Christine L. Gatlin

Metal-ligand redox reactions in gas-phase quaternary peptide-metal complexes by electrospray ionization mass spectrometry

Abstract The dipeptides Phe-Leu, Leu-Phe, Leu-Ala, and Ala-Leu form quaternary complexes of the type [Cu(II)(peptide - 2H+M)bpy]+ in the gas phase when electrosprayed in the presence of Cu(II) salts, 2.2′-bipyridyl (bpy), and an alkali hydroxide (MOH). The gas-phase complexes decarboxylate on collisional activation at low ion kinetic energies. The resulting ions undergo unusual eliminations of neutral Na, K, and Rb, which depend on the peptide structure. The ionization energy of the decarboxylated Phe-Leu-Cu-bpy complex was bracketed at 4.2 eV. Other collision-induced dissociations also depend on the alkali metal ion and the peptide structure. Ab initio calculations on a model system are reported and used to discuss the electronic properties of the peptide complexes.

Quantitative electrospray ionization mass spectrometric studies of ternary complexes of amino acids with Cu2+ and phenanthroline

Electrospray ionization (ESI) mass spectra of ternary complexes of Cu(2+) and 1,10-phenanthroline with the 20 essential amino acids (AA) were investigated quantitatively. Non-basic amino acids formed singly charged complexes of the [Cu(AA - H)phen](+) type. Lysine (Lys) and arginine (Arg) formed doubly charged complexes of the [Cu(HAA - H)phen](2+) type. Detection limits were determined for the complexes of phenylalanine (Phe), glutamic acid (Glu) and Arg, which were at low micromolar or submicromolar concentrations under routine conditions. Detection limits of low nanomolar concentrations are possible for amino acids with hydrophobic side-chains (Phe, Tyr, Trp, Leu, Ile) as determined for Phe. The efficiencies for the formation by ESI of gaseous [Cu(AA - H)phen](+) ions were determined and correlated with the acid-base properties of the amino acids, ternary complex stability constants and amino acid hydrophobicities expressed as the Bull-Breese indices (DeltaF). A weak correlation was found between DeltaF and the ESI efficiencies for the formation of gaseous [Cu(AA - H)phen](+) [Cu(HAA - H]phen](2+) and [AA + H](+) ions that showed that amino acids with hydrophobic side-chains were ionized more efficiently. In the ESI of binary and ternary amino acid mixtures, the formation of gas-phase Cu-phen complexes of amino acids with hydrophobic side-chains was enhanced in the presence of complexes of amino acids with polar or basic side-chains. An interesting enhancement of the ESI formation of [Cu(Glu - H)phen](+) was observed in mixtures. The effect is explained by ion-cluster formation at the droplet interface that results in enhanced desorption of the glutamic acid complex.