Two-dimensional spin-exchange solid-state NMR study of the crystal structure of cellulose II

A two-dimensional (2D) 13C–13C spin-exchange NMR experiment was applied to the uniformly 13C-enriched cellulose II in order to obtain interatomic distance information in the cellulose II crystal. A radio frequency-driven recoupling (RFDR) rotor-synchronized π pulse sequence was incorporated during the mixing time of the 2D experiment. The 2D spin-exchange NMR recorded with a short mixing time (0.80–2.58 ms) provided the correlations between a pair of strongly coupled 13C spins such as neighbor carbon nuclei. This spectrum enabled us to assign all 13C resonance lines of two kinds of anhydroglucose residues A and B in the structure of cellulose II. On the basis of the 13C resonance assignment of residues A and B, the interatomic distances from each C1 to the other carbon nuclei were compared by measuring the 2D spectra recorded with longer mixing times (5.12–20.48 ms). As a result, it was revealed that the respective residues A and B are composed of independent chains (–A–A– and –B–B– repeating units) and that there are no –A–B– repeating units in the chain. This experimental technique is expected to be applicable to conformation analysis of polysaccharides as well as the other cellulose polymorphs.