Cyclization of Peptide b9 Ions

The product ion mass spectra obtained by CID of the b9 ions derived by loss of neutral alanine from the MH+ ion of the peptides Tyr(Ala)9, (Ala)4Tyr(Ala)5, and (Ala)8TyrAla are essentially identical, indicative of full cyclization reaction to a common intermediate before fragmentation. This leads to abundant nondirect sequence ions in the product ion mass spectra of the b9 ions. The product ion mass spectra of the b8 ions from the first two peptides also are essentially identical. The fragmentation of the MH+ ions also leads to low intensity nondirect sequence ions in the product ion mass spectra. N-terminal acetylation blocks the cyclization and eliminates nondirect sequence fragment ions in the product ion mass spectra.

Graphical Abstractb9 Ions undergo cyclization and form nondirect sequence ions. The cyclization is blocked by N-terminal acetylation.Figure optionsDownload high-quality image (62 K)Download as PowerPoint slide