Atomic Structure of Phosphorylated Nanocellulose Revealed by Dynamic Nuclear Polarization-Enhanced Solid-State NMR

Atomic-level details of nanocellulose structure, particularly at its surface, remain difficult to resolve and are therefore poorly understood. Here we apply dynamic nuclear polarization (DNP)-enhanced solid-state NMR to directly probe the surface chemistry of phosphorylated cellulose nanofibers. Multidimensional ¹³C-¹³C and ³¹P-¹³C correlation experiments reveal both mono- and diphosphate substitution, with C2 and C6 identified as the preferred sites. Quantitative multiCP analysis establishes the degree of phosphorylation and the conformational distribution of surface phosphate groups, while ³¹P-³¹P correlations reveal their corresponding spatial distribution. The site-specific phosphorylation derived from DNP-NMR data was used to construct a CNF model for molecular dynamics (MD) simulations, which reproduce the fibril twisting observed by AFM and yield ³¹P-³¹P radial distribution functions consistent with the DNP-NMR data. In addition, the MD-derived C6 phosphorylated and nonphosphorylated conformational distribution both within the fibril core and at its surface is in agreement with the DNP-NMR data. To rationalize the preferred conformations of phosphorylated C6 groups observed in both NMR experiments and MD simulations, DFT calculations were carried out and show that these conformations are governed by facet-dependent hydrogen-bond formation at the nanocellulose surface.