Effects Of Cu ^{+ / 2 +} On IC Base Pair Proton Transfer

In this paper, the effects of Cu²⁺ cation on proton-transfer processes in inosine-cytosine base pair have been studied using the method of B3LYP/6-311++G**and Gaussian 09 application program. After Cu⁺/2+ coordinated to various positions of inosine-cytosine base pair, three stable adducts were obtained: Cu⁺/2+IN4-C, Cu⁺/2+IN11-C and Cu⁺/2+I-CO12. In gas phase, the structure of transition states and products in different proton-transfer processes were optimized, and the corresponding structure in water also been optimized using the same method, then analyze the proton-transfer processes of structure Cu⁺/2+I-C in water.In gas phase, the single proton-transfer which H+ transferred along the path 1occurred when Cu⁺ coordinated to the N4 or N11 of inosine, and the concerted double proton-transfer occurred when Cu⁺ coordinated to O12 site of cytosine and the single proton-transfer of structure Cu⁺IN11-C is more favorable. In water, the energy gap of the structure in proton-transfer processes is decreased, and energy barriers are increased, all of this are not advantageous to the proton-transfer.In gas phase, the stepwise double proton-transfer occurred after Cu²⁺ coordinated to N4 or N11 site of inosine with the same path way: first, H9 transferred from the N7 site of inosine to the N10 site of cytosine along with the hydrogen bond N8-H9…N10; then H8 transferred from N7 site of cytosine to O7 site of inosine along with the hydrogen bond N7-H8 … O7. The proton-transfer process of structure Cu²⁺IN4-C is similar to that of structure Cu²⁺IN11-C, and the first step single proton-transfer is very easy to happen, but it is not good for the second step single proton-transfer. Different single proton-transfer occurred after Cu²⁺ coordinated to O12 site of cytosine and the pathway 1 is more favorable. In water, there is no proton-transfer for structure Cu²⁺IN11-C, and the energy barrier in proton-transfer process of structure Cu²⁺IN4-C is decreased, but the second step single proton-transfer is still not easy to happen. For structure Cu²⁺I-CO12, while the proton-transfer easily occur along the pathway 1, the proton-transfer along pathway 2cannot happen.