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The Substitution Effect on Reaction Enthalpies of Antioxidant Mechanisms of Juglone and Its Derivatives in Gas and Solution Phase: DFT Study.

1. Introduction

5-Hydroxy-1,4-naphthoquinone, ordinarily called juglone, has been isolated from many plants in the walnut family [1, 2]. Investigations demonstrated that the principal chemical responsible for the walnut allelopathy is juglone. Walnut has then been reported to be toxic to both herbaceous and woody plants [3, 4]. Hydrojuglone is found in leaves, stems, fruit hulls, inner barks, and roots. When exposed to air or soil compounds, hydrojuglone is oxidized into the allelochemical juglone, which is highly toxic [5]. Rain washes juglone from the leaves and carries it into the soil. Thus, neighbouring plants of the walnut are affected by absorbing j uglone through their roots [3, 4]. To the best of our knowledge, the literature does not contain any reports of growth-inhibiting effect of juglone. The pretreatment of seeds of plants (tomato and wheat) by the combination of gibberellic acid (GA3) and kinetin (KIN) has been shown to effectively overcome the effects of juglone stress on neighbouring plants of the walnut [6].

Walnut leaves have widely been used in folk medicine for treatment because of many medicinal properties (antidiarrheic, antihelminthic, and depurative) and biological activities (hypotensive, antimicrobial, anticancer, and sedative activities) [7, 8]. It is reported that walnut is one of the most enriched dietary plants in the antioxidants, ranked at the top of the scale just after dog rose and before pomegranate [9]. These higher antioxidant properties have been proven by experimental analysis [10]. This explained why walnut had the greatest antioxidant activity among commonly analyzed foods and drinks in Turkey [11]. The interest in juglone has increased because of several further studies that revealed that juglone has cytotoxic and genotoxic effects against cultured melanoma tumor cell [12]. In addition, juglone has been shown to be a potential anticancer drug due to its strong inhibitor properties of peptidyl-prolyl isomerase (PIN1) (a therapeutic target for anticancer research) with anti-inflammatory and antifibrotic properties [13]. As antioxidant, juglone appears to play an important role in the pathogenesis of many neurodegenerative processes (Alzheimer's and Parkinson's diseases) [14]. Moreover, the literature reports the structure-activity relationship of the multitarget anti-Alzheimer compounds designed by combining a tacrine and a juglone [15]. Besides the potent inhibitory activity, this ligand displays interesting abilities to block amyloid-[beta] aggregation and antioxidant proprieties in vitro [16]. This effectively protects neurons from the oxidative stress damage occurring in Alzheimer's disease. This explains the motivation to investigate the structure-activity relationships of juglone and its derivatives using experimental [17] and theoretical [18] tools. As shown in Figure 1, juglone and its derivatives possess an intramolecular hydrogen bond between hydroxyl and keto groups. The importance of these intramolecular hydrogen bonds has been shown in phenolic constituents [19-22]. The calculations of antioxidant activity of juglone and its derivatives have been earlier performed only on two descriptors: hydrogen atom transfer (HAT) and ionization potential (IP) by Jin [18].

In general, three different mechanisms are involved in the description of the antioxidant action [23]: hydrogen atom transfer (HAT) mechanism (see (1)), electron transfer-proton transfer (ET-PT) mechanism (see (2)-(3)), and sequential proton loss-proton transfer (SPLET) mechanism (see (4)(5)):

RX - H [right arrow] [RX.sup.*] + [H.sup.*] (1)

RX - H [right arrow] RX - [H.sup.*+] + [e.sup.-] (2)

RX - [H.sup.*+] [right arrow] [RX.sup.*] + [H.sup.+] (3)

RX - H [right arrow] [RX.sup.-] + [H.sup.+] (4)

[RX.sup.-] [right arrow] [RX.sup.*] + [e.sup.-] (5)

In the present work, we have performed extensive density functional calculations of thermodynamic energies of reactions (1)-(5). The detailed comparison between the reaction enthalpies of these different antioxidant mechanisms of the juglone and its derivatives and those of the classical antioxidants has been done. The survey of the literature indicates that such an analysis has notbeen done previously for juglone and its derivatives.

2. Computational Details

All the calculations were performed using the Gaussian 09 software package [24]. The geometry of molecular structures (neutral, radical, ionic, and cationic structures) was optimized using DFT method at B3LYP/6-31+G(d,p) level. The optimized structures were confirmed to be real minima by the frequency calculations (no imaginary frequency). The natural bond orbital (NBO) analysis of charge and the spin density of radicals of the molecular system studied were computed at B3LYP/6-31+G(d,p) level of the theory. Solvent contribution at 298.15 K to reaction enthalpies was computed employing integral equation formalism of the polarized continuum model IEF-PCM method [25].

