Doubly charged protonated a ions derived from small peptides.

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Computational Chemistry

Background:

• Protonated peptide backbone ions are valuable for structural elucidation in mass spectrometry.
• Understanding ion formation mechanisms is key to improving analytical techniques.

Purpose of the Study:

• To investigate the formation of doubly charged protonated a(2) and a(3) ions from lanthanum-peptide complexes.
• To determine the influence of proline residue location and aliphatic amino acids on protonated a-ion generation.
• To computationally assess the proton affinity of these ions.

Main Methods:

• Collision-induced dissociation (CID) of [La(III)(peptide)(CH(3)CN)(m)](3+) complexes.
• Systematic variation of peptide sequence, focusing on proline residue placement.
• Density functional theory (DFT) calculations (B3LYP/6-311++G(d,p)) to determine proton affinities.

Main Results:

• Abundant doubly charged protonated a(2) and a(3) ions are formed, particularly from peptides containing proline residues, especially at the N-terminus.
• Triproline (PPP) yields abundant (a(3)+H)(2+) ions.
• DFT calculations show a high proton affinity for the a(3) ion from PPP (167.6 kcal mol(-1)).
• Protonated a(2) ions from diglycine/diproline and a(3) ions from triglycine exhibit lower proton affinities and are less abundant, suggesting proton transfer to water.

Conclusions:

• Peptide sequence, especially the presence and position of proline, significantly impacts the formation of doubly charged backbone ions.
• Lanthanum-mediated CID is an effective method for generating these diagnostic ions.
• Computational analysis supports experimental observations regarding ion stability and formation propensity.