SOLVENT EFFECT ON THE THERMODYNAMICS OF THE OXIDATION OF POTENTIAL ANTIOXIDANT GARLIC COMPOUNDS M F Andrada*†, E G Vega Hissi†, J C Garro Martínez†, Graciela N. Zamarbide†, Mario R. Estrada † and Imre G. Csizmadia†§ †*
Departamento de Química, Universidad Nacional de San Luis, SAN LUIS, 5700 Argentina. Tel: +54-0266-4423789,
Int. 122.
[email protected] §
Department of Chemistry and Chemical Informatics, Faculty of Education, University of Szeged, H-6701, SZEGED,
Hungary and Department of Chemistry University of Toronto, Toronto, Ontario Canada , M5S 3HG.
Abstract Antioxidant capacity of garlic has been attributed to organic sulfur compounds such us allyl methyl disulfide. Using quantum chemical calculations at B3LYP/6-31+G (d) and G3MP2B3/6-31+G (d) levels of theory, we study three possible oxidation reactions of this compound against hydrogen peroxide, a reactive oxygen species, from a thermodynamic point of view. Because these reactions are supposed to occur in biological media, solvent effect was taken into consideration. Oxidation over the double bond that leads to the formation of an epoxide is more thermodynamically feasible, limiting the antioxidant capacity of this compound.
Introduction Oxygen is vital for aerobic life processes. However, about 5% or more of the inhaled
O2 is converted to
reactive oxygen species (ROS) [1]. Thus, cells under aerobic condition are always threatened with the insult of ROS, which however are efficiently taken care of by the highly powerful antioxidant systems of the cell without any untoward effect. When the balance between ROS production and antioxidant defences is lost, ‘oxidative stress’ results. This stress deregulates the cellular functions through a series of events, leading to various pathological conditions including cardiovascular dysfunction, neurodegenerative diseases, gastroduodenal pathogenesis, metabolic dysfunction of almost all the vital organs, cancer, and premature ageing. ROS generation is a physiological process that occurs in every cell of all aerobic organisms. The species such as anion superoxide (•O2–), hydrogen peroxide ( H 2O2 ), and the extremely reactive hydroxyl radical (•OH), are formed in the sequential univalent process by which O2 undergoes reduction [1-5]. If the hydrogen peroxide is not eliminated enzymatically, (e.g. by glutathione peroxidase) it may be accumulate and can cause biological damage. Sulphur containing compounds found in garlic may also act as a peroxide scavenger. Garlic belongs to the plant genus Allium sativam. Many health benefits are attributed to organic compounds extracted from aqueous garlic oil, especially, those compounds containing in their structure one or more sulfur atoms. In particular, antioxidant capacity of allyl methyl disulfide is of great importance. In this context, our research group has previously studied in the gas phase the oxidation reactions of this compound with hydrogen peroxide, classified as a reactive oxygen species. [6] In aqueous medium, garlic sulfur compounds are highly reactive molecules. Despite their widespread culinary and medicinal use, relatively little is known about the cellular and molecular mechanisms through which garlic extracts produce their physiological effects [7-12].
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The solvent effect has received considerable attention of research chemists in both theoretically and experimentally fields. It is well know that the reaction rate, reactivity, molecular structures and charge distribution can be affected by the medium and the reaction conditions. Because of much of the chemical processes of practical interest occur in condensed phase, particularly the biological ones, the role of the environment (solvent, proteins, etc.) takes a crucial importance that can not be neglected at the time to explain and predict properties and behavior of chemical and biochemical systems. As the proposed oxidation reactions occur in a biological environment, in this paper, solvent effect was taken into consideration in the study of the reaction mechanism for the three possible pathways proposed for the oxidation of allyl-methyl disulfide with hydrogen peroxide (Fig.1). We present a thermodynamical study focusing the attention to three different oxidized forms of allyl methyl disulfide, their structures and their thermodynamic stabilities, preceding to any kinetic consideration of the problem. The results are compared with previous studies carried out in the gas phase.
