Experimental and Theoretical Investigations of Infrared Multiple Photon Dissociation Spectra of Aspartic Acid Complexes with Zn 2+ and Cd 2 .

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.