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The average local ionization energy and Fukui function of L-ascorbate, the local reactivity descriptors of antioxidant reactivity

The DFT study of electron donor reactive sites of L-ascorbate, main reactive form of vitamin C for antioxidant reactions in water medium at physiological conditions, is presented. The local reactivity descriptors, average local ionization energy Ī(r) and electron donor Fukui function fˉ(r), for ascorbate conformers were calculated using B3LYP/6-311++G** level of theory in gas-phase. The obtained results show that Ī(r) and fˉ(r) identify nearly the same reactive site toward electrophiles on ascorbate molecule. The most preferred reactive site for donating electron in reaction with radical or oxidizing molecule is associated mostly with the less electronegative C(3) atom below the gama-lactone ring plane of ascorbate. The Ī(r) and fˉ(r) obtained for ascorbate conformers suggests that the side chain conformations or intramolecular H-bonding have no significant contributions to the reactive site position in ascorbate molecule. However, the values of minimum point of average local ionization energy on electron density isosurface ĪS, min calculated for ascorbate conformers suggests that the side chain conformations and intramolecular H-bonding could have a role in modulation of antioxidant reactivity of ascorbate. The results presented in this paper show that electron donor reactive site is different from the proton donor reactive site for ascorbate. The leaving H(9) proton of hydroxyl group on ascorbate gama-lactone ring in antioxidant reaction with radical or oxidizing molecule is not associated with large fˉ(r) values or HOMO orbital suggesting that, at least in early stage of reaction, the proton and electron could be transferred from ascorbate through different orbital set what is of importance in studies of antioxidant reaction mechanisms for ascorbate.

Antioxidant; Ascorbate; Vitamin C; Fukui function; Average local ionization energy.

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