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. 2007 Jul 1;93(1):1-10.
doi: 10.1529/biophysj.106.099986.

Ab initio modeling of glycosyl torsions and anomeric effects in a model carbohydrate: 2-ethoxy tetrahydropyran

H Lee Woodcock  1 , Damian Moran, Richard W Pastor, Alexander D MacKerell Jr, Bernard R Brooks
Affiliations

Ab initio modeling of glycosyl torsions and anomeric effects in a model carbohydrate: 2-ethoxy tetrahydropyran

H Lee Woodcock et al. Biophys J. .

Abstract

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 DeltaE = E(equatorial) - E(axial) = 1.42 kcal/mol. When corrected for solvent (by the IEFPCM model), zero-point vibrations and entropy, DeltaG(298) = 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 DeltaE 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 DeltaE by approximately 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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Figures

FIGURE 1
FIGURE 1
Schematic of axial (left) and equatorial (right) 2-ethoxy tetrahydropyran, including labels showing the glycosyl (φ) and exoglycosyl (ψ) torsion angles.
FIGURE 2
FIGURE 2
Relaxed φ,ψ surfaces for the axial (left) and equatorial (right) 2-Eth-THP glycosyl linkage computed at the MP2/6-31G(d) level of theory, with vacuum (top) and solvent (IPCM) (bottom).
FIGURE 3
FIGURE 3
1-D Axial (top) and equatorial (bottom) φ-surfaces.
FIGURE 4
FIGURE 4
1-D Axial (top) and equatorial (bottom) ψ-surfaces.
See this image and copyright information in PMC

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