Electrospray tandem mass spectrometric studies of phosphopeptides and phosphopeptide analogues

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Abstract
A set of synthetic phosphopeptides and phosphopeptide analogues was studied by tandem nano‐electrospray mass spectrometry. The influence of the collision offset and of the charge state of the molecular ion on phosphate‐specific fragmentation processes was investigated in detail. H–D exchange experiments and structural considerations support a six‐centered transition being present in the neutral loss of H3PO4 from serine, threonine and homoserine phosphopeptides, where the C‐α hydrogen of serine or threonine or the C‐β hydrogen of homoserine is transferred to the protonated phosphate group. Neutral loss of H3PO4 at moderate collision offset potential represents a very abundant fragmentation process for serine, threonine and homoserine phosphopeptides. The most specific feature for discrimination of these phosphopeptides from tyrosine phosphopeptides is the m /z 79:97 ratio in the negative ion product spectra, which is consistently elevated in tyrosine phosphopeptides as compared with serine, threonine and homoserine phosphopeptides. The fragment ions of methylphosphono‐ and H‐phosphonopeptides can be explained by the same mechanisms as are applicable to phosphopeptides. Copyright © 1999 John Wiley & Sons, Ltd.