An untargeted liquid chromatography–mass spectrometry‐based workflow for the structural characterization of plant polyesters

Summary Cell wall localized heterogeneous polyesters are widespread in land plants. The composition of these polyesters, such as cutin, suberin, or more plant‐specific forms such as the flax seed coat lignan macromolecule, can be determined after total hydrolysis of the ester linkages. The main bottleneck in the structural characterization of these macromolecules, however, resides in the determination of the higher order monomer sequences. Partial hydrolysates of the polyesters release a complex mixture of fragments of different lengths, each present in low abundance and therefore are challenging to structurally characterize. Here, a method is presented by which liquid chromatography–mass spectrometry (LC‐MS) profiles of such partial hydrolysates are searched for pairs of related fragments. LC‐MS peaks that show a mass difference corresponding to the addition of one or more macromolecule monomers were connected in a network. Starting from the lowest molecular weight peaks in the network, the annotation of the connections as the addition of one or more polyester monomers allows the prediction of consecutive and increasingly complex adjacent peaks. Multi‐stage MS (MSn) experiments further helped to reject, corroborate, and sometimes refine the structures predicted by the network. As a proof of concept, this procedure was applied to partial hydrolysates of the flax seed coat lignan macromolecule, and allowed to characterize 120 distinct oligo‐esters, consisting of up to six monomers, and containing monomers and linkages for which incorporation in the lignan macromolecule had not been described before. These results showed the capacity of the approach to advance the structural elucidation of complex plant polyesters. Significance Statement Most analyses of plant polyesters have focussed on hydrolysates to determine their composition. Partial hydrolysates release a mixture of oligo‐esters that are too low abundant for purification, hampering any advance in the elucidation of the macromolecular structure. We developed an liquid chromatography–mass spectrometry‐based strategy for the structural characterization of a large number of oligo‐esters in partial hydrolysates. Application to the flax lignan macromolecule shows the relevance of the method in advancing the structural characterization of plant polyesters.