Structure of the [M + H - H2O]+ ion from tetraglycine: a revisit by means of density functional theory and isotope labeling

Collision-induced dissociations of protonated 18O-labeled tetraglycines labeled separately at either the first or the second amide bond established that water loss from the backbone occurs from the N-terminal residue. Density functional theory at B3LYP/6-311++G(d,p) predicted that the low-energy [G4 + H - H2O]+ product ion is an N1-protonated 3,5-dihydro-4H-imidazol-4-one. The ion at the lowest energy, III, is 24.8 kcal mol-1 lower than the protonated oxazole structure, II, proposed by Bythell et al.( J. Phys. Chem A2010, 114, 5076-5082). In addition, structure III has a predicted IR spectrum that provides a better match with the published experimental IRMPD spectrum than that of structure II.