Considering the configuration characteristic, this paper takes K^＋ and one g-tetrad (g-tetrad-K^＋) as studied object, together with K^＋ and two g-tetrads (1,2-g-tetrads-K^＋ and 2,3-g-tetrads-K^＋) in guanine-quadruplex structure (G4) as studied object. We have designed several moving routes for K^＋ in g-tetrad-K^＋, 1,2-g-tetrads-K^＋ and 2,3-g-tetrads-K^＋ three systems to explore the interaction between g-tetrad and K^＋. First, ab initio [MP2/6-31G(d,p)] and ABEEMσπ/MM (Atom Bond Electronegativity Equalization Method at σπ level fused into Molecular Mechanics) methods have been used to investigate the changes of structure and energy, when K^＋ moves in g-tetrad-K^＋. The results indicate that ABEEMσπ/MM can accurately describe the properties of the g-tetrad-K^＋ system structure and energy. To approaching the practical system, ABEEMσπ/MM has been used to explore the bigger system K^＋ and two g-tetrads, which is too big to calculate the interaction energy using MP2. The changes of the interaction energy from ABEEMσπ/MM, when K^＋ moves in g-tetrad-K^＋, 1,2-g-tetrads-K^＋ and 2,3-g-tetrads-K^＋ systems indicate that if absorb enough energy, it is easiest for the K^＋ in G4 moves out of the cavity along the α direction.

Key words: guanine-tetrad, potassium ion, ab initio, ABEEMσπ/MM