A quantum chemical study on the antioxidant properties of aureusidin and bracteatin

A Quantum Chemical Study on theAntioxidant Properties of Aureusidinand Bracteatin

K. SENTHIL KUMAR,1 R. KUMARESAN2

1Department of Physics, Hindusthan College of Engineering and Technology, Coimbatore,Tamil Nadu 641032, India2Department of Physics, Government Arts College (Autonomous), Coimbatore,Tamil Nadu 641018, India

Received 16 August 2010; accepted 9 September 2010Published online 18 January 2011 in Wiley Online Library (wileyonlinelibrary.com).DOI 10.1002/qua.22964

ABSTRACT: The radical scavenging activities of the two flavonoids, aureusidin andbracteatin, have been explored by using density functional theory (DFT) with the B3LYPexchange correlation functional. These compounds are characterized by a highantioxidant activity which confers to them pharmacological properties that are usefulfor the treatment of several diseases. The minimum energy conformations obtainedfrom the energy scan is a further geometry optimization performed at the B3LYP-DFTlevel with 6-311G** basis set. Harmonic vibrational frequencies have been performed onthe optimized structures to ensure that the geometries obtained were real minima. Forradicals, the geometry optimizations and frequency calculations were also done at thesame mode of theory. Single point energy was calculated at the same level of theory inthe gas phase and in two solvents with different polarities (water and benzene) with theaim of computing the bond dissociation enthalpy (BDE) for the OAH bonds and theionization potentials (IPs). The OAH BDE parameter calculated for each OH groupseems to be the best indicator of the antiradical property of these compounds. Thisdemonstrates the importance of the H-atom transfer mechanism to explain theircapacity to scavenge the free radicals. The active sites are identified. BDE for thesesystems do not follow the same trends in gas and solution phases. The values ofreactivity-based descriptors such as total energy, orbital energy gap, maximumhardness principle, and electronegativity are also related with the antioxidantactivity. VC 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111: 4483–4496, 2011

Key words: aureusidin; bracteatin; antioxidants; bond dissociation enthalpies;ionization potentials; density functional theory

Correspondence to: R. Kumaresan; e-mail: [email protected]

International Journal of Quantum Chemistry, Vol 111, 4483–4496 (2011)VC 2011 Wiley Periodicals, Inc.

1. Introduction

F lavonoids belong to a class of naturallyoccurring phenolic compounds, widely dis-

tributed in the plant kingdom [1]. Their structuresare based on the flavan motif and they are classi-fied by the presence of different substituents onthe rings that are generally being the hydroxyl oralkoxyl functional groups. Flavonoids possessanticancer, antiviral, and anti-inflammatory prop-erties [2–4] with low toxicity. They are known fortheir antioxidant abilities, prevention of aging [5],cardiac ailments, and cancer [6]. Their biochemi-cal and physiological activities are well recog-nized and have been intensively studied [7–9].

All polyphenolic flavonoids consist of a ben-zene ring (A) condensed with a six-memberedring or five-membered ring (C) which carries aphenyl group (B) as a substitutent in the secondposition as shown in Figure 1. The structure offlavonoid compounds, aureusidin and bracteatin,are shown in Figure 2 with relevant numbers foratoms, which consists of two aromatic rings(A and C) linked with third ring (B) with a car-bon atom by an unsaturated bond and they areclassified by the substitution of OH groups in theB-ring. Jayaprakasam and coworkers [10] haveisolated the flavonoid compounds named aureusi-din and bracteatin derived from the flowers ofHelichrysm buddleiodes (Fig. 2). This study high-lights antioxidant effect of aureusidin and bractea-tin flavonoids, an extract from flowers of Heli-

chrysm buddleiodes plants distributed in theWestern Ghats of Nilgris, Tamilnadu, India.

An attempt has been made in this study to elu-cidate the structure–radical scavenging activity ofthese flavonoids by means of density functionaltheory (DFT). A catechol moiety is necessary formost natural antioxidants to enhance their activity.DFT has been recently developed and has suc-ceeded in bond dissociation enthalpy (BDE) [11]estimation and in the description of radical species.The choice of functional aspect has significantlyinfluenced the accuracy of the results. Their antiox-idant ability is related to the number and mutualposition of hydroxyl groups [12] and to conjuga-tion and resonance effects [13]. The factors such asconjugation and electron delocalization can con-tribute significantly to the stabilization of the radi-cal form of antioxidants, improving their perform-ance as scavengers [13]. It has been already provedthat the B-ring is the most important site forhydrogen transfer and consequently for the antiox-idant capacity [14], and most of the theoreticalinvestigations have focused only on the B-ring.

In the literature, two main mechanisms bywhich antioxidants can play their protective rolewere proposed and widely analyzed [11]. The firstone is referred as H-atom transfer (HAT) from theantioxidant AroH that becomes itself a radical.

R� þArOH ! RHþArO�

A greater stability of the ArO* radical withregard to R* is required for a good efficiency ofArOH. The second one is referred as one-electronFIGURE 1. Basic structure of flavonoid.

