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 (EST) of the proteins from the H and H-c states in solution. Both states in solution produce the same charge states in EST. 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 similar to 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 (similar to 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. (J Am Soc Mass Spectrom 2008, 19, 1906-1913) (c) 2008 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry