A kinetic approach to the proton affinities of amine bases

The fragmentation of proton‐bound clusters [B1—H—B2]+ of amine bases B1 and B2 were studied at collision energies of 10–50 eV in the quadrupole 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 are reported based on the expanded proton affinity scale of Meot‐Ner and Sieck [J. Am. Chem. Soc. 113, 4448 (1991)]. It is found that the fragmentation of mixed proton‐bound clusters of α‐amino acids and amines can be used to establish the proton affinities of simple amino acids such as glycine, alanine, valine, leucine and phenylalanine. However, 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 intramolecular hydrogen bonding.