FIGURE 2. Structures of aureusidin and bracteatin.

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4484 INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY DOI 10.1002/qua VOL. 111, NO. 15

transfer in which the antioxidant gives an electronto the free radical becoming a radical cation. Theradical cation

R� þArOH ! R� þArOHþ�

arising from the electron transfer must be stable.According to the current knowledge of the radicalscavenging processes of flavonoiod antioxidants,the single step HAT seems to be the simplestmechanism [15]. BDE and ionization potential (IP)values are used as molecular descriptors in aneffort to elucidate the radical scavenging activityof compounds under investigation. A relativelylow value of OAH BDE and IP values indicatesthe expected antioxidant activity.

The contribution of each OH group to the radi-cal scavenging activity is established, supportedby the theoretical BDE and IP values obtained forboth flavonoids in the gas phase and in the pres-ence of solvents (water and benzene) and thedouble bond between C2 and C10 also increasesthe radical scavenging activity [13]. In this study,other electronic properties like electron affinity(EA), electronegativity (v), hardness (g), softness(S), electrophilic index (x), highest occupied mo-lecular orbital (HOMO) and lowest unoccupiedmolecular orbital (LUMO) energies, and energygap were obtained for the two flavonoid com-pounds. To the best of our knowledge, no othertheoretical study has been reported so far aboutthese two phenolic antioxidants.

2. Computational Methods

Quantum chemical calculations at the DFT/B3LYP level of theory together with the 6-311G**basis set were used to determine the geometryand energies of the selected neutral molecules.B3LYP is a hybrid functional of the DFT methodwhich consists of the Becke’s three-parameterexact exchange functional (B3) [16] combinedwith the nonlocal gradient-corrected correlationfunctional of the Lee–Yang–Parr (LYP) [17]. Theminimum energy conformations have beenobtained from the energy scan at the B3LYP-DFTlevel with 6-311G** basis set. The most stable geo-metries were optimized and then the single pointenergies were obtained by the same level oftheory. Harmonic vibrational frequency calcula-tions have been performed on the optimized

structures and the absence of imaginary frequen-cies was found, which ensured the real minima.Solvation calculations on the single point level inwater and benzene were carried out using thepolarizable continuum model of the self-consist-ent reaction field theory. The geometries, energies,and frequency calculations for the phenoxy radi-cals (ArO*) were obtained after the H abstractionfrom the OH group of the optimized structure forthe parent molecule at the same level of theory.The thermodynamic corrections [18] were calcu-lated at 298 K and 1 atm and have been added toelectronic energies.

BDE has been calculated from the formula BDE¼ Hr þ Hh � Hp, where Hr is the enthalpy of theradical generated by H-abstraction, Hh is the en-thalpy of the hydrogen atom (�0.49765 hartree)[19], and Hp is the enthalpy of the parent mole-cule. The IP values were determined from theequation IP ¼ Ec � Ep, where Ec is the energy ofthe cation and Ep is the energy of the parent mol-ecule. The cations and anions were optimized atB3LYP/6-311G** level, and harmonic vibrationalfrequency calculations were performed at thesame level to ensure minimum energy structureand to provide zero point energies (ZPEs). Theelectron affinities were computed as the energydifference between the neutral molecule and itsanion at 0 K. The new scaling factor for 6-311G**basis set is approximately equal to one and hencezero point vibrational energies were not correctedwith scaling factor [20]. According to Koopman’stheorem, the values of EA and IP can be approxi-mately equated to the negative of the LUMO andHOMO eigen values, respectively. The electronicproperties including HOMO, LUMO energies,EA, IP, electronegativity, and chemical hardnesshave been obtained using the following Eqs. (1)–(4) [21–24].

Electronegativity (v)

v ¼ �l ¼ � IPþ EA

2

� �(1)

Hardness (m)

g ¼ IPþ EA

2

� �(2)

Softness (S)

S ¼ 1

2g(3)

ANTIOXIDANT PROPERTIES OF AUREUSIDIN AND BRACTEATIN

VOL. 111, NO. 15 DOI 10.1002/qua INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 4485

Electrophilic index

x ¼ l2

2g

� �(4)

Total spin values hS2i have been checked foreach radical structure, and no spin contaminationhas been found. The unrestricted open shellapproach was used for neutral and radical species.All the calculations were performed by using Gaus-sian 03W, Rev D.01 computational package [25].

