Gaining insight into cell wall cellulose macrofibril organisation by simulating microfibril adsorption
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One of the most important interactions within the paracrystalline matrix of the plant cell wall occurs between cellulose microfibrils to allow for the formation of larger diameter macrofibrils. Here, we have used computational techniques to investigate how different microfibril surfaces might adsorb onto one another. Molecular dynamics simulations show that limited direct adsorption occurs between non-polar surfaces and free energy of desorption calculations suggest this is due to a high energy barrier for the removal of a single layer of water between these surfaces. Further, it is predicted that when microfibril aggregation occurs, significant conformational changes take place at the surfaces of interaction involving O2 dihedral angles, exocyclic C6 conformation, and microfibril chain tilt. It is more likely that direct interactions initially take place between polar (110) surfaces, and that surface interactions occur between the same types of surface, such as 110 to 110, 1–10 to 1–10 or 200 to 100, where hydrogen bonds can be formed, to stabilise the aggregate. Additionally, we have identified that for the exocyclic group of a glucose residue to change conformation in origin layers, the O2 dihedral in residues before and adjacent to the glucose must rotate to a more cis-like conformation, compared to the trans-like conformation observed in crystalline cellulose. This change in exocyclic conformation occurs due to a slight shift in adjacent chains that preferentially stabilises the exocyclic conformation change in a specific glucose residue of each cellobiose repeat.
KeywordsMolecular dynamics Cellulose Microfibril Macrofibril Umbrella sampling Adsorption
This work was funded by a grant from the Australia Research Council to the ARC Centre of Excellence in Plant Cell Walls (MJG, MSD and AB) (CE110001007); and the Victorian Life Sciences Computation Initiative (VLSCI) grant numbers “VR0319” on its Peak Computing Facility at the University of Melbourne, an initiative of the Victorian State Government. We thank Daniel Weber (University of Melbourne) for his assistance in developing scripts to setup and analyse umbrella sampling simulations.
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