocarbons. A new class of metastable metal hydroperoxides is described that involves the rearrangement of the ground-state metal hydroperoxide to its inverted isomeric form with a hydroxyl radical hydrogen bonded to the metal oxide (MO-OH --> MO....HO). The activation energy for this somersault motion of the FeO-OH group is 20.3 kcal/mol for the P450 model porphyrin iron(III) hydroperoxide [Por(SH)Fe(III)-OOH(-)] to produce the isomeric ferryl oxygen hydrogen bonded to an *OH radical [Por(SH)Fe(III)-O....HO(-)]. This isomeric metastable hydroperoxide, the proposed primary oxidant in the P450 hydroxylation reaction, is calculated to be 17.8 kcal/mol higher in energy than the ground-state iron(III) hydroperoxide Cpd 0. The first step of the proposed mechanism for isobutane oxidation is abstraction of a hydrogen atom from the C-H bond of isobutane by the hydrogen-bonded hydroxyl radical to produce a water molecule strongly hydrogen bonded to anionic Cpd II. The hydroxylation step involves a concerted but nonsynchronous transfer of a hydrogen atom from this newly formed, bound, water molecule to the ferryl oxygen with a concomitant rebound of the incipient *OH radical to the carbon radical of isobutane to produce the C-O bond of the final product, tert-butyl alcohol. The TS for the oxygen rebound step is 2 kcal/mol lower in energy than the hydrogen abstraction TS (DeltaE() = 19.5 kcal/mol). The overall proposed new mechanism is consistent with a lot of the ancillary experimental data for this enzymatic hydroxylation reaction.     

The Electrochemical Evaluation of the Antioxidant Activity of Substituted Tetraphenylporphyrins

Oxidative–reductive and antioxidant properties of 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin, 5,10,15,20-tetrakis(4-aminophenyl)porphyrin, and 5,10,15,20-tetrakis(4-pentoxyphenyl)porphyrin in their reaction with the 2,2-diphenyl-1-picrylhydrazile free radical are studied. Two of the three abovelisted compounds, namely, 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin, were found to possess antioxidant activity, the former’s antioxidant activity being higher, while 5,10,15,20-tetrakis(4-pentoxyphenyl)porphyrin showed no antioxidant properties. A probable mechanism of antioxidant activity of the studied porphyrins involves hydrogen homolytic detachment from functional substituent in phenyl ring and the hydrogen radical interaction with 2,2-diphenyl-1-picrylhydrazile.     

密度泛函研究细梗胡枝子提取物中新型抗氧化剂的分子结构和活性氧清除机理

The molecular structure and radical scavenging activity of three novel antioxidants from Lespedeza Virgata, lespedezavirgatol, lespedezavirgatal, and lespedezacoumestan, have been studied using density functional theory with the B3LYP and BhandHLYP methods. The optimized geometries of neutral, radical cation, radical and anion forms were obtained at the B3LYP/6-31G(d) level, in which it was found that all the most stable conformations contain intramolecular hydrogen bonds. The same results were obtained from the MP2 method. The homolytic O-H bond dissociation enthalpy and the adiabatic ionization potential of neutral and anion forms for the three new antioxidants and adiabatic electron affinity and H-atom affinity for hydroxyl radical, superoxide anion radical, and hydrogen peroxide radical were determined both in gas phase and in aqueous solution using IEF-PCM and CPCM model with UAHF or Bondi cavity. The antioxidant activities and reactive oxygen species scavenging mechanisms were then discussed, and the results obtained from different methods are consistent. Furthermore, the antioxidant activities are consistent with the experimental findings of the compounds under investigation.     

Unusual Poly(phenylacetyloxy)‐Substituted 1,1′: 4′,1″‐Terphenyl Derivatives from Fruiting Bodies of the Basidiomycete Thelephora ganbajun

Five new poly(phenylacetyloxy)‐substituted 1,1′: 4′,1″‐terphenyl derivatives, ganbajunins A – E ( 2 – 6 ) were isolated from the fruiting bodies of the Basidiomycete Thelephora ganbajun Zang . Their structures were established by spectroscopic (including 2D‐NMR) and chemical means.     

