Abstract
Recalcitrance of grasses to enzymatic digestion arises to a significant degree from a complex array of phenolic crosslinks between cell wall polysaccharide chains that inhibit their conversion to biofuels and lower their nutritive value for animal feed applications. Polysaccharide esters of ferulic acid are abundant in plant cell walls. Crosslinks between polysaccharides are formed through oxidative dehydrodimerization of ferulates, producing dehydrodiferulates (henceforth termed diferulates). Such ferulates and diferulates further crosslink plant cell walls by radical coupling cross-reactions during lignification. Although cell wall digestibility can be improved by cell wall metabolic engineering, or post-harvest by various pretreatment processes, a more comprehensive understanding of the role and impact of ferulate crosslinking on polysaccharide hydrolysis would be accelerated by availability of analytical methods that can distinguish the various diferulates released during biomass pretreatments, many of which are isomers. In this report, we present an ultrahigh-performance liquid chromatography/tandem mass spectrometry (UHPLC/MS/MS) strategy for comprehensive separation and identification of diferulate isomers. Collision-induced dissociation (CID) mass spectra of [M + H]+ ions distinguished various isomers without requiring derivatization. Characteristic product ions for 8-O-4-, 8-8-non-cyclic, 8-8-cyclic, 8-5-cyclic, 8-5-non-cyclic, and 5-5-linked isomers were identified. All diferulates were identified either as di-acids in extracts of NaOH-hydrolyzed corn stover, or as a diverse group of diferulate mono- and di-amides in extracts of Ammonia Fiber Expansion (AFEX™)-treated corn stover. This approach allows for direct analysis of released diferulates with minimal sample preparation, and can serve as the foundation for high-throughput profiling and correlating pretreatment conditions with biomass digestibility in biorefineries producing biofuels and biochemicals.