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).