Ammonia chemical ionization of some free amino acids and underivatized peptides

Ammonia chemical ionization mass spectra of rapidly heated free amino acids and peptides provide an opportunity for a study of the competition between gas-phase solvation and association reactions of these biochemically interesting species. Compounds investigated include methionine, phenylalanine, tryptophan, and peptides; AlaAla, PheAla, AlaAlaAla, AlaPheGly, GlyAlaPhe, tetraAla and PCAHisProNH 2 (thyrotropin releasing hormone). Extensive formation of solvated-protonated species were observed and associated protonated peptides up to (4 M + 1) + in some cases. The observation of association reactions with rapidly heated samples presents problems in the analytical use of this technique. Investigation of the kinetic order of reactions leading to association of protonated species reveals a significantly more effective energy transfer process in collisions of protonated peptides with parent neutral molecules than with neutral solvent gas.