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Influence of Structural Characteristics of Substituents on the Antioxidant Computational and Theoretical Chemistry xxx (2015) xxx–xxx Contents lists available at ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc Influence of structural characteristics of substituents on the antioxidant activity of some anthraquinone derivatives ⇑ Zoran Markovic´ a, , Svetlana Jeremic´ a, Jasmina Dimitric´ Markovic´ b, Marijana Stanojevic´ Pirkovic´ c, Dragan Amic´ d a Department of Chemical-Technological Sciences, State University of Novi Pazar, Vuka Karadzˇic´a bb, 36300 Novi Pazar, Serbia b Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia c Faculty of Medical Sciences, University of Kragujevac, Svetozara Markovic´a 69, 34000 Kragujevac, Serbia d Faculty of Agriculture, The Josip Juraj Strossmayer University, Kralja Petra Svacˇic´a 1d, HR-31000 Osijek, Croatia article info abstract Article history: The antioxidant activity of chrysophanol (CH), emodin (EM), aloe-emodin (AE), and 1,3,8- Received 31 August 2015 trihydroxyanthraquinone (THA) was examined in water and pentyl ethanoate by using the M052X/ Received in revised form 4 October 2015 6-311++G(d,p) level of theory. It was shown that hydrogen bonds are of significant importance for the Accepted 4 October 2015 stability of the radical and anionic species obtained in the HAT and SPLET mechanisms. Consequently, Available online xxxx all radicals and anions formed with retention of O9ÁÁÁH1–O1 and O9ÁÁÁH8–O8 hydrogen bonds are more stable than those where the hydrogen bonds are disturbed. The exceptions are the radical and anion of Keywords: AE, the unpaired electron or negative charge is poorly delocalized. Anthraquinones The high IP values for all investigated compounds undoubtedly discredit the SET-PT mechanism. Since Substituents structure DFT the PA values are notably lower than the BDE values, one can conclude that SPLET is more favorable reaction Radical scavenging pathway than the HAT mechanism in both solvents. In the case of CH and AE 1-OH and 8-OH are the most Antioxidative capacity reactive sites for radical inactivation, while 3-OH is the most reactive site for EM and THA. The examined Reaction mechanism compounds are moderate antioxidants. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction effect is primarily reflected in the possibility for the formation of stable free radicals from the neutral molecule. Among molecules that show antiradical activity there are many The real antioxidant activity of a molecule can be successfully compounds with anthraquinone core in their structure. Some of estimated by analyzing the potential energy surface (PES) of the them have natural genesis and can be detected and isolated from reaction with a certain radical. Namely, the PES allows calculation plant extracts, and some of them are obtained as synthetic prod- of the activation and reaction energies, implying that both kinetical ucts. The curative properties of the plant, which have been used and thermodynamical approaches to the examined reaction are in traditional medicine, stem from anthraquinones which are con- provided [7]. On the other hand the computed parameters give tained in them [1]. Moreover some anthraquinones are also syn- useful informations on the antioxidant power without considering thesized, as it is know that they inhibit the progression of some reaction pathway. Theoretical investigations of antiradical potency diseases [2–5]. Due to the large number of anthraquinone mole- of some molecules are mostly based on analyzing the thermody- cules, many of them are not examined. Although all of them have namical or/and kinetical aspects of the reaction of a certain common structural feature they do not show the same scavenging molecule and certain radical specie [8–13]. Thermodynamical sta- capacity. However, the structural characteristics of the sub- bility of the newly formed free radical, which has an anthraqui- stituents on the periphery of the molecule have a significant none as a precursor, is one of the main preconditions for impact on the scavenging activity of individual molecule [6]. This anthraquinone to act as scavenger. There are at last three well described mechanisms through which stable anthraquinone radical can be formed [14–18]. The ⇑ first one means that radical can be obtained directly, via homolytic Corresponding author. Tel.: +381 20 317 754; fax: +381 20 337 669. dissociation of hydroxyl O–H bond. This mechanism is named HAT E-mail addresses: [email protected] (Z. Markovic´), [email protected] (S. Jeremic´), [email protected] (J. Dimitric´ Markovic´), marijanas14@gmail. (‘‘Hydrogen Atom Transfer”), and bond dissociation reaction can be com (M. Stanojevic´ Pirkovic´), [email protected] (D. Amic´). described by Eq. (1). Thermodynamical propitiatory of this reaction http://dx.doi.org/10.1016/j.comptc.2015.10.004 2210-271X/Ó 2015 Elsevier B.V. All rights reserved. Please cite this article in press as: Z. Markovic´ et al., Influence of structural characteristics of substituents on the antioxidant activity of some