Surface-Induced Protein Unfolding in Submicron Electrospray Emitters

The charging of protein ions formed by nanoelectrospray ionization (nanoESl) with tips that are between 1.5 mu m and 250 nm In outer diameter is compared. More charging is obtained with the smaller tip sizes :for proteins that have a net positive charge in solution, and additional high-charge-state distributions are often observed. A single charge-state distribution of holo-myoglobin ions is produced by nanoESl from a slightly acidified aqueous solution with the micron outer diameter tips, but some apo-myoglobin ions are produced with the submicron tips. In contrast, the charge-state distributions for proteins with a net negative charge in solution do not depend on tip size. Both the formation of high charge states and the appearance of higher-charge-state distributions, as well as the loss of the heme group from myoglobin, indicate that a fraction of the protein population is unfolding with the smaller tips: The increased charging with the smaller tip sizes for proteins with a net positive charge but not for proteins with a net negative charge indicates that the unfolding occurs prior to nanoelectrospray ionization as a result of Coulombic attraction between positively charged protein molecules in solution and the glass surfaces of the emitter tips that are negatively charged. These results demonstrate a novel method for producing highly charged protein ions that does not require exposing the proteins to additional chemicals either in solution or in the gas phase.