3. Results and Discussion

3.1. CONFORMATIONAL ANALYSIS

Potential energy curves of the two moleculeshave been characterized as a function of torsionangle around the C20AC10 bond exploring in stepsof 5� from �100� to 260� at B3LYP/6-311G** levelof theory. Potential energy surface scan of theaureusidin has been computed as a function ofthe torsion angle O1C2C10C20 (�100� to 260�)around the C20-C10 bond as shown in Figure 3.Only two minima are found for aureusidin, oneat h ¼ 0� (A-I) and other at h ¼ 180� (A-II) alongthe rotation of catechol moiety. Potential energysurface scan graph for aureusidin at B3LYP/6-311G** level of theory is shown in Figure 4. Themost stable conformer is found at �1029.21262hartree with the torsion angle of O1C2C10C20 as 0�.The energy difference between two conformers

(A-I and A-II) is very low, in the order of0.0029448 hartree (1.85 kcal/mol). Rotation of cate-chol moiety for aureusidin gave the maximumenergy at h ¼ 90� with a barrier of 8.84 kcal/mol.Potential energy surface scan of the bracteatin hasbeen computed as a function of the torsion angleO1C2C10C20 (�100� to 260�) around the C20AC10

bond. Two minima are found for bracteatin, one ath ¼ 0� (B-I) and other at h ¼180� (B-II) along therotation of pyrogallol. Potential energy surface scangraph for bracteatin at B3LYP/6-311G** level oftheory is presented in Figure 5. Rotation of pyrogal-lol moiety for bracteatin gave the maximum energyat h ¼ –90� with a barrier of 7.60 kcal/mol. Themost stable conformer is found at –1104.46277 har-tree with the torsion angle O1C2C10C20 as 0�. Theenergy difference between the two conformers (B-Iand B-II) is very low, of the order of 0.0025621 har-tree (1.61 kcal/mol). At this torsion angle and com-putational level, the two neutral molecules showplanar structure and the coplanar conformation ofthe rings A, B, and C allows a good electronicdelocalization among the rings.

Van Acker et al. [26] have indicated that thetorsion angle between rings B and C is relatedwith the free radical scavenging activity of flavo-noids. The smaller the angle, the better the reso-nance between the rings B and C; the phenoxylradical in ring B becomes more stable, and the fla-vonoids become more active. These findings arein agreement with the case of quercetin and luteo-lin in which quercetin is more active [27] as therings B and C of the former are more planar thanthe latter owing to the existence of the intramolec-ular hydrogen bond in the former. The molecular

FIGURE 3. Potential energy surface scan foraureusidin and bracteatin.

FIGURE 4. Potential energy surface scan graph foraureusidin at B3LYP/6-311G** level of theory.

KUMAR AND KUMARESAN


planar structure of quercetin and luteolin is pre-sented in Figure 6.

The results shown in Tables I and II give opti-mized bond distances and bond angles for neutralmolecule and its radicals of the two compounds cal-culated at B3LYP/6-311G** level of theory. Apartfrom the absolute minimum conformation of aureu-sidin molecule, a hydrogen atom removal from the4-OH, 6-OH, and 40-OH and 50-OH yields four radi-cal forms. The energies of two flavonoid moleculesand their radicals are enlisted in the Table III.

The order of stability for aureusidin radicals are asfollows: 40-OH > 50-OH > 6-OH > 4-OH. The 40-OHradical is the most stable one by an energy differenceof 13.12 kcal/mol from the least stable 4-OH radical.All these radicals retain planarity and consequentlyconjugation is similar to the parent molecule. The 40-OH radical is characterized by intramolecular hydro-gen bond which contributes to the stability.

Hydrogen atom removal from the 4-OH, 6-OH,and 40-OH, 50-OH, and 60-OH sites gives five radi-cals for bracteatin molecule. The order of stabilityfor bracteatin radicals are as follows: 50-OH > 60-OH > 40-OH > 6-OH > 4-OH.

The most stable radical at 50-OH site has anenergy difference of 31.94 kcal/mol with the leaststable 4-OH radical. These results also confirmthe conclusion of Martins et al. who have stated[28] the 5’-OH group to be the most acidic andmost favored deprotonation site.

3.2. ELECTRONIC PROPERTIES

In Table IV, the dipole moment of two flavo-noids and their radicals at B3LYP/6-311G** level

of computation have been reported. Dipolemoment values lie in the range of 2.085–9.062 Dand of 2.581–9.491 D for aureusidin and bracteatinradicals, respectively. These values are ratherhigh, reflecting the numerous polarized hydroxylor carbonyl functions distributed over the struc-tures. The two structures of flavonoids differ onlyby the presence of hydroxyl groups in the B-ring,and there is a little difference in the dipolemoment value. The molecular dipole moment isused to measure bond polarities and charge den-sities in a molecule. Bond polarity is one of thefactors that determines the physiochemical prop-erty for molecules. The calculated values of thetotal dipole moments are 6.1056 D for aureusidinand 6.1236 D for bracteatin, which signifies therelatively polarized nature of the systems andthey are soluble in polar solvents like water. Thepolarizability is one of the important quantities ofthe molecule which can provide some informationon the intramolecular interaction [29]. For aureu-sidin, the calculated average polarizability is218.27 au and for bracteatin it is 220.74 au, whichconfirms that both the molecules have thecapacity to polarize other atoms or molecules and

FIGURE 5. Potential energy surface scan graph forbracteatin at B3LYP/6-311G** level of theory.

FIGURE 6. Molecular structures of quercetin andluteolin.



TABLE IGeometry parameters of aureusidin and radicals at B3LYP/6-311G** (bond distances A and bond angle 8).