anthraqui- none derivatives, Comput. Theoret. Chem. (2015), http://dx.doi.org/10.1016/j.comptc.2015.10.004 2 Z. Markovic´ et al. / Computational and Theoretical Chemistry xxx (2015) xxx–xxx in appropriate environmental conditions can be established by cal- quinone) are registered as natural antioxidants with numerous culating bond dissociation enthalpy (BDE) value. Lower BDE values pharmacological and biological activities [3,22–26]. There are no indicate easier bond dissociation, and more stable anthraquinone data about activity and sources of THA, but its structure is suitable radical specie. The other two mechanisms describe heterolytic for comparison with the structures of other three molecules. bond cleavage and forming radical through ionic intermedier spe- cies. Both mechanisms are two-step reactions. In the first step of 2. Methodology section the mechanism known as SET-PT (‘‘Single-Electron Transfer fol- lowed by Proton Transfer”) electron leaves neutral anthraquinone The equilibrium geometries of all investigated anthraquinones molecule forming radical-cation, which loses its proton in the sec- and corresponding radicals, anions, and radical cations were ond step of the reaction giving radical specie (Eq. (2)). Whether the optimized using the Gaussian 09 program package [27] at the reaction is possible or not, in the present environmental condi- M05-2X/6-311++G(d,p) level of theory. The M05-2X functional, tions, it is governed by the values of IP (ionization potential) and developed by the Truhlar group [28], has been successfully used PDE (proton dissociation enthalpy). Finally, ‘‘Sequential Proton in numerous calculations involved in investigations of thermody- Loss Electron Transfer” (SPLET) is mechanism in which in the first namical and kinetical properties of the reactions of different radical step the anthraquinone-anion is formed upon losing the proton scavenger compounds [29,30]. from neutral moiety. Further, this anion loses an electron giving To investigate scavenger potency of the selected molecules in rise to a radical form (Eq. (3)). The possibility of radical formation real environment, all calculations were performed in water and via this mechanism can be established on the basis of the PA (pro- pentyl ethanoate. Water was selected as a widespread solvent. ton affinity) and ETE (electron transfer energy) values. Calculation The role of pentyl ethanoate is to imitate lipids as environment of the energy requirements for each mechanism [15–19] may indi- in cell membranes, and other relevant molecular surroundings in cate the radical scavenging mechanism that is thermodynamically living organisms. In order to include the influence of solvents, preferred and point out the active sites for radical inactivation of the SMD model was used [31]. All geometries of molecules, radi- four selected anthraquinone molecules: chrysophanol (CH), aloe- cals, anions and radical cations were optimized, and frequency cal- emodin (AE), emodin (EM) and 1,3,8-trihydroxyanthraquinone culations were performed in the presence of appropriate solvent. (THA). The optimized geometries of the investigated species were Å Å obtained by using the restricted (close shell) and unrestricted A-OH ! A-O þ H ð1Þ (open shell) calculations. All minima were verified by the absence þ ÀeÀ Åþ ÀH Å A-OH ! A-OH ! A-O ð2Þ of imaginary frequencies. The NBO analysis [32,33] of all anthra- þ À quinones and the corresponding radicals and anions was per- ÀH À Àe Å A-OH ! A-O ! A-O ð3Þ formed using the M05-2X density matrix. Thermodynamical parameters that describe here investigated This gives information not only about which of the investigated mechanisms are calculated using the following equations [16–19]: molecules has better scavenging potency, but how structure of the Å Å formed radical influence the easiness of radical generation, and BDE ¼ HðA-O ÞþHðH ÞHðA-OHÞð4Þ how the structure of the substituents influence the radical stabil- þÅ À IP ¼ HðA-OH ÞþHðe ÞHðA-OHÞð5Þ ity. The structures of four selected molecules are similar: all of ¼ ð ÅÞþ ð þÞ ð þÅÞðÞ them have anthraquinone core with hydroxyl groups in positions PDE H A-O H H H A-OH 6 À þ 1 and 8 (Fig. 1) and different substituents in positions 3 (S1) and PA ¼ HðA-O ÞþHðH ÞHðA-OHÞð7Þ Å À 6 (S2). ETE ¼ HðA-O ÞþHðeÀÞHðA-O Þð8Þ CH, EM and AE are the components of extracts of some herbs from the family of Polygonaceae. They are used in traditional In order to calculate the thermodynamical quantities in Eqs. (4)–(8), Chinese medicine as laxative agents or liver cleaners and protec- it is necessary to calculate the enthalpies of the radicals, anions, and tors [20]. CH (1,8-dihydroxy-3-methylanthraquinone) is known radical cations of anthraquinones, as well as of the hydrogen atom, for its antimicrobial activity [21]. EM (1,3,8-trihydroxy-6-methy- proton, and electron. The solvation enthalpies of the proton, elec- lanthraquinone) and AE (1,8-dihydroxy-3-hydroxymethylanthra- tron and hydrogen atom in appropriate solvents have been earlier calculated [34] using M05-2X level of theory.
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