Disulfide-intact and -reduced lysozyme in the gas phase: Conformations and pathways of folding and unfolding

The conformations of gaseous lysozyme ions (+5 through +18) produced by electrospray ionization have been studied in the gas phase using ion mobility mass spectrometry techniques. When solutions containing the disulfide-intact and disulfide-reduced lysozyme are electrosprayed, the gas-phase ions that are produced have distinctly different collision cross sections. Disulfide-intact ions favor two conformer types: a highly folded conformer with a cross section near that calculated for the crystal structure and a partially unfolded conformer that is formed when the ions are injected into the drift tube at high injection voltages. Ions formed from the disulfide-reduced solution have collision cross sections that are much larger than any observed for the disulfide-intact protein, showing that these ions are largely unfolded. Gas-phase proton-transfer reactions in the ion source can be used to favor lower charge states for both solutions. When protons are removed from disulfide-intact lysozyme ions, highly folded compact conformations are favored. Exposing the disulfide-reduced lysozyme ions to proton-transfer reagents causes the protein to fold up, and several of-the new conformations have cross sections that are indistinguishable from those measured for the disulfide-intact protein. It appears that an array of gas-phase folding intermediates or misfolded metastable states are stable because of the well-defined interplay between attractive-folding and repulsive-Coulombic interactions.