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

Electrospray ionization (ESI) mass spectra of ternary complexes of Cu2+ and 1,10-phenanthroline with the 20 essential amino acids (AA) were investigat ed quantitatively. Non-basic amino acids formed singly charged complexes of the [Cu(AA - H)phen](+) type. Lysine (Lys) and arginine (Arg) formed doubl y charged complexes of the [Cu(HAA - H)phen](2+) type. Detection limits wer e 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 p ossible for amino acids with hydrophobic side-chains (Phe, Tyr, Trp, Leu, I le) as determined for Phe, The efficiencies for the formation by ESI of gas eous [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 (Delta F) . A weak correlation was found between Delta F 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 wer e ionized more efficiently. In the ESI of binary and ternary amino acid mix tures, the formation of gas-phase Cu-phen complexes of amino acids with hyd rophobic side-chains was enhanced in the presence of complexes of amino aci ds with polar or basic side-chains. An interesting enhancement of the ESI f ormation of [Cu(Glu - H)phen](+) was observed in mixtures. The effect is ex plained by ion-cluster formation at the droplet interface that results in e nhanced desorption of the glutamic acid complex. Copyright (C) 2000 John Wi ley & Sons, Ltd.