Negative-charge driven fragmentations for evidencing zwitterionic forms from doubly charged coppered peptides

Area of Science:

• Analytical Chemistry
• Biochemistry
• Mass Spectrometry

Background:

• Amino acids and peptides typically exist as zwitterions in aqueous solutions.
• In the gas phase, such as during electrospray ionization (ESI), zwitterionic forms are less stable due to the lack of solvation.
• Metallic cations can stabilize zwitterionic forms in the gas phase, impacting fragmentation.

Purpose of the Study:

• To investigate the influence of copper(II) ions on the zwitterionic character of pentapeptides in the gas phase.
• To analyze the fragmentation patterns of [M + Cu(II)](2+) peptide complexes using collision-induced dissociation (CID).
• To differentiate between isobaric pentapeptides based on their fragmentation behavior.

Main Methods:

• Electrospray ionization (ESI) mass spectrometry was used to generate gas-phase peptide ions.
• A 3D ion trap mass spectrometer with an orthogonal ESI source was employed.
• Low-energy collision-induced dissociation (CID) was performed on [M + Cu(II)](2+) pentapeptide complexes.

Main Results:

• Negative-charge driven fragmentations were observed on positively charged peptide ions, suggesting a zwitterionic character.
• Specific fragment losses, such as (CH(3)--C(6)H(4)--O)(*) and styrene, differentiated isobaric pentapeptides (YGGFK/YGGFQ and KYGGF/QYGGF).
• An unusual loss of NH(4)(+) from the N-terminus and the reduction of Cu(II) to Cu(I) were also noted.

Conclusions:

• Copper(II) ions can stabilize zwitterionic forms of peptides in the gas phase, influencing their fragmentation.
• Mass spectrometry with CID and cation adducts provides a method for distinguishing between structurally similar peptides.
• The observed fragmentation patterns offer insights into peptide structure and stability in the absence of solvation.