Solution-state structures of the cellulose model pullulan in lithium chloride/N,N-dimethylacetamide.

Pullulan is soluble in both N,N-dimethylacetamide (DMAc) and LiCl/DMAc. Here, the solution-state structures of pullulan in these two solvents were studied by size-exclusion chromatography with multi-angle laser light scattering (SEC/MALLS) and nuclear magnetic resonance (NMR) analyses. SEC/MALLS and off-line refractive index analyses revealed that the hydroxy groups of pullulan interact with LiCl in DMAc, as observed previously for cellulose. The nine different COH protons of pullulan in DMAc and LiCl/DMAc appeared independently in NMR spectra and were successfully assigned. When LiCl was present, the degree of dissociation of all COH protons of pullulan was increased by Li+ and Cl- ions, and the conformations of the COH groups became stable and restricted in DMAc. Based on the obtained results, the solution-state structures of cellulose in LiCl/DMAc are hypothesized to contain similar interactions between the cellulose hydroxy groups, and Li+ and Cl- in LiCl/DMAc to those observed for pullulan.