d-Amino Acid Detection in Peptides by MALDI-TOF-TOF

Detection of a d-amino acid residue in natural peptides by mass spectrometry remains a challenging task, as this post-translational modification does not induce any change in molecular mass. To our knowledge, the present article is the first report using matrix-assisted laser desorption/ionization (MALDI) for the discrimination and the quantification of peptide isomers. In this work, we used synthetic hepta- and decapeptides of biological relevance and their isomers. All-l sequences and some isomers containing a d-residue in various positions were analyzed.