Solid-State Selective 13C Excitation and Spin Diffusion NMR to Resolve Spatial Dimensions in Plant Cell Walls

The average spatial dimensions between major biopolymers within the plant cell wall can be resolved using a solid-state NMR technique referred to as a ¹³C cross-polarization (CP) SELDOM (selectively by destruction of magnetization) with a mixing time delay for spin diffusion. Selective excitation of specific aromatic lignin carbons indicates that lignin is in close proximity to hemicellulose followed by amorphous and finally crystalline cellulose. ¹³C spin diffusion time constants (T _SD) were extracted using a two-site spin diffusion theory developed for ¹³C nuclei under magic angle spinning (MAS) conditions. These time constants were then used to calculate an average lower-limit spin diffusion length between chemical groups within the plant cell wall. The results on untreated ¹³C enriched corn stover stem reveal that the lignin carbons are, on average, located at distances ∼0.7-2.0 nm from the carbons in hemicellulose and cellulose, whereas the pretreated material had larger separations.