The scavenging activity of juglone and its derivatives is related to O-H bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), and electron transfer enthalpy (ETE). These five thermodynamic parameters are, respectively, defined according to the following relations:

BDE = H ([H.sup.*]) + H ([R.sup.*]) - H (RX - H) (6)

IP = H (RX - [H.sup.*+]) + H ([e.sup.-]) - H (RX - H) (7)

PDE = H ([RX.sup.*]) + H ([H.sup.+]) - H (RX - [H.sup.*+]) (8)

PA = H ([RX.sup.-]) + H ([H.sup.+]) - H (RX - H) (9)

ETE = H ([RX.sup.*]) + H ([e.sup.-]) - H ([RX.sup.-]) (10)

The calculated gas enthalpies of a proton (H([H.sup.+])) and an electron (H([e.sup.-])) are, respectively, 6.197 kJ/mol and 3.145 kJ/mol [26]. The reaction free energies of reactions (1)-(5), ordinary denoted BDFE (bond dissociation free energy), IPFE (ionization potential free energy), PDFE (proton dissociation free energy), PAFE (proton affinity free energy), and ETFE (electron transfer free energy) are, respectively, calculated according to relations (6)-(10). In the same vein, the calculated gas free energies of a proton (H([H.sup.+])) and an electron (H([e.sup.-])), are respectively, -26.28kJ/mol [27] and -3.72kJ/mol [28]. Proton and electron solvation enthalpies and free energies were taken from the literature [29, 30]. The hydrogen atom solvation reaction enthalpies and free energies were also taken from earlier researches [30].

3. Results and Discussion

3.1. General Considerations. Seventeen derivatives (J2-18) with electron donating or withdrawing substituents on the phenyl ring of 5-hydroxy-1,4-naphthoquinone as shown in Figure 1 have been designed in order to investigate the substituent effects. The optimized structures of neutral molecules obtained possess 5-hydroxy-1,4-naphthoquinone moiety that is completely planar. These optimized structures are characterized by the formation of hydrogen bond between hydroxyH and oxygen atom of carbonyl group. From Table 1S, it can be seen that the [O.sub.1]-[H.sub.1] ... [O.sub.2] bond's length ranges from 1.621 [Angstrom] to 1.709 [Angstrom] in gas phase. We also investigated the formation of other hydrogen bonds between oxygen atom of carbonyl group and hydrogen atom of the substituent (H-X: X = O, N) using the cut-off definition of X-H ... [O.sub.2] hydrogen bond (H ... [O.sub.2] < 3.0 [Angstrom] and X-H ... [O.sub.2] angles > 110[degrees]) [31]. Our B3LYP results then indicated that another hydrogen bond can only be obtained for J5. Conversely, we registered 104.6[degrees] (J3) and 101.3[degrees] (J4) for X-H ... [O.sub.2] angle (X = N). For J17, two conformations are found, J17A and J17B, corresponding, respectively, to the formation of [O.sub.1]-[H.sub.1] ... [O.sub.2] and [O.sub.1]- [H.sub.1] ... O (O: oxygen atom of N[O.sub.2] substituent). Structure J17A ([E.sub.T] = -814.87830035 hartree) has been shown to be slightly more stable than J17B ([E.sub.T] = -814.87466923 hartree) in gas phase. The minor total energy difference (2.3kJ/mol) explained why we did not take into account the configuration J17 in this paper. In the whole, the substitution on the phenyl ring of 5-hydroxy-1,4-naphthoquinone leads to the sensitive shortening of the [O.sub.1]-[H.sub.1] ... [O.sub.2] bond distances. This fact is in accordance with the elucidation on the enhancement of the interaction strength of the hydrogen bond by the introduction of the electron-donation or electron-withdrawing group in J1 [18]. The difference average is about 0.038 [Angstrom] in gas phase. The comparison between [O.sub.1]-[H.sub.1] bond distances of J2-18 and those in J1 indicated that the introduction of substituent groups provokes an elongation of the [O.sub.1]-[H.sub.1] distances. Although solvation of these molecules does not give any additional information on [O.sub.1]-[H.sub.1] ...[O.sub.2] interaction, it augments this elongation. Because ofthe absence ofexperimental geometrical parameters of the molecular library analyzed, a verification of the optimized structures cannot be done. Nevertheless, we have compared experimental gas phase FT-IR frequencies of some functional group of juglone to theoretical values [32, 33] (Table 2S and Figure 1S). For juglone, we have obtained the following linear dependence between experimental frequencies ([[upsilon].sub.Exp]) and theoretical frequencies ([[upsilon].sub.Theo]):

[[upsilon].sub.Exp] = 0.8908 [[upsilon].sub.Theo] + 238.89. (11)

The correlation coefficient ([R.sup.2] = 0.998) obtained confirms the suitability of the theoretical method employed for geometry optimization.

