Fragmentation characteristics of b(n) (n=2-15) ions from protonated peptides.

The fragmentation reactions of protonated oligoalanines (trialanine, tetraalanine and pentaalanine) and the fragments present in the electrospray ionization (ESI) mass spectrum of polyalanine have been studied by collisionally activated dissociation (CAD) mass spectrometry (MS(2) and MS(3) experiments). The MS(n) experiments provided strong evidence that the m/z 71n+1 ion series in the ESI mass spectrum of polyalanine is a b(n) series. These ions are formed via the b(n) -y(m) pathway of amide bond cleavage, which results in the formation of a proton-bound complex of an oxazolone and a peptide/amino acid. Also, the MS(2) spectra of the b(n) series from polyalanine revealed that the chain length of b(n) ions influences significantly the dissociations taking place. For example, b(n) ions start losing H(2)O at n ≥5 and the losses of CO and CO+NH(3) decrease in intensity from b(2) to b(15). The elimination of H(2)O+NH(3) and the elimination of 61 mass (HN=C=O+H(2)O) commence with b(6); their abundances initially increase up to ~ b(8)-b(9) and then gradually decrease until b(15) (largest fragment studied). The tandem mass spectrometry experiments help to elucidate the dissociation mechanisms of the observed structures of biopolymer fragments.