Gas‐phase H–D exchange reactions of protonated amino acids and peptides with ND3

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AbstractThe gas‐phase H‐D exchange reactions of protonated amino acids and small peptides with ND3 have been studied for reactions occurring in the quadrupole collision cell of a hybrid BEqQ mass spectrometer (B = magnetic sector, E = electric sector, q = quadrupole collision cell, Q = quadrupole mass analyzer). Protonated amino acids RCH(NH2)COOH· H+, including those with hydroxyl, carboxyl, and amine functions in the R group, exchange all active hydrogens readily. The amide hydrogens of protonated glutamine and asparagine exchange less readily as does the N‐bonded hydrogen of the imidazole ring of histidine. The phenolic hydrogen of protonated tyrosine and tyrosine‐containing peptides does not undergo significant exchange. Protonated arginine undergoes only limited exchange with ND3 and arginine‐containing peptides are essentially unreactive with ND3. Although simple protonated di‐ and tripeptides exchange all active hydrogens readily, peptides with amide groups in the side‐chain exchange the last two hydrogens slowly. Mechanisms for exchange of the active hydrogens are discussed in relation to the proton affinities of the various sites of the amino acids and the potential and limitations of the method for counting the labile hydrogens of unknown peptides is assessed.