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A density functional study of antioxidant properties on anthocyanidins
Journal of Molecular Structure 935 (2009) 110–114
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
A density functional study of antioxidant properties on anthocyanidins Rosa Guzmán a, Cristobalina Santiago b, Mario Sánchez c,* a
Instituto de Ciencias Básicas, Universidad Veracruzana, Rafael Sánchez Altamirano S/N, Colonia Industrial Ánimas, Km. 3.5, Carretera Xalapa-Las Trancas, C.P. 91192, Xalapa, Veracruz, Mexico b Instituto de Agroindustrias, Universidad Tecnológica de la Mixteca, Carretera a Acatlima Km. 2.5, C.P. 69000, Huajuapan de León, Oaxaca, Mexico c Centro de Investigación en Materiales Avanzados, S.C., Parque de Investigación e Innovación Tecnológica Alianza Norte 202, Autopista Aeropuerto-Monterrey Km. 10, C.P. 66600, Apodaca, NL, Mexico
a r t i c l e
i n f o
Article history: Received 29 April 2009 Received in revised form 26 June 2009 Accepted 29 June 2009 Available online 4 July 2009 Keywords: Antioxidant capacity DFT BDE Anthocyanidins
a b s t r a c t A density functional theory (DFT) study, using the B3LYP/6-31G(d,p) method, was performed in a attempt to understand the antioxidant properties of some anthocyanidins. This study is based on the H-atom transfer mechanism, which implicates the evaluation of the bond dissociation enthalphy (BDE) of all OH substituents in each structure. The electronic structures studied in this paper are: aurantinidin, cyanidin, delphinidin, malvinidin, pelargonidin and peonidin. Analysis of the computed results suggest that the antioxidant capacity of those structures is in the following order: cyanidin > malvidin > aurantinidin > delphinidin P peonidin > pelargonidin. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Anthocyanins are natural pigments widely distributed in nature [1]. These compounds are flavonoids that belong to the family of polyphenols. Anthocyanins are glycosylated polyhydroxy and polymethoxy derivatives of 2-phenylbenzopyrylium (flavylium) cation. The anthocyanins are glycosides and acylglycosides of anthocyanidins. To date, there are 17 known naturally occurring anthocyanidins, but only a few of them are significant and most common from food point of view [2]. Among most important anthocyanidins are pelargonidin, cyanidin, peonidin, delphinidin, and malvidin. Previous studies of anthocyanidins have showed their effect in the prevention of cardiovascular and neurological diseases [3–5], as well as cancer and diabetes [4,6–10]. Several of these biological effects are related to the radical scavenging (antioxidant) properties of these compounds. In addition, multiple studies have evaluated the antioxidant properties of anthocyanidins (ArOH), however, the relationship of these properties and the chemical structure of anthocyanidins is not yet fully understood. Some reports suggested that an increase in the number of hydroxyl groups have a consequent increase in the antioxidant capacity of the flavonoids, but this is not always true. Interesting experimental data obtained by using oxygen radical capacity activity method (ORAC) indicated that delphinidin, which has three OH on the B ring, showed a lower * Corresponding author. Tel.: +52 81 1156 0812; fax: +52 81 1156 0820. E-mail address: [email protected] (M. Sánchez). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.06.048
antioxidant than pelargonidin, malvidin, cyanidin and peonidin compounds with one or two hydroxyl group on the same ring [11]. To understand the behavior of anthocyanidins before free radicals, it is necessary to know the mechanisms that are taking place. There are two mechanisms for phenolic compounds known so far: the H-atom transfer mechanism and electron-transfer mechanism. In this paper, we will discuss the results obtained with the B3LYP/ 6-31G(d,p) method, using only the H-atom mechanism. The electron-transfer mechanism is not taken into consideration. It has been reported that the scavenging activity of phenolic antioxidants is related to the bond dissociation enthalphy (BDE) of the O–H groups present in the structure. Recently, theoretical methods have been used to evaluate chemical properties, such like BDE and excited states in polyphenolic compounds [12–14]. The main goal in this work is to understand, by the use of electronic methods, the reason why some anthocyanidins show higher antioxidant activity than other ones. The used method is B3LYP/631G(d,p), which combines the density functional theory (DFT) and the 6-31G(d,p) basis set [15–17]. 2. Results and discussion The anthocyanidins studied in this paper are: aurantinidin, cyanidin, delphinidin, malvidin, pelargonidin, and peonidin (Fig. 1). These compounds have been optimized and characterized as global minimum. All the structures shown in Fig. 2 are cationic and planar. Crystal structures of pelargonidin and cyanidin bromide monohydrate have been reported,[18,19] but they do not
111
R. Guzmán et al. / Journal of Molecular Structure 935 (2009) 110–114
3' 2' 8
HO 7
A
6 5
1
5'
2
C 4
B
1'
O+
3
OH 4'
6'
OH
OH
C3'
C5'
C6
-H -OH -OH -OMe -H -OMe
-H -H -OH -OMe -H -H
-OH -H -H -H -H -H
Aurantinidin Cyanidin Delphinidin Malvidin Pelargonidin Peonidin
Fig. 1. Structure of anthocyanidins [2].
