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,COs], 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-,COs](ACN) and [N,CO-,COs][N,CO,COs] coordinations, respectively. Interestingly, there was only minor spectral evidence for the analogous Zn(Asp-H)+[N,CO-,COs] 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-,COs](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.