Kb. Shelimov et Mf. Jarrold, CONFORMATIONS, UNFOLDING, AND REFOLDING OF APOMYOGLOBIN IN VACUUM - AN ACTIVATION BARRIER FOR GAS-PHASE PROTEIN-FOLDING, Journal of the American Chemical Society, 119(13), 1997, pp. 2987-2994
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 whi
ch may reflect the state of the protein in solution. These relatively
compact conformations unfold into more extended conformations when col
lisionally heated. Only extended conformations are observed for the hi
gh (>+10) charge states, and they become more extended as the charge i
ncreases. Proton stripping of the higher (>+7) charge states to produc
e the +4 to +7 charge states results in spontaneous collapse into part
ially folded conformations. Further folding is observed upon collision
al heating of the collapsed structures, indicating the presence of an
activation barrier for protein folding in the gas phase. The barrier p
robably results from Coulomb repulsion and the reorganization of secon
dary structure. For the lower (<+7) charge states, the most stable con
formations 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 protei
ns share common structural motifs in the gas phase, as they do in solu
tion.