Phytochemical and antioxidant properties of some Cassia species

In this study, the antioxidant activity of aqueous and ethanol extracts of four plants from the genus Cassia were evaluated by various antioxidant assays, including ferric reducing antioxidant power (FRAP), DPPH free radical scavenging, metal chelating activity, phosphomolybdenum reducing power, hydrogen peroxide radical scavenging, hydroxyl radical scavenging, deoxyribose degradation and β-carotene bleaching assay. The various antioxidant activities were compared to standard antioxidant such as ascorbic acid. All the extracts showed antioxidant activity in the tested methods. Among the four species, Cassia auriculata has been found to possess highest activity in most of the tested models. In addition to the antioxidant activity, the total phenolics and flavonoids were measured in the extracts. The ethanolic extract exhibited highest phenolics and flavonoid contents and had also shown potent antioxidant activity in comparison to the aqueous extracts. The possible antioxidant mechanism of the ethanol extract can be due to its hydrogen or electron donating and direct free radical scavenging properties. Hence, the ethanol extract represents a source of potential antioxidants that could be used in pharmaceutical industries.     

Trapping of the OH radical by alpha-tocopherol: a theoretical study

The antioxidant activity of alpha-tocopherol against the damaging hydroxyl radical was analyzed theoretically by hybrid density functional theory, following the direct dynamics method, where the thermal rate constants were calculated using variational transition-state theory with multidimensional tunneling. We found that the OH radical is only slightly or not at all selective, attacking by different mechanisms at several positions of the alpha-tocopherol molecule, giving competitive reactions. The most favorable pathways are the hydrogen abstraction reaction from the phenolic hydrogen and the electrophilic addition onto the aromatic ring. We propose a final rate constant, the sum of the competitive hydrogen abstraction and addition reactions, > or =2.7 x 10(8) M(-1) s(-1) at 298 K, where the hydrogen abstraction reaction represents only 20% of the total OH radical reaction. This result indicates that, molecule by molecule, in an apolar environment, alpha-tocopherol is less effective than coenzyme Q (which presents a rate constant of 6.2 x 10(10) M(-1) s(-1) at 298 K) as a scavenger of OH radicals. It was also found that both mechanisms are not direct but pass through intermediates in the entry channel, with little or no influence on the dynamics of the reactions. The hydrogen abstraction reaction also presents another intermediate in the exit channel, which may have a significant role in preventing the pro-oxidant effects of alpha-tocopherol, although less important than with free radicals other than OH.     

单体结构和聚合条件对大体积乙烯基聚合物旋光性质的影响

合成了3种手性大体积烯类单体——(+)-4,4″-二[(S)-2-甲基丁氧基]-2′-乙烯基对三联苯(p-BMVT)、(+)-3,3″-二[(S)-2-甲基丁氧基]-2′-乙烯基对三联苯(m-BMVT)和(+)-2,2″-二[(S)-2-甲基丁氧基]-2′-乙烯基对三联苯(o-BMVT),其中后两个为新化合物.系统研究了单体结构对其聚合反应活性以及单体结构和反应条件对所得聚合物旋光性质的影响.p-和m-BMVT在合适的条件下可以顺利地进行自由基聚合,形成某一旋向占优的手性二级结构;手性取代基在单体分子上移动一个共价键的距离导致聚合物的旋光方向相反.单体o-BMVT的合成产率低且不能进行自由基聚合.提高芳烃类或者降低非芳烃类聚合溶剂的极性、升高反应温度、减少单体浓度有利于得到旋光度大的聚合物.     

Antioxidant potential of styrene pyrone polyphenols from Inonotus obliquus: A combined experimental,density functional theory (DFT) approach and molecular dynamic (MD) simulation

