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

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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.


This article was originally published in The Journal of Physical Chemistry B. The full-text article from the publisher can be found here.


Journal of Physical Chemistry B


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