Bond parameters Neutral molecule 4-OH radical 6-OH radical 40-OH radical 50-OH radical

O(1)AC(2) 1.394 1.394 1.394 1.394 1.380C(2)AC(3) 1.497 1.497 1.497 1.497 1.499C(3)AO(3) 1.216 1.216 1.216 1.216 1.216C(8)AC(9) 1.396 1.396 1.396 1.396 1.395C(8)AO(1) 1.371 1.371 1.371 1.371 1.379C(4)AC(5) 1.394 1. 393 1.394 1.394 1.394C(4)AO(4) 1.349 1.349 1.349 1.349 1.347O(4)AH(4) 0.964 – 0.964 0.964 0.964C(5)AC(6) 1.401 1.401 1.401 1.401 1.401C(5)AH(5) 1.084 1.084 1.084 1.084 1.084C(6)AO(6) 1.359 1.359 1.359 1.359 1.356O(6)AH(6) 0.963 0.963 – 0.963 0.963C(6)AC(7) 1.400 1.400 1.400 1.400 1.403C(7)AH(7) 1.083 1.083 1.083 1.083 1.083C(7)AC(8) 1.383 1.383 1.383 1.383 1.380C(2)AC(10) 1.343 1.343 1.343 1.343 1.357C(10)AC(20) 1.450 1.450 1.450 1.450 1.428C(10)AH(10) 1.087 1.087 1.087 1.087 1.086C(20)AC(30) 1.083 1.083 1.083 1.083 1.083C(30)AC(40) 1.381 1.381 1.381 1.381 1.374C(40)AO(40) 1.377 1.377 1.377 1.377 1.346O(40)AH(40) 0.962 0.962 0.962 – 0.965C(40)AC(50) 1.407 1.407 1.407 1.407 1.480C(50)AO(50) 1.357 1.357 1.357 1.357 1.233O(50)AH(50) 0.966 0.966 0.966 0.966 –C(50)AC(60) 1.390 1.390 1.390 1.390 1.456C(60)AH(60) 1.083 1.083 1.083 1.083 1.083C(60)AC(70) 1.389 1.389 1.389 1.389 1.360C(70)AH(70) 1.084 1.084 1.084 1.084 1.085O(1)AC(2)AC(3) 109.6 109.6 109.6 109.6 109.8C(9)AC(3)AO(3) 131.2 131.2 131.2 131.2 131.6C(9)AC(8)AO(1) 112.5 112.5 112.5 112.5 112.2C(4)AO(4)AH(4) 109.4 – 109.4 109.4 109.6C(4)AC(5)AC(6) 120.4 120.4 120.4 120.4 120.5C(6)AC(5)AH(5) 118.4 118.4 118.4 118.4 118.3C(5)AC(6)AO(6) 116.2 116.2 116.2 116.2 116.2C(6)AO(6)AH(6) 109.6 109.6 – 109.6 109.9C(5)AC(6)AC(7) 121.9 121.9 121.9 121.9 121.9C(5)AC(4)AO(4) 122.7 122.7 122.7 122.7 122.9C(6)AC(7)AH(7) 122.2 122.2 122.2 122. 2 122.3C(6)AC(7)AH(7) 122.2 122.2 122.2 122. 2 122.3C(6)AC(7)AC(8) 116.0 116.0 116.0 116.0 115.8C(7)AC(8)AC(9) 124.1 124.1 124.1 124.1 124.4C(8)AO(1)AC(2) 107.4 107.4 124.1 107.4 107.5O(1)AC(2)AC(10) 124.7 124.7 124.7 124.7 125.0C(2)AC(10)AC(20) 131.1 131.1 131.1 131.1 130.9C(2)AC(10)AH(10) 112.2 112.2 112.2 112.2 112.0C(10)AC(20)AC(30) 123.4 123.4 112.2 123.4 123.7C(20)AC(30)AH(30) 119.8 119.8 119.8 119.8 119.2C(20)AC(30)AC(40) 120.3 120.3 120.3 120.3 121.0C(30)AC(40)AC(50) 121.0 121.0 121.0 121.0 121.7

(continued)

KUMAR AND KUMARESAN


are soluble in polar solvents. The solubilityenhances the antioxidative potency as it improvesthe mobility of antioxidant between membranesand lipoprotein [30, 31].

