Improved analysis of arabinoxylan-bound hydroxycinnamate conjugates in grass cell walls

摘要

Methanolysis of hydroxycinnamoylated arabinoxylan. Top: Schematic arabinoxylan chains shown with the arabinosyl substitutions on every second unit as revealed recently [39], along with pCA and FA substitutions on arabinosyl units, two representative diferulates (8-O-4 and 8-5) formed by radical coupling, and a monolignol ferulate (ML-FA) conjugate, all known to be present in grass cell walls; acetate substitutions, also known, are not shown. The bonds produced by radical coupling are shown in bold-black, and the bonds formed by post-coupling rearomatization reactions (along with the OH from water addition in the case of the ML-FA coupling) are in gray. Bottom: The derived products released for structural analysis and quantification upon methanolysis that, ideally, methanolyzes the arabinosyl unit from the xylan backbone but leaves the arabinosyl esters intact. The original xylan chain is shown with low opacity for easier visualization of the source of the products, and the elemental formulas and exact mass (for MS analysis) are shown. As noted in the text, some actual hydrolysis can occur (producing acids) and some methanolysis of the hydroxycinnamate-arabinosyl ester can occur, producing DFAs with only a single Ara unit; the improved method here produces lower amounts of these undesirable side-products and more of the desired pCA-MeAra 3, FA-MeAra 4, and the various DFA-diMeAra isomers 6 (as well as ML-FA-MeAra cross-products 7); the compound numbers are from Fig. 2