Zwitterionic States in Gas‐Phase Polypeptide Ions Revealed by 157‐nm Ultra‐Violet Photodissociation

摘要: A new method of detecting the presence deprotonation and determining its position in gas-phase polypeptide cations is described. The involves 157-nm ultra-violet photodissociation (UVPD) based on monitoring losses CO2 (44 Da) from electronically excited deprotonated carboxylic groups relative to competing COOH (45 neutral groups. Loss a strong indication zwitterionic [(+)center dot center dot(-)center dot(+)] salt bridge cation. This provides tool for studying, example, nature binding within clusters. Collision-activated dissociation (CAD) decarboxylated localizes deprotonation. Fragment abundances can be used semi-quantitative assessment branching ratio among different acidic sites, however, mechanism fragment formation should taken into account. Cations Trp-cage proteins exist preferentially as zwitterions, with divided between Asp(9) residue C terminus 3:2. majority dications same molecule are not zwitterions. Furthermore, UVPD produces abundant radical M dot+ protonated molecules through loss hydrogen atom. producing ions general applied any peptide abundance molecular produced sufficient further tandem mass spectrometry (MS/MS), which, cases studied, yielded side-chain basic amino acid most fragmentation channel together some backbone cleavages.