Gas phase attachment of water and methanol to Ag(I) complexes with alpha-amino acids in an ion trap mass spectrometer

Electrospray ionization was used to generate gas phase complexes of Ag+ wit h selected alpha -amino acids. Following storage (isolation without collisi onal activation) in an ion trap mass spectrometer, the mass spectra produce d from the complexes of Ag+ with alpha -amino acids such as alanine, valine and tert-leucine contained peaks consistent with the formation of water or methanol molecule adduct ions. The same adduct ions were not present, howe ver, in the mass spectra generated from the Ag+ complexes with phenylalanin e, tyrosine and tryptophan following isolation and storage under similar co nditions. For those complexes that showed reactivity, the uptake of water a nd methanol increased with longer storage times in the ion trap. A prelimin ary molecular modeling study using phenylalanine demonstrated that the arom atic ring coordinates the Ag+ ion, and the interaction between the metal io n and pi -system, in part, is assumed to prohibit the binding of water or m ethanol during isolation in the gas phase. This conclusion is supported by a comparison of the adduct formation by the Ag+ complexes with phenylalanin e, 4-fluorophenylalanine and alpha -aminocyclohexanepropionic acid. In addi tion, collision induced dissociation experiments involving the Ag+ complexe s of phenylalanine, tyrosine and tryptophan suggest that limiting the coord ination of the Ag ion by the complexing molecule (i.e. by loss of a coordin ating functional group and/or change in structure due to dissociation) resu lts in the binding of a water or methanol molecule during storage in the io n trap. Surprisingly, the bare Ag+ ion, when trapped and stored under ident ical experimental conditions, formed neither adduct species, suggesting tha t the attachment of water or methanol may be due to interactions with a mol ecular orbital within the Ag+/molecule complex. Copyright (C) 2001 John Wil ey & Sons, Ltd.