MALDI-TOF based identification and de-novo sequencing. (Top) MALDI-MS spectrum from the ~19 kDa protein of B. garinii ST4 (strain PBi). Relevant mass signals employed for database searching have been labeled and known trypsin and keratin peptide signals have been marked with a black triangle. The precursor ion selected for subsequent MS/MS measurement is indicated by an arrow. (Bottom) MALDI-MS/MS spectrum from the above precursor ion at m/z = 2603.19. Ions ascribed to the main fragmentation series, y (C-terminal series) and b (N-terminal series) are labeled, and C-terminal fragment ions produced by the loss of ammonia (-17 amu) are indicated by an asterisk (*). The amino acid sequence obtained by manual de novo sequencing is displayed showing assignment of y and b fragment ions. The letters K and L are used to indicate ambiguous ascription to glutamine/lysine and isoleucine/leucine pairs, respectively.