Conformations, unfolding, and refolding of apomyoglobin in vacuum: An activation barrier for gas-phase protein folding

Gas-phase ion mobility measurements have been used to characterize the conformations of the +4 to +22 charge states of apomyoglobin. For the +8 to +10 charge states, generated by electrospraying pH approximate to 3 solutions, two relatively compact conformations were resolved which may reflect the state of the protein in solution. These relatively compact conformations unfold into more extended conformations when collisionally heated. Only extended conformations are observed for the high (>+10) charge states, and they become more extended as the charge increases. Proton stripping of the higher (>+7) charge states to produce the +4 to +7 charge states results in spontaneous collapse into partially folded conformations. Further folding is observed upon collisional heating of the collapsed structures, indicating the presence of an activation barrier for protein folding in the gas phase. The barrier probably results from Coulomb repulsion and the reorganization of secondary structure. For the lower (<+7) charge states, the most stable conformations appear to be slightly more compact than the native protein in solution. The collision cross sections per residue for the extended conformations of apomyoglobin and cytochrome c are similar. The cross sections for the compact folded conformations of these proteins also scale with the number of residues. This suggests that different proteins share common structural motifs in the gas phase, as they do in solution.