Intramolecular Hydrogen Transfer from the Alpha-Carbon (Cα) and Backbone Amide Nitrogen (N) to Form c- and y-Ions in Negative-Ion CID of Peptides.

The source of hydrogen in the formation of c- and y-ions produced by intramolecular hydrogen transfer in negative-ion CID experiments with peptides has been examined using Cα-, Cβ-, and backbone amide (N)-deuterated peptides AAA(d3)AA, AAG(d2)AA, AAAG(d2)A, and AAAAA-d7, as well as five other peptides. The c- and y-ions produced by deuterium transfer from the deuterated residues were detected and identified by the exact m/z values obtained with a high-resolution orbitrap mass spectrometer. The rate of deuterium transfer obtained indicates that over 50% of the hydrogen was originated from the backbone amide nitrogen, with the residual hydrogen coming from the backbone Cα.