PROTEIN-STRUCTURE IN-VACUO - GAS-PHASE CONFIRMATIONS 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 cro ss sections are close to those estimated for the native solution-phase conformation. Furthermore, gas-phase BPTI retains its compact structu re when collisionally heated. These results are consistent with the kn own 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 na tive 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 serie s of well-defined intermediates. Only extended conformations are obser ved 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, intram olecular charge ''solvation'', and Coulomb repulsion. For the low char ge states, compact folded conformations have the lowest energy because they maximize intramolecular interactions. For intermediate charge st ates, elongated conformations, which minimize Coulomb repulsion while maximizing intramolecular interactions and intramolecular charge ''sol vation'', become favored. For the high charge states, the elongated co nformations unravel to an extended string as Coulomb repulsion dominat es.