A HILIC-PDA(-MS) profiling approach for the analysis of (methyl esterified) unsaturated galacturonic acid oligosaccharides released from pectins and food by-products

Functional and nutritional properties of pectins and pectic oligosaccharides (OS) depend on their structure. A major structural element of pectins are homogalacturonans, which consist of α-1,4-linked galacturonic acids (GalA). Homogalacturonans can be degraded to unsaturated (u)GalA-OS by pectin lyases. To analyze liberated uGalA-OS, a profiling approach based on hydrophilic interaction chromatography (HILIC) with photodiode array (PDA) and optional mass spectrometric (MS) detection was developed. Determination of molar relative response factors (RRF) of de-esterified uGalA-OS (degree of polymerization (DP) = 2-13) in relation to acarbose (internal standard) at 235 nm allows for (semi-)quantitative estimation of (methyl esterified) uGalA-OS and for application in other laboratories. Single quadrupole-MS enables to verify DP and to determine degree of methylation of uGalA-OS. Positions of methyl groups can be tentatively identified by orbitrap-MS. The developed profiling approach can be useful to analyze the specificity of a pectin lyase by analysis of uGalA-OS released from commercial pectin: The pectin lyase applied here appears to prefer a methyl group at cleavage subsite -1, but probably does not require a methyl group at subsite +1. In another application, various methyl esterified uGalA-OS (DP ≤ 12) were identified in enzymatically treated carrot pomace, a pectin-rich food by-product that can potentially be used to enhance nutritional properties of food products.