Fig. 2. Stable electronic structures for anthocyanidins and their atomic charges in terms of NPA.
show planar geometries. The explanation to this is that crystal structures depend on orientations, which are favorable for the formation of the hydrogen-bond network for crystal stability. In order to observe o determine where exactly the electronic deficiency is found in all those structures, we calculated the natu-
ral charges (Fig. 2). The NBO analysis suggests that the positive charge is located at the oxygen atoms O1 with values from 0.422 to 0.433. These values have more positive character than the rest of the oxygen atoms present in each compound. The two carbon atoms at alpha position to the O1 present a particular
112
R. Guzmán et al. / Journal of Molecular Structure 935 (2009) 110–114
characteristic. For example, the C2 has higher positive values than the rest of the carbon atoms supporting the OH groups. On the other hand, the other nearby carbon atom to O1 is also positive. From Fig. 2, we can highlight the formation of hydrogen bonds (except for pelargonidin) for five of the six structures. These bonds help to stabilize to the structures and play a very important role in stabilizing free radicals (Fig. 4), as discussed later in the text. In order to convert the anthocyanidins into free radicals, it is necessary to remove a hydrogen atom in each OH group. This way, the number of free radicals generated for each structure were: six for delphinidin, five for aurantinidin and cyanidin, and four for malvidin, pelargonidin and peonidin (Table 1). Practically, the free radical structures do not suffer significant changes in comparison with their corresponding parent structures.
The BDE estimated of every O–H bond and the relative energy of every structure is showed in Table 1. Those values were calculated based on the Eq. (1) at 298 K, where ArOH is the parent anthocyanidin and ArO is the corresponding free radical. The energy value for the hydrogen atom computed at the same level of theory is 0.5002728 Hartrees.
ArOH ! ArO þ H
ð1Þ
From Table 1 we can extract the following information: the two anthocyanidins more reactive toward a free radical are aurantinidin and delphinidin, since they have the lowest BDE values with 80.77 and 80.45 kcal/mol, respectively. The rest of structures have higher BDE values for more than 4.50 kcal/mol. A question that comes to mind is: what would happen if each parent structure was placed alone before several free radicals?
(1.20) (17.43) (7.66)
(0.00) OH
(10.36) OH
HO
O
OH HO
HO
O
OH
OH
OH
OH
(0.00)
(9.20)
(4.49)
(2.43)
(6.20)
Aurantinidin
Cyanidin
(8.88) (2.24)
(0.00) OH (16.45)
OMe (10.83)
OH
OH
HO
HO
O
O
OMe
OH OH
OH (8,38)
OH
OH
(0.00)
(5.54) (9.05)
(14.23)
Malvidin
Delphinidin
(9.03)
(11.19) OMe (9.89)
OH
OH
HO
HO
O
OH
O
(8.87)
OH OH
OH (0.00) (7.63) Pelargonidin
(0.00) (5.51) Peonidin
Fig. 3. Relative energies for anthocyanidins computed with the B3LYP/6-31G(d,p) method.
R. Guzmán et al. / Journal of Molecular Structure 935 (2009) 110–114
113
Fig. 4. The more stable free radicals of anthocyanidins stabilized by hydrogen bonds.
