Molecular Dynamics Simulations of Biomimetic Carbohydrate Materials
Sammanfattning: The present thesis honors contemporary molecular dynamics simulation methodologies which provide powerful means to predict data, interpret observations and widen our understanding of the dynamics, structures and interactions of carbohydrate systems. With this as starting point my thesis work embarked on several cutting edge problems summarized as follows. In my first work the thermal response in crystal cellulose Iβ was studied with special emphasis on the temperature dependence of the crystal unit cell parameters and the organization of the hydrogen bonding network. The favorable comparison with available experimental data, like the phase transition temperature, the X-ray diffraction crystal structures of cellulose Iβ at room and high temperatures, and temperature dependent IR spectra supported our conclusions on the good performance of the GLYCAM06 force field for the description of cellulose crystals, and that a cautious parameterization of the non-bonded interaction terms in a force field is critical for the correct prediction of the thermal response in cellulose crystals. The adsorption properties of xyloglucans on the cellulose Iβ surface were investigated in my second paper. In our simulations, the interaction energies between xyloglucan and cellulose in water were found to be considerably lower than those in vacuo. The van der Waals interactions played a prevailing role over the electrostatic interactions in the adsorption. Though the variation in one side chain did not have much influence on the interaction energy and the binding affinity, it did affect the structural properties of the adsorbed xyloglucans. The interaction of the tetradecasaccharide XXXGXXXG in complex with the hybrid aspen xyloglucan endo-transglycosylase PttXET16-34 was studied in the third paper. The effect of the charge state of the “nucleophile helper” residue Asp87 on the PttXET16-34 active site structure was emphasized. The results indicate that the catalysis is optimal when the catalytic nucleophile is deprotonated, while the “helper” residue and general acid/base residue are both protonated. In my forth paper, the working mechanism for a redox-responsive bistable rotaxane based on an α-cyclodextrin ring was investigated. The umbrella sampling technique was employed to calculate the free energy profiles for the shuttling motion of the α-cyclodextrin ring between two recognition sites on the dumbbell of the rotaxane. The calculated free energy profiles verified the binding preferences observed experimentally. The driving force for the shuttling movement of the α-cyclodextrin ring was revealed by the analysis of the free energy components.
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