3.2. HAT Mechanism. In order to clarify the geometries and the energetic aspects of different radical species obtained, we adopted the notation Jn-RaX (n = 1-18) for radical species in order to denote where the abstraction of an H atom from X-H bond occurs. The OH BDE values were calculated in various media (for each substituent) and reported in Table 1 and Table 3S. The spin density plots for analyzed radicals are presented in Figure 2. The diminution of BDE values compared to that of J1 observed for electron donating substituent is in good agreement with previous researches [34]. Lower BDE values are obtained for amino substituents (J2-4). The difference between BDE values of J1 and those of J2-4 ranges from 25 to 47kJ/mol. In the case of J3, the abstraction of hydrogen atom leads to the formation of J3radO1 and J3ranN. The latter is slightly more stable than the former by 2,04 kJ/mol. The BDE values of 392 and 380 kJ/mol, respectively obtained, for J3radO1 and J3radN provide an indication that H atom removal on an amino group is preferred to H atom removal on a 5-hydroxy-1,4-naphthoquinone moiety. The spin distribution shows an almost identical involvement of atoms of 5-hydroxy-1,4-naphthoquinone moiety and those of the amino substituent for both radicals. The [mathematical expression not reproducible] orbital interaction ([E.sup.(2)] = 2.73 kcal/mol) may be the plausible explanation of the higher stability of J3radN. In contrast, the energy difference of 0.55 kJ/mol between the J4radO1 and J4radN radical species in favor of the former suggested that despite the identical BDE values obtained (372 kJ/mol) for both corresponding radical species, the H atom removal on 5-hydroxy1,4-naphthoquinone moiety is almost the most preferred. This former is stabilized by the N-H ... [O.sub.2] intramolecular hydrogen bond with the second-order perturbation energy of 0.35 kJ/mol. The presence of two intramolecular hydrogen bonds N-H ... [O.sub.1] ([E.sup.(2)] = 0.62kJ/mol) and [O.sub.1]-[H.sub.1] ... [O.sub.2] ([E.sup.(2)] = 2.81 kcal/mol) in J4radN radical does not affect the N center. An increase of the total energy difference observed in the same sense with the increase of the dielectric constant of the solvent (3.160 (benzene), 3.280 (toluene), 6.842 (methanol), 6.875 (acetonitrile), and 7.075 kJ/mol (water)) clearly proved that this H atom abstraction site is more preferred in solution phase. Contrary to amino substituents, the difference in OH BDE values (21 kJ/mol) found for the J5radO1 and J5radO radical species revealed that the H abstraction on 5-hydroxy-1,4-naphthoquinone moiety is less favored for OH group (J5). The J5radO radical displays [mathematical expression not reproducible], and [O.sub.1]-[H.sub.1] ... [O.sub.2] intramolecular hydrogen bond with second- order perturbation energy of 2.64 kJ/mol, whereas J5radO1 radical centre is stabilized by [mathematical expression not reproducible], and O-H ... [O.sub.1] hydrogen bond ([E.sup.(2)] = 0.63 kJ/mol). For J5 molecule in gas phase, the [O.sub.1]-[H.sub.1] BDE difference between the value calculated from monodeprotonated form (anion) and that obtained from neutral form equal to 49 kJ/mol demonstrates the fact that the deprotonation ofthe parent molecule severely alters the BDE values. This fact is in accordance with the previous investigation of the impact of the deprotonation of sixteen flavonoids on the OH BDE values carried by Klein et al. [35]. The substitution of OH group by the OMe (J6) leads to a 17 kJ/mol increase in the [O.sub.1]-[H.sub.1] BDE. Similar results have been observed in the case of hydroxyl daidzein and glycitein by Lengyel et al. [36]. The substitution of hydrogen atom (J1) with ethyl group (J8) or with t-butyl group (J7) induces a slight decrease in BDE (8 kJ/mol and 7 kJ/mol, resp., for J8 and J7). The values agree with earlier researches [37] that demonstrated that the influence of the side chain on the antioxidant is more pronounced when the number of carbon atoms of the alkyl substituent exceeds five. But it is difficult to expand such a comparison between linear and branched substituents in ortho position due to the steric effects. The difference that is about 1kJ/mol may therefore be attributed to the error bars of the method used. These two substituents (R = [C.sub.2][H.sub.5] and t-but) do not contribute to the spin density distribution (Figure 2). Close result obtained for F (J9), Cl (J10), and Br (J11) substituents revealed that the variation of the influence of the electronegativity of halogen substituent has a minor effect on the BDE values. Besides the electronegativity, an increase of the C-halogen bond length was also observed when the atomic radius of halogen atom augmented regardless of the media: in water, the C-halogen bond lengths were 1.352 [Angstrom] (C-F), 1.746 [Angstrom] (C- Cl), and 1.846 A (C-Br). In contrast, a sensitive decrease of BDE values is obtained for the unsaturated double bond (CH=C[H.sub.2] (J13) and [C.sub.6][H.sub.5] (J12)) substituents that extend the system conjugation. Figure 2 clearly indicates that spin density distribution for the radicals generated from J12-13 involves all atoms of a ring and those of the substituents around the substitution site. The removal of a hydrogen atom on the J15 molecule gives two radical species: J15radO1 and J15radO. The respective values of BDE obtained, 406 and 472kJ/mol, expose the easier removal of the [H.sub.1] hydrogen atom on 5-hydroxy-1,4-naphthoquinone moiety. The O-[H.sub.1] BDE of structure with COOH (J15) was smaller than BDE value of J1, despite the electron-withdrawing effect. The BDE difference (10 kJ/mol) displays the importance of the presence of [O.sub.1] ... H-O intramolecular hydrogen bond to the stability of the radical. This fact is in agreement with prior researches [38-40]. For other withdrawing substituents, augmentation of O-[H.sub.1] BDE ranging from 3 to 11 kJ/mol compared to that of J1 molecule is obtained. Figure 3 showed that the solvation of molecules provokes a diminution of BDE values with the exception of J15 in methanol and acetonitrile. The BDE difference averages obtained in various media are 7.14 (benzene), 7.33 (toluene), 14.28 (methanol), 14.71 (acetonitrile), and 15.21 kJ/mol (water). This confirms the fact that the higher polarity of the solvent, the easier the HAT mechanism [19]. Fifen et al. [20] explained through the analysis of spin density distribution of polyphenolic deoxybenzoins in solvents that the reduction of BDE in polar media is related to the greater stabilization of the corresponding radical species in polar solvents. The BDE values (Figure 3) confirm that abstraction of the hydrogen atom is easier in polar media. In the whole, one can also observe from Table 4S that the HAT mechanism is endergonic.

