Mechanistic and Kinetic Study on the Reactions of Coumaric Acids

Aug 27, 2014 - ... Universidad de Castilla−La Mancha, Avenida Camilo José Cela ... H atom abstraction from −OH and −COOH groups and addition to...
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Mechanistic and Kinetic Study on the reactions of coumaric acids with reactive oxygen species: a DFT approach Andres Garzon, Ivan Bravo, Antonio J. Barbero, and Jose Albaladejo J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/jf5011148 • Publication Date (Web): 27 Aug 2014 Downloaded from http://pubs.acs.org on September 13, 2014

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Journal of Agricultural and Food Chemistry

Mechanistic and Kinetic Study on the reactions of coumaric acids with reactive oxygen species: a DFT approach

Andrés Garzón,*,1 Iván Bravo1, Antonio J. Barbero2 and José Albaladejo3

1

Departamento de Química Física, Facultad de Farmacia, Universidad de Castilla-La Mancha, Paseo de los

estudiantes, s/n, 02071, Albacete, Spain 2

Departamento de Física Aplicada, Facultad de Farmacia, Universidad de Castilla-La Mancha, Paseo de los

estudiantes, s/n, 02071, Albacete, Spain 3

Departamento de Química Física, Facultad de Ciencias Químicas, Universidad de Castilla-La Mancha,

Avenida Camilo José Cela, 10, 13071, Ciudad Real, Spain

* E-mail: [email protected]

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Abstract

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The mechanism and kinetics of reactions between coumaric acids and a series of

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reactive oxygen species (•OX) was studied through the Density Functional Theory (DFT).

4

H-atom abstraction from –OH and –COOH groups and addition to the non-aromatic double

5

bond were the most representative reaction pathways chosen for which free energy barriers

6

and rate constants were calculated within the Transition State Theory (TST) framework.

7

From these calculations, it was estimated that •OH > •OCH3 > •OOH > •OOCH3 is the order

8

of reactivity of •OX with any coumaric acid. The highest rate constant was estimated for p-

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coumaric acid + •OH reaction, while the rest of •OX species are more reactive with o-

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coumaric acid. On the basis of the calculated rate constants, H-abstraction from –OH group

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should be the main mechanism for the reactions involving •OCH3, •OOH and •OOCH3

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radicals. Nevertheless, the addition mechanism, which sometimes is not considered in

13

theoretical studies on reactions of phenolic compounds with electrophilic species, could play

14

a relevant role in the global mechanism of coumaric acid + •OH reactions.

15 16 17

1. Introduction

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Phenolic acids constitute a family of natural compounds which can be found in some

19

foods such as cereal grains (1−3), virgin olive oil (4−9), wine (10−12) and honey (13, 14), as

20

well as in medicinal plants (15). Antioxidant and radical scavenging activity (16−19), along

21

with antiinflamatory (14, 20) and antiulcer (21) properties have been attributed to phenolic

22

acids. An especially relevant group of phenolic acids are those derivate from

23

hydroxycinnamic acid for which a large number of antioxidant activity studies have been

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carried out (16, 19, 22−26). It is generally assumed that the reactions between phenolic

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compounds and reactive oxygen species (•OX) such as hydroxyl (•OH), alcoxyl (RO•) and 2 ACS Paragon Plus Environment

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peroxyl (ROO•) radicals proceeds through four main mechanisms: direct H-transfer process

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from the antioxidant molecule (eqn. 1), single electron transfer process which can be

28

followed by a subsequent proton transfer process (eqn. 2a and 2b), radical adduct formation

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(eqn. 3) (27−34) and sequential proton loss electron transfer (eqn. 4a and 4b) (27−35):

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OX + ArOH → XOH + ArO•

(1)

31



OX + ArOH → XO– + ArOH•+

(2a)

32

OX– + ArOH•+ → XOH + ArO•

(2b)

33



OX + ArOH→ XOH–ArO•

(3)

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ArOH → ArO– + H+

(4a)

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ArO– + •OX + H+ → ArO• + XOH

(4b)

36

Different theoretical methodologies are being used to study the mechanism of the

37

reactions between radicals and phenolic compounds. The computation of molecular

38

parameters such as the O-H bond dissociation enthalpies, ionization potentials, HOMO-

39

LUMO energies, and charge populations have been employed for different authors to

40

evaluate the radical scavenging activity and reaction mechanism (29−31, 35−41). An

41

approach to the molecular mechanism can also be performed using the analysis of the

42

critical points on the potential energy surface of reaction (32−34). Recently, Transition State

43

Theory (TST) and Canonical Variational Transition Sate Theory (CVT) have been

44

successful employed to study the molecular mechanism of phenolic compound reactions and

45

to estimate the corresponding rate constants (42−44).

