Structural Details of Crystalline Cellulose from Higher Plants

摘要

It is commonly assumed that cellulose from higher plants contains the la and 1beta crystalline allomorphs together with surface and disordered chains.For cellulose la,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 lalpha from algal sources.Algal cellulose la can be converted to the 1beta form by high-temperature annealing.We used this approach to generate cellulose samples differing in1beta 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,1betaand la-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 1beta and la,with slight differences implying increased disorder and minor conformational discrepancies.The FTIR spectrum of the 1beta form was closely similar to its more crystalline counterparts,but the FTIR spectrum of the lalpha 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 1beta but show that the la-like chains,although conformationally quite similar to crystalline algal cellulose lalpha,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.