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Abstract

This thesis presents a computational analysis of cellulose hydrolysis. Density
functional theory (DFT), hybrid quantum mechanics/ molecular mechanics methods (QM/MM), and molecular dynamics (MD) were used to unravel the mechanism of this reaction and of down-stream processes like glucose-fructose isomerization. The main goal was to relate electronic and structural properties to energy barriers and to recommend improvements for the experimental work.
We first studied cellulose hydrolysis in water using a cellobiose model described by
density functional theory with implicit solvation (DFT/CPCM). The calculated
reaction mechanism involved protonation, conformational change, breaking of the glycosidic linkage, and addition of water to the anomeric carbon atom. This four-step mechanism was found to be preferred over an alternative three-step mechanism by 7 kcal mol⁻¹ due to entropic contributions. The total activation energy and the reaction free energy amounted to 31 and -3 kcal mol⁻¹,
respectively. The low basicity of the glycosidic oxygen and the exo-anomeric
effect were identified as the main impediments to hydrolysis.
Next we treated the solvent molecules explicitly (QM/MM) to obtain a more
realistic picture. We chose cellobiose and a 40-unit glucose chain as cellulose models. While we found the same mechanisms as before, the ability of the explicit solvent molecules to undergo hydrogen bonding with the solute led to differences. Protonated cellulose structures and non-chair conformers were found not to be stable intermediates in most cases. Additionally, the anomeric effect that affects the barriers was influenced by intermolecular hydrogen bonding with water molecules.
After realizing the importance of solvation, we went one step further and investigated the impact of different solvents on cellulose hydrolysis. Considering the major role of conformational changes in the computed mechanisms, we applied MD simulations to the QM/MM models. As solvents we chose water and the ionic liquid 1-ethyl-3-methylimidazolium acetate (EmimAc). According to the simulations, EmimAc is capable of breaking structural and electronic barriers to hydrolysis, because the solvent-solute interactions are stronger than in water. The cellulose chain ends are predicted to be hydrolyzed before the center of the
chain because they are better accessible by solvent.
There are many molecules other than glycosides that exhibit the anomeric effect. Spiroaminals are one example. Even though the ring opening reaction of spiroaminals is similar to cellulose hydrolysis, steric effects dominate over the anomeric effect in this case.
The product of cellulose hydrolysis is glucose, which can be converted to fructose.
We investigated the glucose-fructose isomerization with different metal catalysts
in water using DFT with implicit solvation (CPCM). The following criteria for efficient metal-based catalysts were identified: moderate Brønsted and Lewis acidity (pK_a = 4–6), coordination of glucose and either water or weaker σ donors as ligands, and energetically low-lying unoccupied orbitals.