Rearrangement of Proline Complexes with Zn2+: An Infrared Multiple Photon Dissociation and Theoretical Investigation

Complexes of proline (Pro) cationized with Zn²⁺ and Cd²⁺ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free electron laser. Complexes of intact Pro with CdCl⁺, CdCl⁺(Pro), a complex of (Zn+Pro-H)⁺ where a proton has been lost, as well as Zn⁺(Pro-H)(Pro) were formed by electrospray ionization. In order to identify the structures formed experimentally, the IRMPD spectra were compared to those calculated from optimized structures at the B3LYP/6-311+G(d,p) level for zinc complexes and B3LYP/def2-TZVP level with an effective core potential on cadmium for the CdCl⁺(Pro) system. For the latter complex, the main binding motif observed has a zwitterionic proline ligand structure, [CO₂^–]cc, where the metal binds to the two carboxylate oxygens. In contrast, for Zn⁺(Pro-H)(Pro), both ligands interact with zinc via a [N,CO^–][N,CO] binding motif, where binding is observed at the carbonyl oxygens and nitrogens for both ligands, consistent with previous work. In both cases, contributions from different puckers of the proline ring may contribute. For (Zn+Pro-H)⁺, we identify that the structure is actually ZnH⁺(Pro-2H), in which the proline has been dehydrogenated and one of the hydrogens has migrated to form a covalent bond with Zn, which verifies a previous report relying on a single OH stretch band.