Hydrogen bonding influences collision-induced dissociation of Na⁺ -bound guanine tetrads

Na⁺ -bound guanine (G)-tetrads possess square planar structures formed solely by noncovalent interactions including multiple hydrogen bonds. Unlike G-tetrads facilitated by other alkali metal ions, an intriguing behavior in collision-induced dissociation (CID) has been observed in Na⁺ -bound G-tetrads, which features a preferential, simultaneous loss of two G ligands in the low energy regime. To understand this unique behavior, we investigated the CID of Na⁺ -bound G-tetrads with mixed ligands of G and 9-methylguanine (9mG), [Na·G_m ·9mG_n ]⁺ (m + n = 4), and [Li·9mG₄ ]⁺ for comparison. In the CID experiments, the simultaneous losses of two ligands were by far more pronounced than the loss of a single ligand for all five Na⁺ -bound G-tetrads. However, it appeared that the CID of [Li·9mG₄ ]⁺ prefers to lose single ligands sequentially. An analysis of the fragment abundances suggested that the generation of Na⁺ -bound dimeric fragments might have occurred with two adjacent ligands. This theoretical study predicted for [Li·9mG₄ ]⁺ that the loss of a single ligand is more energetically favorable than the production of neutral hydrogen-bonded fragments by 35.5 kJ/mol (ΔG). This contradicts our previous calculations for [Na·9mG₄ ]⁺ that a neutral loss of hydrogen-bonded dimers provides the lowest energy product state of Na⁺ -bound dimeric fragments, which is lower than that of Na⁺ -bound trimeric fragments by 15.6 kJ/mol. From the results, this comparative study suggests that the pronounced generation of Na⁺ -bound dimeric fragments in CID of the G-tetrads is likely promoted by the dissociation pathway associated with neutral loss of hydrogen-bonded dimers. It thus demonstrates that multiple hydrogen bonding participating in formation of Na⁺ -bound G-tetrads may also strongly influence the fate of dissociating complexes of G-tetrads.