Repair Of Oxidizing Hydroxyl Adduct Radicals Of DNA Bases By Hydroxyl-trans-Stilbenes

Repair of oxidizing hydroxyl adduct radicals of DNA base(base-OH-adduct)by eleven hydroxyl-trans-stilbenes via single electron transfer(SET)under physiological conditions(pH=7.4)are investigated using density functional theory.Firstly,the pKa and the molar fraction(f)of anionic form(PSO-)and neutral form(PSOH)of hydroxyl-trans-stilbenes under physiological conditions(pH=7.4)are calculated.The results show that the deprotonated from the 4'-OH group in ring B of hydroxyl-trans-stilbenes is the most favorable among the OHs in ring A and ring B.The values of the calculated pKa of hydroxyl-trans-stilbene are in the range of 7.62 to 8.99.3',4',5,5'-Tetrahydroxy-trans-stibene has the smallest pKa among eleven hydroxyl-trans-stilbenes.Under physiological conditions(pH=7.4),the molar fraction of PSOH is in the range of 0.624 to 0.975 and the molar fraction of PSO-is small and in the range of 0.025 to 0.376.Secondary,repair oxidizing base-OH-adduct radicals by anionic and neutral forms of hydroxyl-trans-stilbenes have been investigated.The Gibbs free energy barriers for the repair reactions of PSO-anions with oxidizing base-OH-adduct radicals are in the range of 1.15 to 12.83 kcal mol-1 and the rate constants of repairing reactions are in the range of 6.09×101-2.82×109 dm3 mol3 L-1 if the molar fractions of PSO-anions are considered,the vertical ionization potential vaules of PSO-anions are in the range of 4.568 to 4.688 eV.3,3',4,4',5,5'-Hexahydroxy-trans-stilbene anion(PSO-d)performs the best repair activity to oxidizing base-OH-adduct radicals and the Gibbs free energy barriers and the rate constants for the repair reactions of PSO-d with oxidizing base-OH-adduct radicals are in the range of 1.37 to 9.68 kcal mol-1 and in the range of 1.13×105?1.75×109 dm3 mol3 L-1,respectively.The vertical electron affinities of oxidizing base-OH-adduct radicals are in the range of 3.603 to 4.249 eV.Among all oxidizing base-OH-adduct radicals,oxidizing cytosine-OH-adduct has the strongest accepting electron capacity and is the easiest to be repaired,however,oxidizing guanine-OH-adduct is the weakest accepting electron capacity and is the hardest to be repaired.The vertical electron affinity of PSOH is in the range of 5.532 to 5.732 eV.The Gibbs free energy barriers for repair reactions of PSOH with oxidizing base-OH-adduct radicals are larger than 18.34 kcal mol-1,and the rate constants for these repair reactions are lower than 1 dm3·mol-1·s-1.Therefore,PSOH can't repair oxidizing base-OH-adduct radicals via SET mechanism.The main factors affecting the repair reactions are the vertical ionization potential,the pKa of PSOHs and the vertical electron affinity of oxidizing base-OH-adduct radicals.This study gives information on repairing oxidizing base-OH-adduct radicals by hydroxyl-trans-stilbene in biological environment for the first time and will help us better understand the repair activity of hydroxyl-trans-stilbene toward base-OH-adduct radicals.