Experimental and Theoretical Investigations of Infrared Multiple Photon Dissociation Spectra of Asparagine Complexes with Zn 2+ and Cd 2+ and Their Deamidation Processes.

Complexes of asparagine (Asn) 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 Asn with Zn ²⁺ , [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 _s ] structure where the metal binds to the backbone amino group and carbonyl oxygens of the carboxylic acid and side-chain amide groups. The Zn ²⁺ system deprotonates at the backbone carboxylic acid and prefers a [N, CO ^- , CO _s ] 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 ₂ , followed by the sequential loss of NH ₃ ) and CdCl ⁺ (Asn) (concomitant loss of NH ₃ + CO).