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o-, m- and p-coumaric acids (CAs) are hydroxycinnamic acids with three different

47

functional groups, i.e. –OH, –COOH, –C=C–, which can react with •OX species (see

48

chemical formula in Figure 1). Previous theoretical studies on homologous phenolic

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compounds such as gallic acid, caffeic acid and sinapinic acid have shown that the single

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electron transfer process (2a and 2b) does not play a relevant role in the reactions with free 3 ACS Paragon Plus Environment

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radicals (30, 32, 41, 42). Besides, even though sequential proton loss electron transfer (eqn.

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4a and 4b) may be a relevant mechanism in ionizing solvents, several studies show that it is

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not favored in vacuum (41, 45, 46). Hence, the present work is only focused on the H-atom

54

abstraction from –OH and –COOH groups and the addition to the non-aromatic double bond

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as the most probable reaction mechanisms. The analysis of the mechanism and kinetics of a

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series of oxidation reactions between CAs and different •OX species (•OH, •OOH, •OCH3

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and •OOCH3) using the Density Function Theory (DFT) is the ultimate goal of this work.

58 59

2. Computational Details

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DFT calculations were carried out using the Gaussian09 (Rev. C.01) set of programs

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(47). The reagent, transition state (TS) and molecular complex (MC) geometries were

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optimized at the M06-2X level (48), along with the 6-311+G** basis set. M06-2X is

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especially recommended for thermodynamic and kinetic calculations (48, 49). An initial

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conformational analysis was carried out for the reagents to obtain the lowest energy

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conformation. The nature of the stationary points was assessed by means of the normal

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vibration frequencies calculated from the analytical second derivatives of the energy. The

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first-order saddle points −which are related to transition states− must show an imaginary

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value for the frequency associated with the eigenvector primarily describing the product

69

formation step; whereas the real minima of the potential energy hypersurface −which are

70

related to stable species− have to show positive real values for all the vibrational

71

frequencies. In addition, Intrinsic Reaction Coordinate, IRC, calculations at M06-2X/6-

72

311+G** level were carried out to assess each reaction pathway.

73

The rate constants (kTST) were calculated through the thermodynamic formulation of

74

the Transition State Theory (TST). Gibbs free energy barriers were computed at 298.15 K.

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The tunneling effect along the reaction coordinate has been accounted by means of the

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transmission coefficient χ(T) given by Wigner’s model, i.e.

|∗ |

 = 1 + 

77





(4)

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where ω* and kB are the TS imaginary frequency and the Boltzmann constant, respectively

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(50−53). Thus, the final expression for the rate constant (kTST/W) becomes: kTST/W = χ(T) kTST

80

(5)

81 82

3. Results and Discussion

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Figure 1S (in supplementary material) summarize the most favored conformers

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obtained from the initial conformational analysis for the three coumaric acids studied in this

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work. As discussed above, the oxidation of hydroxycinnamic acid derivatives with reactive

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oxygen species is expected to carry out via addition to the double bond or H-abstraction. In

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order to simplify the study ―there are a considerable number of chemically different

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hydrogen atoms in the studied compounds― and since the most labile hydrogen atoms

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correspond to the hydroxyl and carboxylic groups we decided to not consider more H-

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abstraction pathways than those related to these two groups. The transition states

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corresponding to the addition pathways were only computed on one side of the reagent due

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to coumaric acids show planar structures (see Figure 2) in which both sides are chemically

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equivalents.

