Design principles for alpha-tocopherol analogues

An (RO)B3LYP/LANL2DZdp//B3LYP/LANL2DZ model for the prediction of the homolytic bond dissociation enthalpy (BDE) and adiabatic ionisation potential (IP) of phenolic antioxidants containing heavy chalcogens has been developed. The model has been used to probe the relationship between geometry, chalcogen substitution and activity for a series of alpha-tocopherol analogues of varying ring size. From this, a series of design principles for cyclic antioxidants has emerged, embodied by the compound 4-hydroxy-2,2,3,5,6-pentamethylbenzoselenete (4c). This compound is predicted to have a BDE comparable to alpha-tocopherol, and should act in a dual chain-breaking and hydroperoxide-decomposing manner, by analogy with other selenide antioxidants. The stability of chalcogen-substituted benzoxetes was considered, and the as yet unsynthesised benzotelluretes are predicted to be stable. Finally, an attempt was made to determine antioxidant mechanism by considering calculated BDE and IP data together with experimental rate data.     

Substituent Effects on the C-H Bond Dissociation Energy of Toluene. A Density Functional Study

The bond dissociation energies of the benzylic C-H bond of a series of 16 para-substituted toluene compounds (p-X-C(6)H(4)CH(3)) have been calculated with the density functional method (BLYP/6-31G). The calculated substituent effects correlate well with experimental rates of dimerization of para-substituted alpha,beta,beta-trifluorostyrenes and rearrangement of methylenearylcyclopropanes. Both electron-donating and electron-withdrawing groups reduce the bond dissociation energy (BDE) of the benzylic C-H bond because both groups cause spin delocalization from the benzylic radical center. The calculated spin density variations at the benzylic radical centers correlate well with both the ESR hyperfine coupling constants determined by Arnold et al. and the calculated radical effects of the substituents. The relative radical stabilities are mainly determined by the spin delocalization effect of the substituents, and polar effect of the substituents are not important in the current situation. The ground state effect is also found to influence the C-H BDE.     

DFT Study on the Antioxidant Activity of a Modeled p-Terphenyl Derivative

Relationships between the structure characteristics of natural p-terphenyl com- pounds isolated from three edible mushrooms (Thelephora ganbajun, Thelephora aeronautical, and Boletopsis grisea) indigenous to China and their mechanism of antioxidant activity were studied. Geometry structures of terphenyl molecule and four corresponding radicals, bond dissociation energy (BDE), frontier orbitals (HOMO and LUMO) and single electron density were calculated using DFT methods (B3LYP/6-311G**). The computational results which are consistent with the experimental data well show that terphenyl molecule scavenges DPPH radical by hydrogen abstract mechanism and the high antioxidant activity depends on the substitution position of hydroxyls. Two active 7-, 8-hydroxyls facilitate the hydrogen abstraction due to the intramolecular hydrogen bond and the resonance effect makes 4-hydroxyl radical more stable.     

Potent 2,2-diphenyl-1-picrylhydrazyl radical-scavenging activity of novel antioxidants, double-stranded tyrosine residues conjugating pyrocatechol

New potent antioxidants conjugating the catechol (=pyrocatechol; pyrCat) group to two N-termini of modified double-stranded tyrosine residues were synthesized and showed radical scavenging activity with 2,2-diphenyl-1-picrylhydrazyl radical (DPPH radical, DPPH˙) as a free radical model, second-order rate constants for the DPPH˙ scavenging reaction, and the results from electron spin resonance (ESR) studies. It was found that the tyrosine (Tyr) residue and pyrCat containing new antioxidants developed in the study have about 3-20 times more potent antioxidative activity than Trolox, pyrCat, and L-ascorbic acid (VC). In order to elucidate the relationship between antioxidant activity and the molecular orbital states, and to design potent antioxidants we present an interesting approach using an absolute hardness (η)-absolute electronegativity (χ) diagram based on chemical hardness. It was shown that quantum chemicals were required to develop potent antioxidants.     

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.     

Density functional theory study of the conformational, electronic, and antioxidant properties of natural chalcones

Chalcones are natural compounds that are largely distributed in plants, fruits, and vegetables. They belong to the flavonoid group of molecules, and some of them exhibit numerous biological activities. The results of quantum chemical calculations (based on density functional theory, using the B3P86 exchange-correlation potential) are reported for 11 chalcones, in the gas phase and in the presence of an implicit solvent (using the conductor-like polarizable continuum model, C-PCM). These results are discussed in regard to the capacity of these chalcones to scavenge the 2,2-diphenyl-1-pycril-hydrazyl (DPPH) free radical. The O-H bond dissociation enthalpy (BDE) parameter, which is calculated for each OH group, seems to be the best indicator of the anti-radical property of these compounds. This demonstrates the importance of the H atom transfer mechanism to explain their capacity to scavenge the free radicals. The active sites are identified as the 6'-OH group and the 3,4-dihydroxy-catechol. The alpha,beta-double bond is influential in determining the activity.     

