Locating and identifying posttranslational modifications by in-source decay during MALDI-TOF mass spectrometry

A technique is described for identifying and locating posttranslational mod ifications (PTMs) in peptides and proteins of known sequence by interpretat ion of c(eta) ion signals generated by in-source decay during delayed ion e xtraction in matrix-assisted laser desorption/ionization time-of-flight mas s spectrometry. Sites of phosphorylation in seven synthetic peptides were d etermined, as was the location of both the hems group and N,N, N-trimethyll ysine in yeast cytochrome c. A semi-automated data analysis process facilit ates the identification of segments of the sequence on each side of the PTM , permitting its placement at the junction of the segments and definition o f the added mass. A graphical display facilitates illustration of both the location and mass of the PTM.