Mass spectrometric analysis of combinatorial peptide libraries derived from the tandem repeat unit of MUC2 mucin

Four 19-member synthetic peptide libraries, based on the TX1TX2T epitope motif of the mucin-2 gastrointestinal glycoprotein (MUC2) and ranging in peptide length from dipeptides to 15-mers (XT, TXT, TQTXT and KVTPTPTPTGTQTXT), were synthesized by combinatorial solid phase peptide synthesis using the portioning-mixing combinatorial approach, and analysed by electrospray ionization mass spectrometry at different (1000-10000) resolutions. Most of the components of the individual libraries could be easily identified in a single-stage molecular mass screening experiment. The resolving power of the instrument becomes an important factor above 800-1000 Da molecular mass, when predominantly multiply charged molecular ions are formed. Approaches to the identification of isobars (glutamine/lysine), isomers leucine/isoleucine) and sequence variations by tandem mass spectrometry, and/or by high-performance liquid chromatography-mass spectrometry are outlined.