Structures of [GPGG + H - H2O]+ and [GPGG + H - H2O - NH=CH2]+ ions; evidence of rearrangement prior to dissociation

Infrared multiple photon dissociation (IRMPD) spectroscopy shows the [GPGG + H - H2O] + ion to have an imidazolone structure. Collision-induced dissociation of this [b(4)](+) ion results in the loss of HN=CH2 from the first residue; the IRMPD spectrum of this MS3 product ion is very similar to that of the [b(4)] + ion itself, strongly indicating that the [b(4) - HN=CH2](+) ion also has an imidazolone structure. Losses of CO and glycine are the dominant dissociation pathways for the [b(4) - HN=CH2] + ion. The latter loss requires tautomerism of the keto-form of the imidazolone ring to become the lower-energy enol-form, prior to dissociation. Isotopic labelling showed that loss of CO occurs from the ring of the keto-form. Density functional theory calculations were performed at both the B3LYP/6-311++G (d, p) and M06-2X/6 -311++G (d, p) levels and the results are in good agreement. (C) 2019 Elsevier B.V. All rights reserved.