An Electrostatic Charge Partitioning Model for the Dissociation of Protein Complexes in the Gas Phase

Abstract

Electrosprayed multi-protein complexes can be dissociated by collisional activation in the gas phase. Typically, these processes follow a mechanism whereby a single subunit gets ejected with a disproportionately high amount of charge relative to its mass. This asymmetric behavior suggests that the departing subunit undergoes some degree of unfolding prior to being separated from the residual complex. These structural changes occur concomitantly with charge (proton) transfer towards the subunit that is being unraveled. Charge accumulation takes place up to the point where the subunit loses physical contact with the residual complex. This work develops a simple electrostatic model for studying the relationship between conformational changes and charge enrichment during collisional activation. Folded subunits are described as spheres that carry continuum surface charge. The unfolded chain is envisioned as random coil bead string. Simulations are guided by the principle that the system will adopt the charge configuration with the lowest potential energy for any backbone conformation. A finite-difference gradient algorithm is used to determine the charge on each subunit throughout the dissociation process. Both dimeric and tetrameric protein complexes are investigated. The model reproduces the occurrence of asymmetric charge partitioning for dissociation events that are preceded by subunit unfolding. Quantitative comparisons of experimental MS/MS data with model predictions yield estimates of the structural changes that occur during collisional activation. Our findings suggest that subunit separation can occur over a wide range of scission point structures that correspond to different degrees of unfolding.

Evaluation of Gallium as a Tracer of Exogenous Hemoglobin–Haptoglobin Complexes for Targeted Drug Delivery Applications

“…This dissociation channel (asymmetric charge partitioning, where a single highly charged polypeptide chain is ejected from the complex ion), is usually a preferred dissociation channel of protein complexes in the gas phase [45,46]. Lastly, a group of lower-abundance ions populate a narrow m/z region of the mass spectrum (4500-5000 u) and partially overlaps with the ionic signal of Hp · (α Ga β Ga ) 2 .…”

“…Since the extent of multiple charging of these species is nearly the same as that of Hp · (α Ga β Ga ) 2 , it seems highly unlikely that these ions represent products of gas-phase dissociation. Even though in some cases asymmetric charge partitioning is not the only channel of gas-phase dissociation of protein assemblies [47], transition from Hp · (α Ga β Ga ) 2 (charge states +27 through +30) to Hp · α Ga β Ga (charge states +27 and +28) in the gas phase would require removal of hemoglobin dimer α Ga β Ga carrying very few charges from the multiply charged assembly, a process that appears to be thermodynamically unfavorable [46]. Although the solution-phase origin of the observed Hp · α Ga β Ga species might seem to contradict the observation that the ionic signal intensity ratio Hp · (α Ga β Ga ) 2 /Hp · α Ga β Ga decreases at higher collisional activation (see Supplementary Material for more detail), this is likely to be a consequence of Hp · (α Ga β Ga ) 2 ions being more prone to gas-phase dissociation (and suffering greater population loss when the collisional energy is increased).…”

Combining tandem mass spectrometry with ion mobility separation to determine the architecture of polydisperse proteins

“…During this unfolding process, charge migration occurs in order to reduce coulombic repulsion over the protein complex surface. This results in the unfolded monomer carrying a disproportionate amount of the original charge in relation to its mass [24][25][26][27][28]. Because charge is conserved in this process, the residual "stripped" complex has a much lower charge than the precursor.…”

Conformational Isomers of Calcineurin Follow Distinct Dissociation Pathways

“…During this dissociation process, the leaving subunit gradually unfolds and attracts a disproportionately high amount of charges relative to its mass. This process leads to a charge dissociation pathway commonly known as “asymmetric charge partitioning” [23, 24]. …”