GASEOUS PROTEIN CATIONS ARE AMPHOTERIC

Singly- and multiply-protonated ubiquitin molecules are found to react with iodide anions, and certain other anions, by attachment of the an ion, in competition with proton transfer to the anion. The resulting a dduct ions are relatively weakly bound and dissociate upon collisional activation by loss of the neutral acid derived from the anion. Adduct ions that behave similarly can also be formed via ion/molecule reacti ons involving the neutral acid. The ion/molecule reaction phenomenolog y, however, stands in contrast with that expected based on the reactio n site(s) being charged. Reaction rates increase inversely with charge state and the total number of neutral molecules that add to the prote in cations increases inversely with cation charge. These observations are inconsistent with the formation of proton-bound clusters but are f ully consistent with the formation of ion pairs or dipole/dipole bondi ng involving the neutral acid and neutral basic sites in the protein. The ion/ion reactions can be interpreted on the basis of conjugate aci d/base chemistry in which the anion, which is a strong gaseous base, r eacts with a protonated site, which is a strong gaseous acid. Adduct i ons can also be formed via ion/molecule reaction which, on the basis o f microscopic reversibility, implies that the neutral acid interacts w ith neutral basic sites on the protein cation. These results suggest t hat acid adduction to gaseous protein cations can be complementary in nature to chemical reactions, such as proton transfer and hydrogen/deu terium exchange, that are strongly mediated by the charge site(s).