Bond dissociation enthalpies calculated by the PM3 method confirm activity cliffs in radical scavenging of flavonoids

Flavonoid Oxidation Potentials and Antioxidant Activities-Theoretical Models Based on Oxidation Mechanisms and Related Changes in Electronic Structure

Herein, I will review our efforts to develop a comprehensive and robust model for the estimation of the first oxidation potential, Ep1, and antioxidant activity, AA, of flavonoids that would, besides enabling fast and cheap prediction of Ep1 and AA for a flavonoid of interest, help us explain the relationship between Ep1, AA and electronic structure. The model development went forward with enlarging the set of flavonoids and, that way, we had to learn how to deal with the structural peculiarities of some of the 35 flavonoids from the final calibration set, for which the Ep1 measurements were all made in our laboratory. The developed models were simple quadratic models based either on atomic spin densities or differences in the atomic charges of the species involved in any of the three main oxidation mechanisms. The best model takes into account all three mechanisms of oxidation, single electron transfer-proton transfer (SET-PT), sequential proton loss electron transfer (SPLET) and hydrogen atom transfer (HAT), yielding excellent statistics (R2 = 0.970, S.E. = 0.043).

Theoretical study of the antioxidant capacity of the flavonoids present in the Annona muricata (Soursop) leaves

A theoretical approach was used to evaluate the antioxidant capacity of 20 flavonoids reported in Annona muricata leaves. The theoretical study was at the GGA level using the wB97XD functional and the cc-pvtz basis set. The calculations were performed in gas phase and implicit solvent phase. The flavonol robinetin (03c) and the flavanol gallocatechin (01c) are species that exhibited the best antioxidant capacity in the HAT, SEPT, and SPLET mechanisms. On the other hand, in the SET I mechanism, flavonol quercetin (03b) was the best, and in the SET II mechanism, the most favored species is the flavanol catechin (01a). However, these species do not achieve to overcome the antioxidant capacity presented by the Trolox.

Interpretation of ANN-based QSAR models for prediction of antioxidant activity of flavonoids

Quantitative structure-activity relationships (QSARs) built using machine learning methods, such as artificial neural networks (ANNs) are powerful in prediction of (antioxidant) activity from quantum mechanical (QM) parameters describing the molecular structure, but are usually not interpretable. This obvious difficulty is one of the most common obstacles in application of ANN-based QSAR models for design of potent antioxidants or elucidating the underlying mechanism. Interpreting the resulting models is often omitted or performed erroneously altogether. In this work, a comprehensive comparative study of six methods (PaD, PaD₂ , weights, stepwise, perturbation and profile) for exploration and interpretation of ANN models built for prediction of Trolox-equivalent antioxidant capacity (TEAC) QM descriptors, is presented. Sum of ranking differences (SRD) was used for ranking of the six methods with respect to the contributions of the calculated QM molecular descriptors toward TEAC. The results show that the PaD, PaD₂ and profile methods are the most stable and give rise to realistic interpretation of the observed correlations. Therefore, they are safely applicable for future interpretations without the opinion of an experienced chemist or bio-analyst. © 2018 Wiley Periodicals, Inc.

Adaptive neuro-fuzzy inference system-applied QSAR with bond dissociation energy for antioxidant activities of phenolic compounds

The aim of this study was to develop quantitative structure-activity relationship (QSAR) models for predicting antioxidant activities of phenolic compounds. The bond dissociation energy of O-H bond (BDE) was calculated by semi-empirical quantum chemical methods. As a new parameter for QSAR models, sum of reciprocals of BDE of enol and phenol groups (X _BDE ) was calculated. Significant correlations were observed between X _BDE and antioxidant activities, and X _BDE was introduced as a parameter for developing QSAR models. Linear regression-applied QSAR models and adaptive neuro-fuzzy inference system (ANFIS)-applied QSAR models were developed. QSAR models by both of linear regression and ANFIS achieved high prediction accuracies. Among the developed models, ANFIS-applied models achieved better prediction accuracies than linear regression-applied models. From these results, the proposed parameter of X _BDE was confirmed as an appropriate variable for predicting and analysing antioxidant activities of phenolic compounds. Also, the ANFIS could be applied on QSAR models to improve prediction accuracy.

Computational studies of free radical-scavenging properties of phenolic compounds

For more than half a century free radical-induced alterations at cellular and organ levels have been investigated as a probable underlying mechanism of a number of adverse health conditions. Consequently, significant research efforts have been spent for discovering more effective and potent antioxidants / free radical scavengers for treatment of these adverse conditions. Being by far the most used antioxidants among natural and synthetic compounds, mono- and polyphenols have been the focus of both experimental and computational research on mechanisms of free radical scavenging. Quantum chemical studies have provided a significant amount of data on mechanisms of reactions between phenolic compounds and free radicals outlining a number of properties with a key role for the radical scavenging activity and capacity of phenolics. The obtained quantum chemical parameters together with other molecular descriptors have been used in quantitative structure-activity relationship (QSAR) analyses for the design of new more effective phenolic antioxidants and for identification of the most useful natural antioxidant phenolics. This review aims at presenting the state of the art in quantum chemical and QSAR studies of phenolic antioxidants and at analysing the trends observed in the field in the last decade.

A novel relationship between the radical-scavenging activity of flavonoids and enthalpy of formation revealed with Hartree-Fock computations and thermochemical deduction

OBJECTIVE

The present study aims to establish the relationship between the reported radical-scavenging activities of flavonoids and some enthalpy changes that may occur during flavonoids' reactions with free radicals.

METHOD

Eight flavonoids were chosen for the study on the basis of their structural merits and reported 1,1-diphenyl-2-picryl-hydrazyl scavenging activities. Enthalpy changes accompanying interconversions between selected conformations (including spin multiplicities) and homolytic dissociations were estimated.