To provide more insight into the antioxidant properties, we analyzed the capacities of J1-18 as a quencher of radicals (R' = *O[R.sub.i], *OO[R.sub.i] (i = 1 (R = H), 2 (R = C[H.sub.3]), 3 (R = C[Cl.sub.3]), and 4 (R = C[H.sub.2]CH=C[H.sub.2])), and *O[O.sup.-]) through a hydrogen atom transfer:

RX - H + R'* [right arrow] RX* + R'H (12)

The reaction free energies values ([DELTA]G) for (O1-H1) HAT (12) calculated at 298.15 K in gas phase are reported in Table 5S. It can be observed that the HAT mechanism towards OR; radicals is exergonic except for reactions towards O[R.sub.3] (J14 ([DELTA]G = 0), J15 ([DELTA]G = 531 kJ/mol), J17 ([DELTA]G = 11 kJ/mol), and J18 ([DELTA]G = 3 kJ/mol)) and that towards O[R.sub.4] (J15 ([DELTA]G = 511 kJ/mol) and J18 ([DELTA]G = 1 kJ/mol)) and that towards O[R.sub.2] (J18 ([DELTA]G = 38 kJ/mol)). In addition, it emerges that the Gibb free energy values are considerably lower towards hydroxyl radicals ([DELTA][G.sub.OH] from -129 to -19 kJ/mol). This finding supports the higher reactivity of *OH [40, 41]. However, the HAT mechanism is endergonic towards all other radicals (except for the *O[O.sup.-] scavenging activity of the analyzed molecules (J1-18) that is less thermodynamically preferred). The free energy value was in the range of 104-214 kJ/mol towards *O[O.sup.-] radical and the range of -1-78 kJ/mol towards *OO[R.sub.i]. This confirms the least reactivity of *O[O.sup.-] radicals. Our data showed that the hydrogen atom abstraction from [O.sub.1]-[H.sub.1] hydroxyl group of the molecules studied (J1-18) is more exergonic than the hydrogen atom abstraction from the substituent for the compounds J4 and J15. Meanwhile, J3 and J5 calculations yielded contrasting results.