Who would have the best capacity to capture more of those free radicals? The answer might come from the relative BDE of every one of the O–H bonds in each structure. The lowest BDE value in the aurantinidin belongs to C6, labeled as ‘‘0.00” (Fig. 3). The energetic closer value is only 2.43 kcal/mol of difference and it belongs to the O–H bond on C5. The third BDE value is higher by 7.66 kcal/mol, regarding to that showed for the substituent on C6. This suggests that if the aurantinidin reacts with free radicals, the first two OH groups attacked will be those located at C6 and C3, consecutively. OH groups that lie on C7 and C’4 will also be attacked, but the reaction will carry out more slowly on these sites. If this reasoning is applicable to the rest of anthocyanidin structures, the compounds with more capacity to absorb free radicals (antioxidant capacity) go in the following order: cyanidin > malvidin > aurantinidin > delphinidin P peonidin > pelarg onidin. From the previous analysis, we can observe that the number of hydroxyl substituents for each structure is important, but not necessarily essential, since the antioxidant capacity depends largely on the BDE of each substituent. This means that a very effective antioxidant should have similar relative BDE values in all its hydroxyl substituents. Obviously, these values must not exceed the BDE values of water (119 kcal/mol) [20]. Since it is the medium where the reactions take place. The same trend can be observed in experimental studies conducted by the group of Wang [21]. They as-
sessed the antioxidant capacity of these compounds (except for aurantinidin) by the ORAC method. Furthermore, the more stable radicals from the parent structures are those that are stabilized by hydrogen bonds. Aurantinidin and delphinidin radicals have two hydrogen bonds (Fig. 4) and these two structures have the lowest values of BDE (80.77 and 80.45 kcal/mol, respectively) compared with the rest of the most stable radical structures of each parent molecule (Table 1). These structures also have the shorter H–O distances. The malvidin radical is also stabilized by a hydrogen bond, but its BDE is higher for about 5 kcal/mol than aurantinidin and malvidin radicals showed in Fig. 4. Malvidin, peonidin and pelargonidin radicals are not stabilized by hydrogen bonds, however, share an interesting feature, the most reactive hydroxyl group is located at C3.
3. Computational methods All calculations were performed using the Gaussian 03 program package [22]. The B3LYP exchange correlation potential was used for optimizing energies in conjugation with the 6-31G(d,p) basis set [15–17]. The gas-phase BDE was calculated as the enthalpy difference at 298 K for the reaction (1). Harmonic vibrational frequencies were computed at the same level of theory for both parent
114
R. Guzmán et al. / Journal of Molecular Structure 935 (2009) 110–114
Table 1 Bond dissociation energies for the free radicals of anthocyanidins.
4. Conclusions
Radical
BDE (kcal/mol)
Relative energies (kcal/mol)
Aurantinidin C3–O C5–O C6–O C7–O C40 –O
89.97 83.20 80.77 88.43 98.19
9.20 2.43 0.00 7.66 17.43
Cyanidin C3–O C5–O C7–O C30 –O C40 –O
91.29 93.00 97.16 88.00 86.80
4.49 6.20 10.36 1.20 0.00
Delphinidin C3–O C5–O C7–O C30 –O C40 –O C50 –O
85.99 94.69 96.90 89.33 80.45 88.83
5.54 14.23 16.45 8.88 0.00 8.38
In this paper we have given a detailed account of how we calculate the gas-phase BDE for some anthocyanidins based on the hydrogen atom transfer mechanism. The calculated BDEs show that the antioxidant capacity of anthocyanidins depends on the proximity of the relative BDE values of every O–H bond found in the structure. The number of OH groups in the structure is very useful, because it increments the probability of finding similar BDE values. On the other hand, structures that have two or more OH neighboring groups will help to stabilize the free radical with hydrogen bonds. It makes that their corresponding parent structures will be more reactive toward free radicals as in delphinidin and aurantinidin.
