Experimental and Theoretical Investigations of Infrared Multiple Photon Dissociation Spectra of Aspartic Acid Complexes with Zn²⁺ and Cd²⁺

Complexes of aspartic acid (Asp) cationized with Zn²⁺: Zn(Asp-H)⁺, Zn(Asp-H)⁺(ACN) where ACN = acetonitrile, and Zn(Asp-H)⁺(Asp); as well as with Cd²⁺, CdCl⁺(Asp), were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free electron laser. A series of low-energy conformers for each complex was found using quantum chemical calculations to identify the structures formed experimentally. The main binding motif observed for the heavy-metal complex, CdCl⁺(Asp)[N,CO,CO_s], is a charge-solvated, tridentate structure, where the metal center binds to the backbone amino group and carbonyl oxygens of the backbone and side-chain carboxylic acids. Likewise, the deprotonated Zn(Asp-H)⁺(ACN) and Zn(Asp-H)⁺(Asp) complexes show comparable [N,CO^–,CO_s](ACN) and [N,CO^–,CO_s][N,CO,CO_s] coordinations, respectively. Interestingly, there was only minor spectral evidence for the analogous Zn(Asp-H)⁺[N,CO^–,CO_s] binding motif, even though this species is predicted to be the lowest-energy conformer. Instead, rearrangement and partial dissociation of the amino acid are observed, as spectral features most consistent with the experimental spectrum are exhibited by a four-coordinate Zn(Asp-NH₄)⁺[CO₂^–,CO_s](NH₃) complex. Analysis of the mechanistic pathway leading from the predicted lowest-energy conformer to the isobaric deaminated complex is explored theoretically. Further, comparison of the current work to that of Zn²⁺ and Cd²⁺ complexes of asparagine (Asn) allows additional conclusions regarding populated conformers and effects of carboxamide versus carboxylic acid binding to be drawn.