Polyphenols containing styrene pyranone skeleton are unique to porous fungi. Inonotus obliquus (IO) is a medicinal and edible porous fungus. Twelve phenolic compounds containing four styryl pyranone polyphenols from IO were isolated and identified in this work. The antioxidant ability of the isolates was characterized utilizing the ferric reducing antioxidant power (FRAP) assay and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) radical scavenging test. Styrylpyranone polyphenols, especially methylinoscavin B, could capture free radicals better than other phenolic compounds, and quantum chemical calculations also confirmed this evaluation. The density functional theory (DFT) calculation data showed that Styrylpyranone polyphenols, especially methylinoscavin B, have a lower energy gap, higher softness and higher electronic chemical potential than other phenolic compounds. The bond dissociation energy values of the bond in C7ʹ O–H of the methylinoscavin B molecule are less than those in C11 and C6ʹ O–H when reacting with ∙OOH (selected as a representative free radical). On the basis of calculations, hydrogen atom transfer (HAT) is supposed to be a preferred mechanism over single electron transfer (SET) when phenolic compounds react with free radicals. Moreover, after treatment with final concentrations of 0.5, 1, 2, 3, 4 and 5 μM phelligridin E (PHE), the activity of SOD1 increased by 70.15%, 11.36%, 145.45%, 172.73%, 205.05% and 275.23%, respectively. The molecular dynamics simulation (MD) study of PHE employed SOD1 (PDB ID: 6FN8). The MD results showed that the hydrogen bonds between ASP147 of SOD1 and PHE promote GLU223-ARG224 to form a stable C coil after combining with PHE. The formation of the C Coil enhanced the stability of the electrostatic loop (EL) of SOD1 and the rate of diffusion of the superoxide anion to the active site. Styrylpyrone polyphenols of Inonotus obliquus origin have the potential to be a source of vigorous free radical scavengers and antioxidant enzyme activators.     

Exploring unusual antioxidant activity in a benzoic acid derivative: a proposed mechanism for citrinin

A mechanism is proposed for the unusual antioxidant activity in citrinin based on computed O-H bond dissociation enthalpies (BDE). These data suggest that citrinin itself is not the active species, but rather a pair of hydrated Michael addition products consisting of substituted 2,6-dihydroxy benzoic acids. These diastereomers act as radical scavengers via O-H bond dissociation with computed BDE's ranging from 78.9-80.9 kcal/mol for the active groups present. These data represent an unusually facile O-H bond dissociation for a phenol containing a strongly electron withdrawing group. This atypical reactivity arises from an intramolecular network of hydrogen bonds that both stabilize the incipient radical and facilitate extended delocalization through atoms external to the aromatic ring. The additional influence of stereochemistry on BDE is computed to be 2.0 kcal/mol. Data presented are for gas phase molecules, but solvents are unlikely to strongly modify these results since most polar groups are involved in intramolecular hydrogen bonds and thus less available for association with solvent. Citrinin and the Michael addition products are likely too toxic for use as antioxidants in organisms but this study clearly identifies specific reaction sites in the active form, thus guiding rational design of synthetic derivatives with more favorable biocompatibility.     

Antioxidant activity of o-bisphenols: the role of intramolecular hydrogen bonding

A kinetic and thermodynamic investigation on the antioxidant activity of 2,2'-methylenebis(6-tert-butyl-4-methylphenol) (2), 2,2'-ethylidenebis(4,6-di-tert-butylphenol) (3), and 4,4'-methylenebis(2,6-di-tert-butylphenol) (4) are reported. EPR studies of the equilibration between 3 or 4 and a reference phenol, and the corresponding phenoxyl radicals, allowed us to determine the O-H bond dissociation enthalpy (BDE) of the O-H bond as 81.2 and 81.1 kcal/mol in 3 and 4, respectively. Despite this similarity, the absolute rate constants for the reaction with peroxyl radicals, determined by autoxidation studies under controlled conditions, indicate that the o-bisphenols 2 and 3 behave as excellent antioxidants while the p-bisphenol 4 is less effective by a factor of 64 and 22, respectively. FT-IR spectroscopy and product studies suggest that the very good antioxidant activity of the o-bisphenols largely arises from both the reduced steric crowding about the hydroxyl group and the stabilization of the aroxyl radical due to the formation of an intramolecular hydrogen bond between the residual OH and the oxygen radical center.     

Formation of the 2,4-pentadiynyl-1 radical (H2CCCCCH, X2B1) in the crossed beams reaction of dicarbon molecules with methylacetylene

