Determination of the dissociation kinetics of a transient intermediate

Abstract

Tandem mass spectrometry provides information on the dissociation pathways of gas-phase ions by providing a link between product ions and parent ions. However, there exists a distinct possibility that a parent ion does not dissociate directly to the observed product ion, but that the reaction proceeds through unobserved reaction intermediates. This work describes the discovery and kinetic analysis of an unobserved reaction intermediate with a quadrupole ion trap. [a ₄−NH₃] ions formed from [YGβFL+H] ions dissociate to [(F*YG−NH₃)−CO] ions. It is expected, however, from previous results, that [F*YG−NH₃] ions should form prior to [(F*YG−NH₃)−CO] ions. Double-resonance experiments are used to demonstrate the existence of intermediate [F*YG − NH₃] ions. Various kinetic analyses are then performed using traditional collision-induced dissociation kinetics and double-resonance experiments. The phenomenological rates of formation and decay of peptide rearrangement ion dissociation products are determined by curve fitting decay and formation data generated with the kinetics experiments. The data generated predict an observable level of the intermediate in a time frame accessible but previously not monitored. By examining early product-ion formation, the intermediate ions, [F*YG−NH₃]⁺, are observed.