INVESTIGATIONS OF GAS-PHASE LITHIUM-PEPTIDE ADDUCTS - TANDEM MASS-SPECTROMETRY AND SEMIEMPIRICAL STUDIES

Tandem mass spectrometry using a hybrid mass spectrometer of BEqQ geometry was used to investigate the gas-phase formation of the [An+ Li - H)+ ion from lithium-peptide adducts. High resolution mass measurements as well as precursor and product ion scans of five peptides indicate that one source of [An+ Li - H) arises from [An+ Li]+. Semiempirical calculations (MNOO) and metastable ion decomposition studies of the peptide Gly-Gly-Gly show that the lithium ion prefers to coordinate to the three internal carbonyls of the neutral molecule to give a species that is energetically more stable than the lithiated zwitterion by 305 kJ/mol. Theoretical and experimental evidence suggest that the monolithiated precursor ion population may be a distribution of structural isomers. (J Am Soc Mass Spectrom 1990, 1, 473-480) © 1990 American Society for Mass Spectrometry.