Molecular conformations at the cellulose–water interface

Abstract

13C-NMR chemical shifts were measured for C-4 and C-6 in a collection of eight crystalline glucoses and glucosides. The influence of the hydroxymethyl conformation was greater at C-4 than at C-6, with mean chemical shifts for gauche–trans molecules displaced 3.1 ppm (C-4) and 2.5 ppm (C-6) relative to gauche–gauche molecules. This information was used to interpret 13C-NMR spectra of crystalline celluloses. Chemical shifts for C-4 in the crystallite cores of celluloses I and II differed by just 0.2 ppm, but the corresponding chemical shifts for well-ordered crystallite surfaces differed by 3.0 ppm. The separation between crystallite-surface signals was attributed to different hydroxymethyl conformations at the cellulose–water interface, i.e., gauche–gauche and gauche–trans on crystallites of cellulose I and cellulose II, respectively. A broad C-4 signal in the spectrum of cellulose II indicated gauche–gauche conformations in disordered cellulose. Chemical shifts for C-6 were consistent with these conformations.