Molecular dynamics (MD) simulations have been applied to study the interactions between different carbohydrates and graphene. In cellulose-graphene complexes, the behaviour of hydrophobic and hydrophilic faces of cellulose chains on a single layer of graphene in aqueous solvent have been investigated. The hydrophobic cellulose face forms a stable complex with graphene and the interface remains solvent-excluded over the course of the simulation. Cellulose chains contacting graphene preserved their intra- and inter-chain hydrogen bonds and maintaining a tg orientation of its hydroxymethyl groups that is similar to that found for the sugar in a vacuum environment. The solvent-exposed cellulose chains of the complex showed more flexibility. By contrast, over the course of the 300 ns MD simulation, the hydrophilic face of cellulose exhibits progressive rearrangement as it seeks to present its hydrophobic face, with disrupted intra- and inter-chain hydrogen bonding; sequential residue twisting to form CH-pie interactions with graphene; and permeation then expulsion of interstitial water. This transition is also accompanied by a more favourable cellulose-graphene adhesion energy as predicted at the PM6-DH2 level of theory. The stability of the cellulose-graphene hydrophobic interface in water reflects the amphiphilicity of cellulose and provides insight into favoured interactions within graphene-cellulose nanocomposites. Furthermore, water is observed to permeate cellulose during rearrangement of the hydrophilic face which may have application in addressing cellulose recalcitrance. In addition, the interaction of six different types of monosaccharide (β/alpha-D-Glc, β/alpha-D-Gal and β/alpha-D-Man) on the surface of graphene has been studied, using PM6-DH2 and PMF calculations in both gas phase and explicit water. The parameters studied included anomer, epimer, saccharide face, hydroxymethyl orientation and solvation. Binding of graphene to monosaccharide is more preferred in vacuum than in water; solvation of the complexes leads to reduction in the number of pie-interactions formed with graphene. In almost all studied complexes, β-anomers bind stronger to graphene compared to alpha-anomers in gas phase and water. Each monosaccharide has two unique faces parallel to the plane of the pyranose ring and these surfaces determine the interaction formed with graphene and water. Binding of graphene with different faces significantly influences the value of the computed interaction and binding free energy. We also find that the interactions between graphene and saccharide are mainly controlled by the number of CH-pie and OH-pie interactions formed between saccharides and graphene. The interaction energy and binding energy values suggest that the a-face of β-D-Glc is the most preferred to bind on graphene in vacuum while the b-face of β-D-Glc is preferred in the aqueous phase.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:713623 |
| Date | January 2017 |
| Creators | Alqus, Rasha |
| Contributors | Bryce, Richard |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/characterisation-of-carbohydrategraphene-interactions-using-molecular-simulation(14685a9e-4497-4aa4-9379-209a9b008cc6).html |
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