The chemical dynamics to synthesize the 2,4-pentadiynyl-1 radical, HCCCCCH(2)(X(2)B(1)), via the neutral-neutral reaction of dicarbon with methylacetylene, was examined in a crossed molecular beams experiment at a collision energy of 37.6 kJ mol(-1). The laboratory angular distribution and time-of-flight spectra of the 2,4-pentadiynyl-1 radical and its fragmentation patterns were recorded at m/z = 63-60 and m/z = 51-48. Our findings suggest that the reaction dynamics are indirect and dictated by an initial attack of the dicarbon molecule to the pi electron density of the methylacetylene molecule to form cyclic collision complexes. The latter ultimately rearranged via ring opening to methyldiacetylene, CH(3)-C triple bond C-C triple bond C-H. This structure decomposed via atomic hydrogen emission to the 2,4-pentadiynyl-1 radical; here, the hydrogen atom was found to be emitted almost parallel to the total angular momentum as suggested by the experimentally observed sideways scattering. The overall reaction was strongly exoergic by 182 +/- 10 kJ mol(-1). The identification of the resonance-stabilized free 2,4-pentadiynyl-1 radical represents a solid background for the title reaction to be included into more refined reaction networks modeling the chemistry of circumstellar envelopes and also of sooting combustion flames.     

Mechanism of the hydrosilylation reaction of alkenes at porous silicon: experimental and computational deuterium labeling studies

The mechanism of the formation of Si-C bonded monolayers on silicon by reaction of 1-alkenes with hydrogen-terminated porous silicon surfaces has been studied by both experimental and computational means. We propose that monolayer formation occurs via the same radical chain process as at single-crystal surfaces: a silyl radical attacks the 1-alkene to form both the Si-C bond and a radical center on the beta-carbon atom. This carbon radical may then abstract a hydrogen atom from a neighboring Si-H bond to propagate the chain. Highly deuterated porous silicon and FTIR spectroscopy were used to provide evidence for this mechanism by identifying the IR bands associated with the C-D bond formed in the proposed propagation step. Deuterated porous silicon surfaces formed by galvanostatic etching in 48% DF/D2O:EtOD (1:1) electrolytes showed a 30% greater density of Si-D sites on the surface than Si-H sites on hydrogen-terminated porous silicon surfaces prepared in the equivalent H-electrolyte. The thermal reaction of undec-1-ene and the Lewis acid catalyzed reaction of styrene on a deuterated surface both resulted in alkylated surfaces with the same C-C and C-H vibrational features as formed in the corresponding reactions at a hydrogen-terminated surface. However, a broad band around 2100 cm(-1) was observed upon alkylating the deuterated surfaces. Ab initio and density functional theory calculations on small molecule models showed that the integrated absorbance of this band was comparable to the intensity expected for the C-D stretches predicted by the chain mechanism. The calculations also indicate that there is substantial interaction between the hydrogen atoms on the beta-carbons and the hydrogen atoms on the Si(111)-H surface. These broad 2100 cm(-1) features are therefore assigned to C-D bands arising from the involvement of surface D atoms in the hydrosilylation reactions, while the line broadening can be explained partly by interaction with neighboring surface atoms/groups.     

Theoretical Study of the Radical Scavenging Activity of Shikonin and Its Derivatives

A series of shikonin derivatives have been designed and their radical scavenging activity has been characterized by the B3LYP/6‐31+G(d) approach. The hydrogen bond properties of the studied structures were investigated using the atoms in molecules (AIM) theory. The calculated results reveal that the hydrogen bond is important for good scavenging activity. The introduction of electron‐drawing (electron‐donating) groups increases (decreases) the scavenging activities of radical and radical cations of shikonin derivatives. Shikonin derivatives appear to be good candidates for the single‐electron‐transfer mechanism, particularly for ? N(CH₃)₂ derivative. Taking this system as an example, we present an efficient method for the investigation of radical scavenging activity from theoretical point of view. With the current work, we hope to highlight the radical scavenging activity of hydroxynaphthoquinones derivatives and stimulate the interest for further studies and exploitation in pharmaceutical industry.     

ESR study on the photosensitized decomposition of polyethylene

Several aromatic compounds were added to low-density polyethylene to determine their effects on the photodegradation of polyethylene. It was shown that some aromatic compounds indeed sensitize the hydrogen abstraction of the allylic hydrogen of unsaturated groups contained in polyethylene as an improper bond. The mechanism of photosensitization was investigated by an ESR method. It was found that the higher photoexcited triplet state of an aromatic molecule produced by the biphotonic ultraviolet light absorption transfers its excess energy to the unsaturated bond to excite it, and the excited unsaturated group releases its allylic hydrogen atom, giving an allylic radical.     

