Enthalpies and Entropies of Activation for <i>sn-</i>1/<i>sn-</i>2 Acyl Chain Loss in Glycerophospholipid Anions via Dipolar DC Kinetics

Glycerophospholipids (GPs) have been observed to prefer losing a particular fatty acyl chain over the other, with the preference for the chain in either the <i>sn-</i>1 or <i>sn-</i>2 position being different between various GP classes. It has been assumed that the <i>sn</i> preference results from the entropic favorability of the transition state conformation; however, this has not been measured previously. Here, we demonstrate the application of our previously established collision-based activation method to GP fragmentation. The method utilizes a dipolar direct current (DDC) potential across a pair of opposing rods to force ions out of the center of the ion trap, causing them to undergo radio frequency (RF) heating by absorbing power from the trapping RF field. We confirmed that the previous assumption holds for some species studied here, wherein the ΔH^‡ values were nearly identical and the ΔS^‡ values showed greater differences between the <i>sn</i> positions. However, some species and ion types seem to be more driven by ΔH^‡. Additionally, the loss of the fatty acyl chains as neutrals rather than ions should also be considered if one is to thoroughly weigh which chain is indeed the preferred loss, as including all forms of acyl chain loss results in an overall favorability for the acyl chain in the <i>sn-</i>2 position to be lost. The driving force behind these different losses seems to be a mixture of entropic and enthalpic reasons, with the identity and presence of the headgroup playing an important role in the observed fragmentation and the measured activation parameters.