Distinguishing Loss of Structure from Subunit Dissociation for Protein Complexes with Variable Temperature Ion Mobility Mass Spectrometry.

The thermal stability and strength of interactions in proteins are commonly measured using isothermal calorimetry and differential scanning calorimetry providing a measurement that averages over structural transitions that occur as the proteins melt and dissociate. Here, we apply variable temperature ion mobility mass spectrometry (VT-IM-MS) to study the effect of temperature on the stability and structure of four multimeric protein complexes. VT-IM-MS is used here to investigate the change in the conformation of model proteins, namely, transthyretin (TTR), avidin, concanavalin A (conA), and human serum amyloid P component (SAP) at elevated temperatures prior, during, and after dissociation up to 550 K. As the temperature of the buffer gas is increased from 300 to 350 K, a small decrease in the collision cross sections ((DT)CCS(He)) of protein complexes from the values at room temperature is observed, and is associated with complex compaction occurring close to the reported solution T(m). At significantly higher temperatures, each protein complex undergoes an increase in (DT)CCS(He) and in the width of arrival time distributions (ATD), which is attributed to extensive protein unfolding, prior to ejection of a highly charged monomer species. This approach allows us to decouple the distinct gas phase melting temperature (T(m)) from the temperature at which we see subunit dissociation. The thermally induced dissociation (TID) mechanism is observed to initially proceed via the so-called "typical" (CID) dissociation route. Interestingly, data collected at higher analysis temperature suggests that the TID process might be adapting more "atypical" dissociation route.