Effects of OH radical addition on proton transfer in the guanine-cytosine base pair

摘要

Double proton transfer (PT) reactions in guanine-cytosine OH radical adducts are studied by the hybrid density functional B3LYP approach. Concerted and stepwise proton-transfer processes are explored between N1(H) on guanine (G) and N3 on cytosine (C), and between N4(H) on C and O6 on G. All systems except GC(6OH) display a concerted mechanism. (8OH)GC has the highest dissociation energy and is 1.2 kcal/mol more stable than the nonradical GC base pair. The origin of the interactions are investigated through the estimation of intrinsic acid-basic properties of the (OH)-O-center dot-X monomer (X = G or C). Solvent effects play a significant role in reducing the dissociation energy. The reactions including (OH)-O-center dot-C adducts have significantly lower PT barriers than both the nonradical GC pair and the (OH)-O-center dot-G adducts. All reactions are endothermic, with the GC(6OH) -> GC(6OH)PT reaction has the lowest reaction energy (4.6 kcal/mol). In accordance with earlier results, the estimated NBO charges show that the G moiety carries a slight negative charge (and C a corresponding positive one) in each adduct. The formation of a partial ion pair may be a potential factor leading to the PT reactions being thermodynamically unfavored.