6-12: Liquid ammonia driven structural crossover in a cellulose Iβ fibril: A molecular dynamics study

Cellulose occurs naturally as hydrogen bonded crystalline nanofibrils (cellulose Iβ) that srongly resit hydrolysis to reactive sugars. Experiments showed that anhydrous ammonia causes subtle alterations within the fibril hydrogen bond network, producing a crystalline allomorph (cellulose III-I) with enhanced cellulase hydrolyzability, yet little is known about the molecular details of the interactions between liquid ammonia and cellulose Iβ fibrils.

In this study, we present the results of extensive molecular dynamics simulations on an ammonia-solvated cellulose Iβ fibril. Both, non-equilibrium and equilibrium molecular dynamics simulations have been used to investigate (i) the time evolution of the relevant fibril's structural and solvation properties (hydroxymethyl group rotational state, cellulose and cellulose-ammonia hydrogen bond networks, crystal unit parameters), (ii) the dynamics of the ammonia molecules percolating through the cellulose fibril and (iii) the equilibrium dynamics of the ammonia molecules within the cellulose fibril. Our simulations reveal the molecular details of the ammonia percolation process responsible for significant changes in both the structural and hydration properties of the cellulose fibril.