Fig.1: Allyl methyl disulfide (C1) and possible products [(C2), C3) and (C4)] obtained by oxidation with hydrogen peroxide. Methods Initial geometries of Fig.1 compounds were taken from gas phase optimized structures calculated in a previous paper [8], where a complete conformational analysis was performed at B3LYP/6-31+G(d) level of theory [13-15]. Briefly, from potential energy surfaces, minimum energy conformers were fully geometry optimized and frequency calculations were performed to obtain the zero-point energies and thermal corrections to convert the internal energies to Gibbs energies at 298.15 K. Then, the quantum chemistry composite method G3MP2B3 was applied to obtain very accurate energies and reliable thermodynamic functions [16-18]. Solvent effects of an aqueous solvent were taken into account by using the self-consistent reaction field (SCRF) polarizable continuum model (PCM) [19-21] approach at the B3LYP/6-31+G(d) level of theory. Solvation free energies in water were calculated [22-24]. Thermodynamical gas phase data and solvation free energies were used to calculate Gibbs free energy change for the reactions proposed through the thermodynamic cycle shown in Fig.2 and according to equation [1], which is
2
written for a case in which all of the species involved are transferred from the gas phase into the solution phase [19,2526].
ΔG aq = ΔG gas + ΔG s (P) + ΔG s (H 2 O) − ΔG s (R) − ΔG s (H 2 O2 )
[1]
Fig.2: Thermodynamic cycle used to calculate free energy of the reaction in solution. ΔGgas and ΔGaq are the Gibbs free energy changes of then reaction in gas phase and solution, respectively; ΔGs is the solvation Gibbs free energy change. (R) refers to compound C1 and (P) to either compound C2, C3 or C4.
Once ΔG(s)are obtained, the equilibrium constants (Ki) for reactions at 298.15 K and 1 atm are calculated with equation 2; and from the latter, the corresponding constants of isomerization (Kij) for oxidized products according to equation 3.
ΔG i K i = e RT −
K ij =
[2]
Kj Ki
[3]
All calculations were carried out using the Gaussian program 03 [13].
Results and Discussion Our previous conformational study involving dihedral angles defined in Fig.3, gave rise to six stable, i.e. minimum energy, conformers of compounds (1), (2) and (4), and four conformers of compound (3)7 which were used in the present study.
Fig.3: Definition of dihedral angles common to all studied species.θ1 (C1S2-S3C4), θ2 (S2S3-C4C5) and θ3 (S3C4-C5C6). The absolute Gibbs free energy, in gas phase, for these structures computed at B3LYP/6-31+G (d) and G3MP2B3 levels of theory are tabulated in Table 1S and Table 2S of Online Resource, respectively. In order to study the oxidation reactions (Fig.1) of compound (1) to compounds (2), (3) and (4), the Gibbs free energy had to be
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computed also at the two levels of theory for H2O and H2O2, in gas phase (Table 3S of Online Resource). Compounds (2), (3) and (4) are structural isomers therefore the numerical values of their thermodynamic functions are comparable. Compounds (2) and (3) are almost isoenergetic while compound (4) is the most stable of the three, for both level of theory. Quantitatively energetic difference between sulfoxides and epoxide is of about 10 kcal/mol and 1 kcal/mol for B3LYP/6-31+G(d) and G3MP2B3, respectively. Using the data of the absolute Gibbs free energy of all species involved (Table 1S, 2S and 3S), the reaction free energy change for the oxidation was calculated in gas phase ( ΔG gas ), for both levels theory, using as reference the most stable conformer of compound (1). These values are tabulated in Table 1. Table 1: Change in the Gibbs free energy in for the oxidation reaction in gas (Fig.1) was computed at B3LYP/6-31+G (d) and G3MP2B3/6-31+G (d) levels of theory. ΔGgas (kcal/mol) B3LYP REACTION 1 -35.963197 g+[g- ac-] -35.225246 g+ [g- ac+] -36.332173 g+ [g+ ac-] -36.279462 g+ [g+ ac+] -36.851750 g+ [a ac-] -35.841460 g+ [a ac+] REACTION 2 -35.527705 g+[ac- ac-] -35.616184 g+ [ac- ac+] -34.856898 g+ [ac+ ac-] -35.993317 g+ [ac+ ac+] REACTION 3 -46.061707 g+[g- sc-] -47.090195 g+ [g- ac+] -45.404077 g+ [g+ sc-] -47.335552 g+ [g+ ac+] -45.412863 g+ [a sc-] -46.866802 g+ [a ac+]
G3MP2B3 -66.065455 -64.694347 -65.319346 -65.186314 -66.123186 -64.725722 -64.536215 -65.457398 -64.086290 -64.917740 -66.472709 -67.504962 -66.069220 -67.100218 -65.066460 -66.157699
An approximation of aqueous solvent effect was considered by PCM model at B3LYP/6-31+G (d) level of theory taking into account that the reactions are supposed to exist in biologic media. In order to calculate the free energy change of the reaction in solution, solvation free energies for all species involved in oxidation reaction of Fig.1 were calculated. Values are tabulated in Table 4S of Online Resource. Solvent effect calculations show that compound (3) has a positive solvation free energy value while compounds (2) and (4) present negative values, being the latter of greater magnitude. Therefore, solvation is thermodynamically more feasible for the epoxide. At B3LYP/6-31+G(d) level of theory Gibbs free energy change in gas phase for the oxidation reactions on either sulfur atom (reactions 1 and 2) are very similar with a difference lower than 1 kcal/mol (Table 1 and Fig.4). However reaction 3 (formation of the epoxide) presents a ΔG value of about 10 kcal/mol lower than the other two reactions, being therefore the most probable thermodynamically. In condensed phase (Table 2 and Fig.4) the trend is similar as occurs in gas phase, with a ΔG value of reaction 3 even more negative (around 18 kcal/mol).