Malvidin C3–O C5–O C7–O C40 –O
85.22 87.46 94.27 96.05
0.00 2.24 9.05 10.83
Pelargonidin C3–O C5–O C7–O C40 –O
87.34 94.97 97.22 98.53
0.00 7.63 9.89 11.19
Peonidin C3–O C5–O C7–O C40 –O
86.77 92.28 95.65 95.80
0.00 5.51 8.87 9.03
Acknowledgement We thank Dr. Claudio Cadena-Amaro for helpful discussions. References
(ArOH) molecule and (ArO) radicals to characterize all their conformations as minima or saddle points and to evaluate the zeropoint energy (ZPE) corrections (no significant changes were found when compared with the total energies). The unrestricted open-shell approach was used for radical species. No spin contamination was found for radicals being the hS2i value about 0.750 in all cases. Natural bond orbital (NBO) analysis implemented in Gaussian 03 was used to evaluate the bond order in all systems. All structures were visualized with Chemcraft program.
[1] F. Delgado-Vargas, O. Paredes-López, Natural Colorants for Food and Nutraceutical Uses, CRC Press, Boca Raton, FL, 2003. [2] J.M. Kong, L.S. Chia, N.K. Goha, T.F. Chia, R. Brouillardb, Phytochemistry 64 (2003) 923. [3] E. Middleton, C. Kandaswami, T.C. Theoharides, Pharmacol. Rev. 52 (2000) 673. [4] W. Ren, Z. Qiao, H. Wang, L. Zhu, L. Zhang, Med. Res. Rev. 23 (2003) 519. [5] C.S. Yang, J.M. Landau, M.T. Huang, H.L. Mewmark, Annu. Rev. Nutr. 21 (2001) 381. [6] D. Amic, D. Davidovic-Amic, D. Beslo, N. Trinajstic, Croatica Chim. Acta 76 (2003) 55. [7] G.M. Blackburn, M.J. Gait, Nucleic Acids in Chemistry and Biology, Oxford University Press, New York, 1996. [8] L.R. Ferguson, Mutat. Res. 475 (2001) 89. [9] B. Halliwell, R. Aeschbach, J. Loliger, O.I. Arouma, Food Chem. Toxicol. 33 (1995) 601. [10] Y. Sakihama, M.F. Cohen, S.C. Grace, H. Yamasaki, Toxicology 177 (2002) 67. [11] H. Wang, G. Cao, R.L. Prior, J. Agric. Food Chem. 45 (1997) 304. [12] M. Leopoldini, T. Marino, N. Russo, M. Toscano, J. Phys. Chem. A 108 (2004) 4916. [13] K. Sakata, N. Saito, T. Honda, Tetrahedron 62 (2006) 3721. [14] J.S. Wright, E.R. Johnson, G.A. Dilabio, J. Am. Chem. Soc. 123 (2001) 1173. [15] A.D. Becke, J. Phys. Chem. 98 (1993) 5648. [16] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Phys. Chem. 72 (1980) 650. [17] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [18] N. Saito, K. Ueno, Heterocycles 23 (1985) 2709. [19] K. Ueno, N. Saito, Acta Crystallogr. B 33 (1977) 114. [20] J. Berkpwitz, G.B. Ellison, D. Gutman, J. Phys. Chem. 98 (1994) 2744. [21] H. Wang, G. Cao, R.L. Prior, J. Agric. Food Chem. 45 (1997) 304. [22] M.J. Frisch, G.W. Trucks, H.B. Schlegel et al., Gaussian 03, Revision E. 01, Gaussian, Inc., Wallingford, CT, 2004.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
A density functional study of antioxidant properties on anthocyanidins Rosa Guzmán a, Cristobalina Santiago b, Mario Sánchez c,* a
Instituto de Ciencias Básicas, Universidad Veracruzana, Rafael Sánchez Altamirano S/N, Colonia Industrial Ánimas, Km. 3.5, Carretera Xalapa-Las Trancas, C.P. 91192, Xalapa, Veracruz, Mexico b Instituto de Agroindustrias, Universidad Tecnológica de la Mixteca, Carretera a Acatlima Km. 2.5, C.P. 69000, Huajuapan de León, Oaxaca, Mexico c Centro de Investigación en Materiales Avanzados, S.C., Parque de Investigación e Innovación Tecnológica Alianza Norte 202, Autopista Aeropuerto-Monterrey Km. 10, C.P. 66600, Apodaca, NL, Mexico
a r t i c l e
i n f o
Article history: Received 29 April 2009 Received in revised form 26 June 2009 Accepted 29 June 2009 Available online 4 July 2009 Keywords: Antioxidant capacity DFT BDE Anthocyanidins