3.3. SET-PT Mechanism. The ionization potentials of the compounds investigated lie in a range from 680 to 884 kJ/mol in the gas phase. Figure 4 shows the spin density distribution for investigated radical cations. For the sake of clarity, we used the abbreviation CatJn (n = 1-18) to denote cation of the corresponding investigated molecules Jn (n = 1-18). The IP values of J1-18 in the solution phase decreased significantly compared with those in the gas phase. The average of the calculated drop in IP values which is 111.94 kJ/mol for benzene, 115.72 kJ/mol for toluene, 266.22 kJ/mol for methanol, 277.27 for acetonitrile, and 285.55 kJ/mol for water corroborates the sensitivity of the cation radical forms of compounds to the polarity of different solvents used [39]. We concluded that the relative order of IP is water < acetonitrile < methanol < toluene < benzene < gas (Figure 5). It emerges that IP values are higher than the BDE values regardless of the solvent considered. The HAT mechanism is clearly dominant thermodynamically over the SET mechanism. Such a result has previously been obtained for phenolic-4-En-1-Yne derivatives isolated from Hypoxis rooperi [22] for natural phenolic acids and their derivatives [38]. For the six environments, the substitution by the electron donating groups leads to the reduction of the IP values. Figure 5 also shows that lowest IP values are obtained for J2 (R = N[Me.sub.2]) in various media. The comparison of the IP values obtained to those of classical antioxidants (vanillic acid, gallic acid, and ascorbic acid) reveals that the more powerful antioxidant can be expected only for electron donating groups. From Table 1, it should be noticed that the compounds with lower IP values than those of the vanillic acid (777 kJ/mol) and gallic acid (763 kJ/mol) at B3LYP/6-311++G(d,p) level are, respectively, J2-4, J6, and J12 and J2-4 [38]. In addition, the IP values for J2-4 and J6-7 substituents are lower than that of the ascorbic acid (804 kJ/mol at B3LYP/6-311++G(2d,2p)//B3LYP/6-31G(d,p)) [19]. It is also noteworthy that IP values calculated for amino substituents are lower than that of Trolox calculated at the same level (763 kJ/mol) [19]. We then concluded that, despite the basis set, the SET mechanism for the amino substituent is stronger in the gas phase than that of the natural phenolic acid derivatives (vanillic acid and gallic acid), ascorbic acid, and Trolox. From Table 1, it is noticeable that, for electron withdrawing substituents (J14-18), the IP values obtained are higher than those of classical antioxidants selected. All the free energies are positive (Table 6S), showing that the ET is not spontaneous regardless of the environment considered.

Table 1 also displays the similarity between the trends observed in PDE values of Jn-RaX radical species and those of BDEs. Such a similarity observed by other authors [19, 34] is attributed to the fact that the second stage of SET-PT mechanism also ends with the formation of JnRaX radical species. It is noticeable that the substitution by electron donating groups in the molecule reference provokes a decrease in 1-OH PDE. The differences are in the 22-88 kJ [mol.sup.-1] range. A notable exception is observed for two halogen substituents (J9 (R = F) and J10 (R = Cl)) and for O-H hydroxyl group (J5). This latter case, regardless of position of the proton abstracted, is confirmed by the augmentation of O-H PDE value of phenyl ring attached to heterocyclic pyran ring of daidzein compared to that of its homolog in 6-hydroxydaidzein [36] in the gas phase. In the whole, the substitution of electron withdrawing group leads to an increase of PDE values in this phase. The solvation causes an enormous decrease of PDEs in comparison to the gas phase values because of high solvation enthalpies [30]. The average decrease in PDEs reached approximately 874.5 kJ/mol. The average decrease in PDEs for different solvents used (benzene (902 kJ/mol), toluene (837 kJ/mol), methanol (886 kJ/mol), acetonitrile (875 kJ/mol), and water (865 kJ/mol)) shows that lowest PDE values are registered in benzene (Table 1 and Table 6S). Our B3LYP data therefore confirm the fact that aprotic solvents have good capability of proton solvation [29, 42]. In the whole, our results showed that, for electron withdrawing substituents (J14-18), PDEs are lower than that of Trolox in benzene (37 kJ/mol) and water (21.2 kJ/mol) at B3LYP/6-311++G(2d,2p)//B3LYP/6-31G(d,p) [19]. We then concluded that proton dissociation ability of J14-18 is significantly higher than that of Trolox in benzene and water. In the same vein, the PDEs in benzene are significantly lower than that of ascorbic acid (8.5 kJ/mol at B3LYP/6-311++G(2d,2p)//B3LYP/6-31G(d,p) in benzene) with the exception of J2-4 and J6-7 [13]. This indicated that the proton donation ability of the molecules analyzed from RX-[H.sup.+] cation is stronger than that of ascorbic acid in benzene. In the whole, Table 7S and Figure 6 showed that PDFE values are positive in the gas phase and in two solvents (toluene and acetonitrile). Consequently, PT mechanism is not spontaneous in these media. Nevertheless, negative values are obtained for other solvents, with exception made for few molecules: J2 (in water) and J4 (in methanol). Subsequently, we concluded that the PT mechanism is then spontaneous in these solvents.

