Amino acid sequence determination of phosphoenkephalins using liquid secondary ionization mass spectrometry

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Abstract
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/z 79 (PO) and 97 (H2PO), 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.

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