10-06: Multiresolution molecular dynamics simulations of crystallline cellulose allomorphs

One way to improve the enzyme degradation process of crystalline cellulose fibrils to glucose is to convert the naturally-occurring crystalline form of cellulose (cellulose I-β) to a different crystalline form (cellulose III-I). The conversion process from crystalline cellulose I-beta to cellulose III relies on a chemical treatment based on anhydrous liquid ammonia. Recent experiments show that the enzymatic degradation rate increases 2-5 times in cellulose III-I respect to cellulose I-β. A physical understanding of how the main structural and thermodynamic differences between these two cellulose crystalline forms affect their different enzyme activity rates could lead to the design of more efficient degradation protocols.To gain a detailed understanding of those differences we introduce a multiresolution computational approach based on molecular dynamics simulations of cellulose I-β and cellulose III-I at both the fully-atomistic and the coarse-grained levels of detail. First, we perform a set of fully-atomistic simulations to gain a detailed understanding of the main structural and hydration differences between cellulose I-β and cellulose III-I, and to relate these differences to their different enzyme degradation rates. Second, we introduce a new coarse-grained model for crystalline cellulose whose relevant degrees of freedom have been identified from the analysis of our fully-atomistic simulations. In particular, our model employs a simplified representation of the cellulose hydroxymethyl group to characterize transitions between different crystalline cellulose allomorphs. We analyze the basic physical properties of these transitions as well as the relative thermal stability of the different allomorphs using molecular dynamics simulations and free energy perturbation calculations.