3.4. SPLET Mechanism. Computational results of PA and ETE in vacuum and in various media are compiled in Table 1 and Table 8S. The substitution of the hydrogen atom by various electron donor substituents leads to augmentation of PA values ranging from 17 to 58 kJ/mol for J2-4 in the gas phase. Such an increase is attributed to donor inductive effect of corresponding substituents (-N[Me.sub.2] and -NHMe) which strengthens the [O.sub.1]-[H.sub.1] bonds. Owing to the mesomeric effect of other donor substituents, a reduction of PAs in the range of 1-64 kJ/mol is observed for J4-13 in the gas phase. The difference of 2 kJ/mol obtained between the N-H PAS and [O.sub.1]-[H.sub.1] homolog in the favor of the latter points out this effect which is due to nearness of the carbonyl function. Nevertheless, contrary result obtained for J3 is related to the simultaneous contribution of the mesomeric effect previously evoked and the donor inductive effect of the methyl group that has substituted the hydrogen atom of the amino group of J4. It is noteworthy to mention that the difference of 16 kJ/mol between the O-H PA and [O.sub.1]-[H.sub.1] PA (J5) in the benefit of the former discloses the fact that the hydrogen atom hampers the proton donation ability. The drop of the PAs in the range of 41-100 kJ/mol for molecular system (J15-J18) with withdrawing substituents is also related to the mesomeric effect that weakens the [O.sub.1]-[H.sub.1] bonds. Our data showed a drastic drop of PAs when passing from gas phase to solution phase. The average drop in PA value approximated to 1107 (benzene), 1039 (toluene), 1039 (methanol), 1234 (acetonitrile), and 1223 kJ/mol (water) sheds light on the intensification ofthe deprotonation process by the solvation and its sensibility to the dielectric constant of the solvent used. This latter point is consistent with the claim of other researchers [42]. Table 1 and Table 8S display the fact that [O.sub.1]-[H.sub.1] PA values are slightly smaller than H-OOC PA ones (J15) in water, toluene, and acetonitrile. This is an indication of easier monodeprotonation of J15 from [O.sub.1]-[H.sub.1] site in these media. Overall, the positive sign of PAs (Table 1 and Table 8S) and PAFEs (Table 9S) discloses the fact that the proton loss (PL) mechanism is endothermic but not spontaneous, independently of the environment. The comparison of the PAs of these molecules with those of classical antioxidants revealed that our data are slightly lower than those of Trolox in water (237kJ/Mol), benzene (457 kJ/mol), and in the gas phase (1450 kJ/mol) at B3LYP/6- 311++G(2d,2p)//B3LYP/6-31G(d,p) level with the exception of J2 in the gas phase [19]. Consequently, the deprotonation of X-H (X = N, O) for molecular systems analyzed is easier than Trolox. In the whole, the majority of PAs in benzene are lower than those of ascorbic acid (367 kJ/mol at B3LYP/6- 311++G(2d,2p)//B3LYP/6-31G(d,p) [19]) and those of a few natural phenolic acids: ferulic acid (388 kJ/mol) and vanillic acid (400kJ/mol) at B3LYP/6-311++G(d,p) level [19]. The deprotonation process has been afterwards shown to be more predominant for these molecules in this medium. In the solution phase, electron transfer enthalpies are higher than the corresponding gas phase values. The average differences between ETE in the gas phase and different solvent are 83.4 (benzene), 72.4 (toluene), 71.6 (methanol), 61.04 (acetonitrile), and 60.4 kJ/mol. This means that electron transfer from anion ([RX.sup.-]) is not favored enough in the solution phase. The positive values obtained for gas phase and solution phase ETFEs (Table 9S) illustrate the nonspontaneity of the ET mechanism from anions.

From Figure 7, it is noticeable that calculated gas phase PAs are higher than corresponding values of IPs (that are approximately twice as high as gas phase BDEs). This demonstrates that the thermodynamically favored mechanism in the gas phase can be classified in the following increasing order: HAT < SET-PT < SPLET. Nevertheless, the drastic drop in solution phase PAs mentioned earlier justifies the fact that these are importantly lower than corresponding solution phase BDEs and IPs. We then assume that SPLET mechanism is more predominant than HAT and its SET-PT homolog in the solution phase. This general trend confirms the importance of this mechanism in polar solvents enlightened by experimental and theoretical studies [38, 43, 44].

4. Conclusions

The antioxidant mechanisms of juglone and its derivatives in gas and solution phase have been studied using computational methods. The main purpose is to evaluate thermodynamically the impact of the substitution effect on these mechanisms. A molecule library of eighteen molecules has then been built. Three major antioxidant mechanisms (HAT, SET-PT, and SPLET) have been analyzed. The optimized geometries obtained are characterized by the formation of hydrogen bond between hydroxy-H and oxygen atom of keto ([O.sub.1]-[H.sub.1] ... [O.sub.2]). The introduction of the electron- donation or electron-withdrawing group in juglone provokes an enhancement of the interaction strength of this hydrogen bond.

In the gas phase, the HAT mechanism is thermodynamically more predominant followed by SET-PT and SPlEt. But the SPLET mechanism is the most preferred in the solution phase. This general trend is in agreement with previous experimental and theoretical studies for phenolic antioxidants. Our data demonstrate that the three mechanisms analyzed are endothermic but not spontaneous with exception of PT mechanism (for benzene, methanol, and water).

https://doi.org/10.1155/2018/1958047

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors are grateful to Benoit Champagne (Universite de Namur, Belgium) for his support. This work has been supported by the Ministry of Higher Education of Cameroon.

