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Ab Initio Modeling of Glycosyl Torsions and Anomeric Effects in a Model Carbohydrate: 2-Ethoxy Tetrahydropyran

H. Lee Woodcock ^*, Damian Moran , Richard W. Pastor ^*, Alexander D. MacKerell, Jr.  and Bernard R. Brooks ^*

^* Laboratory of Computational Biology, National Heart Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland; Department of Chemistry and Biomolecular Sciences, Macquarie University, NSW 2109, Australia; and Department of Pharmaceutical Sciences, School of Pharmacy, University of Maryland, Baltimore, Maryland

Correspondence: Address reprint requests to H. L. Woodcock, E-mail: hlwood{at}nih.gov.

A range of ab initio calculations were carried out on the axial and equatorial anomers of the model carbohydrate 2-ethoxy tetrahydropyran to evaluate the level of theory required to accurately evaluate the glycosyl dihedral angle and the anomeric ratio. Vacuum CCSD(T)/CBS extrapolations at the global minimum yield E = E_equatorial – E_axial = 1.42 kcal/mol. When corrected for solvent (by the IEFPCM model), zero-point vibrations and entropy, G₂₉₈ = 0.49 kcal/mol, in excellent agreement with the experimental value of 0.47 ± 0.3 kcal/mol. A new additivity scheme, the layered composite method (LCM), yields E to within 0.1 kcal/mol of the CCSD(T)/CBS result at a fraction of the computer requirements. Anomeric ratios and one-dimensional torsional surfaces generated by LCM and the even more efficient MP2/cc-pVTZ level of theory are in excellent agreement, indicating that the latter is suitable for force-field parameterization of carbohydrates. Hartree-Fock and density functional theory differ from CCSD(T)/CBS for E by 1 kcal/mol; they show similar deviations in torsional surfaces evaluated from LCM. A comparison of vacuum and solvent-corrected one- and two-dimensional torsional surfaces indicates the equatorial form of 2-ethoxy tetrahydropyran is more sensitive to solvent than the axial.

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