Regenerable chain-breaking 2,3-dihydrobenzo[b]selenophene-5-ol antioxidants

A series of 2,3-dihydrobenzo[b]selenophene-5-ol antioxidants was prepared by subjecting suitably substituted allyl 4-methoxyphenyl selenides to microwave-induced seleno-Claisen rearrangement/intramolecular Markovnikov hydroselenation followed by boron tribromide-induced O-demethylation. The novel antioxidants were assayed for their capacity to inhibit azo-initiated peroxidation of linoleic acid in a water/chlorobenzene two-phase system containing N-acetylcysteine as a thiol reducing agent in the aqueous phase. Antioxidant efficiency as determined by the inhibited rate of peroxidation, Rinh, increased with increasing methyl substitution (Rinh=46-26 microM/h), but none of the compounds could match alpha-tocopherol (Rinh=22 microM/h). Regenerability as determined by the inhibition time, Tinh, in the presence of the thiol regenerating agent decreased with increasing methyl substitution. Thus, under conditions where the unsubstituted compound 5a inhibited peroxidation for more than 320 min, alpha-tocopherol worked for 90 min and the trimethylated antioxidant 5g for 60 min only. Sampling of the aqueous phase at intervals during peroxidation using antioxidant 5a showed that N-acetylcysteine was continuously oxidized with time to the corresponding disulfide. In the absence of the regenerating agent, compounds 5 inhibited peroxidation for 50-60 min only. A (RO)B3LYP/LANL2DZdp//B3LYP/LANL2DZ model was used for the calculation of homolytic O-H bond dissociation enthalpies (BDE) and adiabatic ionization potentials (IP) of phenolic antioxidants 5. Both BDE (80.6-76.3 kcal/mol) and IP (163.2-156.0 kcal/mol) decrease with increasing methyl substitution. The phenoxyl radical corresponding to phenol 5g gave an intense ESR signal centered at g=2.0099. The H-O bond dissociation enthalpy of the phenol was determined by a radical equilibration method using BHA as an equilibration partner. The observed BDE (77.6+/-0.5 kcal/mol) is in reasonable agreement with calculations (76.3 kcal/mol). As judged by calculated log P values, the lipophilicity of compounds 5 increased slightly when methyl groups were introduced into the phenolic moiety (2.9>C log P<4.2). The capacity of compounds 5a (kinh=3.8x10(5) M-1 s-1) and 5g (kinh=1.5x10(6) M-1 s-1) to inhibit azo-initiated autoxidation of styrene in the homogeneous phase (chlorobenzene) was also studied. More efficient regeneration at the lipid-aqueous interphase is the most likely explanation why the intrinsically poorest antioxidant 5a can outperform its analogues as well as alpha-TOC in the two-phase system. Possible mechanisms of regeneration are discussed and evaluated.     

Is HOMO Energy Level a Good Parameter to Characterize Antioxidant Activity

As free radicals cause degradation of many industrial materials, degeneration of foods,and scores of diseases, such as inflammation, neurodegeneration, and tUmors, selectinghighly efficient antioxidants with low toxicity is of great importance and in fact, has beenpaid much attention'-3. Moreover, it has been pointed out that theoretical methods willaccelerate the selection of new antioxidants4.5. Indeed, various theoretical parametershave been found appropriate to characterize the free radica…     

Predicting the activity of phenolic antioxidants: theoretical method, analysis of substituent effects, and application to major families of antioxidants.

A procedure based on density functional theory is used for the calculation of the gas-phase bond dissociation enthalpy (BDE) and ionization potential for molecules belonging to the class of phenolic antioxidants. We show that use of locally dense basis sets (LDBS) vs full basis sets gives very similar results for monosubstituted phenols, and that the LDBS procedure gives good agreement with the change in experimental BDE values for highly substituted phenols in benzene solvent. Procedures for estimating the O--H BDE based on group additivity rules are given and tested. Several interesting classes of phenolic antioxidants are studied with these methods, including commercial antioxidants used as food additives, compounds related to Vitamin E, flavonoids in tea, aminophenols, stilbenes related to resveratrol, and sterically hindered phenols. On the basis of these results we are able to interpret relative rates for the reaction of antioxidants with free radicals, including a comparison of both H-atom-transfer and single-electron-transfer mechanisms, and conclude that in most cases H-atom transfer will be dominant.     

