Figure 2.

LC chromatogram of the four isomers of DAEFR (a) before and (b) after modification by 4-iodo-benzoic acid. The four isomers are easily separated after modification. (c) LC chromatogram of a peptide mixture containing ^IDVGSNK-NH₂, ^IDVGSNK-NH₂, and ^ILDLAGR. The ^IDVGSNK epimers cannot be completely separated. (d) MS³ (RDD) spectra extracted from points marked by blue asterisks in panel c. The two spectra are significantly different, indicating partial separation of the two peptides. (e) LC chromatogram of a single peptide from the same run at a later elution time. (f) MS³ (RDD) spectra corresponding to the blue asterisks from panel e. The two spectra are almost identical. ^IX represents the 4-iodo-benzoic acid modification of X where X is any amino acid. Underlined residues correspond to d-amino acids, and bold residues correspond to isoaspartic acid.