A Computational Study of Cellulose Regeneration: All-Atom and Coarse-Grained Molecular Dynamics Simulations

Processing natural cellulose requires its dissolution and regeneration. It is known that the crystallinity of regenerated cellulose does not match that of native cellulose, and the physical and mechanical properties of regenerated cellulose can vary dependent on the technique applied. In this paper, we performed all-atom molecular dynamics (MD) simulations attempting to simulate the regeneration of order in cellulose.¹ Cellulose chains display an affinity to align with one another on the nanosecond scale; single chains quickly form clusters, and clusters then interact to form a larger unit, but the end results still lack that abundance of order. Where aggregation of cellulose chains occurs, there is some resemblance of the 1–10 surfaces found in Cellulose II, with certain indication of 110 surface formation. Concentration and simulation temperature show an increase of aggregation, yet it appears that time is the major factor in reclaiming the order of “crystalline” cellulose.<br/><br/>Available experimental data indicates that regeneration of cellulose is a sluggish process. While we tried to push all-atom MD simulations to their limits in terms of the system size and simulation time, our findings demonstrate that to capture the order of the regenerated crystalline cellulose, evolution of much larger systems should be traced for much longer times. For this purpose, coarse-grained (CG) MD simulations based on Martini 3 force-field were developed and applied to study cellulose regeneration at a scale comparable to the experiments.² The X-ray diffraction (XRD) curves were monitored to follow the structural changes of regenerated cellulose and trace formation of cellulose sheets and crystallites. The calculated coarse-grained morphologies of regenerated cellulose were backmapped to atomistic ones. After the backmapping we find that the regenerated coarse-grained cellulose structures calculated for both topology parameters of cellulose I_β and cellulose II/III, are transformed to cellulose II, where the calculated XRD curves exhibit the main peak at approximately 20–21 degrees, corresponding to the (110)/(020) planes of cellulose II. This result is in good quantitative agreement with the available experimental observations.<br/><br/>Our results demonstrate that coarse-grained molecular dynamics simulations represent a powerful tool to provide a microscopic insight into the process of cellulose regeneration, which is not available by conventional experimental means.<br/><br/>(1) Heasman, P.; Mehandzhiyski, A. Y.; Ghosh, S.; Zozoulenko, I. A Computational Study of Cellulose Regeneration: All-Atom Molecular Dynamics Simulations. <i>Carbohydr. Polym.</i> <b>2023</b>, <i>311</i>, 120768. https://doi.org/10.1016/j.carbpol.2023.120768.<br/>(2) Pang, J.; Mehandzhiyski, A. Y.; Zozoulenko, I. A Computational Study of Cellulose Regeneration: Coarse-Grained Molecular Dynamics Simulations. <i>Carbohydr. Polym.</i> <b>2023</b>, <i>313</i>, 120853. https://doi.org/10.1016/j.carbpol.2023.120853.