The electron donating ability of the molecule ischaracterized by the HOMO eigen values; ahigher HOMO eigen value always increases theelectron donating ability which in turn acceleratesthe antioxidant activity. The radical scavengingability of the flavonoids is related to the IPs of theanions. EAs and IPs of the different radicals havebeen computed as the energy (scaled by ZPE) dif-ference between the neutral molecule and its ion.The electronegativity measures the tendency toattract electrons in a chemical species, while hard-ness is a measure of resistance to charge transfer.A high IP indicates that the system does not loseelectrons easily, and a molecule with a greaterelectronic affinity tends to take electrons easily.Considering the IP values in the gas phase thatare presented in Table V, aureusidin was foundto be less active than bracteatin and bracteatindonate an electron easily than aureusidin. Theelectron donating ability of flavonoids seems tobe related to an extended electronic delocalizationover the whole molecule. According to Koop-man’s theorem the value of EA and IP can beapproximately equated to the negative of theLUMO and HOMO eigen values, respectively.The calculated EAs from ELUMO are larger than thevalues obtained from En – Ea for both flavonoids.The IPs given by the negative of the HOMO ener-gies are smaller than the values obtained from Ei –En. The HOMO eigen values are shifted downfrom the –IP, and LUMO eigen values are shiftedup from –EA [32–35]. Molecular orbital theory pro-vides just an approximation and the approxima-tions are not quantitatively reliable. The Koop-man’s theorem does not take into account electroncorrelations and orbital relaxation effects [36].

Based on the orbital considerations, the hardnessvalues were found to be giving controversialresults. The validity of Koopman’s theorem wasverified [37], and the two results of electronegativ-ity obtained by orbital and energy considerationagree very well for both the molecules as summar-ized in Table VI (with a difference of 0.073 eV foraureusidin and 0.173 eV for bracteatin).

The value of frontier orbital energy gap forbracteatin was found to be higher than aureusidinwhich denotes that the former is more stable thanthe latter by 0.15 eV. The results of energy consid-eration show that aureusidin has a higher hard-ness than bracteatin. Based on the maximumhardness principle, the aureusidin should bemore stable than bracteatin, which is in agree-ment with the minimum energy criterion. Theresults of orbital consideration on the hardnessvalue give controversial results. The validity ofKoopman theorem was again verified. In aureusi-din, the charge density is mainly localized on thering B with a lesser involvement in aromatic ringA. The HOMO orbital for aureusidin 4-OH radicaland 50-OH radical are delocalized over threerings. The HOMO orbital for aureusidin 6-OHradical and 40-OH radical are localized over the Aand C rings and C2AC10 bond, whereas theLUMO for all aureusidin radicals are delocalizedover all the three rings as shown in Figure 7. Inbracteatin radicals, the electron density is distrib-uted in all the rings, with a sensible increase onthe ring B, due to nearly planar arrangement.

3.3. BDE AND IP EVALUTION

BDE and IP values are used as moleculardescriptors in an effort to elucidate the radicalscavenging activity of compounds under investi-gation. BDE is very sensitive to correlation poten-tial. LYP correlation potential with the three-

TABLE I(Continued)

Bond parameters Neutral molecule 4-OH radical 6-OH radical 40-OH radical 50-OH radical

C(30)AC(40)AO(40) 124.4 124.4 124.4 124.4 123.7C(40)AO(40)AH(40) 109.9 109.9 109.9 – 109.3C(40)AC(50)AO(50) 120.5 120.5 120.5 120.5 121.7C(50)AO(50)AH(50) 107.8 107.8 107.8 107.8 –C(40)AC(50)AC(60) 119.1 119.1 119.1 119.1 115.2C(70)AC(60)AH(60) 121.4 121.4 121.4 121.4 122.0C(70)AC(60)AH(60) 119.2 121.4 119.2 119.2 118.1C(50)AC(60)AC(70) 119.2 121.4 119.2 119.2 118.1



TABLE IIGeometry parameters of bracteatin and radicals at B3LYP/6-311G** (bond distances A and bond angle 8).

Bond parameters Neutral molecule 4-OH radical 6-OH radical 40-OH radical 50-OH radical 60-OH radical

