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 ope rated 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 ra tionalized on the basis of competition between unimolecular decomposit ion versus cooling of the ion-ion collision complex. In the case of pr otonated quinoline, no significant barriers are expected along the rea ction coordinate for proton transfer. However, the [C9H8N](+) ion-mole cule reaction product is not expected to transfer a proton without und ergoing rearrangement, as is consistent with ion trap collisional acti vation results. The rearrangement reaction introduces a significant ba rrier along the reaction coordinate, thereby increasing the lifetime o f the ion-ion collision complex. RRKM modeling for a polypeptide of co mparable size suggests that a barrier of 0.6 eV or greater will allow for the observation of cation attachment whereas the lifetimes of coll ision complexes with well depths less than similar to 0.6 eV are too s hort for collisional cooling by the bath gas to be effective.