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Fig.4: Comparison of Gibbs free energy of oxidation reactions in the gas phase and solvated phase at level of theory B3LYP/6-31+G (d) Table 2: Change in the Gibbs free energy in solution for the oxidation reactions (Fig.1), computed at B3LYP/6-31+G (d) and G3MP2B3/6-31+G (d) levels of theory. ΔGaq (kcal/mol) B3LYP G3MP2B3 REACTION 1 -30.419 -25.308 g+[g- ac-] -28.521 -24.346 g+ [g- ac+] -30.618 -25.035 g+ [g+ ac-] -29.716 -25.129 g+ [g+ ac+] -30.628 -25.092 g+ [a ac-] -29.567 -25.107 g+ [a ac+] REACTION 2 -28.264 -23.494 g+[ac- ac-] -28.372 -24.124 g+ [ac- ac+] -27.613 -22.902 g+ [ac+ ac-] -28.679 -23.798 g+ [ac+ ac+] REACTION 3 -46.078 -44.673 g+[g- sc-] -46.936 -44.869 g+ [g- ac+] -46.480 -44.468 g+ [g+ sc-] -48.011 -45.781 g+ [g+ ac+] -47.079 -44.852 g+ [a sc-] -47.973 -45.786 g+ [a ac+]
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G3MP2B3/6-31+G(d) level of theory results show similar ΔG values for the three reactions in gas phase (Table 1 and Fig.5). But when solvent effect is taken into consideration reaction 3 is more feasible (i.e. ΔG value of about 8 kcal/mol lower) than reactions 1 and 2 (Table 2 and Fig.5). Besides, free energy changes of all oxidation reactions are closer when G3MP2B3/6-31+G (d) level of theory is used, in both gas and condensed phases. Therefore the proposed reactions become more competitive.
Fig.5: Comparison of Gibbs free energy of oxidation reactions in the gas phase and solvated phase at level of theory G3MP2B3/6-31+G (d)
Compounds (2), (3) and (4) are structural isomers; therefore the numerical values of their thermodynamic functions are comparable. Equilibrium constants and the corresponding isomerization constants were calculated from equations (2) and (3). Isomerization constants (Table 3) of the conversion of compounds (2) and (3) to compound (4) present the higher values. Table 3: Constants for the isomerization of the oxidized products.
k2,3 K3,2 K3,4 K4,3 k2,4 K4,2
B3LYP/6-31G+(d) Gas Phase Solvated Phase 0.2350 0.0372 4.2558 26.839 2.063x108 1.3933x1014 4.846x10-9 7.177x10-15 7 4.849x10 5.1913x1012 2.06210-8 1.926x10-13
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G3MP2B3/6-31G+(d) Gas Phase Solvated Phase 0.3249 0.0203 3.0777 49.198 31.721 1.024x107 0.0970 9.762x10-8 10.3066 2.082x104 0.09702 4.803x10-6
The equilibrium constants for the isomerization of the oxidized product are illustrated in Fig.6 and as expected: K2→4 = K2→3 . K3→4 or
ΔG2→4 = ΔG2→3 + ΔG3→4
Fig.6: Interconversions and equilibrium constants for isomers 2, 3 and 4
Conclusions Reaction free energy change values and isomerization constants values obtained in gas and condensed phases reveal that oxidation reaction on the double bond is thermodynamically preferred, generating a new oxidative species which could be more dangerous than hydrogen peroxide itself. These results indicate a limited role as antioxidant of allyl methyl disulfide and perhaps other sulfur-containing garlic compounds with carbon-carbon double bonds in their structures. Nevertheless a study of the reaction mechanism of the three proposed oxidations should provide insights into which aspects, either thermodynamic or kinetic, will prevail. Online Resource. Tables of absolutes Gibbs free energies and solvation free energies of all species involved in the proposed reactions. References
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