a b s t r a c t A density functional theory (DFT) study, using the B3LYP/6-31G(d,p) method, was performed in a attempt to understand the antioxidant properties of some anthocyanidins. This study is based on the H-atom transfer mechanism, which implicates the evaluation of the bond dissociation enthalphy (BDE) of all OH substituents in each structure. The electronic structures studied in this paper are: aurantinidin, cyanidin, delphinidin, malvinidin, pelargonidin and peonidin. Analysis of the computed results suggest that the antioxidant capacity of those structures is in the following order: cyanidin > malvidin > aurantinidin > delphinidin P peonidin > pelargonidin. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Anthocyanins are natural pigments widely distributed in nature [1]. These compounds are flavonoids that belong to the family of polyphenols. Anthocyanins are glycosylated polyhydroxy and polymethoxy derivatives of 2-phenylbenzopyrylium (flavylium) cation. The anthocyanins are glycosides and acylglycosides of anthocyanidins. To date, there are 17 known naturally occurring anthocyanidins, but only a few of them are significant and most common from food point of view [2]. Among most important anthocyanidins are pelargonidin, cyanidin, peonidin, delphinidin, and malvidin. Previous studies of anthocyanidins have showed their effect in the prevention of cardiovascular and neurological diseases [3–5], as well as cancer and diabetes [4,6–10]. Several of these biological effects are related to the radical scavenging (antioxidant) properties of these compounds. In addition, multiple studies have evaluated the antioxidant properties of anthocyanidins (ArOH), however, the relationship of these properties and the chemical structure of anthocyanidins is not yet fully understood. Some reports suggested that an increase in the number of hydroxyl groups have a consequent increase in the antioxidant capacity of the flavonoids, but this is not always true. Interesting experimental data obtained by using oxygen radical capacity activity method (ORAC) indicated that delphinidin, which has three OH on the B ring, showed a lower * Corresponding author. Tel.: +52 81 1156 0812; fax: +52 81 1156 0820. E-mail address: [email protected] (M. Sánchez). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.06.048
antioxidant than pelargonidin, malvidin, cyanidin and peonidin compounds with one or two hydroxyl group on the same ring [11]. To understand the behavior of anthocyanidins before free radicals, it is necessary to know the mechanisms that are taking place. There are two mechanisms for phenolic compounds known so far: the H-atom transfer mechanism and electron-transfer mechanism. In this paper, we will discuss the results obtained with the B3LYP/ 6-31G(d,p) method, using only the H-atom mechanism. The electron-transfer mechanism is not taken into consideration. It has been reported that the scavenging activity of phenolic antioxidants is related to the bond dissociation enthalphy (BDE) of the O–H groups present in the structure. Recently, theoretical methods have been used to evaluate chemical properties, such like BDE and excited states in polyphenolic compounds [12–14]. The main goal in this work is to understand, by the use of electronic methods, the reason why some anthocyanidins show higher antioxidant activity than other ones. The used method is B3LYP/631G(d,p), which combines the density functional theory (DFT) and the 6-31G(d,p) basis set [15–17]. 2. Results and discussion The anthocyanidins studied in this paper are: aurantinidin, cyanidin, delphinidin, malvidin, pelargonidin, and peonidin (Fig. 1). These compounds have been optimized and characterized as global minimum. All the structures shown in Fig. 2 are cationic and planar. Crystal structures of pelargonidin and cyanidin bromide monohydrate have been reported,[18,19] but they do not
111
R. Guzmán et al. / Journal of Molecular Structure 935 (2009) 110–114
3' 2' 8
HO 7
A
6 5
1
5'
2
C 4
B
1'
O+
3
OH 4'
6'
OH
OH
C3'
C5'
C6
-H -OH -OH -OMe -H -OMe
-H -H -OH -OMe -H -H
-OH -H -H -H -H -H
Aurantinidin Cyanidin Delphinidin Malvidin Pelargonidin Peonidin
Fig. 1. Structure of anthocyanidins [2].
