A Multi-Step Chromatographic Approach to Purify Radically Generated Ferulate Oligomers Reveals Naturally Occurring 5-5/8-8(Cyclic)-, 8-8(Noncyclic)/8-O-4-, and 5-5/8-8(Noncyclic)-Coupled Dehydrotriferulic Acids

Abstract

Ferulate-mediated cross-linking of plant cell wall polymers has various implications on the quality of plant based food products, forage digestibility, and biomass utilization. Besides dehydrodiferulic acids (DFA), dehydrotriferulic acids (TriFA) gained increasing interest over the past two decades, because they potentially cross-link up to three polymers. Here, we describe a separation strategy to obtain several TriFA as analytical standard compounds from a reaction mixture after radical coupling of ethyl ferulate. By using silica flash chromatography, Sephadex LH-20 chromatography, and reversed phase HPLC, six known TriFA as well as three previously unidentified ferulic acid trimers were obtained, and their structures were characterized by mass spectrometry and NMR spectroscopy (1H, HSQC, COSY, HMBC, and NOESY). The novel trimers were identified as 5-5/8-8(cyclic)-, 8-8(noncyclic)/8-O-4-, and, tentatively, 5-5/8-8(noncyclic)-TriFA. Natural occurrence of these TriFA in plant cell walls was demonstrated by LC-MS/MS analyses of alkaline cell wall hydrolyzates.

Hydroxycinnamic Acids and Their Derivatives in Broa, a Traditional Ethnic Maize Bread

“…Compounds 31, 61, 109, and 113 (TFA 1, 2, 3, and 4) also presented a precursor ion at m/z 577. Other minor TFAs that have been described in maize are 8-O-4′/5-8″ C , 8-8′/4-O-8″-and, possibly, 5-5′/8-8″-TFA [ 4 , 42 ]. Additionally, TFAs may also exhibit cis or trans configurations, therefore increasing their structural diversity.…”

“…5′/8-8′′-TFA [4,42]. Additionally, TFAs may also exhibit cis or trans configurations, therefore increasing their structural diversity.…”

Hydroxycinnamic acid‐modified xylan side chains and their cross‐linking products in rice cell walls are reduced in the Xylosyl arabinosyl substitution of xylan 1 mutant

“…Xylans are linked via radical–radical cross‐coupling of the ferulate moieties present on the xylan A side chains to lignin and other xylan chains. The cross‐coupling versatility is illustrated by the detection of a wide variety of diferulate isomers, i.e., 8–O‐4’, 8–8’, 8–5’, 5–5’, and 5–O‐4’, with the 5–5’‐linkage being the most frequent (Bento‐Silva et al., 2018; Ralph, 2010; Ralph et al., 2004; Waterstraat and Bunzel, 2018). Our findings further substantiated this through the detection of (i) different linkages, i.e., 8–O‐4’ and 8–5’, in the G‐FA‐A cross‐coupling products, (ii) lignin units other than G units (H units) attached to FA‐A, (iii) cross‐coupling products between ferulates and minor lignin monomers such as tricin, i.e., T‐FA‐A 32 , and (iv) detection of dicaffeates and several G‐CA‐A isomers, representing dicaffeates as potential novel lignin–xylan bridging structures.…”

“…Specifically, FA has been described to cross‐link xylan chains through oxidative coupling with other FA groups (Ishii, 1997). 5–5‐, 8–O–4‐, 8–5‐, and 8–8‐coupled ferulate dehydrodimers as well as dehydrotrimers and ‐tetramers have been identified (Bento‐Silva et al., 2018; Burr and Fry, 2009; Ralph, 2010; Waterstraat and Bunzel, 2018). The diversity of coupling products highlights that these ferulate structures are synthesized in a combinatorial radical cross‐coupling process similar to that which occurs during lignification (Ralph, 2010; Ralph et al., 2004).…”

Probing the role of cell wall feruloylation during maize development by differential expression of an apoplast targeted fungal ferulic acid esterase

“…Ferulic acid (FA) acylates a portion of arabinose residues via an ester-linkage at the C5-hydroxyl group of α-ʟ-arabinozyl residues of arabinoxylan and some ferulates are also ether- or C-C linked to lignin [ 7 – 9 ]. Upon oxidative coupling, mediated by peroxidase/H 2 O 2 , over 50% of cell wall esterified ferulates form different dimers [ 10 – 12 ] and trimers [ 13 , 14 ] cross-linking AX by creating inter and intra-molecular bonds between previously separated AX chains. Ferulate coupling with lignin also leads to cross-linking polysaccharides and lignin [ 8 ].…”