Unravelling the factors that drive separation in differential mobility spectrometry: A case study of regioisomeric phosphatidylcholine adducts

Abstract

Differential mobility spectrometry (DMS) has shown promise as an analytical tool in the field of lipidomics. However, the underlying mechanism that drives DMS-based lipid separations is still somewhat unclear. Here, we investigate the finer details in the separability of the regioisomeric lipids 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) from 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC), including the effect of cation choice, chemical modifier, and temperature. We conduct DMS-MS studies that are supported by a hybrid molecular dynamics and quantum mechanical approach to explore the conformations and energetics of the [OPPC···X]+ and [POPC···X]+ (X = Ag, K) constructs. Computational models evaluated using density functional theory reveal structural differences between low energy regioisomeric silver adducts, which translates to unique collision cross sections. Structural differences in regioisomers, as reflected through collision cross section evaluations, are not retained in potassiated adducts. Population weightings suggest coalescence of [OPPC···Ag]+ and [POPC···Ag]+ collision cross sections as higher energy species become populated at elevated temperatures. This effect presents itself experimentally, revealing diminished resolving power as the temperature of the DMS cell is increased. The results outlined here provides atomistic insight into how dynamic ion collision cross sections affect separations and guidance for future DMS-driven lipidomics applications.