Figure 2.

Intermolecular interactions of cell wall polysaccharides and proteins from SSNMR. (a) Polysaccharide-polysaccharide through-space contacts from ¹H-driven ¹³C spin diffusion (PDSD) buildup curves [36]. Representative buildup curves are shown for intra-residue iC4–iC6 and inter-chain iC4-s/G/AC4 cross peaks within cellulose (black), intermolecular s/G/AC4-R/GAC1 cross peaks between surface cellulose and pectins (blue), and between interior cellulose iC4 and pectin R/GAC1 (red). (b) Hydration of wall polymers obtained from water-polysaccharide ¹H spin diffusion [39]. Water polarization transfer to pectins GAC2/RC5 (green) is faster than to cellulose iC4 in both intact and extracted cell walls. However, partial extraction of pectins significantly slows down water polarization transfer to all polymers, which can be attributed to the removal of Ca²⁺ ions, which immobilize water in the intact cell wall. (c) Expansin binding to dicot cell walls from ¹³C-¹⁵N filtered ¹³C spin diffusion experiments [25]. Representative 1D ¹³C spectra without spin diffusion, which show only the protein ¹³C signals, and with 3 s spin diffusion, which show both protein and polysaccharides signals, indicate that cellulose signals are enhanced by polarization transfer from wild-type expansin and a hyperactive mutant expansin. Representative protein-cellulose (iC4) magnetization transfer curves are shown for an active expansin mutant versus wild-type expansin. These data indicate that the functional binding target of the expansin is cellulose. (d) Structural model of the expansin-cellulose complex [25].