Submitted to: Rapid Communications in Mass Spectrometry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2011
Publication Date: 2/1/2011
Citation: Bowman, M.J., Dien, B.S., O Bryan, P.J., Sarath, G., Cotta, M.A. 2011. Selective chemical oxidation and depolymerization of switchgrass (Panicum virgatum L.) xylan with oligosaccharide product analysis by mass spectrometry. Rapid Communications in Mass Spectrometry. 25(8):941-950.
Interpretive Summary: Biomass from dedicated energy crops (e.g., switchgrass) represents a source of renewable material for conversion to ethanol and other liquid biofuels. The conversion of biomass to simple monosaccharides, required for fermentation, is a complex process typically involving chemical and enzymatic steps. The focus of this research is on the xylan component of switchgrass comprising approximately 30% of the plant's biomass. Xylan has been suggested to have a structural role as substitutions of the chain are not random, but rather are grouped into domains. A deeper understanding of the domain structure can lead to more efficient pretreatments or optimized enzyme blends to hydrolyze xylan and allow for more efficient conversion of cellulose to glucose. This work represents method development for the determination of xylan structural features of herbaceous energy crops and how these features effect conversion to biofuels.
Technical Abstract: Xylan is a barrier to enzymatic hydrolysis of plant cell walls. It is well accepted that the xylan layer needs to be removed to efficiently hydrolyze cellulose and consequently pretreatment conditions are in part optimized for maximal xylan depolymerization or displacement. Xylan consists of a long chain of beta 1,4-linked xylose units substituted with arabinose (usually alpha-1,3-linked in grasses) and glucuronic acid (alpha-1,2-linked). Arabinose units can be further substituted with acids (acetic, uronic, and phenolic acids) and/or hexose. It has been proposed that the substitutions are not random along the xylan chains but rather occur as domain structures suggesting they have a structural function and therefore may play a role in determining reactivity of xylan to pretreatment. Analysis of intact xylan is problematic because of its chain length and heterogeneous side groups; therefore, we explored conditions to partially hydrolyze alkaline-extracted switchgrass xylan, with and without prior modification, for analysis of the resultant multimeric fragments by high performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) to determine conditions producing a distribution of information-rich arabinoxylooligomers. A two-step process of oxidation followed by acid depolymerization provided oligosaccharides containing the desired arabinose branches intact. Selected samples were derivatized with a reducing end label and analyzed by hydrophilic interaction chromatography-liquid chromatography-mass spectrometry/mass spectrometry (HILIC-LC-MS/MS) to demonstrate the potential utility of this chemistry for determining the sequence of arabinoxylooligosaccharides present in plant cell wall material.