The possibility of chemical modification of peptides and proteins under the
condition of proteolytic digestions and FABMS analysis was investigated. The
results ;indicate that among the amino acid constituents of peptides and proteins:
serinefcysteine, and cystine are the most sensitive residues which undergo chemical
modificadons under the exprimenta1 conditions. The chemical modification of these
amino acids which is governed by the intrinsic properties of the peptides will result
in the formation of a molecular ion mass which is 16 mu lower than the molecufitir
ion of the parent peptide, MHf. Exact mass measurements of (MH-16)
molecular ions indicate that these ions may cornspond to(MH-H O +H ) but
not(MH-CH ) molecules. In addition, the results indicate the presence of no
inter-and /or intrachain disulfide bond rearrangements uhder the experimental
conditions of degradations and FABMS analyses of Lysozyme and Rihonuclease A.