Mass spectrometric and computational methods for the analysis of copper(II)-2,2'-bipyridine amino acid complexes

Dissociations of [Cu(AA-H)bpy]+· complexes begin with decarboxylation followed by beta-cleavage of the amino acid side chain. The resulting fragment ions appear at different m/z ratios for both structural isomers and isobars. This unique response for different amino acids allowed quantification of mixtures of the amino acid isomers Leucine and Isoleucine, tripeptides containing Leucine and Isoleucine, and the isobars Glutamine and Lysine. High level density functional theory calculations were performed to validate the proposed dissociation mechanisms for Leucine, Isoleucine, and both singly and doubly charged Lysine complexes. Computations were a powerful method for the analysis of these Cu(II) complexes, as they offered explanations for the relative dissociation energetics, stabilities, and reactivities for various amino acids. Density functional theory results showed that doubly charged lysine complexes underwent a unique form of dissociation, where the dissociation was driven solely by charge separation and the spin was remote from the site of bond lysis.; Copper(II) complexes were again used for the analysis for amino acid stereoisomers; however, the 2,2'-bipyridine auxiliary ligand, was replaced with a chiral derivative, called a chiragen. A three pronged approach to determining stereoisomers was chosen, so that by using mass spectrometry, nuclear magnetic resonance, and density functional theory calculations, all areas of gas phase, solution phase, and theory could be analyzed. It was determined that D and L forms of a given amino acid isomer will bind to a Cu(II)chiragen complex differently based on the structure and solvent environment.; To investigate the neutral chemistry of Cu(II) complexes, an electrospray source was built for the existing tandem quadrupole neutralization-reionization mass spectrometer. Using this source, neutral spectra of the auxiliary ligands 2,2'-bipyridine and 1,10-phenanthroline were acquired for the first time. Copper(Il)-2,2'-bipyridine complexes were also used to examine the curious system of 3-NO2-tyrosine, which exhibited a significantly different dissociation spectrum upon higher levels of mass spectrometry. The combination of mass spectrometry with density functional theory provided a possible explanation for the differences observed. A new, energetically favorable structure for [Cu(3-NO2-Y-H)bpy] +· is proposed where the copper center is bound between the hydroxy group and the nitro group of the amino acid side chain.