Supplementary Materials

From Crystallographic parameters obtained from the optimized structures of juglone and its derivatives (Table 1S), it is clearly shown that the length of the [O.sub.1]-[H.sub.1] ... [O.sub.2] hydrogen bonding (HB) decreases with the substitution. Such an observation corroborates the increment observed in [O.sub.1]-[H.sub.1] bond length due to the substitution of the H atom. The agreement between vibrational frequencies of optimized structures (J1-18) and that obtained experimentally (Table 2S) is confirmed by the value of the correlation coefficient ([R.sup.2] = 0.998) obtained between the two sets of data (Figure 1S). The diminution of BDE values for [O.sub.1]-[H.sub.1] bond is presented in Table 3S when the polarity of the study medium increases. This fact is in good agreement with the increase in [O.sub.1]-[H.sub.1] bond length observed in polar solvents (methanol, acetonitrile, and water). The positive BDFE values of the compounds (Table 4S) show that the HAT from all the molecules is nonspontaneous. Table 5S clearly shows that all reactions involving hydroxyl radical are spontaneous. This fact corroborates with the high reactivity of that radical. In addition, a huge decrease in IP values is observed (Table 6S) for polar media, agreeing with low values of electron solvation enthalpy in polar solvents IP. PDE values for molecules containing electron withdrawing groups are almost all negative (Table 6S) (except in toluene), showing that the proton transfer reaction from those radicals is exothermic. For all study compounds, PDFE values (Table 7S) are negative in benzene, meaning that the proton transfer reaction from the molecules is thermodynamically spontaneous in that solvent. In water and methanol, the same observation is made for almost all molecules herein studied. PA and ETE values (Table 8S) are positive for all molecules and in all the media. This means that the involved reactions are exothermic. The free energy values (PAFE and ETFE, Table 9S) for these reactions are also positive, meaning that the reactions are nonspontaneous in standard conditions. Table 1S: optimized geometrical parameters (bond lengths ([Angstrom]) and bond angles ([degrees])) in various media at B3LYP/6- 31+G(d,p) level. Table 2S: experimental and calculated vibrational frequencies ([cm.sup.-1]) at the B3LYP/6-31+G(d,p) level. Table 3S: B3LYP/6-31+G(d,p) total energies of radical ([E.sub.T] in a.u) and bond dissociation enthalpies (BDE in kJ/mol) in solution phase. Table 4S: bond dissociation free energy of Jn (in kJ/mol) at B3LYP/6-31+G(d,p) level in solution phase. Table 5S: thermochemical parameters for interaction of investigated juglone derivatives with some of oxygen species obtained ([Angstrom][G.sub.R'] in kJ/mol) at B3LYP/6-31+G(d,p) level ([Angstrom][G.sub.R'] in kJ/mol). Table 6S: thermodynamic energies of Jn (in kJ/mol) at B3LYP/6-31+G(d,p) level in various media: ionization potentials (IP) and proton dissociation enthalpies (PDE). Table 7S: thermodynamic energies of Jn (in kJ/mol) at B3LYP/6-31+G(d,p) level in various media: ionization potential free energies (IPFE) and proton dissociation free energies (PDFE). Table 8S: thermodynamic energies of Jn (in kJ/mol) at B3LYP/6-31+G(d,p) level in various media: proton affinities (PA) and electron transfer enthalpies (ETE). Table 9S: thermodynamic energies of Jn (in kJ/mol) at B3LYP/6-31+G(d,p) level in various media: proton affinity free energies (PAFE) and electron transfer free energy (ETFE). Figure 1S: dependence of experimental and calculated gas phase FT-IR frequencies ([cm.sup.-1]) at B3LYP/6-31+G(d,p) level. (Supplementary Materials)

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Aymard Didier Tamafo Fouegue, (1) Desire Bikele Mama (iD), (2) Julius Numbonui Ghogomu (iD), (1) Younang Elie, (3) and Marie-Annie Etoh (2)

(1) Department of Chemistry, Faculty of Science, University ofDschang, P.O. Box 67, Dschang, Cameroon

(2) Department of Chemistry, Faculty of Science, University of Douala, P.O. Box 24157, Douala, Cameroon

(3) Department of Inorganic Chemistry, Faculty of Science, University of Yaounde I, P.O. Box 812, Yaounde, Cameroon

Correspondence should be addressed to Desire Bikele Mama; bikelemama@yahoo.fr

Received 21 October 2017; Revised 27 December 2017; Accepted 9 January 2018; Published 16 April 2018

Academic Editor: Dario Pasini

Caption: Figure 2: Plots of spin density for various hydrogen atom-abstracted radicals of juglone and selected derivatives.

Caption: Figure 3: Bond dissociation energies of Jn (in kJ/mol) at B3LYP/6- 31+G(d,p) level in various media.

Caption: Figure 4: Plots of spin density for various hydrogen atom-abstracted cation radicals of juglone and derivatives.

