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

Abstract

The DFT study of electron donor reactive sites of l-ascorbate, the main reactive form of vitamin C for antioxidant reactions in water medium and in physiological conditions, is presented. The local reactivity descriptors, average local ionization energy Ī(r) and electron donor Fukui function f−(r), for ascorbate conformers were calculated using B3LYP/6-311++G** level of theory in the gas-phase. The obtained results show that Ī(r) and f−(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 γ-lactone ring plane of ascorbate. The Ī(r) and f−(r) obtained for ascorbate conformers suggest 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 the minimum point of average local ionization energy on electron density isosurface ĪS,min calculated for ascorbate conformers suggest that the side chain conformations and intramolecular H-bonding could play a role in modulation of antioxidant reactivity of ascorbate. The results presented in this paper show that the electron donor reactive site is different from the proton donor reactive site for ascorbate. The leaving H(9) proton of hydroxyl group on ascorbate γ-lactone ring in antioxidant reaction with radical or oxidizing molecule is not associated with large f−(r) values or HOMO orbital suggesting that, at least in the 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.