Multiresolution Molecular Dynamics Simulations of Crystalline Nanofibrils

We introduce a multiresolution computational approach for the study of crystalline nanofibrils.Our multiresolution approach integrates fully-atomistic and coarse-grained levels of detail and it's particularly suited for the study of structural transitions between crystalline allomorphs. First, fully-atomistic simulations are used to gain a detailed understanding of the main structural differences between the crystalline phases under consideration. Second, we introduce a new coarse-grained, off-lattice model for the crystalline fibrils whose relevant degrees of freedom have been identified from the analysis of our fully-atomistic simulations. Both the structural transition and the relative thermal stability of the two allomorphs are studied at the coarse-grained level by means of Replica exchange molecular dynamics. The structural transition is analyzed within the framework of the Ginzburg-Landau formalism. As an example application of our method we consider two different allomorphs of crystalline cellulose nanofibrils, namely cellulose I-beta (the naturally--occurring form of cellulose) and cellulose III(I) (obtained from cellulose I-beta via ammonia pretreatment). Recent experiments show that the enzymatic degradation rate increases 2-5 times in cellulose III(I) respect to cellulose I-beta. Understanding the factors that regulate enzyme degradation of crystalline cellulose is a major challenge in the context of biofuels production from cellulosic biomass. Our multiresolution computational approach sheds new light on how the main structural and thermodynamic differences between these two cellulose crystalline forms affect their different enzyme activity rates.