2-33: Structural and thermodynamic properties of a cellulose I bundle in explicit water

We investigate the self-assembly of a dozen cellulose chains into a bundle in explicit aqueous environment. Cellulose occurs in the woody cell wall as microfibrils of two distinct crystal phases, namely I-alpha and I-beta. We are using all-atom replica exchange molecular dynamics (REMD) simulations to understand the molecular aspects that lead to cellulose adopting these crystalline forms where cellulose chains oriented parallel to each other. REMD simulations provide a microscopic picture of aggregation propensities of these multiple cellulose chains towards a cellulose bundle and reveal the thermostability of the structural assembly, the flexibility of conformations and other thermodynamic and mechanical properties. Conformational biases upon assembly are captured and compared with those of individual soluble cellulose chains. Young's modulus of this solvated bundle is 81 GPa., a value comparable with experiments, and an order of magnitude larger than of a single solvated cellulose chain. The cellulose bundle undergoes an order-to-disorder transition and disassembles at high temperature. Interestingly, below 310 K, the configurations transforms to an ensemble of less packed assembly, featuring extended layered structures that may mimic the initial structures during the biosynthesis. This work was supported in part by a LANL-LDRD grant under the auspices of the US Department of Energy.