Cation attachment to multiply charged anions of oxidized bovine insulin A-chain

Multiply charged anions of oxidized bovine insulin A-chain react with protonated quinoline exclusively by proton transfer in a Paul trap operated with helium bath gas at a pressure of 10(-3) Torr. The isomeric [C9H8N]+ ions formed from the reaction of [C4H4]+ with pyridine, on the other hand, react largely by attachment to the multiply charged anions of oxidized bovine insulin A-chain. This observation can be rationalized on the basis of competition between unimolecular decomposition versus cooling of the ion-ion collision complex. In the case of protonated quinoline, no significant barriers are expected along the reaction coordinate for proton transfer. However, the [C9H8N]+ ion-molecule reaction product is not expected to transfer a proton without undergoing rearrangement, as is consistent with ion trap collisional activation results. The rearrangement reaction introduces a significant barrier along the reaction coordinate, thereby increasing the lifetime of the ion-ion collision complex. RRKM modeling for a polypeptide of comparable size suggests that a barrier of 0.6 eV or greater will allow for the observation of cation attachment whereas the lifetimes of collision complexes with well depths less than approximately 0.6 eV are too short for collisional cooling by the bath gas to be effective.