Mechanistic and Kinetic Study on the Reactions of Coumaric Acids

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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]

1 ACS Paragon Plus Environment


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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).

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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

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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

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

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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

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

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

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

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

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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

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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)

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•

OX + ArOH → XO– + ArOH•+

(2a)

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OX– + ArOH•+ → XOH + ArO•

(2b)

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•

OX + ArOH→ XOH–ArO•

(3)

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ArOH → ArO– + H+

(4a)

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ArO– + •OX + H+ → ArO• + XOH

(4b)

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

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reactions between radicals and phenolic compounds. The computation of molecular

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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

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

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

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

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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

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

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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

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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

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

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

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

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

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

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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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•

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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

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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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•

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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

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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

•

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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

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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

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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

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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


•

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

Page 21 of 25


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.



Page 22 of 25

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


Page 23 of 25


TS1 (•OH)

TS2 (•OOH)

TS3 (•OCH3)

TS4 (•OOCH3)

Figure 2



a)

b)

Figure 3


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Page 25 of 25


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

ACS Paragon Plus Environment

12

14

Mechanistic and Kinetic Study on the Reactions of Coumaric Acids

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