To characterize how subunit interfaces were possibly rearranged by isCA, we employed top-down electron capture dissociation (TD ECD). ECD cleaves the peptide N-Cα bond, typically resulting in c/z product ion types. For native-like proteins, the released product ions can be indicative of protein secondary, tertiary, and even quaternary structure. Additionally, the obtained products can be sensitive to pre-ECD collisional activation, thus this technique can also diagnose rearranged protein structures Figures S9 and S10 show SA's ECD sequence coverage at cone voltages of 20 V and 160 V, respectively. We note that the depicted sequence includes an N-terminal methionine residue. The SA commercially available from Thermo Scientific does not have all N-terminal methionine residues cleaved, which is evident in the mass spectrum. Because the tetramer likely forms in a stochastic manner, there are mass spectral peaks that correspond to 0, 1, 2, 3, and 4 conserved methionine residues on the complex, with individual monomer subunits carrying 0 or 1 Met. Thus, we used both possible sequences, with and without the leading Met, but choose to display the ECD data featuring the leading Met. Cleavage sites were similar between the two protein sequences.