Ion mobility mass spectrometry for lipopeptides analysis

Cyclic lipopeptides (CLPs) are produced by bacteria in a ribosome independent manner and consists in a hydrophilic peptide linked to a B-hydroxy fatty acid chain. Despite CLPs have relatively low molecular weight (between 800 and 2,000 Da), the analysis of lipopeptides remains challenging due to the wide variety of synthetized isoforms differing in fatty acid chain length, in methyl group branching position, and in the nature of the amino-acids residues. These isoforms are suspected to have different biological activities requiring development of reliable methods for CLPs identification and characterization. In this work, we present an original approach combining UPLC and ion mobility - mass spectrometry at high resolving powers to separate and identify the different species.
Lipopeptides were separated by UPLC (I-class, Waters, U.K.) on a C18 BEH column and identified by CID MS/MS mass spectrometry. Ion mobility – mass spectrometry (IM-MS) measurements were performed on a trapped Ion Mobility Mass Spectrometer (TimsTOF, Bruker Daltonics, U.S.A.). Accurate Collison Cross Section (CCS) were obtained in both positive and negative mode and compared with theoretical CCS obtained by the trajectory method from IMoS V2.
Separation of lipopeptides such as surfactins, iturins, or mycosubtilins was successfully performed by reverse phase liquid chromatography. Lipopeptides were separated according to lipid chain length and according to the branching position of the methyl group (iso/anteiso/linear).
Ion mobility analysis with computational chemistry support gave additional information on the lipopeptides structure and family. Structural information are obtained by experimental and theoretical CCS comparison while the lipopeptides family could be determined with CCS/mass trends. These results pave the way to a new strategy for fast lipopeptides identification and could potentially contribute to lipopeptides structure elucidation.