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Different transition states were named as TSn, where n indicates the position

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attacked by the •OX species. Thus, TS1 and TS4 correspond to the transition states in which

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addition mechanism occurs through both TS2 and TS3. Figure 2 shows some examples of

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the different transitions states found for the reactions of p-coumaric acid with •OX. Detailed

99

information on the main geometrical parameters for all the computed transition states can be

OX abstracts the H atom from the –OH and –COOH groups, respectively, while the

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found in the supplementary material. For H-abstraction mechanism, TSs involving •OH

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radicals show some differences with respect to the rest of TSs. TS1s involving •OH show

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geometries close to the reagents in which the breaking O…H bond length is within 1.02 –

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1.03Å, while the forming H…O bond is significantly longer (1.40 – 1.42 Å). For the rest of

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TS1s, differences between the lengths of the breaking and forming bonds are significantly

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smaller, lying within 0.01 – 0.18Å. The geometries of TS4s involving •OH are also different

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to the rest of TS4s. In the first ones, the difference between the lengths of the breaking and

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forming bonds are ≤ 0.09 Å, while for the other TS4s are within the range 0.32 – 0.61Å.

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Besides, for TS4s involving •OH and •OOH radicals, the short lengths (≤ 2.05 Å) between

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the hydrogen atom from •OX and the carbonyl oxygen from the acid group, compared to the

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sum of van der Waals radii (2.6 Å) (54), suggest possible stabilizing interactions. This fact

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may lead to the formation of 5 and 6-membered pseudo-rings for TS4s involving •OH and

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113

were observed for TS2 and TS3 with respect to the sort of •OX species chosen. •OX interacts

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with an unsaturated carbon with a C=C…O angle within 87 – 110 degrees and with a O…C

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bond length within 1.88 – 2.18 Å. Two examples of IRC profiles for H-abstraction and

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addition pathways of the m-coumaric acid + •OH reaction can be seen in Figure 3.

OOH radicals (see Tables 1S – 6S in supplementary material). No significant differences

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Table 1 summarizes the activation (∆G≠) and reaction (∆rG) Gibbs free energies

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calculated for each considered reaction pathway. ∆G≠ increases following the order of •OH
•OCH3 > •OOH > •OOCH3). For coumaric acid + •OH reactions, kTST

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values for the addition mechanism through TS3 are at least 60% higher than the

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corresponding TS1 H-abstraction pathway (for m-coumaric acid + •OH reaction the

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difference both kTST values becomes an order of magnitude). However, these differences are

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reduced or even inverted with the use of tunneling coefficient corrections since the small

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imaginary frequencies (see Table 2) computed for the different TS1s (υ < 1600i cm-1) leads

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to χ(T) coefficients from 3 to 4 times larger than those calculated for the addition pathways.

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Hence, both addition through TS3 and H-abstraction through TS1 can be considered 7 ACS Paragon Plus Environment

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pathways with a comparable weight in the global reaction mechanism. Here, it is worth

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noting that each addition pathway presents two equivalent reaction points because of the

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planarity of the molecule. For the reactions involving the rest of •OX species studied, H-

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abstraction at the hydroxyl group should be the main reaction mechanism. In these cases,

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values estimated for kTST/W corresponding to TS1 are 2 or more orders of magnitude higher

155

than kTST/W values estimated for the rest of pathways. The lowest rate constants were estimated for m-coumaric acid reactions with any

156 157



158

work, the highest radical scavenging activity (RSA), using the 2,2-diphenyl-1-

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picrylhydrazyl (DPPH•) assay, was measured for both o-coumaric and p-coumaric acid with

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respect to the m-isomer. Nevertheless, the trolox antioxidant activity (TEAC) —using the

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2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS•+) assay— determined for p-

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coumaric acid highlights on the rest of studied compounds (TEAC is about 2 times higher

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for p-isomer than for o- and m-isomers). Comparable TEAC values have been reported for

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o- and m-coumaric acids. These differences in the antioxidant activity could not be

165

explained by Nenadis et al. (55) on the basis of bond dissociation energies and ionization

166

energies. Thus, the kind of •OX species seems to play a role on the order of reactivity of

167

coumaric acids, which is consistent with our results. According to our calculations,

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p-coumaric acid may be the most reactive with •OH within the studied compounds, being the

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addition to an unsaturated carbon the main reaction mechanism. In contrast, o-coumaric

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should be the most reactive with •OOH, •OCH3 and •OOCH3, and in this case, the H-

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abstraction from the hydroxyl group seems to be the main reaction mechanism. As

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mentioned before, previous theoretical studies on the reactions of related hydroxycinnamic

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acids with radicals also suggest that the reaction mechanism is controlled by addition for the

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reactions with •OH and by H-abstraction for the reactions with •OOH (42). Unfortunately,

OX which agrees with the experimental observations reported by Nenadis et al. (55). In that

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addition mechanism was not considered in other previous theoretical works on the

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antioxidant activity of hydroxycinnamic acids such as p-coumaric, caffeic and sinapinic

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acids (27, 30, 41). Mantzavinos et al. determined the reaction intermediates of p-coumaric

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acid oxidation in different conditions by high-performance liquid chromatography (HPLC).

