Structural characterization of gas-phase ions of cysteine (Cys) and cysteine methyl ester (CysOMe) complexed to zinc and cadmium is investigated by infrared multiple photon dissociation (IRMPD) action spectroscopy using a free electron laser in combination with density functional theory calculations. (with empirical dispersion corrections), B3P86, and MP2(full) levels.

Complexes of glutamine (Gln) cationized with Zn(2+) and Cd(2+) were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free-electron laser. Electrospray ionization yielded complexes of deprotonated Gln with Zn(2+), [Zn(Gln-H)](+), and intact Gln with CdCl(+), CdCl(+)(Gln). For each complex, the spectra obtained were compared with those for low-energy conformers found using quantum chemical calculations to identify the structures present experimentally. Calculations were performed at the B3LYP/6-311+G(d,p) level for [Zn(Gln-H)](+) and at the B3LYP/def2-TZVP level with an SDD effective core potential on cadmium for CdCl(+)(Gln). The main binding motif observed for the Cd(2+) complex was a charge-solvated, tridentate [N,CO,COsc] structure in which the metal binds to the backbone amino group and the carbonyl oxygens of the carboxylic acid and side-chain amide groups. The Zn(2+) system similarly preferred a [N,CO(-),COsc] binding motif, where binding was observed at the carboxylate site along with the backbone amino and side-chain carbonyl groups. In both cases, the theoretically determined lowest-energy conformers explain the experimental [Zn(Gln-H)](+) and CdCl(+)(Gln) spectra well.

Complexes of asparagine (Asn) cationized with Zn and Cd were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free electron laser. Electrospray ionization yielded complexes of deprotonated Asn with Zn, [Zn(Asn-H)], and intact Asn with CdCl, CdCl(Asn). Series of low energy conformers for each complex were found using quantum chemical calculations in order to identify the structures formed experimentally. The experimentally obtained spectra were compared to those calculated from optimized structures at the B3LYP/6-311+G(d,p) level for [Zn(Asn-H)] and the B3LYP/def2-TZVP level with an SDD effective core potential on cadmium for the CdCl(Asn) system. The main binding motif observed for the CdCl complex is a charge solvated, tridentate [N, CO, CO] structure where the metal binds to the backbone amino group and carbonyl oxygens of the carboxylic acid and side-chain amide groups. The Zn system deprotonates at the backbone carboxylic acid and prefers a [N, CO, CO] binding motif, where binding was observed at the carboxylate site along with the backbone amino group and side-chain carbonyl groups. In both cases, the theoretically determined lowest-energy conformers explain the experimental [Zn(Asn-H)] and CdCl(Asn) spectra well. Additionally, complete mechanistic pathways were found for each of the major dissociation reactions of [Zn(Asn-H)] (primary loss of CO, followed by the sequential loss of NH) and CdCl(Asn) (concomitant loss of NH + CO).