Structural and Thermochemical Investigations of Multiply Charged Transition Metal Ion Complexes with Water and with Amino Acids

Abstract

Guided ion beam tandem mass spectrometry is used to obtain the kinetic energy dependent cross sections for collision-induced dissociation of hydrated transition metal complexes of Ni2+(H2O) x, x = 4 - 11, Co2+(H2O) x, x = 5 - 11, and CoOH+(H2O) x, x = 1 - 4. The resulting cross sections obtained for each reactant ion are analyzed using statistical models to yield 0 K bond dissociation energies (BDEs). The primary dissociation pathway for these three systems is the loss of a single water followed by sequential water loss at higher energies. The charge separation process, M2+(H 2O)x → MOH+(H2 O)m + H+(H 2O)x-m-1, is also observed for particular sized complexes in the M2+(H2O)x studies. Threshold analyses determine the charge separation pathway to be energetically favored over water loss at x = 4 for Ni2+ and x = 6 for Co2+. For collision induced dissociation of CoOH +(H2O)x complexes, the loss of OH becomes a competitive dissociative pathway to the loss of water at x = 1. Combining the experimental HO-Co+(H2O) and water loss BDEs from CoOH+(H2O)x with those for Co+(H 2O)y from literature, BDEs for the loss of OH from CoOH+(H2O)x are also derived. Theoretical geometry optimizations and single point energy calculations are performed on reactant and product complexes using several levels of theory to obtain thermochemistry for comparison to experiment. Structural characterization of gas-phase ions of cysteine (Cys), cysteine methyl ester (CysOMe) and serine (Ser) complexed to zinc and cadmium dications is investigated by infrared multiple photon dissociation (IRMPD) action spectroscopy using a free electron laser in combination with theoretical calculations. IRMPD spectra are measured for [Zn(CysH)]+, [Cd(CysH)] +,[Zn(CysOMeH)]+, [Cd(CysOMeH)]+, CdCl +(CysOMe), [Zn(SerH)CH3CN]+and CdCl+(Ser) complexes. The measured action spectra are compared to linear absorption spectra calculated at the B3LYP/6-311+G(d,p) level (Zn2+ complexes) and B3LYP/def2-TZVP levels (Cd2+ complexes) to identify the structures of the experimentally observed species. On the basis of these experiments and calculations, all complexes examined here adopt a tridentate binding coordination of the amino acid to the metal.