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

Collision-induced dissociations of isotopically labeled protonated tetraglycines establish that the [b ₄ ] ⁺ ion formed by loss of water from the second amide bond (structure II) rearranges to form N ₁ -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=CH ₂ ) from ion I with a barrier of 167.1 kJ mol ^-1 , giving [GlyGlyGlyGly + H - H ₂ O - HN=CH ₂ ] ⁺ , ion III; a minor channel, loss of NH ₃ , has a slightly higher barrier (181.5 kJ mol ^-1 ). Using labeled glycine ( ¹³ C _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 D ₂ ), 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.