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 (Delta H 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 (C-13(alpha)) 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 alpha-hydrogens that are from the same residue as that from which the water had been eliminated.