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| - Disulfide-Intact and -Reduced Lysozyme in the Gas Phase: Conformations and Pathways ofFolding and Unfolding
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| - The conformations of gaseous lysozyme ions (+5 through +18)produced by electrospray ionization havebeen studied in the gas phase using ion mobility mass spectrometrytechniques. When solutions containingthe disulfide-intact and disulfide-reduced lysozyme are electrosprayed,the gas-phase ions that are producedhave distinctly different collision cross sections.Disulfide-intact ions favor two conformer types: ahighlyfolded conformer with a cross section near that calculated for thecrystal structure and a partially unfoldedconformer that is formed when the ions are injected into the drift tubeat high injection voltages. Ions formedfrom the disulfide-reduced solution have collision cross sections thatare much larger than any observed forthe disulfide-intact protein, showing that these ions are largelyunfolded. Gas-phase proton-transfer reactionsin the ion source can be used to favor lower charge states for bothsolutions. When protons are removedfrom disulfide-intact lysozyme ions, highly folded compactconformations are favored. Exposing the disulfide-reduced lysozyme ions to proton-transfer reagents causes the protein tofold up, and several of the newconformations have cross sections that are indistinguishable from thosemeasured for the disulfide-intactprotein. It appears that an array of gas-phase foldingintermediates or misfolded metastable states are stablebecause of the well-defined interplay between attractive−folding andrepulsive−Coulombic interactions.
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