CONFORMATIONAL STABILITY OF HEME-PROTEINS IN VACUO

The function and conformational stability of myoglobin were studied in vacuo by using an electrospray mass spectrometer. The electrospray technique gently transfers protein molecules from the solution phase to the gas phase, and solvent-free protein ions are produced in the mass spectrometer. Horse myoglobin was dissolved at neutral pH, Fe3+ in the heme was reduced to Fe2+ to produce the biologically active oxymyoglobin in solution, and then the protein was isolated in vacuo. A molecular ion (17 601 Da) corresponding to the molecular mass of oxymyoglobin (17 600.0 Da) was observed in the mass spectrum. This demonstrates that the protein retains a heme and an oxygen molecule in the gas phase. Since the biological function of myoglobin is to carry an oxygen molecule, this is the first observation that a protein is functional in the absence of solvent. Gas-phase ''unfolding'' of myoglobin was also studied. Collisions of accelerated protein ions with nitrogen curtain gas at a quadrupole guidance lens or argon gas introduced at a second quadrupole increase the ''molecular temperature'' of myoglobin, resulting in release of the heme from myoglobin. Apomyoglobin produced at the quadrupole guidance lens showed a larger collisional cross section than that of myoglobin, revealing conformational disordering of the protein. The gas-phase unfolding of horse and whale myoglobins and the alpha-chain of human hemoglobin induced at the second quadrupole were studied as a function of the argon gas thickness. Horse and whale myoglobins showed the same gas-phase stability, whereas the alpha-chain of human hemoglobin was less stable. Consequently, the amino acid sequence by itself has sufficient information to stabilize the gas-phase conformation of the heme proteins.