Intramolecular Hydrogen Transfer from the Alpha-Carbon (Cα) and Backbone Amide Nitrogen (Nb) 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 (Nb)-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. It is clear that the hydrogen does not originate from the side chain Cβ. It is hypothesized that the intramolecular hydrogen transfer to form negative c- and y-ions takes place via 3-, 4-, 6-, 7-, 8-, and 9-membered ring transition states.
KeywordsPeptide Negative ion CID c-ion y-ion Hydrogen transfer
The reviewer’s suggestion about the use of a deuterated alanine pentamer AAAAA-d7 is highly acknowledged. MT gratefully acknowledges the support from the fund for Creation of Innovation Centers for Advanced Interdisciplinary Research Area Program in the Project for Developing Innovation Systems from the Ministry of Education, Culture, Sports, Science and Technology.
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