Location: Plant Polymer ResearchTitle: Validating empirical force fields for molecular-level simulation of cellulose dissolution Author
Submitted to: Computational and Theoretical Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2012
Publication Date: 2/21/2012
Citation: Bazooyar, F., Momany, F.A., Bolton, K. 2012. Validating empirical force fields for molecular-level simulation of cellulose dissolution. Computational and Theoretical Chemistry. 984(1):119-127. Interpretive Summary: It has been shown by numerous scientists that the structures of various chemical compounds can be successfully defined using computer programs. These programs can generate a valid structure much more quickly than when using historical chemical techniques. However, some types of programs are preferred over others when modeling certain families of chemical compounds. Cellobiose, the main unit in cellulose, is a compound whose structure has been determined using classical chemical techniques. By comparing the structures generated using a variety of computation structure programs, a preferred program can be selected. Three programs were used: COMPASS, Dreiding and Universal. After performing numerous computations, the COMPASS method was found to have better agreement with experimental results. The COMPASS program can be used to model other similar systems, where the results will be useful in converting cellulose into ethanol.
Technical Abstract: The calculations presented here, which include dynamics simulations using analytical force fields and first principles studies, indicate that the COMPASS force field is preferred over the Dreiding and Universal force fields for studying dissolution of large cellulose structures. The validity of these force fields was assessed by comparing structures and energies of cellobiose, which is the repeating unit of cellulose, obtained from the force fields with those obtained from MP2 and DFT methods. In agreement with the first principles methods, COMPASS is the only force field that yields the anti form of cellobiose in the vacuum. This force field was also used to compare changes in energies when hydrating cellulose with 1-4 water molecules. Although the COMPASS force field does not yield the change from anti to syn minimum energy structure when hydrating with more than two water molecules – as predicted by DFT – it does predict that the syn conformer is preferred when simulating cellobiose in bulk liquid water and at temperatures relevant to cellulose dissolution. This indicates that the COMPASS force field yields valid structures of cellulose under these conditions. Simulations based on the COMPASS force field show that, due to entropic effects, the syn form of cellobiose is energetically preferred at elevated temperature, both in vacuum and in bulk water. This is also in agreement with DFT calculations.