Quenching Reactions of Triplet State Riboflavin by Vitamin C: A Theoretical Investigation

Abstract

Sir, Vitamin C (ascorbic acid), the water-soluble, smallmolecule antioxidant, is of great biological significance [1, 2]. Previous studies [3, 4] demonstrated that vitamin C can inhibit the photo-oxidative damage caused by riboflavin which can induce Type-I and II photosensitized oxidation mechanisms [5–7]. Considering the fact that many antioxidants can inhibit the riboflavin’s photosensitizing damage through deactivating its triplet state [7–9], it is of significance to investigate the reactions of vitamin C with triplet state riboflavin, which may contribute to the protective effect of vitamin C. In the present study, the quenching mechanisms of triplet state riboflavin by vitamin C were explored by means of density functional theory (DFT) calculations, which have been widely used in investigating both the antioxidant and photosensitization mechanisms of various bioactive compounds [7, 11–14]. The calculation methods are as follows. Firstly, molecular structures of vitamin C (monoanion form in neutral solution) and riboflavin were fully optimized by hybrid functional B3LYP with 6-31G(d,p) Gaussian basis set in vacuo. Then, the lowest triplet excitation energy (ET1) of the two molecules was estimated by time-dependent (TD)DFT with the same basis set in solvent (water). Moreover, as the diffuse functions are essential for treatment of anion and cation radicals, the vertical electron affinities (VEA) and vertical ionization potentials (VIP) of vitamin C and riboflavin were estimated using a combined DFT method labeled as B3LYP/6-31+G(d,p)//B3LYP/6-31G(d,p), which means that B3LYP/6-31+G(d,p) was used to perform a single-point calculation on the basis of B3LYP/631G(d,p)-optimized structures. The O-H bond dissociation enthalpy (BDE) of vitamin C and H-atom affinity (HAA) of riboflavin were calculated by (RO)B3LYP/6311+G(2d,2p)//AM1 method, which takes advantages of accuracy and economy [11]. During the single point calculations, the solvent (water) effect was taken into consideration by employing the self-consistent reaction field (SCRF) method with the polarizable continuum model (PCM) of Tomasi and coworkers [15–17]. All the calculations were accomplished with the Gaussian 03 package of programs [18]. During the photosensitization, riboflavin is initially excited to the singlet state upon irradiation and then may intersystem cross to the relatively long-lived triplet state. Triplet state riboflavin can react with molecular oxygen to photogenerate various reactive oxygen species [5–7] and at the same time, it can be deactivated by vitamin C, theoretically through the following possible pathways. Firstly, triplet state riboflavin (RF) may react with vitamin C (VC) through direct energy transfer (Eq. 1).