AMINO-ACID-SEQUENCE DETERMINATION OF PHOSPHOENKEPHALINS USING LIQUID SECONDARY IONIZATION MASS-SPECTROMETRY

Liquid secondary ionization mass spectrometry (LSIMS) operating in the positive- and negative-ion modes was used to study fragmentation profiles and to obtain the amino acid sequences of a set of seven phosphoenkephalin peptides. The use of glycerol as the liquid matrix led to increase in fragmentation of phosphopeptides, The prominent amino acid sequence-determining ions in the positive-ion mode are y-type C-terminal ions; the N-terminal sequence-specific ions are observed sporadically. The most dominant ions in those mass spectra, however, are the immonium ions and a few low-mass side-chain cleavage products. The mass spectra in the negative-ion mode are more information-rich, and provide data complementary to that from the positive-ion mode. The phosphate group marker ions, m/z79 (PO3-) and 97 (H2PO4-), are prominent and both N- and C-termini sequence ions are formed with equal facility in this mode of analysis. Both positive- and negative-ion mass spectral data are useful in determining the amino acid sequence of all of the seven phosphoenkephalins. Thus, LSIMS alone can be a viable option to the tandem mass spectrometry approach when sufficient quantities (>50 nmol) of phosphopeptides are available.