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The authors suggested that the oxidation at pH 3.5 starts through two main routes: (i)

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oxygen attack to the non-aromatic double bond to form glyoxylic acid and

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p-hydroxybenzoic acid (see Figure 1); (ii) decarboxylation reaction leading to p-

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vinylphenol. Under acidic conditions (pH = 1), and in presence of homogenous catalyst ions

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such as Fe2+, Cu2+, Zn2+ and Co2+, decarboxylation mechanism is favored (56, 57).

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Andreozzi et al. also determined the reaction intermediates of p-coumaric acid oxidation

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with ozone by HPLC, and proposed that the initial oxidation step arises from the

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electrophilic attack of ozone to the non-aromatic double bond (58). Therefore, the addition

187

mechanism should be considered in theoretical studies on oxidation reactions of phenolic

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acids containing non-aromatic double bonds with reactive oxygen species.

189 190 191 192

Acknowledgments The authors gratefully thank Supercomputing Service of Castilla-La Mancha University for allocation of computational resources.

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Table 1. Activation (∆G≠) and reaction (∆rG) Gibbs free energies calculated for the most relevant pathways of the studied reactions. TST rate constants estimated at 298.15 K are also appended including (kTST/W) or not (kTST) tunneling coefficient corrections. The level of theory used was M06-2X/6-311+G**. Compound Radical Pathway ∆G≠ ∆rG kTST χ(T) kTST/W (kcal mol-1) (kcal mol-1) (L mol-1 s-1) (L mol-1 s-1) Individual Global reaction mechanism pathway • o-coumaric TS1 6.25 -30.58 1.63×108 4.35 7.07×108 OH acid TS2 8.44 -21.63 4.03×106 1.22 4.93×106 1.30×109 TS3 5.98 -24.35 2.59×108 1.13 2.92×108 TS4 10.79 -4.85 7.60×104 3.89 2.95×105 • TS1 16.95 0.74 2.34 5.81 1.36×101 OOH TS2 22.66 4.47 1.51×10-4 1.55 2.35×10-4 1.36×101 TS3 20.20 2.27 9.69×10-3 1.43 1.39×10-2 TS4 34.99 26.47 1.39×10-13 2.14 2.98×10-13 6 • TS1 9.24 -16.20 1.05×10 4.49 4.74×106 OCH3 TS2 16.35 -9.34 6.46 1.36 8.80 4.74×106 TS3 13.92 -9.75 3.88×102 1.28 4.94×102 TS4 21.59 9.54 9.34×10-4 1.81 1.69×10-3 • TS1 17.26 2.51 1.38 6.10 8.43 OOCH3 1.54 8.63×10-6 TS2 24.62 8.18 5.62×10-6 8.65 TS3 18.93 4.14 8.25×10-2 1.34 1.11×10-1 -15 -15 TS4 37.31 28.25 2.77×10 2.07 5.72×10 7 • m-coumaric TS1 7.28 -26.93 2.88×10 3.68 1.06×108 OH 6 acid 1.18 5.45×106 TS2 8.36 -22.92 4.61×10 9.09×108 8 TS3 5.79 -25.55 3.55×10 1.12 3.96×108 TS4 11.16 -4.12 4.10×104 3.74 1.54×105 -1 • TS1 18.68 4.39 1.27×10 5.98 7.56×10-1 OOH -4 TS2 22.46 3.69 2.12×10 1.51 3.21×10-4 7.57×10-1 1.46 2.81×10-4 TS3 22.52 1.54 1.93×10-4 TS4 36.34 27.20 1.44×10-14 2.06 2.97×10-14 5 • TS1 10.31 -12.55 1.73×10 4.53 7.85×105 OCH3 1.32 2.06×101 TS2 15.83 -11.04 1.56×101 7.89×105 TS3 13.12 -11.75 1.49×103 1.29 1.92×103 TS4 22.09 10.27 4.02×10-4 1.67 6.73×10-4 -2 • TS1 19.77 6.16 2.02×10 6.37 1.29×10-1 OOCH3 -5 TS2 24.18 7.12 1.17×10 1.50 1.76×10-5 1.29×10-1 -4 1.46 3.37×10-4 TS3 22.42 5.27 2.30×10 TS4 38.26 28.97 5.65×10-16 1.95 1.10×10-15 8 • p-coumaric TS1 6.24 -29.97 1.66×10 4.04 6.70×108 OH 6 acid TS2 8.14 -22.10 6.73×10 1.19 8.03×106 2.05×109 8 1.10 6.82×108 TS3 5.46 -25.08 6.22×10 TS4 10.90 -4.62 6.37×104 3.85 2.45×105 -1 • TS1 17.55 1.35 8.54×10 6.22 5.31 OOH TS2 22.25 3.95 3.02×10-4 1.53 4.62×10-4 5.34 TS3 20.22 2.50 9.33×10-3 1.41 1.31×10-2 -13 -13 TS4 35.11 26.70 1.14×10 2.08 2.37×10 • TS1 9.58 -15.59 5.87×105 4.60 2.70×106 OCH3 1 TS2 15.88 -10.40 1.42×10 1.33 1.89×101 2.71×106 TS3 12.68 -11.64 3.13×103 1.26 3.94×103 TS4 21.64 9.76 8.52×10-4 1.74 1.49×10-3 -1 • TS1 17.88 3.12 4.87×10 6.72 3.27 OOCH3 TS2 24.35 7.47 8.78×10-6 1.51 1.33×10-5 3.27 TS3 22.04 5.44 4.36×10-4 1.43 6.23×10-4 -15 -15 TS4 37.61 28.47 1.69×10 2.02 3.42×10