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

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.     

Optimization of the antioxidant activity of hydroxy-substituted 4-thiaflavanes: a proof-of-concept study

The design and the synthesis of a new family of hydroxy-4-thiaflavanes, in which the reactive phenolic OH is ortho to the sulfur atom of the benzofused oxathiin ring, allowed to prepare antioxidants that show rate constants for the reaction with peroxyl radicals (k(inh)), and bond dissociation energies (BDE), of the ArO-H group identical to those of α-tocopherol, the main component of vitamin E and the most effective lipophilic antioxidant known in nature. The peculiar conformation of the six-membered heterocyclic ring prevents the formation of an intramolecular hydrogen bond between the OH group and the S atom, while ensuring a good stabilization by electron donation of the phenoxyl radical formed after the reaction with peroxyl radicals. The preparation of these compounds was achieved through an inverse electron demand hetero Diels-Alder reaction of styrenes with o-thioquinones, in turn prepared from accurately designed 1,3-dihydroxy arenes. Properly arranging the substitution pattern on the aromatic ring, as in derivatives 9 and 11, allowed to reach values of k(inh) up to 4.0×10(6) M(-1) s(-1) and BDE((OH)) of 77.2 kcal mol(-1). This approach represents an innovative way to obtain highly active antioxidants without using strongly electron donating alkylamino groups which are associated with adverse toxicological profiles.     

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 …     

Theoretical studies on the structures, thermodynamic properties, detonation properties, and pyrolysis mechanisms of spiro nitramines

Density function theory (DFT) has been employed to study the geometric and electronic structures of a series of spiro nitramines at the B3LYP/6-31G level. The calculated results agree reasonably with available experimental data. Thermodynamic properties derived from the infrared spectra on the basis of statistical thermodynamic principles are linearly correlated with the number of nitramine groups as well as the temperature. Detonation performances were evaluated by the Kamlet-Jacobs equations based on the calculated densities and heats of formation. It is found that some compounds with the predicted densities of ca. 1.9 g/cm3, detonation velocities over 9 km/s, and detonation pressures of about 39 GPa (some even over 40 GPa) may be novel potential candidates of high energy density materials (HEDMs). Thermal stability and the pyrolysis mechanism of the title compounds were investigated by calculating the bond dissociation energies (BDE) at the B3LYP/6-31G level and the activation energies (E(a)) with the selected PM3 semiempirical molecular orbital (MO) based on the unrestricted Hartree-Fock model. The relationships between BDE, E(a), and the electronic structures of the spiro nitramines were discussed in detail. Thermal stabilities and decomposition mechanisms of the title compounds derived from the B3LYP/6-31G BDE and the UHF-PM3 E(a) are basically consistent. Considering the thermal stability, TNSHe (tetranitrotetraazaspirohexane), TNSH (tetranitrotetraazaspiroheptane), and TNSO (tetranitrotetraazaspirooctane) are recommended as the preferred candidates of HEDMs. These results may provide basic information for the molecular design of HEDMs.     

Novel 1,3,4-thiadiazole conjugates derived from protocatechuic acid: Synthesis,antioxidant activity,and computational and electrochemical studies

A series of 15 novel 1,3,4-thiadiazole amide derivatives containing a protocatechuic acid moiety were synthesized and structurally characterized. In addition, the corresponding imino (4) and amino (5) analogues of a phenyl-substituted 1,3,4-thiadiazole amide derivative 3a were prepared to compare the effects of the structural changes on the radical-scavenging activity. The obtained compounds were examined for their antioxidative potential by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assays. In addition, selected compounds were studied by density functional theory (DFT) and cyclic voltammetry experiments. The tested compounds showed high potential to scavenging DPPH radical and ABTS radical cation compared with the referent antioxidants ascorbic acid and nordihydroguaiaretic acid (NDGA). On the basis of the calculated thermodynamic parameters, it can be concluded that the sequential proton loss electron transfer (SPLET) mechanism represents the most probable reaction path in a polar solvent for DPPH radical–scavenging activity. On the other hand, the single electron transfer followed by proton transfer (SET-PT) can be a likely mechanistic pathway in the case of an ABTS radical cation.     

