FAST ATOM BOMBARDMENT MASS SPECTROMETRY (FABMS) ANALYSIS OF AN N- TERMINAL - BLOCKED PEPTIDE

FABMS analysis of T-lb peptide before and after one cycle of Edman degradation indicated an unblocked N-terminal Thr residue for this tryptic peptide. In contrast , our data showed a molecular protonated ion, MH + for T- la peptide at 655 mass units (mu) which is 42 mu higher than the MH ion of T- 1b peptide. In addition, T- la peptide was not amenable to one cycle of manual Edman degradation. These observations suggest that the N-terminal residue of T- 1 a peptide is blocked by a chemical group having a mass of 42 mu. This observation is consistent with the presence of an N-terminal acetyl group. This prediction was further investigated by complete characterization of the gas phased fragment ions, originated from the individual peptides, using link-scanning approach. The fragment ions again suggest an acetyl group attached to an N-terminal threonine. In addition, the fragment ions provide evidence that the two peptides (T-la and T-lb) have identical structures except for the N-acetyl group