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

Abstract Infrared multiple photon dissociation (IRMPD) spectroscopy shows the [GPGG + H – H 2 O] + ion to have an imidazolone structure. Collision-induced dissociation of this [b 4 ] + ion results in the loss of HN CH 2 from the first residue; the IRMPD spectrum of this MS 3 product ion is very similar to that of the [b 4 ] + ion itself, strongly indicating that the [b 4 – HN CH 2 ] + ion also has an imidazolone structure. Losses of CO and glycine are the dominant dissociation pathways for the [b 4 – HN CH 2 ] + 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.