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