Oxidized and Reduced Dimeric Protein Complexes Illustrate Contrasting CID and SID Charge Partitioning

Area of Science:

• Biophysical Chemistry
• Mass Spectrometry
• Protein Chemistry

Background:

• Gas-phase dissociation of protein complexes is complex, influenced by factors like protein structure, flexibility, and dissociation technique.
• Understanding charge partitioning is crucial for interpreting mass spectrometry data of biomolecular assemblies.

Purpose of the Study:

• To investigate charge partitioning behaviors during collision-induced dissociation (CID) and surface-induced dissociation (SID) of two cysteine-containing homodimer proteins.
• To elucidate the influence of disulfide bond reduction and charge state on dissociation pathways.

Main Methods:

• Utilized collision-induced dissociation (CID) and surface-induced dissociation (SID) on $\beta$-lactoglobulin and $\alpha$-lactalbumin.
• Analyzed charge partitioning, collision cross section (CCS), stability, and unfolding/restructuring properties.
• Compared dissociation outcomes for intact and reduced disulfide bonds, and across different charge states.

Main Results:

• CID predominantly induced restructuring, while SID favored symmetric charge partitioning and monomer separation.
• Charge-reduced CID precursors exhibited smaller CCS, greater stability, and more symmetric charge distribution compared to higher charge-state CID.
• Gas collisions revealed charge-dependent restructuring, whereas surface collisions promoted symmetric monomer separation.

Conclusions:

• CID involves sequential reorganization of bonds via multiple low-energy collisions, favoring restructuring.
• SID utilizes a large energy jump, cleaving intermolecular bonds preferentially over intramolecular ones, leading to symmetric dissociation.
• Both methods generate restructured precursors, but SID is more effective for symmetric charge partitioning.