ACTIVATION PARAMETERS OF SMALL BIOMOLECULE IONS MEASURED VIA DIPOLAR DC KINETICSIN A QUADRUPOLE ION TRAP

Measuring the dissociation kinetics of ions enables us to better understand their structure and behavior. When a dissociation method allows for the calculation of activation parameters such as the Arrhenius or Eyring parameters, we can gain even more insight into how an ion is fragmenting and why there might be a preference for the formation of one fragment over another. However, not all dissociation methods that enable these measurements are widely available and easy to implement in all laboratories. Here, we have expanded on the work done by Tolmachev et al. to develop a collision-based method that allows for the measurement of activation parameters on nearly any quadrupole-based instrument via the application of a dipolar direct current (DDC) across a pair of opposing rods. When this offset voltage is applied, all ions in the trap are moved from the center toward one of the rods and undergo radio frequency-heating. This applied offset voltage can be converted into an “effective temperature” and thus can be used to determine activation energies, preexponential factors, ΔH^‡, and ΔS^‡. We have expanded the original model by Tolmachev to account for factors such as the use of round rods and the use of polyatomic collision gases, as well as accounting for complications such as measuring the survival yield of a multiply charged precursor and accounting for detector efficiency based on the mass-to-charge (m/z) of an analyte.

We applied this model to a series of glycerophospholipids to determine the driving factor behind the preference for losing the fatty acyl chain in the sn-2 position as well as to determine the influence of the headgroup present on the ion. For phosphatidylcholines, we confirmed the assumption that the main factor for the sn-2 preference was the difference in ΔS^‡ values between the two acyl chain positions. For different headgroups such as ethanolamine, serine, and glycerol, this is not necessarily the case and there are contributions from the ΔH^‡ as well. For phosphatidylethanolamine and phosphatidylserine species the ΔH^‡ seems to be the main driving factor and for the phosphatidylglycerol there are influences seen from both ΔH^‡ and ΔS^‡. Additionally, we have preliminarily confirmed the behavior of these intact glycerophospholipids by also looking at a few lysophospholipids where the sn-2 chain has been cleaved off using phospholipase A₂. The values we obtained matched those for the intact glycerophospholipids, confirming that the sn-2 chain does not affect the dissociation of the sn-1 chain from the structure. We want to expand this to investigate the opposite, whether the sn-1 chain influences the dissociation of the sn-2 chain, as well as look at species we were not previously able to look at due to isomeric impurities, which the use of phospholipase A₂ is able to effectively circumvent via the exclusive cleavage at the sn-2 position.