RESULTS

A novel relationship exists between the total enthalpy of reaction for the abstraction of two hydrogen atoms from flavonoids, their reported radical-scavenging activities and the enthalpy of the homolytic dissociation of hydrogen molecule (104.206 kcal mol(-1)). Only those flavonoids which could give up two hydrogen atoms with total enthalpy changes well below 104.206 kcal mol(-1) were active radical scavengers.

DISCUSSION

By appealing to equilibrium dynamics, we demonstrated that, for flavonoids to be able to donate hydrogen atoms, the change in enthalpy accompanying the abstraction of two hydrogen atoms needs to be less than 104.206 kcal mol(-1). This condition does not seem to be restricted to flavonoids only but rather generally applicable to chian-breaking antioxidants.

CONCLUSION

Thermodynamical relationships may be the most important factors governing the radical-scavenging reactions of flavonoids and possibly other compounds as well. Nevertheless, a more complete characterization of antioxidants would necessitate kinetic analysis.

A DFT STUDY ON THE ROLE OF DIFFERENT OH GROUPS IN THE RADICAL SCAVENGING PROCESS

The molecular properties of robinetin and melanoxetin which are the two naturally occurring flavonoid compounds have been studied theoretically by means of density functional theory approach (DFT) at the level of B3LYP/6-311G(d,p). The analysis of computed bond dissociation enthalpy (BDE), proton dissociation enthalpy (PDE), proton affinity (PA), electron transfer enthalpy (ETE) values for both the flavonoid compounds indicate the role of B-ring for the significant antioxidant characteristics and the instability of the A-ring. It also concerns the dominant role of BDE mechanism for antioxidant activity than PDE, PA and ETE mechanisms. Ionization potential (IP) is also found to be trustworthy in the study of antioxidant activity and the computed IP magnitudes are in agreement with the values of synthetic food additives. Further, the various molecular descriptors along with the plot of frontier molecular orbitals and Mulliken spin population analysis have been obtained and the validity of Koopmans' theorem is also verified with reference to antioxidant behavior.

Computation of the bond dissociation enthalpies and free energies of hydroxylic antioxidants using the ab initio Hartree-Fock method

INTRODUCTION

A new method for calculating theoretical bond dissociation enthalpy (BDE) and bond dissociation free energy (BDFE) of hydroxylic antioxidants is forwarded. BDE and BDFE may be understood as activation energies accompanying the formation of transition states, which may undergo downhill homolytic dissociation. The new method does not involve the complete fission of O-H bonds.

METHOD

Theoretical gas phase BDE values were calculated with the ab initio unrestricted Hartree-Fock (UHF) method, as changes in enthalpy between ground singlet states (GS) and triplet dissociative states (DS). Similarly, gas phase BDFEs were estimated from the corresponding changes in Gibbs free energy. The results were then compared with reliable experimental reports.

RESULTS

The proposed theoretical approach of BDE and BDFE determination was tested using 10 simple phenols, 5 flavonoids, and l-ascorbic acid derivatives. The agreement between our calculated gas phase results and the adopted experimental values were generally within 0.5 kcal mol(-1), with a very few exceptions.

DISCUSSION

Generally, steric interactions as well as intramolecular hydrogen bonding involving the dissociating OH group should be minimized in the GS. The DS are both electronically and vibrationally exited transition states. They have one unpaired electron on the carbon atom, which bears the homolytically dissociating OH group and are second order saddle points with a fixed

CONCLUSION

It was concluded that ab initio UHF was well suited for the estimation of gas phase BDE and BDFE. The method presented has a good potential for application across a range of hydroxylic antioxidants. Currently, work is underway to extend its application in other class of antioxidants.

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Key Words

• antioxidants
• bond dissociation enthalpy
• bond dissociation free energy
• thermodynamics

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MESH Headings

• Algorithms
• Antioxidants/chemistry
• Ascorbic Acid/chemistry
• Flavonoids/chemistry
• Gases
• Hydroxides/chemistry
• Isomerism
• Models, Theoretical
• Molecular Structure
• Phenols/chemistry
• Quantum Theory
• Thermodynamics

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Affiliation(s)

Ameha Seyoum Woldu Department of Pharmaceutical Chemistry and Pharmacognosy, School of Pharmacy, College of Health Sciences, Addis Ababa University, Addis Ababa, Ethiopia.
Joachim Mai

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Reliability of bond dissociation enthalpy calculated by the PM6 method and experimental TEAC values in antiradical QSAR of flavonoids

The applicability of the newly developed RM1 and PM6 methods implemented in the semiempirical quantum chemistry mopac2009 software package in modeling free radical scavenging activity of flavonoids was examined. Bond dissociation enthalpy (BDE) of OH groups could be calculated much faster than with DFT method but with similar quality. Despite the known shortcomings of the Trolox equivalent antioxidant capacity (TEAC) assay, we show that taking into account the hydrogen atom transfer (HAT) mechanism of free radical scavenging of flavonoids encoded by minimal BDE values (BDE(min)) and the number of OH groups (nOH), as well as experimental data, reasonable QSAR models could be developed. For TEAC values of 38 flavonoids measured by the ABTS free radical, a model based on BDE(min) and nOH was developed, having very good statistical parameters (r=0.983, r(cv)=0.976). The applicability of this model to three different data sets of flavonoids and reliability of TEAC values measured in distinct laboratories were discussed. Finally, a reasonably good model of experimental vitamin C equivalent antioxidant capacity (VCEAC) of 36 flavonoids was obtained (r=0.954, r(cv)=0.947), involving BDE(min) and nOH as descriptors. Additionally, all presented models have comparable fit and cross-validated statistical parameters, as well as significant regression coefficients.