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. ...