Why Static O-H Bond Parameters Cannot Characterize the Free Radical Scavenging Activity of Phenolic Antioxidants: ab initio Study

Recently, selecting high efficient phenolic antioxidants with low toxicity was paid much attention1-4. Moreover, quantitative structure-activity relationships (QSAR) for phenolic antioxidants have been investigated to accelerate the selection process5-7. Hence, how to theoretically characterize the free radical scavenging activity of phenolic antioxidants is important and significant. Although the parameters characterizing O-H bond dissociation energy or enthalpy (BDE) correlate well with …     

Predicting how polyphenol antioxidants prevent DNA damage by binding to iron

Prevention of oxidative DNA damage due to hydroxyl radical is important for the prevention and treatment of disease. Because of their widely recognized antioxidant ability, 12 polyphenolic compounds were assayed by gel electrophoresis to directly quantify the inhibition of DNA damage by polyphenols with Fe(2+) and H2O2. All of the polyphenol compounds have IC50 values ranging from 1-59 microM and inhibit 100% of DNA damage at 50-500 microM concentrations. Gel electrophoresis results with iron(II)EDTA and UV-vis spectroscopy experiments confirm that binding of the polyphenol to iron is essential for antioxidant activity. Furthermore, antioxidant potency of polyphenol compounds correlates to the pKa of the first phenolic hydrogen, representing the first predictive model of antioxidant potency based on metal-binding. Understanding this iron-coordination mechanism for polyphenol antioxidant activity will aid in the design of more-potent antioxidants to treat and prevent diseases caused by oxidative stress, and help develop structure-activity relationships for these compounds.     

Polypyrroles as antioxidants: kinetic studies on reactions of bilirubin and biliverdin dimethyl esters and synthetic model compounds with peroxyl radicals in solution. Chemical calculations on selected typical structures

[reaction: see text] Rate constants for hydrogen-atom transfer (HAT) from bilirubin dimethyl ester (BRDE) and biliverdin dimethyl ester (BVDE) to peroxyl radicals during inhibited autoxidation of styrene initiated by azo-bisisobutyronitrile (AIBN) were k(inh)(BRDE) = 22.5 x 10(4) and k(inh)(BVDE) = 10.2 x 10(4) M(-1) s(-1), and the stoichiometric factors (n) were 2.0 and 2.7, respectively. A synthetic tetrapyrrole (bis(dipyrromethene)) containing the alpha-central (2,2') CH2 linkage gave k(inh) = 39.9 x 10(4) M(-1) s(-1) and n = 2.3, whereas the beta-linked (3,3') isomer was not an active antioxidant. Several dipyrrinones were synthesized as mimics of the two outer heterocyclic rings of bilirubin and biliverdin. The dipyrrinones containing N-H groups in each ring were active antioxidants, whereas those lacking two such "free" N-H groups, such as N-CH3 dipyrrinones and dipyrromethenes, did not exhibit antioxidant activity. Overall, the relative k(inh) values compared to those of phenolic antioxidants, 2,6-di-tert-butyl-4-methoxyphenol (DBHA) and 2,6-di-tert-butyl-4-methylphenol (BHT), were 2,2'-bis(dipyrromethene) > BRDE > DBHA > dipyrrinones > BVDE > BHT. This general trend in antioxidant activities was also observed for the inhibited autoxidation of cumene initiated by AIBN. Chemical calculations of the N-H bond dissociation enthalpies (BDEs) of the typical structures support a HAT mechanism from N-H groups to trap peroxyl radicals. Intramolecular hydrogen bonding of intermediate nitrogen radicals has a major influence on the antioxidant activities of all compounds studied. Indeed, chemical calculations showed that the initial nitrogen radical from a dipyrrinone is stabilized by 9.0 kcal/mol because of H-bonding between the N-H remaining on one ring and the ground-state pyrrolyl radical of the adjacent ring in the natural zusammen structure. The calculated minimum structure of bilirubin shows strong intramolecular H-bonding of the N-H groups with carbonyl groups resulting in the known "ridge-tile" structure which is not an active HAT antioxidant. The calculated minimum structure of biliverdin is planar. BRDE is readily converted into BVDE by reaction with the electron-deficient DPPH* radical under argon in chlorobenzene. An electron-transfer mechanism is proposed for the initiating step in this reaction, and this is supported by the relatively low ionizing potential of a model dipyrrole representing the two central rings of bilirubin.