Interconversion between 4-Imidazolone Ions; Isomers of [b₄]⁺ Derived from Protonated Tetraglycine

Collision-induced dissociations of isotopically labeled protonated tetraglycines establish that the [b₄]⁺ ion formed by loss of water from the second amide bond (structure II) rearranges to form N₁-protonated 3,5-dihydro-4H-imidazol-4-one (structure I), the product of water loss from the first amide bond. Structure II is slightly higher in energy than I (ΔH at 0 K is 5.1 kJ mol^-1, as calculated at M06-2X/6-311++G-(d,p)), and the barrier to interconversion is 139.8 kJ mol^-1 above I. The dominant dissociation pathway is the loss of methanimine (HN=CH₂) from ion I with a barrier of 167.1 kJ mol^-1, giving [GlyGlyGlyGly + H - H₂O - HN=CH₂]⁺, ion III; a minor channel, loss of NH₃, has a slightly higher barrier (181.5 kJ mol^-1). Using labeled glycine (¹³C_α) it was determined that loss of the imine is from the same residue as that from which water was initially lost. The collision-induced dissociation spectra of ion III derived from both I and II were identical, and their energy-resolved curves were also very similar. Ion III fragments by losses of a glycine molecule (the dominant channel), a water molecule, and a glycine residue (57 Da), giving ions IV, V, and VII, respectively. Isotopic labeling established the origins of each of the neutral molecules that are lost. Using glycine (2,2 D₂), rapid deuterium exchange was observed for both ions I and II for the α-hydrogens that are from the same residue as that from which the water had been eliminated.