Conformational Dynamics of Hemoglobin in Solution and the Gas Phase Elucidated by Mass Spectrometry

Solution and gas-phase measurements can provide valuable insights into biomolecular conformational dynamics. By comparing the data from such experiments, it is possible to elucidate the nature of the interactions governing a biomolecule's stability. Here, we measured human, bovine, and porcine hemoglobin stability in solution and the gas phase using collision-induced dissociation, collision-induced unfolding, surface-induced dissociation, and temperature-controlled nanoelectrospray mass spectrometry. Hemoglobin dimer and tetramer stability in solution and gas phases did not correlate, likely due to differences in the composition of positive and negative amino acids on the surface of these molecules. Specifically, the absence of Lys-116 on the β-subunit makes it easier for the human hemoglobin dimer to dissociate in the gas phase. However, the presence of Lys-60 makes the subunit more rigid thus it cannot unfold to the same extent as the other hemoglobin. Hemoglobin tetramers of different origins had similar stability in the gas phase, as there was no difference in the composition of charged amino acids at the tetramer interface. These results highlight how temperature-controlled mass spectrometry and collision-induced unfolding can elucidate the structural reasons behind differences in the gas-phase and solution stability of protein complexes.