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