Caption: Figure 5: Ionization potentials of Jn (in kJ/mol) at B3LYP/6- 31+G(d,p) level in various media.

Caption: Figure 6: Proton dissociation free energies of Jn (in kJ/mol) at B3LYP/6-31+G(d,p) level in various media.

Caption: Figure 7: Thermodynamic energies of Jn: bond dissociation energy (in kJ/mol), ionization potential of Jn (in kJ/mol), and proton affinities (in kJ/mol) at B3LYP/6-31+G(d,p) level in gas phase.
Table 1: Thermodynamic energies in kJ/mol: bond dissociation
energies, ionization potential, proton dissociation energies, proton
affinities, and electron transfer energies in gas phase and in water
at B3LYP/6-31+G(d,p) level.

Molecules                      BDE (kJ/mol)      IP (kJ/mol)

Denotations   Substituents   Gas       Water     Gas   Water

J1                 H         417       400       834   538
J2            N[Me.sub.2]    370       344       680   422
J3                NHMe       392       343       721   439
                             380 (a)   377 (a)
J4             N[H.sub.2]    372       350       743   452
                             372 (a)   377 (a)
J5                 OH        380       367       805   510
                             359 (b)   348 (b)
J6                OMe        397       387       773   495
J7                t-bu       409       394       798   519
J8             [C.sub.2]     410       394       806   521
               [H.sub.5]
J9                 F         410       393       845   547
J10                Cl        412       396       834   542
J11                Br        412       397       827   539
J12               Phen       405       390       763   500
J13             C.sub.2]     401       386       786   501
               [H.sub.3]
J14               CHO        420       407       863   565
J15               COOH       406       398       883   575

J16                CN        472       477       874   571
J17            N[O.sub.2]    421       406       884   581
J18            C[F.sub.3]    428       410       867   567

Molecules                      PDE (kJ/mol)        PA (kJ/mol)

Denotations   Substituents   Gas       Water     Gas        Water

J1                 H         899       41        1422       192
J2            N[Me.sub.2]    987       100       1480       194
J3                NHMe       988       82        1448       219
                             986 (a)   108 (a)   1439 (a)   229 (a)
J4             N[H.sub.2]    946       76        1413       184
                             946 (a)   103 (a)   1415 (a)   221 (a)
J5                 OH        892       36        1377       163
                             871 (b)   16 (b)    1361 (b)   151 (b)
J6                OMe        940       71        1423       188
J7                t-bu       928       53        1417       200
J8             [C.sub.2]     921       50        1423       196
               [H.sub.5]
J9                 F         881       24        1398       174
J10                Cl        888       31        1395       175
J11                Br        902       35        1394       176
J12               Phen       959       68        1408       192
J13             C.sub.2]     934       63        1408       189
               [H.sub.3]
J14               CHO        873       19        1403       179
J15               COOH       840       1         1322       127
                                                            129c
J16                CN        864       13        1366       161
J17            N[O.sub.2]    861       7         1358       155
J18            C[F.sub.3]    870       18        1381       171

Molecules                      ETE (kJ/mol)

Denotations   Substituents   Gas       Water

J1                 H         312       386
J2            N[Me.sub.2]    266       328
J3                NHMe       261       302
                             263 (a)   330 (a)
J4             N[H.sub.2]    276       343
                             277 (a)   334 (a)
J5                 OH        319       382
                             314 (b)   374 (b)
J6                OMe        297       372
J7                t-bu       308       375
J8             [C.sub.2]     303       397
               [H.sub.5]
J9                 F         434       398
J10                Cl        336       399
J11                Br        335       376
J12               Phen       314       375
J13             C.sub.2]     309       405
               [H.sub.3]
J14               CHO        333       448
J15               COOH       401       423

J16                CN        371       432
J17            N[O.sub.2]    387       414
J18            C[F.sub.3]    359       372

(a) Relative to N-H thermodynamic parameter values. (b) O-H
thermodynamic parameter values. (c) Related to H-OOC thermochemical
parameter values.

Figure 1: Numbering system used to designate specific atom of
juglone and its derivatives followed by specification of electron
donating or withdrawing substituent used.

R                    Name

H                    J1
N[Me.sub.2]          J2
NHMe                 J3
N[H.sub.2]           J4
OH                   J5
OMe                  J6
t-bu                 J7
[C.sub.2][H.sub.5]   J8
F                    J9
Cl                   J10
Br                   J11
Phen                 J12
[C.sub.2][H.sub.3]   J13
CHO                  J14
COOH                 J15
CN                   J16
N[O.sub.2]           J17
C[F.sub.3]           J18
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Title Annotation:Research Article
Author:Fouegue, Aymard Didier Tamafo; Mama, Desire Bikele; Ghogomu, Julius Numbonui; Elie, Younang; Etoh, M
Publication:Journal of Chemistry
Date:Jan 1, 2018
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