O(1)AC(2) 1.394 1.394 1.394 1.394 1.394 1.394C(8)AO(1) 1.375 1.375 1.375 1.375 1.375 1.375C(9)AC(3) 1.441 1.441 1.441 1.441 1.441 1.441C(9)AC(4) 1.402 1.402 1.402 1.402 1.402 1.402C(5)AC(4) 1.391 1.391 1.391 1.391 1.391 1.391C(5)AC(6) 1.403 1.403 1.403 1.403 1.403 1.403C(6)AC(7) 1.410 1.410 1.410 1.410 1.410 1.410C(8)AC(9) 1.392 1.392 1.392 1.392 1.392 1.392C(3)AO(3) 1.229 1.229 1.229 1.229 1.229 1.229C(4)AO(4) 1.344 1.344 1.344 1.344 1.344 1.344C(6)AO(6) 1.357 1.357 1.357 1.357 1.357 1.357C(7)AH(7) 1.083 1.083 1.083 1.083 1.083 1.083C(5)AH(5) 1.081 1.081 1.081 1.081 1.081 1.081O(6)AH(6) 0.963 0.963 – 0.963 0.963 0.963O(4)AH(4) 0.974 – 0.974 0.974 0.974 0.974C(2)AC(10) 1.342 1.342 1.342 1.342 1.342 1.342C(10)AC(20) 1.456 1.456 1.456 1.456 1.456 1.456C(20)AC(30) 1.407 1.407 1.407 1.407 1.407 1.407C(30)AC(40) 1.389 1.389 1.389 1.389 1.389 1.389C(40)AC(50) 1.398 1.398 1.398 1.398 1.398 1.398C(60)AC(70) 1.390 1.390 1.390 1.390 1.390 1.390C(70)AC(20) 1.407 1.407 1.407 1.407 1.407 1.407C(40)AO(40) 1.360 1.360 1.360 1.360 1.360 1.360C(50)AO(50) 1.400 1.400 1.400 1.400 1.400 1.400C(60)AO(60) 1.360 1.360 1.360 1.360 1.360 1.360C(10)AH(10) 1.087 1.086 1.086 1.086 1.086 1.086C(30)AH(30) 1.079 1.079 1.079 1.079 1.079 1.079C(70)AH(70) 1.083 1.083 1.083 1.083 1.083 1.083O(40)AH(40) 0.967 0.967 0.967 – 0.967 0.967O(50)AH(50) 0.966 0.966 0.966 0.966 – 0.966O(60)AH(60) 0.967 0.967 0.967 0.967 0.967 –C(8)AO(1)AC(2) 107.5 107.5 107.5 107.5 107.5 107.5C(9)AC(8)AO(1) 111.5 111.5 111.5 111.5 111.5 111.5C(9)AC(8)AC(3) 108.4 108.5 108.5 108.5 108.5 108.5C(8)AC(9)AC(4) 120.9 120.9 120.9 120.9 120.9 120.9C(5)AC(4)AC(9) 118.2 118.2 118.2 118.2 118.2 118.2C(4)AC(5)AC(6) 119.4 119.4 119.4 119.4 119.4 119.4C(5)AC(6)AC(7) 123.1 123.1 123.1 123.1 123.1 123.1O(1)AC(2)AC(10) 125.0 125.0 125.0 125.0 125.0 125.0C(2)AC(10)AC(20) 130.9 130.9 130.9 130.9 130.9 130.9C(10)AC(20)AC(30) 122.8 122.8 122.8 122.8 122.8 122.8C(20)AC(30)AC(40) 119.6 119.6 119.6 119.6 119.6 119.6C(30)AC(40)AC(50) 120.0 120.0 120.0 120.0 120.0 120.0C(60)AC(70)AC(20) 120.2 120.2 120.2 120.2 120.2 120.2C(70)AC(20)AC(30) 119.9 119.9 119.9 119.9 119.9 119.9C(5)AC(6)AO(6) 116.0 115.9 115.9 115.9 115.9 115.9C(5)AC(4)AO(4) 121.2 121.3 121.3 121.3 121.3 121.3C(9)AC(3)AO(3) 128.4 128.4 128.4 128.4 128.4 128.4C(30)AC(40)AO(40) 120.0 120.0 120.0 120.0 120.0 120.0C(40)AC(50)AO(50) 119.5 119.5 119.5 119.5 119.5 119.5C(70)AC(60)AO(60) 120.2 120.2 120.2 120.2 120.2 120.2

(continued)

KUMAR AND KUMARESAN


parameter scheme for exchange has been used toinvestigate the aromatic hydroxyl groups attachedto flavonoid nuclei. The OAH groups of attachedsugars, as well as hydrogens attached to flavo-noid core do not contribute to scavenging potency[38]. Structure–activity relationship studies on fla-vonoids suggest that the substitution pattern inthe A-ring correlates poorly with antioxidant ac-tivity [39]. On the basis of gas phase BDE values,it is evident that pyrogallol and secondly catecholare predicted to be the most efficient radical scav-

engers among the simple phenols in-line with ex-perimental findings [40–44]. Such efficiency isrelated to further stabilization of the derived

TABLE II(Continued)

Bond parameters Neutral molecule 4-OH radical 6-OH radical 40-OH radical 50-OH radical 60-OH radical

C(6)AO(6)AH(6) 109.7 109.7 – 109.7 109.7 109.7C(4)AO(4)AH(4) 108.1 – 108.0 108.0 108.0 108.0C(40)AO(40)AH(40) 107.3 107.3 107.3 – 107.3 107.3C(50)AO(50)AH(50) 108.7 108.7 108.7 108.7 – 108.7C(60)AO(60)AH(60) 107.3 107.3 107.3 107.3 107.3 –C(2)AC(10)AH(10) 112.5 112.5 112.5 112.5 112.5 112.5C(6)AC(7)AH(7) 122.0 122.0 122.0 122.0 122.0 122.0C(6)AC(5)AH(5) 119.6 119.6 119.6 119.6 119.6 119.6C(20)AC(70)AH(70) 121.0 121.0 121.0 121.0 121.0 121.0C(20)AC(30)AH(30) 121.4 121.4 121.4 121.4 121.4 121.4

TABLE IIIElectronic energies for neutral molecule and radicalsin gas phase at B3LYP/6-311G** level of theory.

Molecule/radicalAureusidin(hartree)

Bracteatin(hartree)

Neutral molecule �1029.2126 �1104.46294-OH radical �1028.5612 �1103.79986-OH radical �1028.5643 �1103.812040-OH radical �1028.5821 �1103.834450-OH radical �1028.5732 �1103.850760-OH radical – �1103.8350

TABLE IVDipole moment for neutral molecule and radicals ingas phase at B3LYP/6-311G** level of theory.

