Arrhenius parameters of singly- and doubly-protonated bradykinin measured via dipolar DC kinetics on a quadrupole/time-of-flight platform

Abstract

Gas-phase fragmentation has long been used to obtain ion structure information, thereby significantly expanding the information that can be obtained via mass spectrometry. Experimental approaches for obtaining quantitative measures of the energies and entropies of unimolecular reactions of ions can be valuable for providing insights into mechanistic aspects of ion fragmentation. Such measurements can be particularly challenging for large polyatomic ions. Approaches that vary the temperatures of ions in a known way can be used to determine Arrhenius or Eyring parameters. However, such approaches require careful variation of the temperature of the vacuum system. Ion trap collisional activation provides another approach to varying ion temperatures provided the experimental conditions used can be connected with an effective ion temperature. Tolmachev et al. have described such a model using dipolar DC as a means for inducing controlled rf-heating. We have evaluated this model using the well-studied protonated leucine enkephalin as a thermometer ion and found that, with a few adjustments, the model is useful in establishing effective temperatures to associate with DDC kinetics measurements. In this work, we evaluate the adjusted model using another singly-protonated thermometer ion, protonated bradykinin, and extended the approach to doubly-protonated bradykinin. The latter ion allowed us to determine how to conduct ion survivor measurements using a channelplate detector under the scenario of a doubly-charged precursor giving rise to complementary pairs of singly charged products.