New insights into the structure of cellulose in plant cell walls

Abstract

The structure of plant cellulose microfibrils remains elusive, despite the abundance of cellulose and its utility in industry. Using 2D solid-state NMR of 13C labelled never-dried plants, six major glucose environments are resolved which are common to the cellulose of softwood, hardwood and grasses. These environments are maintained in isolated holo-cellulose nanofibrils, allowing more detailed microfibril characterisation. We show there are only two glucose environments that reside within the microfibril interior. These have the same NMR 13C chemical shifts as tunicate cellulose I{beta} centre and origin chains, with no cellulose I being detected. The third major glucose site with a carbon 4 chemical shift near 89 ppm, previously assigned to the crystalline microfibril interior, is now shown to be one of four surface glucose environments. The NMR peak widths of all four surface glucose environments are similar to those of the core indicating that their glucose local order is comparable; there is no significant amorphous cellulose in the microfibrils. Consequently, the ratio of the carbon 4 peaks at [~]89 and [~]84 ppm, which has often provided a sample cellulose crystallinity index, is not a meaningful measure of crystallinity or the interior to surface ratio. The revised ratio for poplar wood microfibrils is estimated to be 1:2, which is consistent with a cellulose I{beta} 18-chain microfibril having 6 core and 12 surface chains, although other microfibril sizes are possible. These advances change substantially both the interpretation of solid-state NMR studies of cellulose and the understanding of cellulose microfibril structure and crystallinity.