Bicarbonate buffers can promote crosslinking and alternative gas-phase dissociation pathways for multiprotein complexes

Abstract Previously, we have reported the stabilization effect of Hofmeister salts for multiprotein complexes (MPC) in the absence of bulk solvent (J. Am. Chem. Soc. 2011, 133 (29), 11,358–11367; Angew. Chem. Int. Ed. 2012, 51 (23), 5692–5695; Angew. Chem. Int. Ed. 2013, 52 (32), 8329–8332.). Our efforts sought to bridge the gap between gas-phase protein structures and those found in solution. To reveal more detailed MPC topology information, native ion mobility-mass spectrometry (IM-MS) measurements are often combined with gas-phase activation methods. Conventional activation methods, including collision induced dissociation/unfolding (CID/CIU), however, primarily report information focused on monomeric subunits within the MPC, limiting the topological information obtained. Herein, we describe a simple buffer-doping method that promotes an alternative MPC CID pathway which readily produces product ions that correspond to larger sub-complexes from within some parent assemblies. Interestingly, tetramers exhibiting a dimer of dimers quaternary structure (e.g. hemoglobin and concanavalin A) produce dimeric product ions upon collisional activation following ionization from bicarbonate buffer, in contrast to the commonly observed monomer-ejection CID pathway. In order to both further investigate and validate our native IM-MS, we performed bottom-up proteomics experiments on MPCs housed in bicarbonate buffer. Our efforts revealed evidence of bicarbonate-mediated disulfide bond formation in proximal Cystine residues. We close by discussing the applications for these observations in the context of MPC structure determination by native IM-MS.