表征O－H解离能参数的AM1计算

对17种酚类化合物用半经验量子化学方法AM1计算了表征O－H解离能的参数△HOF值,即酚类化合物与其经抽氢反应产生的自由基生成热之差。经过与实验测定的17种酚类化合物的O－H解离能比较,评价了AM1方法在计算△HOF值方面的有效性。发现AM1计算的△HOF值与O－H解离能有很好的相关性(γ=0.9495),优于经验方法,比如加和规则对O－H解离能的预测。虽然AM1方法在计算间位取代对O－H解离能的贡献方面是无效的,但将用于计算解离能类的参数以预测抗氧化剂活性还是可行的。     

The effect of ring nitrogen atoms on the homolytic reactivity of phenolic compounds: understanding the radical-scavenging ability of 5-pyrimidinols

Six substituted 5-pyrimidinols were synthesized, and the thermochemistry and kinetics of their reactions with free radicals were studied and compared to those of equivalently substituted phenols. To assess their potential as hydrogen-atom donors to free radicals, we measured their O-H bond dissociation enthalpies (BDEs) using the radical equilibration electron paramagnetic resonance technique. This revealed that the O-H BDEs in 5-pyrimidinols are, on average, about 2.5 kcal mol(-1) higher than those in equivalently substituted phenols. The results are in good agreement with theoretical predictions, and confirm that substituent effects on the O-H BDE of 5-pyrimidinol are essentially the same as those on the Obond;H BDE in phenol. The kinetics of the reactions of these compounds with peroxyl radicals has been studied by their inhibition of the AIBN-initiated autoxidation of styrene, and with alkyl and alkoxyl radicals by competition kinetics. Despite their larger O-H BDEs, 5-pyrimidinols appear to transfer their phenolic hydrogen-atom to peroxyl radicals as quickly as equivalently substituted phenols, while their reactivity toward alkyl radicals far exceeds that of the corresponding phenols. We suggest that this rate enhancement, which is large in the case of alkyl radical reactions, small in the case of peroxyl radical reactions, and nonexistent in the case of alkoxyl radical reactions, is due to polar effects in the transition states of these atom-transfer reactions. This hypothesis is supported by additional experimental and theoretical results. Despite this higher reactivity of 5-pyrimidinols towards radicals compared to phenols, electrochemical measurements indicate that they are more stable to one-electron oxidation than equivalently substituted phenols. For example, the 5-pyrimidinol analogues of 2,4,6-trimethylphenol and butylated hydroxytoluene (BHT) were found to have oxidation potentials approximately 400 mV higher than their phenolic counterparts, but reacted roughly one order of magnitude faster with alkyl radicals and at about the same rate with peroxyl radicals. The 5-pyrimidinol structure should, therefore, serve as a useful template for the rational design of novel air-stable radical scavengers and chain-breaking antioxidants that are more effective than phenols.     

Evaluation of a Combined Quantum Chemical Method Used in Calculating O?H Bond Dissociation Enthalpy

O? H bond dissociation enthalpies (BDE) for a variety of substituted phenols were calculated using a combined quantum chemical method. It is found that the calculated O? H BDE correlated well with the recommended values, except for ortho‐tert‐butyl substituted phenols. For the electron‐donating group substituted phenols the calculated O? H BDE are slightly higher than the recommended values, however, for the electron‐withdrawing group substituted phenols the calculated O? H BDE are slightly lower than the recommended values.     

烷烃中碳氢键离解能的估算及其应用

将烷烃中的C－H键看成氢原子H与烷基Ri相连接而成的Ri－H键，以烷基的 HOMO能级和氢原子的轨道能来关联Ri－H键的离解能BDE。研究表明，烷烃分子中 Ri－H键的离能BDE与烷基Ri的极化效应指数PEI（Ri）有良好的线性关系：BDE＝ c+dPEI(Ri)。所得方程具有良好的估算精度。烷基Ri极化效应指数PEI（Ri）在羟 基自由基与烷烃反应速度常数的定量相关中，也得到良好的应用。     

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.     

Borane–Lewis Base Complexes as Homolytic Hydrogen Atom Donors

Radical stabilization energies (RSE)s have been calculated for a variety of boryl radicals complexed to Lewis bases at the G3(MP2)‐RAD level of theory. These are referenced to the B? H bond dissociation energy (BDE) in BH₃ determined at W4.3 level. High RSE values (and thus low BDE(B? H) values) have been found for borane complexes of a variety of five‐ and six‐membered ring heterocycles. Variations of RSE values have been correlated with the strength of Lewis acid–Lewis base complex formation at the boryl radical stage. The analysis of charge‐ and spin‐density distributions shows that spin delocalization in the boryl radical complexes constitutes one of the mechanisms of radical stabilization.