Unusual behavior of gem-diamino peptides in fast atom bombardment and electrospray ionization mass spectrometry

Gem-diamino peptides were studied by fast atom bombardment (FAB) and electrospray ionization (ESI) mass spectrometry to evidence specific fragmentations involving the labile modified bonds. Base peaks recorded in the positive mode were attributed to fragment ions even with soft atmospheric pressure ionization. The criterion that one ion corresponds to one molecule in ESI mass spectra commonly encountered in high throughput analysis of combinatorial libraries must be considered with caution when fragile structures are analyzed.