Identification of isoforms of aspartic acid residues in peptides by 2D UV-MS fingerprinting of cold ions

We use 2D UV-MS cold-ion spectroscopy for the identification of L-Asp, D-Asp, L-isoAsp and D-isoAsp residues in a fragment peptide derived from the hormone protein amylin. Relative solution concentrations of all four isoforms in an equimolar quaternary mixture have been determined within 4% error. This method demonstrates that for binary mixtures of the peptides an accuracy of 2.5% can be reached in few-second measurements.