Structural Details of Crystalline Cellulose from Higher Plants

Abstract

It is commonly assumed that cellulose from higher plants contains the Iα and Iβ crystalline allomorphs together with surface and disordered chains. For cellulose Iα, the evidence for its presence in higher plants is restricted to the C-4 signals in the solid-state ¹³C NMR spectrum, which match those of crystalline cellulose Iα from algal sources. Algal cellulose Iα can be converted to the Iβ form by high-temperature annealing. We used this approach to generate cellulose samples differing in Iβ 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, Iβ and Iα-like phases from linear combinations of the observed ¹³C NMR and FTIR spectra. The ¹³C NMR spectra resembled those of highly crystalline tunicate or algal cellulose Iβ and Iα, with slight differences implying increased disorder and minor conformational discrepancies. The FTIR spectrum of the Iβ form was closely similar to its more crystalline counterparts, but the FTIR spectrum of the Iα 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 Iβ but show that the Iα-like chains, although conformationally quite similar to crystalline algal cellulose Iα, 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.