Conformations of gas-phase ions of ubiquitin, cytochrome c, apomyoglobin, and beta-lactoglobulin produced from two different solution conformations.

At low pH in solutions of 50% methanol, proteins form expanded denatured states (the "H" state). In 90% methanol, proteins form expanded helical denatured states with artificial alpha-helices (the "H(c)" state). Gas-phase ions of ubiquitin, cytochrome c, apomyoglobin, and native and disulfide-reduced beta-lactoglobulin were formed by electrospray ionization (ESI) of the proteins from the H and H(c) states in solution. Both states in solution produce the same charge states in ESI. The conformations of the ions were studied with cross section measurements and gas-phase H/D exchange experiments. The cross sections show that the ions retain considerable folded structure. For a given protein and given charge state, ions produced from the H and H(c) states showed the same cross sections (within approximately 1%). Ions of cytochrome c, apomyoglobin, and native and reduced beta-lactoglobulin of a given charge state showed no differences in H/D exchange level when produced from the H or H(c) state. However, ubiquitin ions produced from the H(c) state consistently exchange fewer ( approximately 13%) hydrogens than ions produced from the H state, suggesting that in this case the gas-phase protein ions retain some memory of their solution conformations.