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·9mG 4 ] + 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·9mG 4 ] + 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·9mG 4 ] + that the loss of a single ligand is more energetically favorable than the production of neutral hydrogen‐bonded fragments by 35.5 kJ/mol (Δ G ). This contradicts our previous calculations for [Na·9mG 4 ] + 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.