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

Abstract

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.