Abstract
Graminaceous arabinoxylans are distinguished by decoration with feruloylated monosaccharidic and oligosaccharidic side-chains. Although it is hypothesized that structural complexity and abundance of these feruloylated arabinoxylan side-chains may contribute, among other factors, to resistance of plant cell walls to enzymatic degradation, quantitative profiling approaches for these structural units in plant cell wall materials have not been described yet. Here we report the development and application of a rapid and robust method enabling the quantitative comparison of feruloylated side-chain profiles in cell wall materials following mildly acidic hydrolysis, C18-solid phase extraction (SPE), reduction under aprotic conditions, and liquid chromatography with diode-array detection/mass spectrometry (LC-DAD/MS) separation and detection. The method was applied to the insoluble fiber/cell wall materials isolated from 12 whole grains: wild rice (Zizania aquatica L.), long-grain brown rice (Oryza sativa L.), rye (Secale cereale L.), kamut (Triticum turanicum Jakubz.), wheat (Triticum aestivum L.), spelt (Triticum spelta L.), intermediate wheatgrass (Thinopyrum intermedium), maize (Zea mays L.), popcorn (Zea mays L. var. everta), oat (Avena sativa L.) (dehulled), barley (Hordeum vulgare L.) (dehulled), and proso millet (Panicum miliaceum L.). Between 51 and 96% of the total esterified monomeric ferulates were represented in the quantified compounds captured in the feruloylated side-chain profiles, which confirms the significance of these structures to the global arabinoxylan structure in terms of quantity. The method provided new structural insights into cereal grain arabinoxylans, in particular, that the structural moiety α-l-galactopyranosyl-(1→2)-β-d-xylopyranosyl-(1→2)-5-O-trans-feruloyl-l-arabinofuranose (FAXG), which had previously only been described in maize, is ubiquitous to cereal grains.
Highlights
Ferulic acid is found ester-linked to arabinoxylans in the primary and secondary cell walls of grasses
The reducing sugar moiety of the native standard compounds rules out their chromatographic separation under the primarily aqueous conditions applied in C18-based HPLC, where oncolumn mutarotation between the reducing sugar’s α- and βanomers, which differ slightly in their retention factors, produces broad, tailing, split peaks
This was especially true for FAX, which spawned a peak over 4 min wide
Summary
Ferulic acid is found ester-linked to arabinoxylans in the primary and secondary cell walls of grasses. The effects of these monomeric ferulate sidechains have not been as thoroughly researched as ferulate crosslinking, both increased appearance and complexity of feruloylated side-chains are hypothesized to reduce enzymatic digestibility and slow fermentation of feruloylated arabinoxylans (Yang et al, 2013; de Vries et al, 2014; Snelders et al, 2014), which has implications for plant protection mechanisms against pathogens, the prebiotic and (potential) antioxidative human health benefits of dietary fibers from different cereal grains, livestock nutrition, and biofuel production. To date, a direct quantitative comparison of the feruloylated side-chain profiles from different grain or forage materials has not been performed, and estimations of profile complexity have been based on results from time-consuming preparative isolations (Saulnier et al, 1995; Bunzel et al, 2002; Allerdings et al, 2006). We have developed a liquid chromatography with diode-array detection/mass spectrometry (LC-DAD/MS) screening method enabling quantitative comparison of the feruloylated side-chain profiles of plant cell wall materials following mildly acidic hydrolysis, C18-solid phase extraction (SPE), and reduction
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