Proton mobility in protonated amino acids and peptides

The MD+ ions of a variety of amino acids and small peptides have been prepared using CD4 and (CD3)(2)CO as chemical ionization reagents. Using tandem mass spectrometry the fragmentation reactions of these MD+ ions have been studied, both those occurring unimolecularly on the metastable ion time scale (CD4 CI) and those occurring following collisional activation ((CD3)(2)CO CI). The results show that the added DC has undergone extensive interchange (leading to H/D scrambling) with all labile hydrogens including carboxylic hydrogens, hydroxylic hydrogens, amidic hydrogens and amino hydrogens. The results indicate that the proton added to amino acids and simple peptides is very mobile and samples all positions bearing labile hydrogens prior to fragmentation of the protonated species. (C) 1997 Elsevier Science B.V.