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Modeling of cellulose crystals

Modeling of cellulose crystals. Alfred D. French and Glenn P. Johnson Southern Regional Research Center New Orleans, Louisiana Robert J. Woods Complex Carbohydrate Research Center Athens, Georgia Karim Mazeau Center for Research on Plant Macromolecules Grenoble, France.

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Modeling of cellulose crystals

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  1. Modeling of cellulose crystals Alfred D. French and Glenn P. Johnson Southern Regional Research Center New Orleans, Louisiana Robert J. Woods Complex Carbohydrate Research Center Athens, Georgia Karim Mazeau Center for Research on Plant Macromolecules Grenoble, France

  2. Goals of presentation • Clarify that results of Matthews et al. cannot apply to cellulose I • Show some our molecular dynamics simulations of cellulose • Discuss the issue of two-fold screw symmetry in cellulose

  3. Carbohydrate Research 341 (2006) 138–152Computer simulation studies of microcrystalline cellulose IbJames F. Matthews, Cathy E. Skopec, Philip E. Mason, Pierfrancesco Zuccato, Robert W. Torget, Junji Sugiyama, Michael E. Himmel,* and John W. Brady*

  4. Carbohydrate Research 341 (2006) 138–152

  5. Carbohydrate Research 341 (2006) 138–152 “It is always the case in simulations of this type that differences between the model and the actual behavior will arise as the result of deficiencies in the energy functions and other approximations employed in the simulations. However, it must be remembered that fiber-diffraction structures may also contain errors, as has often been the case in previous attempts to characterize the structure of cellulose. The structures found here may be a good model for cellulose IVI, but the disordered nature of this allomorph make the experimental data difficult to interpret.”

  6. -110 110 200 Cotton fiber bundle from CAMD synchrotron by Zakhia Ford.

  7. Our approach • MD studies carried out with AMBER 8 and GLYCAM-04 force field • Goal to find minimum size adequate system for longest time simulations (10 ns) • Learn how vacuum simulations differ from solvated MD simulations

  8. b g a Our 19-chain model for MD studiesa-b projection corner chain central chain Design maximizes the number of neighbors for each chain

  9. c a Our 19-chain model, a-c projection

  10. Molecular dynamics trajectories for different size models

  11. After 10 ns of simulation, a-b projection

  12. Central 7 chains of 19-chain model, after 10 ns

  13. Experimental and simulated diffraction patterns Modeled from Crystal Structure Experimental (Z. Ford, CAMD Synchrotron) Modeled by Our Molecular Dynamics

  14. Final frame of MD results (X-ray results)

  15. O6 trajectories, 8 residues of center chain

  16. O6 trajectories, 8 residues of corner chain

  17. 2-fold helical screw axis symmetry

  18. Minimum energy cellobiose structure at B3LYP/6-31+G* level A local minimum in the region of crystal structures. The QM global minimum is a completely different structure.

  19. Conclusions • Results depend on modeling software • Cellulose I x-ray diffraction pattern is not compatible with proposed unit cell of Matthews et al. • Unit cell from our MD studies was more similar but intensities were not correct • O6 orientation was mostly tg with AMBER/GLYCAM-04 (Matthews got tg/gg) • Both AMBER/GLYCAM-04 and CSFF gave molecular and crystal twists but QM studies of isolated cellobiose did not give molecular twist • Results are similar to Yui et al, who used AMBER/GLYCAM-04, but with water surrounding the crystallite

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