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

Complexes of threonine (Thr) cationized with Zn and Cd 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 CO-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] 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 Zn complex), and side-chain hydroxyl oxygen (OH). For the Zn system, there also appears to be a population of a higher-energy species in which the side chain is deprotonated, either [N, O, CO] or [N, O, OH]. CO-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.