Investigating the antioxidant mechanism of violacein by density functional theory method
Introduction
Violacein, (3-[1,2-dihydro-5-(5-hydroxy-1H-indol-3-yl)-2-oxo-3H-pyrrol-3-ilydene]-1,3-dihydro-2H-indol-2-one), is the major pigment isolated from Chromobacterium violaceum. Structure of violacein (Fig. 1) has three heterocyclic moieties named as 5-hydroxylindole (5HI), 2-pyrrolidone (2PYR) and oxindole (OX). Violacein has attracted much attention recently for its pharmacological properties [1], [2]. Antioxidant activity of violacein has been studied by scavenging nitrogen reactive species, DPPH radical, hydroxyl radical and inhibiting lipid peroxidation in different models [3], [4]. However, there is no further report on its antioxidant mechanism. Also, the violacein has one hydroxyl group and three N–H groups, which are believed to play an important role in antioxidant activity [5], [6]. In order to give a better insight into violacein`s antioxidant activity, the antioxidant mechanism is investigated by using density functional theory method, which is useful and economic to explore antioxidant mechanism of organic molecular [7], [8], [9].
The antioxidant mechanisms of organic molecular include hydrogen atom transfer and electron coupling proton transfer. The former is controlled by bond dissociation energy (BDE) of antioxidant and the latter is controlled by ionization potential (IP). The lower the BDE or IP is, the higher the hydrogen atom or electron transfer ability will be. Thus, based on the structure optimized, the O–H and N–H BDEs and IP of violacein and its isomers are calculated.
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Computational methods
The initial geometries of violacein and its isomers are optimized by AM1 level, then a combined method labeled as ROB3LYP/6-311+G (2d, 2p)//B3LYP/6-31(d) [10] is employed in this study to give a precise BDE value. The geometry of violacein is optimized by B3LYP/6-31(d) method first, and the zero-point vibrational energy (scaled by a factor of 0.9804) is obtained by frequency calculation at the same level. Then, the single-point energy is calculated at the ROB3LYP/6-311+G (2d, 2p) level. The
Electron coupling proton transfer ability
The geometry of single ground state violacein is optimized by B3LYP/6-31(d) in gas phase. The results exhibit the OX and 2PYR rings are coplanar with the dihedral angle of C(11)–C(12)–C(15)–(16) close to 0°. While the 5HI ring is twisted out of the planar because the dihedral angle of C(8)–C(9)–C(10)–(11) is 23.6°, which is slightly higher than the 21° of previous report calculated by AM1 method [12]. The IP of Violacein is 146.88 kcal/mol, lower than that of α-tocopherol (154.90 kcal/mol) [13]
Hydrogen atom transfer ability
At the same time, the O–H and N–H bond BDEs of violacein have been calculated in gas phase to estimate their hydrogen transfer ability. As seen from Table 2, the N7–H7 BDE of violacein is 79.49 kcal/mol, lower than N14–H14, N17–H17 and O25–H25 BDE, so N7–H7 maybe take an important role in its hydrogen transfer process. Also, it is slightly lower than that of the N–H in DPPH-H (80 kcal/mol [14]), therefore, the violacein can scavenge DPPH radical as experimental testified [3]. According to our
Conclusions
Based on above analysis, we can draw following conclusions: violacein has a strong electron donating ability, but a weak hydrogen atom transfer ability. However, its coplanar isomers exhibit a strong hydrogen atom donating ability as well as its electron- donating ability. For violacein, the N7–H7 band plays an important role in its antioxidant activity. The violacein`s hydrogen atom transfer ability will be improved if it changes to isomer I or II.
Acknowledgment
This research is supported by Shanghai Supercomputer Center (Dawning 4000A).
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