Theoretical explanation for the DNA cleavage by GO with cation: anti-cooperativity effect among the π⋯π, cation⋯π/σ and H-bonding interactions in cytosine⋯GO⋯Mⁿ⁺ (Mⁿ⁺ = Na⁺, Mg²⁺, Al³⁺)

In order to reveal the nature of DNA cleavage by inorganic intercalator GO (graphene oxide) with cation, the cooperativity effects among the π⋯π, cation⋯π/σ and H-bonding interactions were evaluated in the cytosine⋯GO⋯Mⁿ⁺ (Mⁿ⁺ = Na⁺, Mg²⁺, Al³⁺) model systems using the M06-2X, MP2 and <i>ω</i> B97X-D methods with the 6-311++G(2d,p) and 6-311++G(3df,3pd) basis sets. The Mⁿ⁺⋯O (ether) and N–H⋯O interactions induce the formation of the π⋯π stacking between cytosine and GO, and the anti-cooperativity effect are dominant in controling of the aggregation process of cytosine, GO and Mⁿ⁺, which was confirmed by the AIM (atoms-in-molecules) and RDG (reduced density gradient) analyses. Furthermore, the solvent effects of H₂O weaken greatly the anti-cooperativity effects. Thus, a deduction on the DNA cleavage by GO⋯cation with the intercalation mode is put forward: due to the anti-cooperativity effect and solvent effect, the π⋯π stacking is weakened in the complexes with Na⁺ or broken in those with Mg²⁺ or Al³⁺. Then the GO⋯Mg²⁺ moiety is squeezed out from the intercalating sites, leading to an invalid cleavage of DNA, while Na⁺ or Al³⁺ is bound tightly to cytosine, with a notable DNA cleavage. This deduction was used to explain reasonably the previous experimental phenomena.