Solution thermochemistry of concanavalin A tetramer conformers measured by variable-temperature ESI-IMS-MS

Variable-temperature nano-electrospray ionization coupled with ion mobility spectrometry-mass spectrometry is used to investigate the thermal denaturation of the tetrameric protein concanavalin A. As the solution temperature is increased, changes in mass spectra and collision cross section distributions provide evidence for discrete structural changes that occur at temperatures that are ~40 to 50 degrees below the temperature required for tetramer dissociation. The subtle structural changes are associated with four distinct tetramer conformations with unique melting temperatures. Gibbs-Helmholtz analysis of the free energies determined with respect to the most abundant "native" state yields heat capacities of ΔC_p = 1.6 ± 0.3, -2.2 ± 0.4, and -2.9 ± 1.6 kJ·K^-1·mol^-1, and temperature dependent enthalpies and entropies for the three non-native conformations. Analysis of the thermochemistry indicates that the high-temperature products are entropically stable until the threshold for tetramer dissociation, and changes in heat capacity are consistent with increases in solvation of polar residues. Our findings suggest these high-temperature non-native states result from an increase in disorder at surface exposed regions. Such studies provide valuable insight towards the structural details of non-native states.