A KINETIC APPROACH TO THE PROTON AFFINITIES OF AMINE BASES

The fragmentation of proton-bound clusters [B1-H-B2]+ of amine bases B 1 and B2 were studied at collision energies of 10-50 eV in the quadrup ole collision cell of a hybrid tandem mass spectrometer. The relative abundances of the fragment ions B1H+ and B2H+ were correlated with the proton affinities of the amines. The proton affinities of 21 amines a re reported based on the expanded proton affinity scale of Meot-Ner an d Sieck [J. Am. Chem. Soc. 113, 4448 (1991)]. It is found that the fra gmentation of mixed proton-bound clusters of alpha-amino acids and ami nes can be used to establish the proton affinities of simple amino aci ds such as glycine, alanine, valine, leucine and phenylalanine. Howeve r, low values are obtained by this method for the proton affinities of amino acids containing a functional group (additional to the carboxyl group) capable of stabilizing the protonated species by intramolecula r hydrogen bonding.