Molecule/radicalAureusidin(Debye)

Bracteatin(Debye)

Neutral molecule 6.1056 6.12364-OH radical 9.0622 9.49066-OH radical 2.0851 6.913540-OH radical 3.0934 2.580750-OH radical 7.5646 3.119460-OH radical – 4.6705

TABLE VITotal energies, frontier orbital energies, energy gap,softness, electronic chemical potential, andelectrophilic index in gas phase calculated atB3LYP/6-311G** level of theory.

Electronicparameter/molecule Aureusidin Bracteatin

ET (hartree) �1028.98865 �1104.23383EHOMO (hartree) �0.20918 �0.22557ELUMO (hartree) �0.07870 �0.08972Egap (eV) 3.5506 3.6967Softness (eV) 0.1408 0.1352Electronic chemicalpotential (eV)

�3.9168 �4.2898

Electrophilic index (eV) 2.1604 2.49

TABLE VProperties related with the chemical potential in thegas phase for aureusidin and bracteatin moleculesat B3LYP/6-311G** level of theory.

Properties

Aureusidin Bracteatin

E O E O

Electronicaffinity (eV)

0.9426 2.1415 1.2557 2.4414

Ionizationpotential (eV)

7.0370 5.6921 6.9781 6.1381

Hardness (eV) 3.0472 3.5506 2.8612 3.6967Electronegativity(eV)

3.9168 3.9898 4.1169 4.2898

E, energy vertical; O, orbital vertical.



phenoxy radical via formation of intramolecularhydrogen bond. A stable radical of the antioxi-dant generated after the H-abstraction reaction[45, 46] decreases the toxicity of the antioxidant.As water is the main element in physiologicalliquids, BDE has been computed in water also. Inaddition, another possible site of action for antiox-idant is the biological membrane, where unsatu-rated lipids are present, and BDE has also beencalculated in a nonpolar solvent like benzene. Thenonpolar solvent benzene has a small dielectricconstant and the values are probably close tothose obtained in the gas phase.

Antioxidant reactions with free radicals thatare proposed in most cases are found to be HATdominant instead of single electron transfer [47].The fastest and more direct mechanism for theo-retical BDE calculation is through the differencesin reaction enthalpy between parent moleculesand their radicals. The OAH BDE has been calcu-lated from the equation

BDE ¼ Hr þHh �Hp (5)

where Hr is the enthalpy of the radical generatedby H abstraction, Hh is the enthalpy of the

FIGURE 7. HOMO and LUMO shapes for the two flavonoids and radicals computed at the B3LYP/6-311G** level oftheory. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

KUMAR AND KUMARESAN


hydrogen atom (�0.49765 hartree) [19] and Hp isthe enthalpy of the parent molecule.

The abstraction of a hydrogen atom from eachOH group present in the considered moleculesproduces four radicals for aureusidin moleculeand five radicals for bracteatin molecule. The cal-culated BDE values for the four radicals formedby hydrogen abstraction on aureusidin and fiveradicals formed by hydrogen abstraction onbracteatin in gas phase, water, and benzene byapplying B3LYP/6-311G** method are given inTable VII.

From the gas phase BDE values, it can befound that lesser amount of energy is required forbreaking the 40-OH, 50-OH groups with respect tothe 4-OH and 6-OH, and in the case of 40-OH and50-OH radicals, the hydrogen abstraction producestwo most stable radicals. This clearly confirmsthat HAT from the B-ring is easier than that of A-ring. This result is consistent with structure–activ-ity relationships of antioxidant flavonoids [48].The catechol moiety in the B-ring of flavonoidshas the advantage of scavenging free radicals dueto relatively low BDE value of OAH [49, 50]. Theexperimental results show the ring B to be theactive moiety [51]. Ring B is considered as the pri-mary target for radical attack in flavonoids andits hydroxylation pattern is of major importancefor the scavenging activity [52]. The calculatedBDE values from liquid phase were not correctedfor ZPE. The BDE sequence for the OH groups ofaureusidin is as follows: 40-OH < 60-OH < 6-OH< 4-OH.

The BDEs computed in the solvent phase (ben-zene) are almost the same as the gas phase but

small difference is observed from the values inwater phase. The trend of BDE in water is differ-ent from that of the trend obtained in the gasphase. The more stable nature of aureusidin radi-cal at 40-OH site is retained in gas phase andin benzene, whereas the stability of aureusidinradical at 40-OH site is reversed in water. No ex-perimental data is available in literature for thismolecule and hence a comparison is not possible.