Fig. 2. Stable electronic structures for anthocyanidins and their atomic charges in terms of NPA.
show planar geometries. The explanation to this is that crystal structures depend on orientations, which are favorable for the formation of the hydrogen-bond network for crystal stability. In order to observe o determine where exactly the electronic deficiency is found in all those structures, we calculated the natu-
ral charges (Fig. 2). The NBO analysis suggests that the positive charge is located at the oxygen atoms O1 with values from 0.422 to 0.433. These values have more positive character than the rest of the oxygen atoms present in each compound. The two carbon atoms at alpha position to the O1 present a particular
112
R. Guzmán et al. / Journal of Molecular Structure 935 (2009) 110–114
characteristic. For example, the C2 has higher positive values than the rest of the carbon atoms supporting the OH groups. On the other hand, the other nearby carbon atom to O1 is also positive. From Fig. 2, we can highlight the formation of hydrogen bonds (except for pelargonidin) for five of the six structures. These bonds help to stabilize to the structures and play a very important role in stabilizing free radicals (Fig. 4), as discussed later in the text. In order to convert the anthocyanidins into free radicals, it is necessary to remove a hydrogen atom in each OH group. This way, the number of free radicals generated for each structure were: six for delphinidin, five for aurantinidin and cyanidin, and four for malvidin, pelargonidin and peonidin (Table 1). Practically, the free radical structures do not suffer significant changes in comparison with their corresponding parent structures.
The BDE estimated of every O–H bond and the relative energy of every structure is showed in Table 1. Those values were calculated based on the Eq. (1) at 298 K, where ArOH is the parent anthocyanidin and ArO is the corresponding free radical. The energy value for the hydrogen atom computed at the same level of theory is 0.5002728 Hartrees.
ArOH ! ArO þ H
ð1Þ
From Table 1 we can extract the following information: the two anthocyanidins more reactive toward a free radical are aurantinidin and delphinidin, since they have the lowest BDE values with 80.77 and 80.45 kcal/mol, respectively. The rest of structures have higher BDE values for more than 4.50 kcal/mol. A question that comes to mind is: what would happen if each parent structure was placed alone before several free radicals?
(1.20) (17.43) (7.66)
(0.00) OH
(10.36) OH
HO
O
OH HO
HO
O
OH
OH
OH
OH
(0.00)
(9.20)
(4.49)
(2.43)
(6.20)
Aurantinidin
Cyanidin
(8.88) (2.24)
(0.00) OH (16.45)
OMe (10.83)
OH
OH
HO
HO
O
O
OMe
OH OH
OH (8,38)
OH
OH
(0.00)
(5.54) (9.05)
(14.23)
Malvidin
Delphinidin
(9.03)
(11.19) OMe (9.89)
OH
OH
HO
HO
O
OH
O
(8.87)
OH OH
OH (0.00) (7.63) Pelargonidin
(0.00) (5.51) Peonidin
Fig. 3. Relative energies for anthocyanidins computed with the B3LYP/6-31G(d,p) method.
R. Guzmán et al. / Journal of Molecular Structure 935 (2009) 110–114
113
Fig. 4. The more stable free radicals of anthocyanidins stabilized by hydrogen bonds.
