Protein Structure in Vacuo:  Gas-Phase Conformations of BPTI and Cytochrome c

Ion mobility measurements have been used to examine the geometries of naked BPTI (bovine pancreatic trypsin inhibitor) and cytochrome c ions in the gas phase, as a function of charge. For BPTI, the measured cross sections are close to those estimated for the native solution-phase conformation. Furthermore, gas-phase BPTI retains its compact structure when collisionally heated. These results are consistent with the known stability of BPTI, where the three-dimensional structure is partly locked into place by three covalent disulfide bridges. For cytochrome c, geometries with cross sections close to those estimated for the native solution phase structure were observed for the low charge states. For intermediate charge states, the compact geometries are metastable, and when collisionally heated they gradually unfold, through a series of well-defined intermediates. Only extended conformations are observed for the higher charge states, and they become more extended as the charge increases. The gas-phase conformation of a protein ion results from a balance between attractive intramolecular interactions, intramolecular charge “solvation”, and Coulomb repulsion. For the low charge states, compact folded conformations have the lowest energy because they maximize intramolecular interactions. For intermediate charge states, elongated conformations, which minimize Coulomb repulsion while maximizing intramolecular interactions and intramolecular charge “solvation”, become favored. For the high charge states, the elongated conformations unravel to an extended string as Coulomb repulsion dominates.