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Table 2. Calculated imaginary frequencies (cm-1) for the studied transition states at the M06-2X/6311+G** level. Compound

Pathway

o-coumaric acid

TS1 TS2 TS3 TS4 TS1 TS2 TS3 TS4 TS1 TS2 TS3 TS4

m-coumaric acid

p-coumaric acid

Radical •

OH -1857.3 -478.8 -360.3 -1724.5 -1662.3 -434.2 -344.9 -2352.3 -1771.1 -445.7 -316.4 -1715.0



OOH -2227.3 -755.2 -666.6 -1081.7 -2264.6 -727.3 -685.3 -1044.9 -2318.4 -737.6 -647.9 -1054.8

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OCH3 -1897.2 -611.6 -533.1 -912.1 -1908.3 -573.1 -544.7 -832.8 -1925.9 -580.1 -516.2 -875.3



OOCH3 -2293.5 -743.0 -593.1 -1049.2 -2352.3 -721.1 -692.1 -991.8 -2428.8 -727.7 -665.5 -1026.5

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Figure Captions Figure 1. Chemical formula of coumaric acids and related compounds. Figure 2. Some of the different transitions states found for the reactions of p-coumaric acid with reactive oxygen species. Geometries were optimized at the M06-2X/6-311+G** level. Figure 3. IRC calculations for both a H-abstraction and an addition channel of the m-coumaric acid + OH reaction computed at the M06-2X/6-311+G** level. (a) Pathway through TS1; (b) Pathway through TS2.

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OH COOH

HO

COOH

COOH

HO

o-coumaric acid

m-coumaric acid H3CO

HO

p-coumaric acid

COOH

COOH

COOH HO OCH 3

HO

Sinapinic acid

Caffeic acid HO

HO

p-hydroxybenzoic acid

COOH

HO

Dihydrocaffeic acid Figure 1

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TS1 (•OH)

TS2 (•OOH)

TS3 (•OCH3)

TS4 (•OOCH3)

Figure 2

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a)

b)

Figure 3

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5 TS2

Relative Energy / kcal mol-1

0

-5

-10

-15

-20

-25

-30 0

2

4

6 8 10 Reaction Coordinate / amu1/2 Bohr

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