Zinc and Cadmium Complexation of _L-Threonine: An Infrared Multiple Photon Dissociation Spectroscopy and Theoretical Study

Complexes of threonine (Thr) cationized with Zn²⁺ and Cd²⁺ were examined by infrared multiple photon dissociation action spectroscopy using light generated from a free electron laser. Low-energy conformers for Zn(Thr-H)⁺(ACN) (where ACN = acetonitrile), Zn(Gly-H)⁺(ACN) (formed via CO₂-laser irradiation of intact Zn(Thr-H)⁺(ACN)), and CdCl⁺(Thr) complexes were found using quantum chemical calculations in order to identify the structures formed experimentally. For all species, the predicted ground structures reproduce the experimental spectra well, where tridentate [N, CO, OH_s] binding motifs were dominantly observed for the intact Zn(Thr-H)⁺(ACN) and CdCl⁺(Thr) complexes. In both of these cases, the metal center binds to the backbone amino group (N), carbonyl oxygen (CO, where this site is deprotonated in the and side-chain hydroxyl oxygen (OH_S). For the Zn2⁺ system, there also appears to be a population of a higher-energy species in which the side chain is deprotonated, either [N, O_s^-, CO] or [N, O_s^-, OH]. CO₂-laser irradiation of Zn(Thr H)⁺(ACN) leads to loss of its side chain via elimination of acetaldehyde, yielding a bidentate Zn(Gly H)⁺[N, CO^-](ACN) complex. Overall, this work explores the binding interactions between Thr and biologically relevant metals in a prototypical environment. Comparison of current work with previous analyses allows for the elucidation of important metal dependent trends associated with physiologically important metal amino acid binding.