Comparative LCMS2 Analysis of Synthetic Ether- and Ester-Linked Polyacids Refines Structural Boundaries for Small Molecules in Dissolved Organic Matter

Dissolved organic matter (DOM) is a ubiquitous component of aqueous systems that contributes to both nutrient transport and carbon sequestration. However, its complexity limits accurate analysis, restricting insight into how it affects and is affected by environmental processes. The bulk of DOM contains molecular signatures indicative of chemical structures extensively functionalized with carboxyl groups, yet few similar molecules have been isolated from either natural product or synthetic chemistry. Recently, we showed that carbocyclic carboxylate-rich alicyclic molecules (CRAM) were inconsistent with the tandem mass spectrometry (MS2) data of DOM, proposing instead that ethers and esters are likely to link subunits of common scaffolds in natural samples. Here, we have synthesized 38 ether- or ester-linked polycarboxylic acids with systematically varied functional group compositions and orientations to test whether their liquid chromatography (LC) and MS2 properties better match those of DOM than carbocyclic CRAM analogues. LC data showed that the relative placement of chemical functionality around central cores is important in determining retention times of natural isomers but is still secondary to functional group composition. MS2 experiments highlight that aliphatic polyacids linked by ethers and esters show features consistent with DOM. Conversely, aromatic carboxylic acids were found to be poor matches to DOM's MS2 data, contradicting common paradigms in the field that ascribe elemental ratios to lignin or tannin derivatives. This work reiterates the need for compounds that accurately represent the functional group composition and carbon skeletons of not only DOM, but also theoretical proxies such as CRAM.