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Complexes of glutamine (Gln) cationizedwith Zn2+andCd2+were examined by infrared multiple photon dissociation(IRMPD) action spectroscopy using light generated from a free-electronlaser. Electrospray ionization yielded complexes of deprotonated Glnwith Zn2+, [Zn(Gln–H)]+, and intact Glnwith CdCl+, CdCl+(Gln). For each complex, thespectra obtained were compared with those for low-energy conformersfound using quantum chemical calculations to identify the structurespresent experimentally. Calculations were performed at the B3LYP/6-311+G(d,p)level for [Zn(Gln–H)]+and at the B3LYP/def2-TZVPlevel with an SDD effective core potential on cadmium for CdCl+(Gln). The main binding motif observed for the Cd2+complex was a charge-solvated, tridentate [N,CO,COsc]structure in which the metal binds to the backbone amino group andthe carbonyl oxygens of the carboxylic acid and side-chain amide groups.The Zn2+system similarly preferred a [N,CO–,COsc] binding motif, where binding was observed at thecarboxylate site along with the backbone amino and side-chain carbonylgroups. In both cases, the theoretically determined lowest-energyconformers explain the experimental [Zn(Gln–H)]+and CdCl+(Gln) spectra well. [ABSTRACT FROM AUTHOR]