Structural Details of Crystalline Cellulose from Higher Plants

Abstract

It is commonly assumed that cellulose from higher plants contains the Ialpha and Ibeta crystalline allomorphs together with surface and disordered chains. For cellulose Ialpha, the evidence for its presence in higher plants is restricted to the C-4 signals in the solid-state (13)C NMR spectrum, which match those of crystalline cellulose Ialpha from algal sources. Algal cellulose Ialpha can be converted to the Ibeta form by high-temperature annealing. We used this approach to generate cellulose samples differing in Ibeta content from flax fibers and celery collenchyma, which respectively are representative of textile (secondary-wall) and primary-wall cellulose. It was then possible to isolate the detailed spectral contributions of the surface, Ibeta and Ialpha-like phases from linear combinations of the observed (13)C NMR and FTIR spectra. The (13)C NMR spectra resembled those of highly crystalline tunicate or algal cellulose Ibeta and Ialpha, with slight differences implying increased disorder and minor conformational discrepancies. The FTIR spectrum of the Ibeta form was closely similar to its more crystalline counterparts, but the FTIR spectrum of the Ialpha form was not. In addition to increased bandwith indicative of lower order, it showed substantial differences in the profile of hydroxyl stretching bands. These results confirm that higher plants synthesize cellulose Ibeta but show that the Ialpha-like chains, although conformationally quite similar to crystalline algal cellulose Ialpha, sit in a different hydrogen-bonding environment in higher plants. The differences are presumably occasioned by the small diameter of the crystallites and the influence of the crystallite surface on chain packing.