Structural investigation of cellulose Iα and Iβ by 2D RFDR NMR spectroscopy: determination of sequence of magnetically inequivalent d-glucose units along cellulose chain

Abstract

In our previous studies of the crystal structure of native cellulose (cellulose I) by solid-state two-dimensional (2D) 13C–13C INADEQUATE, it was revealed that cellulose Iα contains two kinds of β-d-glucose residues (A and A′) in the unit cell and that cellulose Iβ contains another two kinds of residues (B and B′). In the present study, the sequence of residues A and A′ along the same chains in cellulose Iα and that of residues B and B′ in Iβ were investigated by 2D 13C–13C rotor-synchronized radiofrequency-driven recoupling (RFDR) experiments using, respectively, uniformly 13C6-labeled (U−13C6) bacterial cellulose and the same [U−13C6] cellulose sample after thermal treatment at 260 °C. The RFDR spectra recorded with a short mixing time (1.0 ms) showed dipolar-coupled 13C spin pairs of only the neighboring carbon of the both phases, while those recorded with a longer mixing time (3.0–15 ms) provided correlations between weakly coupled 13C spin pairs as well as strongly coupled 13C spin pairs such as neighboring carbon nuclei. In the RFDR spectrum of the [U−13C6] cellulose recorded with a mixing time of 15 ms, the inter-residue 13C–13C correlation between C4 of residue A and C2 of residue A′ and that between C3 of residue A and C4 of residue A′ were clearly observed. In the case of cellulose Iβ, however, inter-residue 13C–13C correlations between residues B and B′ could not be detected in the series of RFDR spectra recorded with different mixing times of annealed [U−13C6] cellulose. From these findings, that cellulose Iα was revealed to have the –A–A′– repeating units along the cellulose chain. For cellulose Iβ, it was revealed that the respective residues B and B′ are composed of independent chains (–B–B– and –B′–B′– repeating units) and that there are no –B–B′– repeating units in the chain.