NMR of Fully and Partially $^{13}$C-Enriched Biomass Enhances Pendent Group Structural Characterization

Traditional solution-state NMR experiments may either fail or yield unsatisfactory results when employing fully-$^{13}$C-labeled biomass due to complications arising from $^{13}$C–$^{13}$C coupling. Constant-time analogs of HSQC experiments mitigate such issues and deliver enhanced sensitivity. A rarely reported CT-HSQC-TOCSY experiment allows the proton coupling network to deliver much of the same value as the parent experiment on unlabeled or 10–15%-$^{13}$C-labeled biomass polymers but with enhanced sensitivity. In the absence of a viable HMBC analog for long-range correlations, a relayed C–C experiment, i.e., via directly bonded $^{13}$C-labeled networks, enables the reliable assignment of coupled carbons, with the added advantage of correlating the more elusive quaternaries. A C–C-FLOPSY experiment takes advantage of fully-$^{13}$C-labeled materials for mapping extensive carbon networks in the complex polymer mixtures inherent in biomass. Various pendent groups (tricin units, cis- and trans-p-coumarates, and p-hydroxybenzoates) that adorn lignins, and the cis- and trans-ferulates on arabinoxylan polysaccharides, are exquisitely revealed in spectra from isolated lignins or whole-cell-wall materials from maize, sorghum, and poplar.