Characterization of phosphorylated amino acids by fast-atom bombardment mass spectrometry

Positive- and negative-ion fast-atom bombardment (FAB) mass spectrometry and linked-field scan techniques at constant B/E are used to characterize phosphorylated serine, threonine, and tyrosine amino acids. Abundant molecular ions are formed for all three amino acids in both modes of ionization. The dominant fragmentation is cleavage of the phosphate ester bond with charge retention in positive-ion FAB by the amino acid backbone and in the negative-ion mode by the phosphate group. The unique feature of positive-ion FAB mass spectra of phosphoserine and -threonine is the loss, from the ion [M + H]+, of a molecule of phosphoric acid (98 Da), whereas the corresponding tyrosine expels a HPO4 (96 Da) moiety to yield a stable phenylalanine ion.