Who would have the best capacity to capture more of those free radicals? The answer might come from the relative BDE of every one of the O–H bonds in each structure. The lowest BDE value in the aurantinidin belongs to C6, labeled as ‘‘0.00” (Fig. 3). The energetic closer value is only 2.43 kcal/mol of difference and it belongs to the O–H bond on C5. The third BDE value is higher by 7.66 kcal/mol, regarding to that showed for the substituent on C6. This suggests that if the aurantinidin reacts with free radicals, the first two OH groups attacked will be those located at C6 and C3, consecutively. OH groups that lie on C7 and C’4 will also be attacked, but the reaction will carry out more slowly on these sites. If this reasoning is applicable to the rest of anthocyanidin structures, the compounds with more capacity to absorb free radicals (antioxidant capacity) go in the following order: cyanidin > malvidin > aurantinidin > delphinidin P peonidin > pelarg onidin. From the previous analysis, we can observe that the number of hydroxyl substituents for each structure is important, but not necessarily essential, since the antioxidant capacity depends largely on the BDE of each substituent. This means that a very effective antioxidant should have similar relative BDE values in all its hydroxyl substituents. Obviously, these values must not exceed the BDE values of water (119 kcal/mol) [20]. Since it is the medium where the reactions take place. The same trend can be observed in experimental studies conducted by the group of Wang [21]. They as-
sessed the antioxidant capacity of these compounds (except for aurantinidin) by the ORAC method. Furthermore, the more stable radicals from the parent structures are those that are stabilized by hydrogen bonds. Aurantinidin and delphinidin radicals have two hydrogen bonds (Fig. 4) and these two structures have the lowest values of BDE (80.77 and 80.45 kcal/mol, respectively) compared with the rest of the most stable radical structures of each parent molecule (Table 1). These structures also have the shorter H–O distances. The malvidin radical is also stabilized by a hydrogen bond, but its BDE is higher for about 5 kcal/mol than aurantinidin and malvidin radicals showed in Fig. 4. Malvidin, peonidin and pelargonidin radicals are not stabilized by hydrogen bonds, however, share an interesting feature, the most reactive hydroxyl group is located at C3.
3. Computational methods All calculations were performed using the Gaussian 03 program package [22]. The B3LYP exchange correlation potential was used for optimizing energies in conjugation with the 6-31G(d,p) basis set [15–17]. The gas-phase BDE was calculated as the enthalpy difference at 298 K for the reaction (1). Harmonic vibrational frequencies were computed at the same level of theory for both parent
114
R. Guzmán et al. / Journal of Molecular Structure 935 (2009) 110–114
Table 1 Bond dissociation energies for the free radicals of anthocyanidins.
4. Conclusions
Radical
BDE (kcal/mol)
Relative energies (kcal/mol)
Aurantinidin C3–O C5–O C6–O C7–O C40 –O
89.97 83.20 80.77 88.43 98.19
9.20 2.43 0.00 7.66 17.43
Cyanidin C3–O C5–O C7–O C30 –O C40 –O
91.29 93.00 97.16 88.00 86.80
4.49 6.20 10.36 1.20 0.00
Delphinidin C3–O C5–O C7–O C30 –O C40 –O C50 –O
85.99 94.69 96.90 89.33 80.45 88.83
5.54 14.23 16.45 8.88 0.00 8.38
In this paper we have given a detailed account of how we calculate the gas-phase BDE for some anthocyanidins based on the hydrogen atom transfer mechanism. The calculated BDEs show that the antioxidant capacity of anthocyanidins depends on the proximity of the relative BDE values of every O–H bond found in the structure. The number of OH groups in the structure is very useful, because it increments the probability of finding similar BDE values. On the other hand, structures that have two or more OH neighboring groups will help to stabilize the free radical with hydrogen bonds. It makes that their corresponding parent structures will be more reactive toward free radicals as in delphinidin and aurantinidin.
Malvidin C3–O C5–O C7–O C40 –O
85.22 87.46 94.27 96.05
0.00 2.24 9.05 10.83
Pelargonidin C3–O C5–O C7–O C40 –O
87.34 94.97 97.22 98.53
0.00 7.63 9.89 11.19
Peonidin C3–O C5–O C7–O C40 –O
86.77 92.28 95.65 95.80
0.00 5.51 8.87 9.03
Acknowledgement We thank Dr. Claudio Cadena-Amaro for helpful discussions. References
(ArOH) molecule and (ArO) radicals to characterize all their conformations as minima or saddle points and to evaluate the zeropoint energy (ZPE) corrections (no significant changes were found when compared with the total energies). The unrestricted open-shell approach was used for radical species. No spin contamination was found for radicals being the hS2i value about 0.750 in all cases. Natural bond orbital (NBO) analysis implemented in Gaussian 03 was used to evaluate the bond order in all systems. All structures were visualized with Chemcraft program.
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