The BDE sequence for the OH groups of brac-teatin is as follows: 50-OH < 60-OH < 40-OH < 6-OH < 4-OH. This confirms that the HAT fromthe B-ring (50-OH, 60-OH, and 40-OH) is easierthan that of the A-ring (6-OH and 4-OH) and itclearly proves that the ring B is to be consideredas the primary target for radical attack in flavo-noids. The lowest BDE is obtained in the 50-OHsite in gas phase, in water, and also in benzene.The trend of BDE values for bracteatin radicals ingas phase and solvent phase (water and benzene)are the same. The reactivity of the B-ring is higherthan that of the A-ring and this result is similar tothose found for quercetin, taxifolin, luteolin, anderiodictyol [44].

3.4. SPIN DENSITY ANALYSIS

The spin density is often considered to be amore realistic parameter and provides a betterrepresentation of the reactivity. The importance ofthe spin density for the description of flavonoidshas been pointed out by Leopoldini et al. [53].The analysis of the spin density on the aureusi-din and bracteatin radicals is undertaken to ra-tionalize the differences in reactivity of the OH

TABLE VIIThe BDE of aureusidin and bracteatin at B3LYP/6-311G** level of theory.

CompoundMethod/position ofhydroxyl group

Gas phaseDFT (kcal/mol)a

Gas phaseDFT (kcal/mol)b

Water DFT(kcal/mol)b

Benzene DFT(kcal/mol)b

Aureusidin radical 1 A-ring 4-OH 87.77 96.51 97.19 96.75Aureusidin radical 2 A-ring 6-OH 85.93 94.56 98.75 95.87Aureusidin radical 3 B-ring 40-OH 75.46 83.36 88.00 84.73Aureusidin radical 4 B-ring 50-OH 80.80 88.89 86.99 88.02Bracteatin radical 1 A-ring 4-OH 94.76 103.76 97.97 101.58Bracteatin radical 2 A-ring 6-OH 87.35 96.12 99.94 97.42Bracteatin radical 3 B-ring 40-OH 74.04 82.11 87.17 84.01Bracteatin radical 4 B-ring 50-OH 64.14 71.87 78.50 74.40Bracteatin radical 5 B-ring 60-OH 73.70 81.72 87.01 83.78

a Total energy with zero point vibrational energy.b Total energy without zero point vibrational energy.



sites in flavonoids and consequently the differen-ces in BDE.

The spin density for the most stable radical (40-OH radical for aureusidin and 50-OH radical forbracteatin) of the compounds indicated that theunpaired electron is delocalized over the entirearomatic ring. The more delocalized the spin den-sity in the radical, easier the radical formed andthus lower is the BDE. The spin populationappears to be slightly more localized for radicalson the B-ring than for those located on the A-ring. The spin density is 0.289 on the O-atom inthe 50-OH aureusidin radical, whereas it is 0.384for the 6-OH aureusidin radical as presented inFigure 8. As a consequence, the BDE is lower inthe B-ring than in the A-ring. The BDE of 4-OHaureusidin radical is about 1.84 kcal/mol higherthan that of 6-OH radical, whereas spin density

on the O atom of the 4-OH radical is lower thanthat of the O-atom of the 6-OH radical. The BDEof 40-OH aureusidin radical is 5.34 kcal/mollower than that of 50-OH radical but the spindensity is higher for the former than that of thelatter. The BDE values in the water phase foraureusidin radical is minimum in the 50-OH site,whereas the spin density is more delocalized in50-OH site. The relation between BDE and spindensity follows the same trend in gas phase andin benzene. The intramolecular hydrogen bondexists between the oxygen in the 40-OH site withhydrogen in the 50-OH leads to stability of thephenoxyl radical that leads to a higher antioxi-dant activity. The spin density is actually moredelocalized in the site 50-OH bracteatin radicalhaving least BDE value compared with the otherradicals.

FIGURE 8. Spin density distribution of nine radical flavonoids computed at theB3LYP/6-311G** level of theory.[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

KUMAR AND KUMARESAN


4. Conclusion

In this article, DFT method has been applied tostudy naturally occurring antioxidant compounds.The study has concerned about the determination ofthe BDE and IP as per the mechanism proposed inthe literature for the radical scavenging activity.The electronic properties such as the dipolemoment, polarizability, total energy, ZPE, HOMOand LUMO energies, energy gap, electronic affin-ity, electronegativity, and chemical hardness havealso been presented. On the basis of obtainedresults in the gas phase, water, and benzene thefollowing conclusions could be drawn.

& The flavonoids under investigation areexpected to be efficient hydrogen atomdonors. The order of activity for the corre-sponding compounds is defined by the sub-stitution pattern in the B-ring according tothe OAH BDE values.

& The BDE sequence for the different radicalsof aureusidin in gas and benzene phaseshave the same trend but a small differenceis observed in water phase, whereas for theradicals of bracteatin it has the same trendin all the phases.

& Aureusidin and bracteatin appear to begood candidates for the one-electron transfermechanism. Their planar conformation andextended electronic delocalization betweenadjacent rings determine low IP values.

& A comparison between two considered mol-ecules indicates bracteatin to be the flavo-noid that requires a lowest energy for bothH-atom and electron transfer mechanismpossessing high antioxidant capacity.

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A quantum chemical study on the antioxidant properties of aureusidin and bracteatin

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