Salt Bridge Rearrangement (SaBRe) Explains the Dissociation Behavior of Noncovalent Complexes

Abstract

Native electrospray ionization-mass spectrometry, with gas phase activation and solution compositions that partially release subcomplexes, can elucidate topologies of macromolecular assemblies. That so much complexity can be preserved in gas phase assemblies is remarkable, although a long-standing conundrum has been the differences between their gas and solution phase decompositions. Collision-induced dissociation of multimeric noncovalent complexes typically distributes products asymmetrically; i.e., by ejecting a single subunit bearing a large percentage of the excess charge. That unexpected behavior has been rationalized as one subunit “unfolding” to depart with more charge. We present an alternative explanation based on heterolytic ion-pair scission and rearrangement, X-COO−…H3+N-Y→X-COO−+H3+N-Y a mechanism that inherently partitions charge asymmetrically. Excessive barriers to dissociation are circumvented in this manner, when local charge rearrangements access a lower-barrier surface. An implication of this ion pair consideration is that stability differences between high- and low-charge state ions usually attributed to Coulomb repulsion may, alternatively, be conveyed by attractive forces from ion pairs (salt bridges) stabilizing low-charge state ions. Should the number of ion pairs be roughly inversely related to charge, symmetric dissociations would be favored from highly charged complexes, as observed. Correlations between a gas phase protein’s size and charge reflect the quantity of restraining ion pairs. Collisionally-facilitated salt bridge rearrangement (SaBRe) may explain unusual size “contractions” seen for some activated, low charge state complexes. That some low-charged multimers preferentially cleave covalent bonds or shed small ions to disrupting noncovalent associations is also explained by greater ion pairing in low charge state complexes.

Compaction of Duplex Nucleic Acids upon Native Electrospray Mass Spectrometry

“…Globular proteins can rearrange by relaxing their side chains 44 and undergo minimal salt bridge rearrangement. 45 Briefly optimized structures often have CCS values matching well with the experiments. 42,[46][47][48] Fabris and co-workers have recently underlined the difficulties in transposing to DNA the MD and CCS calculation protocols traditionally used for proteins.…”

Defining Gas-Phase Fragmentation Propensities of Intact Proteins During Native Top-Down Mass Spectrometry

“…Our approach for the analysis of protein complexes [58, 60, 64] uses energy to eject monomers in the source region of the mass spectrometer. Often, these ejected monomers will be observed with a disproportionately high number of charges as compared to the intact precursor in a process known as asymmetric charge partitioning [46, 65], which can attributed to partial or complete unfolding of the monomer [46, 66, 67] or to heterolytic scission of ion pairs [68]. Because the preferred fragmentation pathways may be affected by this increased protonation, monomers that were ejected from complexes were not included in this study.…”

193 nm Ultraviolet Photodissociation Mass Spectrometry of Tetrameric Protein Complexes Provides Insight into Quaternary and Secondary Protein Topology

“…Although none of the basic residues in these salt bridges were found to be protonated, asymmetric partitioning of the salt bridges could in the formation of a 6+ monomer that undergoes charge relocation to the interfacial region in response to coulombic interactions. 21 …”

“…based on heterolytic cleavage of interfacial salt bridges instead of subunit unfolding to rationalize asymmetric charge partitioning. 21 …”

“…examined CID and SID data and proposed that scrambling and heterolytic cleavage of interfacial salt bridges rather than subunit unfolding could account for asymmetric charge partitioning. 21 …”