Molecular recognition in the gas phase ligand switching reactions of the proton bound dimer of sarcosine and glycylglycine

Ion-molecule reactions of the proton bound dimer of [Sar+H+GlyGly]+ (where Sar = sarcosine and GlyGly = glycylglycine) proceed via two main reaction channels, i.e. association and ligand switching. The association reaction, which involves formation of an adduct between the protonated dimer and neutral base, occurs more readily for the oxygen containing bases and those with a lower gas phase basicity. Molecular recognition was demonstrated for the ligand switching reactions in which nitrogen containing bases preferred to switch out sarcosine while the oxygen containing bases preferred to switch out glycylglycine. Molecular dynamics followed by semiempirical PM3 calculations for the ligand switching reactions of [Sar+H+GlyGly]+ with methylamine directly correlated with the experimental findings by predicting that the most stable product ion arises from switching out sarcosine. These calculations reveal that the most stable adduct structure and the most stable ligand switched structure arise from proton transfer to methylamine to yield ions of the type [(Sar)(GlyGly)(LigH+)] and [(GlyGly)(LigH+)].